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Harmonics and VFDs
22

Harmonics and VFDs

Harmonics and VFDs

(OP)
One of the areas that I operate in, has a high density of VFDs on pumps on relatively weak supplies. The result is that the high levels of harmonics on the VFD inputs has accumulate in the supply and is causing a high THD of the supply voltage. While we do have supply regulations covering harmonics, in this instance, the harmonics are higher than they should be.

There is an option of using zig zag transformers and six phase rectifiers as a means of reducing the harmonics drawn by drives however in this case, the drives are already installed.

There is a transformer for each drive and sizes range up to 200KW.

One thought that I had, was that for future installations, and there are new installations going in all the time, that the new supply transformers be designed with a zig or a zag winding to give a phase shift, and install equal loading on the leading and lagging phase shifts. This should act like a twelve pulse input on one drive, only it will be across two drives.

Any thoughts on this??

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

2
We used to do basically the same thing with drive isolation transformers for multiple pump installations.  Order with delta-wye transformers with opposite 30 degree phase shift and or use combination of delta-delta and delta-wye.  It works well.  

Once we stopped isolation transformers on every drive we got away from this, I guess.  

RE: Harmonics and VFDs

7
If you want to fix harmonics in an existing system you can us an active harmonic filter similar to an Accusine.  http://www.squared.com/us/products/power_management.nsf/unid/4CF540FFB16F65D385256C7D006D6717/$file/activepfcpage.htm

The active filters work but designing a solution into the system from the beginning like you suggest is probably better if you can convince the client to pay for it up front.  If you need to fix it afterwards this is less expensive.

RE: Harmonics and VFDs

A question related to this thread.  Are drive manufacturers using switching patterns to minimize harmonic distortion?

RE: Harmonics and VFDs

2
Harmonic distorsion is a matter between mains and the input section of the inverter. It is usually a passive rectifier, so switching pattern does not have any influence.

But, there are inverters with an active front end (AFE). They use power factor correcting techniques (PFC, not power factor compensation) to draw a perfect sine shaped current. There, of course, the switching pattern of the input system is optimized for lowest possible harmonics.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

2
Mark
Your proposal is an economic method of harmonic mitigation but being able to guarantee equal loading with pumps is not so easy, and if the loading is off on one leg, then problems within the transformer windings will ensue.
Active filters should not always be seen as a problem solver after the event. If you know the problem harmonics before installing the drives, then designing an active filter solution isn't always as expensive as you think as you simply design the AF for the harmonic load you are looking at unlike an AFE that has to be designed for the FLC of the drive connected.
Hey guess what, we've just launched a brand new range of AF....
I'll be in NZ late Feb if you want to catch up Mark!

RE: Harmonics and VFDs

3
(OP)
The current situation is a rural area with 22KV overhead lines. Each pump has it's own transformer 22KV/400V. There are around 2000 pumps in this area of which probably half are controlled by VFDs. The harmonic distortion on the supply is greater than 7% and there are more pumps being installed all the time. The pumps all run almost continuously from October through March so balancing would not be an issue.
All pumps are three phase.
Active filters are a problem because we would need to add one to each installation, or use a large transformer to step down to 400V and install one large unit to correct the main supply.
My thought was to install all new pumps with a lead or a lag phase shift to help to a) cancel the effects of harmonics on new installations and to reduce the effects of existi9ng installations.
The existing transformers used are all delta to star. An advantage of this configuration is that it cancels the triplen harmonics. We could use star to star transformers on new installations, giving the effect of a 12 pulse input, but we would get much higher 3rd harmonic on the 22KV lines from this configuration.
We could also use active front ends for all new drives, but this would have no effect on the existing harmonics. and would drive up the price.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

Is that THDV or THDI? If volts, then it is on the high side, but still not too bad. If it is amperes, then I would just laugh at it.

But, knowing you, I suspect it is THDV. The question is then if it is on the 400 V side or if it is also on the 22 kV side. If on the 22 kV side, then one should definitely not laugh at it.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

(OP)
Hi Gunnar

It is THDV and on the 22KV side. There are some concerned faces.
Because the load is increasing, it is preferable that we specify the additional load to reduce rather than increase the THDV.

Have a good day,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

wow, 7% THVD on the primary, that is a problem. What is it like on the secondary? I'm surprised the network operator allows you to connect at all. If this was the UK using G5/4, they would enforce that certainly for the future but also retrospectively for all the existing issues.
I'll send you some info offline that might be of use.

RE: Harmonics and VFDs

Just one more question. You did measure via VT? And it was connected line-line?

I once had a panic job where someone had measured THD using a VT connected from line to ground. That produces lots of "imaginary harmonics" on a non-grounded 22 kV line.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

4
OK, since this is a pump application, I have to throw in my two cents worth.  You say that of the 2,000 pumps, about ½ are running on VFD controls.  I can assure you that the other ½ of the pumps that are not running on VFD controls, are not causing harmonic problems.  And since the pumps run continuously, there is no problem with high currents from multiple starts.  Therefore, it stands to reason that if you do not use Drives to control the pumps, you also will not have the problems with harmonics.

I have seen this many times.  The electronics or drive experts are left to try and solve problems that should not have been present to start with.  By knowing how to size and spec the right  pumps, and by using mechanical means of varying the flow if necessary, there are NO energy savings by using VFDs.   Since there are no energy savings on most pump applications by using a VFD, the addition of a VFD just complicates the issue by adding problems like harmonics, bearing currents, resonance frequencies, and voltage spikes to the motor.  VFD controls also make the supply of water less dependable than if the pumps were controlled by simple mechanical means.

The more VFD’s added to a system, the worse the problems with harmonics.  There are many other good uses for Drives such as machine tools, conveyor belt systems, and positive displacement pumps.  If you do not use Drives on centrifugal pumps, that could be just as easily and as efficiently controlled by ATL or Soft Start only, then in this case you would have 2,000 less VFD’s adding increasing amounts of harmonics to the system, and 2,000 less problems.  It would stand to reason that if you do not cause a problem by using a VFD, then you do not have to find a complicated and expensive way to solve the problems later.

I do not understand why there are so many intelligent people, like the ones on this thread, who understand all the complications of electronics in use today, and so few people who understand pumps well enough to know that VFD’s do not save energy, and are usually more trouble than they are worth.

I know there are a lot of you who would like a VFD on every machine for your own job security.  However, the first indication of real knowledge about Drives, should be when and when they are not the best control for the particular application.

Many end users are having the carrot dangled in front of them claiming that the VFD can save 20% to 50% in energy.  Many articles and papers are being written on how this hospital or that high rise building, has saved tremendous energy by switching pumps to VFD controls.  If you read these articles carefully, you will see that the energy savings actually came from downsizing the pumps, eliminating dump valves, and cutting waste, rather than the implementation of a Drive.  Anytime you vary the speed of a pump, you are using more energy per gallon than if the pump were running at it’s designed BEP.  Knowing this, how can anybody say varying the speed of a centrifugal pump with a VFD can save energy?

The use of filters, extra grounding, zig zag transformers, six phase rectifiers, and countless other band aids, would not be needed if you did not install 2,000 harmonic generators (or VFD’s) to a system that could have more easily and effectively be controlled by other means.

RE: Harmonics and VFDs

(OP)
Hello Valvecrazy

Quote:

By knowing how to size and spec the right  pumps, and by using mechanical means of varying the flow if necessary, there are NO energy savings by using VFDs.
In a constant flow situation, I would agree with you, but in a variable flow situation, I can not agree with you and there is sufficient evidence to show that using variable speed does save considerable energy on pumps operating under variable flow situations. For example, a farmer has three irrigators of different sizes fed from one pump. The flow requirement varies depending on which irrigators he has on. One solution is to use correctly sized pump per irrigator, but that means three wells, three pumps etc and the single solution with a VFD is more attractive.
If you reduce flow by throttling the pump, using a valve, then the pump operates at a much lower efficiency and thus wastes energy, so using valves or similar to reduce the flow is not efficient.
If you have a better option, I would be very interested to hear it.

Gunnar, the harmonic voltages have been measured on the output of a three phase transformer.

Ozmosis, the THDV level kind of crept up on the network operator, they knew it was not good, but did not realize how bad it had got.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

As someone said in an earlier thread, the well depth plus the pressure required by the sprinkler sets the Minimum Possible Head or TDH for the pump.  This also limits the minimum speed that the pump can spin that will still produce the head required.  In the case of the 100 HP and 1200 GPM pump in the attached curve, minium speed is only 3290 RPM.  The power required still falls off further as the flow required decreases, the same as it does when the pump is running at a constant of 3550 RPM.  Notice at 100 GPM flow the variable speed curve shows this pump dropped from 100 HP to 38 HP.  However, the power required by the full speed pump dropped from a100 HP to 42 HP simply by restricting the flow to 100 GPM.  This shows only 4% difference at low flow between VFD and NO VFD.  Add back in the parasitic losses of the Drive and reduced efficiency of the motor compared to running the motor on Across The Line, and there is no difference in power consumption of VFD or NO VFD.

Moving the sweet spot of the curve and maintaining maximum efficiency is just VFD propaganda, when the electric meter is still spinning at the same rate regardless.  It is a common misconception that a VFD can slow a properly sized pump down enough to save energy.

Using your example of the variable flow situation, 1200 GPM, 800 GPM, or 400 GPM, the two curves follow each other so closely that all things considered, restricting with a valve reduces energy consumption as much as varying the speed.

The reason the problem crept up on the operator is because the number of VFD's continues to increase.  Changing to VFD's on pump applications does not decrease energy consumption, it  increases the THDV level and causes other problems.

RE: Harmonics and VFDs

This is very interesting!

I do not have the time to do my own comparison, but you seem to have a valid case here. Anyone done the same comparison with same or different results?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

(OP)
Hello Valvecrazy

That curve is what we refer to as a flat curve pump and they are difficult to get good results with VFDs because the curve of flow against pressure is essentially flat across quite a range of flow, but if we look at that pump and consider the situations that you nominated with a flow of 1200GPM and 100GPM. If we assume that the pressure is relatively constant, then there is a lot of energy that is being wasted in the pump when it is throttled back to 100GPM.
Hydraulic power is the product of flow, pressure and a constant K.
Taking the case of 1200GPM and 230ft, we have a hydraulic power of 1200 x 230 x K HP = 276000 x K HP.
If we assume that the pump efficiency is 75% at this point on the curve, then the value for K will be about 0.00028

Now look at the case of 100GPM. The head is about 270, so the hydraulic power is 100 x 270 x 0.00028 = 7.56HP
The absorbed power at this flow is around 40HP, so the losses in the pump are 32.5HP (24KW) The efficiency of the pump is 18.9%

If you were able to slow the pump down and stil have sufficient head, then the efficiency of the pump would rise towards 75% and you would save nearly 30HP (22.3KW)

In the case of the pump curves that you have suggested, then it will be difficult to operate the the pump at a head of around 240 ft and achieve an energy saving with a VFD. If however, we used this same pump at a head of say 120 ft, then we could us a VFD to achieve an energy saving at 100GPM.

Most pumps used in this region for irrigation are submersible and have a very definite slope on the flow/head curve and these definitely allow the VFD to slow the pump down to achieve appreciable energy savings.

So, yes, I agree, with the pump used in your example, a VFD would not achieve energy savings under the nominated conditions, but there is still considerable loss when the flow is reduced.
A different pump with the same operating conditions would enable the pump to achieve the required head at reduced flow and speed, and the operating efficiency of the pump is much enhanced.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

(OP)
This curve would be more typical of the type of curves that are commonly found on submersible pumps.
http://www.lmphotonics.com/images/pump1.htm
At low flow, the power lost in the pump when it is throttled, is higher than a flat curve pump because the head is much higher, but it is able to be controlled with a VFD to gain a significant improvement in efficiency.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

2
Mark;
I ran across a line of transformers about a year ago that may be of interest.
The transformers were called "Mitigators".
They were three phase dry types in the range of 50KVA to 100KVA.
There were two types installed.
The first type had three windings on each leg of the secondary. The first winding was about 80% and the second and third winding were each about 20%.
The connections were: A Phase, the 80% winding plus a 20% winding from B phase and a 20% winding from C phase.
The configuration was repeated for the other phases.
The second type was similar except that the main windings were about 60% and the second and third windings were about 40%.
At the time I was not able to get any information from the contractor doing the installation. I spent some time Googleing with no joy.
I have again spent over an hour googleing with no positive results. Possibly this technique may be of use to you and possibly someone will recognize the transformers and be able to give us more information.
On pumps, I am on the fence here. I suspect that the answer may be, "It depends!"
I haven't had to check pump curves for a few years but as I remember the pumps that I was concerned with, the motor loading and the flow dropped off as the head increased. However I would not assume that this applied to all pumps.
I believe that there are instances where a VFD is beneficial and instances where the advantage of a VFD is marginal.
It depends!
Respectfully

RE: Harmonics and VFDs

Very informative discussion. LPS for all.

* I would go green if only I were not yellow *

RE: Harmonics and VFDs

I have a question. How much effect will using separate transformers to create the 12-pulse have? Any time I have seen that done a single 3-winding transformer was used and the harmonics canceled in the transformer. Will the harmonics cancel in the high voltage line between a Y-Y and a Y-Delta transformer?

There's a company in Canada called Mirus that makes a line filter called a Lineator. It is a 3-lead in/3-lead out filter that installs like a line reactor. That thing works amazingly well for reducing the harmonics of a drive. Unfortunately, Mirus knows they do and asks a premium for them. Regardless, these would be a very reasonable solution to use due to the ease of installation and the fact that they would cancel the harmonics at the source.

RE: Harmonics and VFDs

Interesting point LionelHutz.

I wonder if a really aggressive negotiation with them for several hundred might not get the price down substantially.  They could also then prep them so they are a pull-out-of-the-box-screw-in-place-wire-in-series-close-lid kind of convenience.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Harmonics and VFDs

The individual transformers are effective at reducing harmonics.  The down side is that both motor have to be running at similar loads to get the biggest benefit.  It won't help at all if only one motor is running, so it certainly is not as widely applicable as 12-pulse drives.  

If individual isolation transformers are being used anyway, there isn't much downside to buying transformers with different phase shifts.

But I do agree that a good filter can also be effective.  I'm familiar with Mirus.  Their broadband filter seems to have some good features and they are fairly popular in some industries.  

RE: Harmonics and VFDs

Of course a VFD works better with a pump that has a steep curve.  This usually means that you have to oversize the pump to be able to (save energy?) by slowing it down with a drive.  With the pump you picked the VFD will vary the output from 100 HP at 1200 GPM to 38 HP at 100 GPM.  If you add back in the parasitic losses and the loss of motor efficiency from running on a VFD, the load on this pump will actually be about 103 HP at max flow and 40 HP at low flow.

Attached are two more curves for Submersible pumps.  

With the curves I picked, the SUBMERSIBLE pump only uses about 85 HP to pump 1200 GPM.  Restricting the flow with a valve on these pumps will still reduce the required horse power from 85 HP to about 45 HP.

The pump you picked for the VFD will use 18 HP more at max flow, than the pump I picked to work with a valve.

The pump you picked for the VFD will only save about 5 HP at minimum flow over the pump I picked to work with a valve.

Therefore, if the pump is used the same amount of time at high flow as it is at low flow, the VFD will cause the pump to use an average of 10 HP more power.  

The way so called efficiency of the pump is figured for the VFD, is deceiving and makes a VFD look better than it really is.  The curve being flat or steep has much less to do with anything than how fast the electric meter is actually spinning.

Submersibles are about 80% of my business, and if the head requirement is 230', it will always be 230'.  The well is not going to change from 230' deep to 120' deep just to help a VFD be more efficient.  When the head is constant, as it is with nearly all pumping applications, it doesn't matter if it is submersible, centrifugal, or turbine, I can nearly always pick a pump that will save just as much energy by using a valve, as when using a VFD.

Look at it this way.  When the pump has been up-sized to work with a VFD, high flow of 1200 GPM at 100 HP is using .0833 HP per gallon. This is not even counting parasitic and other losses from the VFD.  When slowed with a VFD to 100 GPM and using 40 HP, that is .4 HP per gallon produced.  That means when the VFD is being used for low flow, it is actually burning .3167 extra horse power per gallon produced.   

When the pump has been picked properly, it only uses 85 HP to pump 1200 GPM, which is only .0708 HP per gallon produced.  That means that the VFD is burning .0125 HP at high flow when compared to a properly sized pump running Across The Line and using a valve for control.

The valve I am referring to only has 7 PSI friction loss at 1200 GPM, which is the only time a valve is actually burning energy.  When a valve is used to further reduce the flow, the excess head produced is actually a free by-product, as throttling causes a reduction in power.

In that respect, a VFD always burns just as much energy, and in many cases more than a valve.

Anytime you can do the job without using a VFD, there are a multitude of problems, which harmonics are just one of, that are never present to start with.

RE: Harmonics and VFDs

valvecrazy

You should put a post in either the pump engineering (under Mechanical) or energy engineering general (under Energy) if you are this passionate about the subject.  I see how you came up with your handle!

I believe that you have some valid points which you have carried to an extreme.  I would be happy to point out what I believe are the errors in your logic in another forum.

RE: Harmonics and VFDs

(OP)
Hello valvecrazy

Quote:

With the pump you picked the VFD will vary the output from 100 HP at 1200 GPM to 38 HP at 100 GPM.
I am curious as to how you got this figure. Can you please enlighten me.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

Thanks gepman!  I studied electronics, including VFD's in school.  I sold and serviced VFD equipment until about 1993, and became increasingly frustrated with all the problems with VFD controls.  I was using valves as a bypass for the VFD's, and observed that the power consumption was basically the same when the valves where being used, as when the Drives were actually working.  I then started pulling out any VFD I had installed and replaced them with across the line panels and control valves.  With valves, my installations became more reliable, less expensive, just as efficient, and nearly all my problems went away.  Since then I have installed many thousands of valve controlled systems. I would be more than happy for you to point out any errors in my logic.  I have been accused of many things for my stance on this subject.  I believe the only thing I am guilty of, is knowing how to actually read and understand a pump curve.  We may should start another thread however, the question was how to solve harmonic problems with VFD's.  I believe I have the best solution.

Marke, I used the 100 HP and 1200 GPM right off your pump curve.  For the 38 HP at 100 GPM I used the affinity law.  For your pump to be producing anything at 230' of head, it has to still be spinning at 72% of full speed.  Because head drops off by the square of the speed, you will only have 50% of the shut off head at 72% speed, which gives me 230'.  Because HP drops off by the cube of the speed, the pump is still pulling 38 HP at the minimum possible speed of 72%.

RE: Harmonics and VFDs

Marke, I presently consult on energy efficiency to the two largest electric utilities in California and haven't heard of this being a problem (although I am just in the process of obtaining a power quality analyzer) but I doubt that even in our rural areas we are quite as rural as in NZ.  The utilities here believe in VFD's and give (cash) incentives to put them in, of course the application has to be correct.  Please post any solution which works because I am sure that with the increasing use of VFD's in California's rural areas that this could become a problem.

valvecrazy

Cut and paste your posts into a new thread in one of the two suggested forums that I gave and I will respond point by point.

RE: Harmonics and VFDs

(OP)
Hello valvecrazy

If you insert a valve in the line to reduce the flow, what happens to the pressure on the output side of the valve?
i.e. the head across the load.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

"The valve I am referring to only has 7 PSI friction loss at 1200 GPM, which is the only time a valve is actually burning energy. When a valve is used to further reduce the flow, the excess head produced is actually a free by-product, as throttling causes a reduction in power."

In other words, there is 7 PSI of friction loss across the valve at maximum flow rate.  The output of the valve stays at a "constant pressure", say 50 PSI regardless of the flow rate.  The pump must produce max flow rate at 57 PSI to accomplish this.  This is as long as the valve stays loaded and ready.  I have several ways of dumping the water from the control chamber of the valve, which reduces the friction loss across the valve to about 1 PSI.  This is rarely needed as the 7 PSI is usually not a problem.  I always get quite a bit more pressure and flow from the pump when it is running ATL than when it is controlled by a drive.  I find that the loss of motor efficiency and the parasitic losses of the Drive itself, cause more of a loss on pump performance than the 7 PSI friction loss across the valve.

The whole idea of choking a pump back to conserve energy is "counter intuitive".   Our minds have no trouble understanding that slowing a pump down reduces energy consumption.  The fact that power is reduced almost exactly the same by simply restricting flow with a valve, is very difficult to grasp.  Most people make the false assumption that throttling waste or burns energy.  I have several working demonstrators that can easily be switched from VFD to ATL and valve control.  Every pump engineer that has seen this demonstration, is bewildered and you can actually see the bulb light up over their head, when they see for themselves that throttling with a valve reduces energy consumption just as much as a VFD.

I am also working with one of the largest utilities in California.  At least one of the main guys there told me that he understands that throttling can reduce energy consumption of a pump as much as a VFD.  As he said, the soft start may be the only advantage of a VFD, which could easily be accomplished with simple Auto Transformer and other means of soft starting.  He has requested, and we are working on getting a university to do a test.  Although, I have done this test myself many thousands of times, and it can easily be seen on a pump curve as well.  He said a test by a university would help people to understand that the "cash incentives" should also be given for choosing the right pump and using a valve as control.

Over the years I have had many people try to go point by point and show me the error of my ways.  I show them how to read a pump curve, and what I look for to choose the right pump.  Then after a few minutes switching between a VFD and ATL with a valve on a working pump system, their eyes get big as saucers and the words "counter intuitive" become a regular part of their vocabulary.  It really does work.  It is just very difficult for people to understand when they already have incorrect preconceived notions about VFD's, and how valves "supposedly" burn energy.

I believe this is the correct thread and forum to discuss this.  Harmonics are just one of many negative side effects of VFD control.  I believe the best way to solve these problems is to not cause them in the first place.  When a system can be controlled just as effectively and as efficiently without using a VFD, then all the problems associated with a VFD are never present to start with.  Spend a little more time choosing the right pump, and a VFD cannot save any additional energy over simple valve controls.

Even on systems with existing and incorrectly sized pumps, a valve can simplify the controls and eliminate all the problems associated with VFD's.  My customers in this position tell me that a little more energy use is a small price to pay, for the simplicity, dependability, and longevity of a pump system controlled by a valve rather than a VFD.

RE: Harmonics and VFDs

(OP)
Hello valvecrazy

I am not sure that I am following your description.

Quote:

the excess head produced is actually a free by-product
There is no free energy, the valve must have an increased pressure across it when it is restricting flow. It is essentially a hydraulic resistor and if you are operating into the same outlet such as an irrigator, it is going to restrict the flow by increasing the pressure across the valve and reducing the pressure at the irrigator. The pressure at the output of the pump will also increase, but this is dependent on the curve of the pump. If there is pressure drop across the valve, and flow through it, there must be a power loss in it.I do not understand where this free byproduct comes from.

I certainly agree that the addition of a VFD to a system does introduce extra losses, typically in the order of 3% and anyone who uses a VFD on a fixed speed application is not doing their customers any favours, choosing the correct impeller for a fixed flow application and running at a fixed speed is sound engineering practice.

In a variable flow situation, it is difficult to optimize an impeller design/selection for all flow conditions. The use of a VFD effectively gives a similar result to an optimized impeller at each flow setting.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

valvecrazy

Since you did not post a separate thread I thought that I would respond here.
 
#1

Quote:

By knowing how to size and spec the right  pumps, and by using mechanical means of varying the flow if necessary, there are NO energy savings by using VFDs.

This sentence does not quite make sense but I think that I know what you mean.  For constant flow applications I don't think that anyone would argue with you that sizing the pump correctly and choosing a pump with a high efficiency at the expected pumping point is the best option.  VFD's are used where variable loads or variable flows are expected.  You state that one should use mechanical means for varying the flow if necessary.  I assume that you would mean a valve in a liquid fluid system and a discharge damper in a fan system.  Across every valve there is a flow (Q) and a pressure drop (H).  The energy required to pump this flow across the valve is (Q*H)/(3960*%eff) where Q is flow in gpm, H is pressure drop across the valve, and %eff is the percent efficiency of the pump and motor combination.  Therefore whenever you put a mechanical means to vary the flow it increases the amount of energy required to pump the fluid.  It is true that if you increase the pressure drop across the valve the flow will decrease however the valve is still absorbing energy that does not need to be absorbed if you varied the flow with another method.  
 
#2

Quote:

Anytime you vary the speed of a pump, you are using more energy per gallon than if the pump were running at its designed BEP.   Knowing this, how can anybody say varying the speed of a centrifugal pump with a VFD can save energy?

The first sentence is just not true AND it neglects the fact that when you vary the flow of a pump by "mechanical means", i.e. a valve, you are not running at the BEP and the energy per gallon will very likely be less with the pump that has had its speed varied than the pump that has its flow varied by a valve.
 
#3

Quote:

Moving the sweet spot of the curve and maintaining maximum efficiency is just VFD propaganda, when the electric meter is still spinning at the same rate regardless.  It is a common misconception that a VFD can slow a properly sized pump down enough to save energy.

The first part of the sentence may be true but you are not moving the "sweet spot" (BEP) with a VFD you are moving the curve.  The "sweet spot" as you call it stays in the same relative position on the pump curve.  When you vary the flow with a valve the "sweet spot" stays in the same position on the pump curve but the system curve moves out of the sweet spot.  Therefore with a VFD you at least have a chance to remain at or near the BEP while with a valve you are almost guaranteed to move out of it (unless you were to the right of the sweet spot to begin with and then you wouldn't have sized you pump correctly as you advocate).
 
#4

Quote:

Using your example of the variable flow situation, 1200 GPM, 800 GPM, or 400 GPM, the two curves follow each other so closely that all things considered, restricting with a valve reduces energy consumption as much as varying the speed.

In your first example at 100 gpm you are correct that the energy use is very close between a VFD and a valve throttled system.  This was due to the low volume being pumped.  Since energy is (as stated above) proportional to (Q*H)/(3960*%eff), when Q is low the energy being used is low.  However look at your curves for 800 gpm, there is about a 15hp difference which is over a 17% savings.  
 
#5

Quote:

Of course a VFD works better with a pump that has a steep curve.  This usually means that you have to oversize the pump to be able to (save energy?) by slowing it down with a drive.

A VFD works the same no matter what the slope of the pump curve.  However the steeper pump curve will allow you to slow the pump down more in applications with high static head if that is what you mean.  The slope of the pump curve does NOT mean that you have to oversize the pump or necessarily affect the efficiency of the pump.  You choose the pump with the shape of the pump curve that you desire based upon your application.  If you had an application where the static head varied considerably you would want a steeper pump curve so you at least still pump some water when the static head increased.  
 
#6

Quote:

Submersibles are about 80% of my business, and if the head requirement is 230', it will always be 230'.  The well is not going to change from 230' deep to 120' deep just to help a VFD be more efficient.  When the head is constant, as it is with nearly all pumping applications, it doesn't matter if it is submersible, centrifugal, or turbine, I can nearly always pick a pump that will save just as much energy by using a valve, as when using a VFD.

The water level in many wells will change depending on the weather.  Some wells will easily change their standing water level by 100'.  Then there is something call "drawdown".  The well will have a replinishment rate usually measured in "ft of drop per 100 gpm".  For a value of 10 this means that the well pumping water level drops 10 feet for every 100 gpm produced.  Therefore at 1000 gpm the water level would drop 100' and at 500 gpm the water level would drop 50'.  Therefore at 500 gpm output your static head would drop 50' from the 1000 gpm output.  Also it is not true that almost all pumping applications are constant head.  Most farmers (and other systems) really only care about the flow, not the head.  The head is kept high so that the desired flow will occur in all situations.  Another example of changing head requirements is a recent VFD application that I analyzed.  The farmer had 200 acres of strawberries.  When first planted he would sprinkler irrigate the strawberries which required a high flow and high head.  Once established he would drip irrigate the strawberries which required a low flow and low head.  A perfect VFD application.  He got a $5,000 incentive from the local power utility to install the VFD based upon a 62,500 kWh savings for a year.  Goulds (the largest pump manufacturer in the world and someone that makes their money in pumps, not VFD's) has introduced some rebranded VFD's (from ABB and A-B) with special software that really makes them quite clever and energy efficient.  The link to this information is http://www.ittmc.com/files/PumpSmart_Bulletin.pdf.   I highly recommend that you read it.  You are correct in your thinking that the higher the percentage of static head/total head in your application, the less energy the VFD can save.  However any time that you can slow the speed of the pump down instead of throttling you will save energy (neglecting VFD efficiency).

A true constant head system only exists in two scenarios, both of which do not occur very often.  The first is if there is no flow.  If there is no flow there is no friction loss and therefore head is constant although there is no need for a pump.  The second is if there is no piping system.  You don’t see that very often either.  Actually if you had a pump between two tanks without piping there still is losses due to the contraction of the flow into the suction and expansion of the flow from the discharge.  If it didn’t expand or contract then you would have to have a pipe on the suction and discharge.  If you had a constant flow single circuit piping system it would be constant head.  Then of course you would NOT need a valve to control flow.  Any time you have a valve in place to control flow then it will not be a constant head system.  Whatever pressure drop occurs across the valve could just as easily been achieved by reducing the speed of the pump to reduce the flow (and the pressure).
 
#7

Quote:

Look at it this way.  When the pump has been up-sized to work with a VFD, high flow of 1200 GPM at 100 HP is using .0833 HP per gallon. This is not even counting parasitic and other losses from the VFD.  When slowed with a VFD to 100 GPM and using 40 HP, that is .4 HP per gallon produced.  That means when the VFD is being used for low flow, it is actually burning .3167 extra horse power per gallon produced.

The pump is NOT upsized to work with a VFD.  You can add the VFD to the pump that has been sized perfectly by you (or has already been installed by someone else).  If you ever want to reduce the flow, you slow the pump down, you DON'T throttle the pump.  I am attached some pump curves with system curves drawn on them.  I think that the problem that you have in conceptualizing a VFD pumping system is that you do NOT see the system curve on the pump curve.  The system curve is the head loss of the piping system as a function of flow.  The head loss is proportional to the flow squared and therefore the system curve will be rising curve from left to right.  The flow is ALWAYS were the pump curve and the system curve intersect.  Closing a valve (throttling) changes the shape of the system curve and pushes it to the left.  Slowing a pump down keeps the shape of the pump curve but reduces it size towards the origin of the pump curve (the same way that reducing the size of the impeller is shown on a pump curve).  

I have taken the pump curve that you scanned and marked it up.  I have assumed that the static head is 120’.  If you try to state that the static head is 231’ then I will say that you could not have gotten 1200 gpm since 1200 gpm would have had at least 4.4’ of head loss in 200’ of pipe (out of the well).  I will state again that the higher the percentage of static head/total head the less energy can be saved by using a VFD.  Once static head/total head is greater than 80% it is very difficult to save any energy.  Back to the example, at 100 gpm that you used previously, the pressure in the system would actually be about 121’ of head (see system curve) and therefore the VFD could be turning at about 72% speed or 2569 rpm.  Assuming the same efficiency as that pumped at 1200 gpm (it won’t be but it will be more efficient than throttling) you would have the following horsepower (100*121)/(3960*.7) = 4.5 hp.  Even if you assume the same efficiency as the throttled pump (about 13.88%) the horsepower used with the VFD is 22hp, about half that used with the valve.

If you don't understand my examples take a look at http://www.maintenanceresources.com/referencelibrary/acdrives/vfd.htm.  In this article if you increased the static head the red line would just move upward but in its same relative position.  If the pump curve was flatter you could not drop the speed of the VFD quite as much but you can still drop it more than you normally calculate because the shape of the system curve slopes downward to the left.  You need to realize that only the static head is constant, the friction loss due to flow varies with the square of the flow.
 

Quote:

The valve I am referring to only has 7 PSI friction loss at 1200 GPM, which is the only time a valve is actually burning energy.  When a valve is used to further reduce the flow, the excess head produced is actually a free by-product, as throttling causes a reduction in power.

I think that you should re-examine this statement to see if you really want to stand by it.  Let's examine it very closely.  First let us assume a static head of 120', a little more than half of the 231' that you have at 1200 gpm.  At full 1200 gpm your valve is burning (1200*7*2.31)/(3960*.7)= 7hp.  At 100 gpm your valve has a pressure drop of 270-121=149’ (remember the system curve shows a total head of 121 feet at 100 gpm).  Therefore the valve consumes (100*149)/(3960*.1388)=27.1hp.  Yes your pump uses less energy than when it was pumping at 1200 gpm but it still wastes energy across the valve.  Think of it this way, you could put a small turbine in place of the valve to produce the pressure drop and generate electricity.
 
I recommend for you the VFD savings calculator by ABB.  It takes into account the static head, the efficiency of the VFD, the slope of the pump curve, and the present or proposed method of controlling flow (throttling valve, on/off, etc.).  It then calculates the savings available for installing a VFD.  You can find this software at http://www.abb.us/product/ap/seitp322/24b03100d005c31ac1256e040043f4c1.aspx.  Play around with it and you can see how static head and pump curve shutoff head affect energy savings.  I will vouch for the accuracy of the software.  

I also see that you made a post since I started this humongous post.  If you actually work with the either utility you know that they have a fleet of over 40 pump testers who test efficiencies of pumps, that these pump testers recommend VFD’s in certain specific cases, and that what you describe does not even follow the SPC (Standard Performance Contract) energy savings software that both utilities REQUIRE to be used for energy savings calculations (you can download it at http://www.aesc-inc.com/download/spc/).  I don’t expect this post to sway you much but I would be happy to meet you at either PG&E’s Pacific Energy Center in San Francisco, SCE’s CTAC in Irwindale, or SCE’s AgTAC in Tulare if you need to see how VFD's work in pumping systems.  The latter would probably be best so we could do a pump test right on the site (they already have it set up to prove everything that you say isn’t so).
 

RE: Harmonics and VFDs

Marke
Here is something that has been investigated in California as a replacement for VFD's.  It is still a VSD (Variable Speed Drive) so it won't be of any interest to valvecrazy but since it is non-electrical it won't create any harmonics.  I don't see how it could be used on a submersible pump but it obviously will work for a horizontal pump and I think that it could be adapted for a vertical mount turbine or lineshaft pump.  

RE: Harmonics and VFDs

(OP)
Hello gepman

As I read that paper, it appears to be a form of magnetic coupling where the input torque and output torque are equal. In a sense, this is similar to using a valve in that it is a series element introducing losses.
If the output shaft was spinning at 75% speed relative to the input shaft, and there is equal input and output torque, then there will be 25% slip losses in the coupling. If I am wrong, and the output toque increases with the slip then this could be worthwhile looking at.

From all the information that I have gleaned so far, it would appear that using different supply transformers on the individual pumps such that there is a balance of transformer load on two groups of transformers, one having an additional phase shift will result in cancellation of the 5th and 7th harmonics. This is good because it means that if new installations are all done with the additional phase shift transformers, the net result will be a reduction in the existing harmonics.

Alternative considerations are fitting filters to all drives, adding active filters to the supply, converting existing drives to active front end etc. All of which are more expensive and disruptive to existing installations.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

Wow Gepman! smarty
Thanks for making the effort.
Most interesting.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Harmonics and VFDs

This is easily one of the most valuable threads in the Motor and Controls forum. Even if the true truth always lies just out of (at least my) reach, it has been a refreshing experience to see dedicated experts discuss these matters so in-depth. I am not to judge, even if I am more in favour of sound physical reasoning than gut-feeling, but I think that gepman backs up his claims more solidly than his opponent.

The magnetic coupler, that was thrown in late in this discussion, is a very good illustration that many engineers and economists tend to forget that slip always is equivalent to losses. As is breaking or any attempt to reduce a movement or a flow without reducing the input power. It is only by reducing input power, while maintaining design goals, that any energy can be won.

The main techniques for reducing input power is:
1. To select correctly sized components (Valvecrazy is absolutely correct there)
2. To select the right operating speed. Be it with the aid of gears, pulleys, using a VFD or any other LOSSLESS means.
3. To select piping and armatures with low resistance to flow.
4. To select motors with high efficiency - and supply them with the right voltage and frequency for the task at hand. Which may be variable.
5. To avoid any device that introduces losses. Like valves, dampers, magnetic couplings or introducing extra slip in motors (the WRIM does the same thing as the magnetic coupler - it increases slip and burns energy).

As a last suggestion, what about hydraulic drives? I have worked with Hägglund drives. They are sometimes superior to any other drive system - although I do not think that they fit very well in Mark's pump systems. But they certainly do not produce any harmonics or EMI of any frequency.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

That magnetic coupling is nothing more than a clever repackaging of the old Eddy Current Clutch, long ago discredited except on affinity rule loads like centrifugal pumps and centrifugal fans.

Since they do nothing to reduce motor excitation at light loads, you can reasonably expect that they will save some energy but not as much as a properly spec'ed VFD/motor system.

I've seen similar magnetic couplings marketed in HVAC for fan loads and, other than the harmonics issues, have little to make them attractive, in my opinion.

RE: Harmonics and VFDs

I have found the elusive eddy current drive thread which both Dick and I looked for a while ago. Seems the 'search' function really is a 'hide' function!

The thread in which we were looking for the missing thread: thread237-107895: Coyote VSD's

and the missing thread which disappeared for a while: thread237-98040: VSD DAMAGED - WHY?
 

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If we learn from our mistakes I'm getting a great education!

RE: Harmonics and VFDs

The systems I work with have a much flatter system curve.  The well is 200' to water.  Static pulls down very little and very quickly, so the feet of lift is constant.  I am also a long time licensed well driller. The drip system on the strawberry patch needs 10 PSI constant.  The underground lines are large enough to have only a couple of PSI of friction loss at 1200 GPM, or max flow.  The variable is that we irrigate the field in thirds.  Sometimes at 400, sometimes 800, and other times 1200 GPM.  So the head is always 230', I just have to choose a pump that has good brake horsepower characteristics for variable flows.  If I spend my time choosing the correct pump, the horse power difference is similar to the first curve I posted.  I do not argue that there are a few points along that curve where the VFD shows a little lower horse power requirements.  You say 15 HP at 800 GPM, it looks more like 12 HP to me.  Add back in the 3% losses from a drive, which I think is low, and you are saving 9 HP or 9% at that flow rate.  At 100 GPM is only 4 HP, with losses is really 1 HP in savings with a VFD.  At 1200 GPM the VFD is actually wasting at least 3% to drive losses compared to ATL controls.  Average all this up, depending on the run time at the various flow rates, and you might average 4% energy savings with VFD.  I cannot argue that.

Now will that 4% ever pay for the added expense of the Drive itself, before the electronics or drive must be replaced?  Is that 4% worth the problems of harmonics, which is actually dirty power and causes everything connected to the grid to be less efficient?  Then we could talk about bearing currents, resonance frequencies, voltage spikes and other things.  One of the biggest problems is longevity.  Motors would not have tags on them bragging about "Voltage Spike Resistant Inverter Ready Windings" if the VFD where not harder on motors than running Across The Line.  So now motors don't last as long and drives don't last as long as equipment running ATL.  How much energy does it take to mine, manufacture, transport, install, and recycle the equipment that did not last at long as it should have?

Then if energy efficiency is that important or the system curve varies as dramatically as yours, I would use a multiple pump set up.  It is very easy to stagger the pressure up or down as needed, and switch between different pumps when using valves for control.  If I needed a 100 HP maximum load, the most efficient way would be to use a 60 HP, 30 HP, and a 10 HP.  You can't get anymore efficient than a 10 HP pumping 100 GPM, or a 10 HP and a 30 HP running together to pump 400 GPM.  800 GPM would take a 60 and 30 running together, while max of 1200 GPM would require all three pumps adding up to 100 HP.

A Flow Based Pressure Management system would use two pumps to produce different pressures.  When the system curve varies from 120' at 400 GPM to 230' at 1200 GPM   I would have a 100 HP doing the high flow high pressure part of the curve.  Then a 20 HP could do the low pressure low flow part much more efficiently.  

I have read the PumpSmart thing and have written AB about their so called calculator.  After all they are trying to sell this product.  The main argument I have is that people rarely give the centrifugal pump credit for it's "magical", "counter intuitive" properties, that causes it to reduce it's power consumption when flow is throttled.  If you know how to take advantage of this, you can stay within a percent or two of what a VFD can do, without all the complications that a VFD adds to the scenario.  For goodness sake, if you don't think a VFD adds complications, just look at this thread!!!!!

RE: Harmonics and VFDs

(OP)
Hello valvecrazy

Yes, agreed there are situations where the correct selection of pump and impeller can give admirable results and a VFD adds nothing. There are many of these installations out there where a VFD has been used, perhaps with a less optimized pump.

There are however, situations where the applications can not be adequately catered for using a single fixed speed pump.

Quote:

A Flow Based Pressure Management system would use two pumps to produce different pressures.  When the system curve varies from 120' at 400 GPM to 230' at 1200 GPM   I would have a 100 HP doing the high flow high pressure part of the curve.  Then a 20 HP could do the low pressure low flow part much more efficiently.
A multi pump installation is one solution and a VFD with one pump is another. I believe that there is merit in both solutions with the VFD effectively scaling the size of the impeller in performance.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

I appreciate the efforts and expertise everyone in this thread had given. I ma very much interested on this thread and I keep on reading all the technical inputs several times. Pumps and undertanding pump curves is way out of my league but I do have the interest to learn how to undertand them...so at least i could get a clearer picture myself on this thread which i find very informative. Is there any link or reading that would give me some simolified way of reading the pump curves? Thanks

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

RE: Harmonics and VFDs

Thank you very much gepman!!!

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

RE: Harmonics and VFDs

I had some pump adventures years ago. Two pumps on alternating service pumping sewage uphill. Changing regulations resulted in the discharge being moved much further up the hill. This put the pumps so far up their curves that one pump could no longer keep up with the flow. Tried both pumps in parallel ad the slight increase in dynamic head lowered the flow through each pump enough that the water boiled inside the pump housings from excess shear.
Higher pressure pumps meant bigger pumps and motors and a couple of hundred feet of buried conduit to upgrade to handle larger feeders.
The solution was to purchase a foot mounted pump and belt drive it over speed for the main pump. The other two pumps were run in series as the back-up system. This still meant replacing and adding feeders and special permission from the AHJ to install the conductors in the existing conduits.
If VFDs had been available then, I could have just installed two on the existing motors, run them up a litle over speed, and gone home. Torque, and Hp at over speed,- not a problem that far up the curves. Efficiency,- so what. But better than what we had. Compare optimum run times with kludge fixes.
respectfully

RE: Harmonics and VFDs

For those of you who are following this with interest:
For a dramatic illustration of the effect of speed on pump curves, many performance booklets give curves for the same pump at 1800 RPM and 3600 RPM. Take a quick look and you will see that speed has a dramatic effect on pump performance and why many pumps run at 3600 RPM.
Respectfully

RE: Harmonics and VFDs

An observation for comment by others. Current measurements are common when setting up pumps, whether VFD or valve controlled.
Because of power factor issues, current is an unreliable indication of real load particularly at light loading. (Remember we are talking about savings or losses of real power in the range of 5% or less.)
What is not so common is accurate flow measurements.
If reduced flow results in 10% or 20% longer running times, an apparent 2% gain in efficiency may be misleading.
Rather than comparing relative efficiencies and energy consumption of motors with various drive methods, it may be more accurate to try to compare the total energy required move at given mass of liquid from point A to point B.
As an example, consider a hypothetical deep well where the draw-down is great and is proportional to the pumping rate.
The greatest efficiency may be approached by pumping continuously at a rate that results in the least draw-down.
It will take more energy to lift 1000 gallons 250 feet than it will to lift the same mass 200 feet.
In regards to pump curves we must not confuse efficiency with effectiveness. As the head on the pumps in question is increased, the effectiveness (and the flow) drop off faster than the head rises. The result, less energy required, but also much less flow or delivery.
The wholistic approach is to consider the total energy required to do the job, not the rate at which the energy is consumed.
Respectfully

RE: Harmonics and VFDs

Mark, on your original post, am I correct in assuming that these are individual customers?
Will the power company consider delta/delta connections on new installations?
Respectfully

RE: Harmonics and VFDs

I have studied the Energy Savings Calculator in the Standard Performance Contract at the following link www.aesc-inc.com/download/spc/) given by gepman.  If the utilities are giving cash incentives based on this "calculator", they are paying out a lot of money for nothing.

This so called "calculator" has no provision for even looking at the pump curve.  It has no provisions for figuring the minimum possible pump speed.  It recommends that you plug in 50% as the minimum pump speed and 96% as the efficiency, which are both bogus numbers.  Anytime you plug in the "recommended" numbers, it shows a significant savings in energy.  When I plug in the real numbers, it shows that there are NO SAVINGS with a VFD.

Since head drops off by the square of the speed, the Minimum Possible Speed is the most important part of the calculation.  Everybody seems to think that you can just keep slowing a pump down to decrease the flow rate, even though this is not possible when a static head or constant pressure is required.  From the example curves given in this thread, 10% to 15% is the most you can slow a pump down and still produce the TDH required.  Even the curve from the oversized pump given by Marke, can only be slowed down by 28%.  None of these real numbers show any energy savings when plugged into the so called Energy Savings Calculator.  

This "Calculator" also recommends that you use 96% as the efficiency.  This is a fictitious number concocted by VFD manufactures.  The true Wire to Water efficiency should be figured and is usually more like 80%.

If they really have a fleet of 40 pump testers, shouldn't there be at least one of them who knows how to read a pump curve and figure minimum speed using the Affinity Law?

If you would like to see how pumps really work, I have several systems on my pump test stand in Texas that can easily be switched between VFD and Valve control.  They easily prove everything I am saying is correct.  If you have a pump test stand set up that proves otherwise, then you have either way oversized the pump for the application, or no one even looked at the Brake Horse Power curve when they were choosing the pump.

As for the sewage pumps that could have had there speed increased to solve the problem, horse power increases by the cube of the speed.  A slight increase in pump speed nearly always means that a larger horse power motor is required.  If these motors were not already maxed out, then a larger diameter impeller could also have been used to solve the problem without needing to vary the speed.

In all my years, I have yet to see a situation where a VFD could save enough energy to even pay for itself, when compared to a correctly chosen pump being throttled by a valve.  The propaganda that says VFD's save energy is so widespread, that not only do most people blindly believe it but, they also get very angry when I try to show them the error of their ways.  Even the "Energy Savings Calculator" shows no savings with a VFD when you plug in the correct numbers instead of the "recommended" numbers.

RE: Harmonics and VFDs

Hello Valvecrazy;
Thank you for considering my post. Changing impellers was not an option and that far up the curve, so little work was being done that the cube law was not a problem.
The drive gurus will point out that VFDs on old existing motors is not a good idea. Rewinding to VFD quality would still have been an economical option.
Respectfully

RE: Harmonics and VFDs

gepman and Valvecrazy;
I understand that both of you have access to pump test stands.
Can either or both of you set up a test to measure the energy consumption (KWHr) to raise a given volume to a given head?
Filling an elevated tank or wasting the discharge out an elevated pipe if the volume can be accurately measured would be the most acceptable test.
Then, apply in turn, a VFD and valves to reduce the flow 20%, 30% or 50% and measure the total KWHr required to pump the same quantity against the same head.
This test should also reflect any savings from reduced dynamic head, although dynamic head issues are much dependent on site conditions.
Respectfully

RE: Harmonics and VFDs

valvecrazy
I agree with your assessment of the SPC calculator and that is why I do not use it.  I have pointed out to the utilities the problems with this portion of the software.  I only mentioned it since you stated you work with these utilities.  I do not know when it will be corrected since it has to go through at least two large utilities (PG&E and SCE) and the CPUC (California Public Utilities Commission).  However the ABB savings calculator which I recommended in my previous post does not have any of these problems.  It is the best calculator that I have seen which does not have the actual pump curve built into the calculation procedure.  The calculator is available at http://www.abb.us/product/ap/seitp322/24b03100d005c31ac1256e040043f4c1.aspx.  Since this calculator has inputs for both static head (which it uses to calculate minimum pump speed) and pump shutoff head (which it uses to estimate the shape of the pump curve between the nominal operating point and the no flow condition) it takes into account all of the issues that you mention.  It also takes into account the efficiency of the VFD (if you use a VFD at 100% flow for 100% of the time it will show a negative savings).  Another good feature is that it calculates savings based upon several different existing control strategies including pump on/off control and control valves.

Again I will state that I do not disagree that pumping systems with a high percentage of static head/total head are not good candidates for VFDs and a lot (if not most) of well pumping applications have this high static head characteristic.  Play around with ABB calculator to see what the break even point is.  Even when you break even on energy you then have to consider the capital and maintenance cost of the VFD.  I also disagree with your previous statement that most pumping applications have a high percentage of static head.  Most process applications, which have a far greater number of installations that well pumps, are not high static head.   I also totally disagree with you that throttling the flow does not consume energy across the valve.  A pump system operates at the intersection of the system curve and the pump curve.  When you control via a valve you are adjusting the system curve and when you control via a VFD you are adjusting the pump curve.  If you have a centrifugal pump curve which is either flat or rising towards the shutoff point (which is almost all centrifugal pump curves) then when you reduce flow with a valve you are always pumping at a higher pressure than when you reduce flow with a VFD.  It is also most likely that the efficiency at this reduced flow point will be better with the VFD than with the valve.  Your comments show that you do not have a good understanding of hydraulics.  Remember that VFD savings come when you VARY the flow not keep it constant.  If the flow is constant you should try to design your system without any throttling.  However it is common for the required flow to be known however in sizing the pump a 10 to 20 percent safety factor is added to the head.  This is done because it is difficult to calculate accurately the exact head loss in a system.  For pumps that operate continuously it can pay to install a VFD to reduce the pump curve to where a valve is not throttling this 10-20% head loss.  I encourage you to try this out.

You are also wrong on your efficiency numbers for VFDs.  I have instrumentation which is capable of measuring power consumption on both the line and load side of VFDs and I have never seen efficiencies below 90% even at extremely low loading of the VFD and much higher is typical.  

I would like you to produce some articles or research that show that throttling flows with valves is more energy efficient than controlling flow with a VFD.
I will try to setup a test at the following test facility http://www.sce.com/RebatesandSavings/EnergyCenters/AGTAC/exhibitsanddisplays/.  See the bottom of the page, "Pumping Technology Station".  I don't think that anything will change your mind because you have too much invested into believing your sermon on valves.  It is nice to think that you know something that the rest of the world does not understand.

Also I would not belittle the pump testers.  Each of these professionals accurately test more pumps in a year than you do in a lifetime.  I have been trying to upload a 6.5MB file on their pump testing which might help you understand wells, ag pumps, hydraulics, efficiencies, etc. however my guess is that this file is larger than the limit for this site.  There is no website link to this paper.

Regarding your comment that you have yet to see a system where a VFD could pay for itself, take a look at http://www.bpa.gov/Energy/N/Utilities_Sharing_EE/Workshop2007/pdf/DonCasada.pdf.  This is the kind of work that I do for SCE and PG&E.  I don't know if you understand the concept of varying load or capacity.  These systems were not oversized, they just have to deal with varying flow or loads.

I am not going to waste my time responding to you anymore however I will try to set up the test I mentioned above or maybe another test out in the field.

RE: Harmonics and VFDs

valvecrazy
Post a pump curve (either flat or rising upward at shutoff) and indicate the static head of the system, the nominal operating point (flow and head with valve wide open), and the throttled operating point.  I will post hand calculations and ABB calculator calculations.  

Remember VFDs are applied where there is substantial flow variation so the throttled point must be significantly less than the nominal point.  Also the static head can't be the system head but if you give me that value then I will also calculate it at 80%, 40%, and 0% static head/total head values.

RE: Harmonics and VFDs

I don't think that anything will change your mind because you have too much invested into believing your sermon on "VFD's". It is nice to think that you know something that the rest of the world does not understand.

I guess you don't know how many pumps I have tested in my lifetime.  It will take me a little time but, I will try to video a test and get it on line.  

If you want to do the calculations with the ABB system, use the curve I posted earlier.  Remember that this system needs 231' of head at low flow as well as high flow, as most pumping applications I am familiar with do.  Check it at 100, 600, and 1200 GPM for equal amounts of time.  Although, I can see these numbers on the curve.  The only thing not included on the curve is the loss from the drive itself.  If you say a drive is 96% efficient, then that means you are losing 4% just for using the drive, and that should be added back to the numbers from the variable speed curve.

I don't know if you understand the concept of varying load or capacity.  I do know there have been too many VFD salesmen and engineers involved in making these Energy Saving Calculators.  Feathering their own nest, so to speak.

RE: Harmonics and VFDs

(OP)
Hi Bill

Quote:

am I correct in assuming that these are individual customers?
Will the power company consider delta/delta connections on new installations?
Yes, all individual customers and yes, I am exploring options with the Power Co, and phase shifting transformers are an option. We would probably have to go for star/star rather than delta/delta because we operate an MEN system out here, plus delta driving VFDs is not a good move.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

(OP)
Hello gepman

Quote:

I have been trying to upload a 6.5MB file
Contact me (via my website) and I can arrange to host that file for you so that all can see.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

valvecrazy

As promised here is my reply.  I don’t expect to convince you since I can only offer engineering principles, calculations, manufacturers literature, articles, research publications, and similar material while you offer your unsupported statements that valves save more energy than VFDs.

I have taken the curve that you posted for the Goulds 12WAHC pump (by the way I could not find that particular pump on the web site) but they are a very good manufacturer.  Now you stated that the system needs 230’ of head at low and high flows.  Please see item #6 in my large post above to see why constant head requirements are basically impossible in a real hydraulic system.  To summarize the flow causes a head loss through the piping system which is proportional to the square of the flow.  Your requirements would necessitate no piping system including the valve which you advocate.  What I have done is take a minimum piping system with the valve (that you described previously) which has a head loss of 7 psig (16.2’), 4.8’ of head loss through the pump discharge pipe in the well, and another 9’ of miscellaneous head loss through foot valves (check valves), elbows, and other fittings in the system.  This gives a total head loss of 30’ at 1200 gpm.  I have used this system data in the ABB calculator.  I have used annual operating hours of 4000 which is probably high for many farmers irrigation system but it wouldn’t be high for a processing plant or a large farm with one well (unlikely).

The first calculation (page 1 of attachment) shows that at 1200 gpm, just by pulling out your valve, the energy savings would be 56,911 kWh per year.  (When a value is entered in the "head over open throttling valve" it will calculate the savings eliminating this valve).  At 600 gpm  (50% flow) (page 2 of attachment) the savings is only 61,253 kWh per year.  If you had lower static head the savings would have been much higher.  At 240 gpm (20% flow) (page 3 of attachment)the savings would be 97,373 kWh per year.  This is due to the slight rise in the curve near the zero flow point which allowed the pump to be slowed down more than a perfectly flat curve.  If you would have selected a pump with a more sharply rising curve (see page 4 of the attachment with the same efficiency at the original 1200 gpm @ 230’ pump, the Goulds 9TLC) the savings on all of the above would have been even more, 187,833 kWh at 240 gpm. (page 5 of the attachment, all that changed is the value for the maximum head).

I am only going to calculate the energy savings by hand for one example to compare it to the ABB calculator.  (See page 6 of the attachment).  Let us use the original pump curve  at 600 gpm.  With a valve the pump would have to pump 600 gpm, approximately 260’ head, and 69% efficiency.  With a VFD the flow would be 600 gpm, the head would be the 200’ static head plus .25*13.8’ (600/1200 squared times the friction loss without the valve) which is 200+3.45=203.45’.  The point on the corresponding 3450 rpm curve has an efficiency of approximately 73%.  I would estimate this point as 684 gpm @ 260 gpm.  Using 88% speed this would give flow at 602 gpm (.88 * 684) and head at 201’ (.88^2 * 260).  Using the horsepower equation (Q*H)/(3960*eff) the bhp of a throttled pump is (600*260)/(3960*.69)=57.09.  Converting this to kWh for 4000 hours of pumping including motor efficiency of 95.4% is 4000*57.09*.746/.954= 178,570 kWh.  For the VFD pump this is (600*200)/(3960*.73)=41.5 bhp.  Converting this to kWh we have 4000*41.5*.746/.935=132,444 kWh.  Note that this includes a 98% VFD efficiency (.98*.954).  Therefore we have a savings of 46,126 kWh which compares reasonably well to the ABB calculator value of 61,253.  The difference in the calculator value is probably due to estimating the efficiency both for the throttled pump curve and the VFD pump curve.  The estimating algorithms in the software would probably work more accurately for systems which had a lower percentage of static head.  The percentage of static head in this system is 200/230=87% which is very high.  The savings on the original system with 50% static head (115’) would be much greater, 114,609 kWh  (see page 7 of the attachment).

I believe that I have validated the ABB pump save calculator (at least as a preliminary tool).  You can always calculate using the pump curve if you have it.  I have yet to run into a farmer that has his pump curve.  Most don’t even know what pump is installed especially if it is a submersible or a line shaft pump.

Here is what you should learn from this:

1)    Just adding the valve costs you energy.
2)    You should like VFDs because using a VFD is like trimming the impeller, it just shifts the pump curve slightly downward as you reduce speed.  It helps to keep the BEP at the operating point while a valve shifts the operating point away from the BEP.
3)    You can save even more money using a VFD if you select a pump curve with a rising slope towards the shutoff point with NO loss of efficiency at your nominal operating point.  Of course if you are going to use a valve this will just make things worse.
4)    Even in high static head systems a VFD can save you energy although it is more difficult to save energy in these systems.  
5)    A reasonable savings calculator is available which takes into account all the issues that you have raised regarding static head, VFD efficiency, etc.

If you care to see more propaganda regarding VFDs saving energy in pumping systems see the study at http://www1.eere.energy.gov/industry/bestpractices/pdfs/variable_speed_pumping.pdf

I will post another paper regarding wells, pumps, etc. if I can get it onto Marke’s web site which he has graciously allowed me to use.

I don’t know if you took a look at the link that I gave regarding SCE’s demonstration pumping facility.  If you are really willing to travel to it I will try to setup a test.  If not I don’t want to waste my time.  This will most likely be my last post in response to you because I have better things to do than try to prove the obvious.

RE: Harmonics and VFDs

The pages in my attachment above are not in the correct order since I uploaded the file before I saved the changes in page order.  The first page (a pump curve) should actually be shifted to be page 6 in order to match my post above.

Sorry.

RE: Harmonics and VFDs

I was informed that putting a throttling valve on the suction side of a centrifugal pump oftenly produces vapor which can harm the pump as well.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

RE: Harmonics and VFDs

See the following link for a very good brochure on pump testing and wells.  There is also some information on hydraulics and pumps in general.  It was written so that a farmer could understand it but it is technically accurate also.  Thanks very much to Marke for hosting the site containing this brochure.

http://www.lmphotonics.com/gepman/Edison%20Pumping%20Guide%2012-27.pdf

Valvecrazy

As you can see you need to accurately measure the water level in the well, flow, pressure, and power in order to calculate the efficiency of a pump (and the effect of using a VFD versus a valve).

RE: Harmonics and VFDs

For those who are interested in Mitigators Transformers,
here's what I found in my old books.

Using the Symetrical Components, one can show that multiplying the secondaries is a way to eliminate harmonics.

I don't have any references about these type of transformers.

I suspect the cost to build one of these may be... ah... out of range.

RE: Harmonics and VFDs

Thanks unclebob.
Diagram "B" in your posts is the vector representation of the "mitigators" that I was trying to describe. I saw two versions.
In the first version the zig-zag was about 10%. In the second version the zig-zag was about 50%.
Respectfully

RE: Harmonics and VFDs

It is nice to know that some things transcend language.  I could see it even though my French is non-existant.

I wonder what the prices of these units are versus an active power correction unit for a given amount of distortion.

RE: Harmonics and VFDs

"I don't expect to convince you since I can only offer engineering principles, calculations, manufacturers literature, articles, research publications, and similar material while you offer your unsupported statements that valves save more energy than VFDs."

I was doing the math and my own research 20 years ago, when I was hyped up about VFD's like you are now.  As you have already proven, there are many ways to doctor the math to make a VFD look really good.  The math has not proven itself accurate with the many thousands of installations I have seen in real world applications.  

I know that a VFD could be more economical if the head lowers to 200' or 115' instead of 231'.  However, the well has a 231' pumping level, or the golf course sprinklers need 100 PSI form the booster pump no matter if running 1 sprinkler or all of them at once.  The well is not going to make itself more shallow just to make a VFD look better.  The sprinklers at the golf course are not going to give the coverage they should at 87 PSI to make a VFD more efficient, they need 100 PSI all the time.

If you want to add in another 14' of head from friction loss, then the pump now needs to deliver 1200 GPM at 245' of head.  If you slow the pump down where it can only produce 200' of head, then the water is not even getting to the top of the well as the lift is 231'.  The 14' of friction loss is going to be reduced as the flow rate is reduced.  So the head can vary a little form 1200 GPM at 245' to 100 GPM at 231'.  Other than that, you cannot reduce the head requirement just to make the power requirements of a VFD look better.  If you are going to run a 1200 GPM pump at 240 GPM all the time, as per your example, then yes a VFD will save considerable energy.  However, if that is all you are going to use, then replacing the 100 HP pump with a correctly sized 25 HP will cut the electric bill in half compared to using the VFD.

What I find in the field, is that most pumps are run at maximum capacity about 80 to 90% of the time, and the variable flow is only used when filling a spray tank or something else intermittent.  This makes the electric bill increase for installing a VFD, after the math had already promised energy savings.  

On the systems that I work with, if the system curve makes much difference, then somebody messed up and put in way too small of a pipe line.  As per the ABB calculator, a reduction in head is really what makes a difference in power consumption, not the VFD.  In the section labeled "head above an open valve", it should read, how much head can be reduced.  Using pipe with less friction loss or going to a lower pressure irrigation system should be what the cash incentive is paid for.  This will decrease the cost per gallon pumped, while decreasing the pump speed with a VFD increases the cost per gallon pumped.

"Efficiency will be reduced when a motor is operated on a bus with harmonic content. The harmonics present will increase the electrical losses which, in turn, decrease efficiency. This increase in losses will also result in an increase in motor temperature, which further reduces efficiency."

Then when transformers or line filters are added to reduce the harmonics, the input voltage is decreased and the amperage and heat increase even more.

I have been playing with the ABB calculator, and I find it to be as bad as the calculator for PG&E, that you already agreed was flawed.  The ABB calculator also makes no provision for  looking at the pump curve.  How can you make a comparison if you don't know what the pump will do naturally?  Considering the source, this calculator is designed to make a VFD look good no matter what.  

I don't believe a Drive can be 98% efficient.  I always see at least a 5% loss in pump output when I use a Drive.  This usually causes more of a loss than the 7 PSI friction loss across a valve.  This means at full flow there would be substantial savings for just pulling out the VFD.  

I did calculate an actual savings of $800.00 per year when flow rates below the maximum are used for a majority of the time.  This is no where near the $4,600.00 savings calculated by the ABB software.  At $800.00 actual savings per year, the VFD would have to last many years to pay for itself.  You might get a year or two of savings if the VFD last longer than it takes to payout, which is not likely.

If the flow required varies widely, even when the head remains constant, there are some energy savings by using a VFD.  It is just never as much as exaggerated by the math or any Energy Savings Calculator concocted by VFD manufacturers.  Even when a VFD can save $1,000.00 per year in energy, if you have to replace a $5,000.00 Drive every 5 years, you are not really saving anything.  I know there are a few old drives out there but, very few last long enough to pay out.  See the attachment below of an ABB drive that only lasted a couple of months.  This is what I am used to seeing in the field, and why I replace VFD's with valves every chance I get.

Then lets not forget what started this thread, which was harmonics.  Harmonics is "dirty power", which causes everything else on the same electric grid to become less efficient.  I have even seen several installations where the air-conditioning for the VFD uses more power than the pump itself.  How can your math or these calculators figure the losses elsewhere in the grid and add them back into the calculation? Harmonics are just one of many negative side effects of VFD control.  Resonance frequencies, bearing currents, voltage spikes, RFI, nuisance trips, required technical assistance, and a several other problems go away when you eliminate a VFD.

Here is what you should learn from this;
1) Just adding the VFD without decreasing the head costs you more energy per gallon.
2) You should like Valves.  Valves let you use the full range of the pumps capacity without any of the negative side effects of VFD.
3) You can save money using a Valve if you select a pump curve with a steep Brake Horse Power slope towards the shutoff point with NO loss of efficiency at your nominal operating point.
4) In high static head systems, which is most pump applications, it is more difficult to save energy with a VFD.
5) A savings calculator is available from the manufacturer of VFD's, which does not take into account the natural characteristics from the pumps curve.



I would like this to be my last post in response to you because I have better things to do than to prove the obvious.  This was written so that a farmer could understand it but, it is technically accurate.  It is just hard to see the truth through VFD colored glasses.

We could all be more green, if manufactures didn't try so hard to keep us in the red.

RE: Harmonics and VFDs

I am amazed with the dedication of both gepman and valvecrazy in support of their respective claims. I do wish though that these highly technical discussion won't just be left with bigger questions to spectators like myself and hope that both gentlemen would work each other and do the necessary actual tests...who knows, this may lead to a very important revelation and improvements...(sigh)

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

RE: Harmonics and VFDs

I hear you nightfox1925.
I was thinking the same when I suggested;

Quote:

Can either or both of you set up a test to measure the energy consumption (KWHr) to raise a given volume to a given head? Filling an elevated tank or wasting the discharge out an elevated pipe if the volume can be accurately measured would be the most acceptable test.
Then, apply in turn, a VFD and valves to reduce the flow 20%, 30% or 50% and measure the total KWHr required to pump the same quantity against the same head.
I want to know the KWHr to lift a given volume to a given head with each system.
I think that everyone will agree that a well designed system running at full output will probably not benefit from a VFD.
however we want to know what happens when the system is asked to perform at 50% capacity and the choice is between a VFD and a valve.
We would like to see a comparison test showing
HEAD x MASS / KWHr
Thanks fellows.

RE: Harmonics and VFDs

I hope gepman doesn’t take it personally.  I have had this argument with many worthy adversaries before.  It may take a few years but, I nearly always hear, “you know you were right”.  
Quotes like;
“We haven’t touched that pump system with the valves controlling it in 14 years.  The VFD system next to it is always needing maintenance, and the drive has been replaced 3 times.”
“The pump system with the Drive has used considerably less energy, not because it is more efficient but, because it has been down a lot of the time.”

Gepman has done an excellent job with the math.  I would not argue with his math for a minute.  Don’t just do the math though, test which one works best in the real world.  Doing the math (especially using the Handy Calculators), is probably going to get you to try a VFD first.  When you have had as many problems with the VFD as you can stand, try a valve.  You may not get quite as good in energy savings as you did with the VFD but, having all those problems go away is worth a lot in itself.  If a valve controlled system last two or three times longer than the VFD system, that has to be added back into the energy savings calculation, and can make much more difference than a few points of efficiency.  The best tests are the ones you do yourself, because it takes time to make these decisions.  If it works best for you in the long run, then it has won the test.  Just don’t let the math keep you from trying a valve.  A good centrifugal impeller does have some natural power reducing qualities, that can easily be taken advantage of, by using a control valve.

I have a couple of 5 HP, 150 GPM pump system demonstrators that can easily be switched back and forth between drive control and valve control.  Of course I have chosen pumps with steep Brake Horse Power curves.   They show basically the same power consumption with VFD as with  the Valve.  One of them stays in my shop in Texas, as I have learned this is something that is easier to show than explain.  I also have several smaller and larger pumps in the test stand.  They are always available for anyone who wants to come to Texas.  The other VFD/Valve demonstrator will be at the World Ag Expo show in Tulare, California on February 12,13,14, Pavilion B, Booth 2128.  I will not be there myself but, several qualified people will be.  I would like to invite Gepman and anyone else who wants to see a side by side demonstration to come.  Being as it is in California, maybe some of you can make it.

My thanks to Gepman and everyone else for this interesting discussion.  I am afraid now you must make your own decisions.  I only ask that you realize you have other options.  BTW, the valve goes on the discharge side of the pump.

RE: Harmonics and VFDs

I said, about half-way down this thread, that this is one of the most interesting and valuable threads ever going in the motor and controls forum. After that, gepman and Valvecrazy have both added a wealth of information, which probably needs several days to absorb, understand, verify and relate to. I am sorry that I do not have that time now either. But doing that is high on my to-do list.

I think that this has been a great discussion. I still cannot say what is right and what is wrong. But I am sure that I will have a closer look at the non-VFD alternatives before deciding blindly that a VFD is needed in pump applications next time.

I make my living out of VFD problems. Bearing problems, EMI, harmonics and also plain fires and explosions in VFDs. Keeping a working knowledge is also a problem with VFDs. It is not unusual that even the manufacturer's HQ has a problem finding support people for drives being 10+ years old.

So, even if VFDs are wonderful devices from an automation point of view, they also do bring problems. Problems that a lowly valve seldom has.

Having said that, I need to add that many VFDs run without problems for many years. It is when problems occur unexpectedly that the situation gets critical.

Thanks to all for a great discussion. I am sure we haven't heard the last word from the participants. This theme merits an own thread next time.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

I guess valvecrazy will only be a few booths down at the World Ag Expo from where the company that I work for will be giving out $1250 (or $0.08 per kWh saved on an annual basis) to any farmer that wants to install a VFD on a well pump.  I will stop by your booth.  I hope that your Texas company is not quite like that other Texas energy company, Enron.  I also invite you to apply for a utility rebate for your energy saving valves, however calculations instead of anecdotal testimonials will be required.

As valvecrazy says the math is correct so the problem will be in his demonstration device.  Please post a P&ID, BOM, and layout drawings so I can tell you what is wrong with your setup.  I will need to know the pump curve, the valve Cv, VFD model number, motor model number, measurement equipment model numbers, etc.  

I have advised many farmers NOT to install VFDs because the static head/total head percentage was too high and/or the amount of hours that the pump ran was not sufficient to justify a VFD.  As I have said previously VFDs save energy by reducing the pressure at the given flow so if the pressure does not change there can be no savings.

See you in Tulare, maybe we can have a drink!

RE: Harmonics and VFDs

And I hope after the Tulare exhibit, since I won't be able to avail myself for a  visit, both/either gentlemen would come back and post new thread of what technically transpired in these demontrations so it may also help us make up our minds as applicable. Many thanks to everybody!!!  

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

RE: Harmonics and VFDs

It's been quite a few years since I've visited this forum and to find a discussion like this on my return is a joyfull thing; finding that I remembered my password and user name was more of a miracle :)

What a great thread. Not having ever been involved with motors or pumps much of it goes whoooosh over my head, but after 20 odd years in the UPS industry the harmonic problems are something I can get my head around.

I see that it has been suggested to use a Dy transformer at the front end of some of the drives to increase the pulse number to 12.

I would suggest that this is taken a step further by the use of different phase shift transformers.

If for example you used different phase shifts of (in degrees) +7.5, +15, +22.5, +30, 0, -7.5, -15, -22.5 and -30, then you efectively end up with a 54 pulse rectifier - 6 pulses x 9 different phase shifts.

Theoretically, if the drives were balanced in load across these 9 different phase shifts, then the input current harmonics would be down to about 2.4% compared to a 6 pulse input which is approximately 27%.

I'm also very interested in the great valve vs drive shootout and I hope the experimental results are posted here.

RE: Harmonics and VFDs

Valvecrazy
I have set up a test at the SCE Ag Pump Demonstration unit at AgTAC, which is across the street from the Ag Expo.  Be there at 1:30PM, February 14.  See http://www.sce.com/RebatesandSavings/EnergyCenters/AGTAC/exhibitsanddisplays/ for exact location.

This is their standard VFD versus valve test unit which they show to all the farmers who want to see it.  Come with a nametag that says "Valvecrazy" and I will recognize you.

RE: Harmonics and VFDs

Gepman, I would love to get to meet you and see your demonstration.  However, as I said earlier, I will not get to come to California for this show.  We have several going on at the same time so, only my guys from the west cost will be at that show.  I will make sure they come by and see you.

I know that the power consumption of throttling a pump, being the same as the power consumption of a VFD, is something most people have to see to believe.  That is why I will have a 5 HP demonstrator at Booth B 2128.  However, I am attaching a curve of the pump that is on my demonstrator.  You can see that at 200 GPM it is pulling a 5 HP load, then when throttled to 20 GPM the power required drops to a 2 HP load.  I don't know why some people cannot see what I am talking about from simply looking at a pump curve.

As you said earlier;
"As I have said previously VFDs save energy by reducing the pressure at the given flow so if the pressure does not change there can be no savings."

Therefore, when maintaining a constant pressure or TDH, which is the case with almost every system I work with, there can be no savings with a VFD.

It is all about choosing the right pump.  A VFD can make a badly designed pump use a little less energy but, it can never do better than a properly chosen pump.

Then there are the things that don't show up on a pump curve, and are hard to add back into the equation.  

1) Parasitic losses of the drive when running and when in the standby mode
2) Environment control for the Drive itself
3) Heat losses from active or passive filters
4) Efficiency loss from stray voltage for everything else in the area
5) Shorter life of the equipment with VFD than with ATL controls
6) And now extra tariffs on the electric bill for VFD induced harmonics

If throttling means 5HP - 3HP = 2HP
Then VFD means   5HP - 3HP + (1,2,3,4,5,6) = ?????????

RE: Harmonics and VFDs

Back to the original question, regarding the use of phase-shift transformers to reduce harmonics, check out http://www.mirusinternational.com .  They make various configurations of harmonic-mitigating transformers which do exactly what you're looking for.

From a quick GoodSearch on "harmonic mitigating transfomer", it appears that Square-D and other players have also gotten into this game now.  Click here for more:  http://www.goodsearch.com/Search.aspx?Keywords=harmonic+mitigating+transformer or Google it here:  http://www.google.com/search?q=harmonic+mitigating+transformer


 

RE: Harmonics and VFDs

As promised here is the report (at least my report) from the World Ag Expo and the VFD vs. Valve shootout.  I have attached a picture of the valvecrazy's test setup.

The test setup consists of a pool or reservoir from which eight solenoid controlled nozzles spray up into the air and back into the pool (similar to an irrigation sprinkler).  The water in the pool is brought into the suction of the pump in the lower left of the picture.  The pump discharge goes through a check valve and then through the red pilot actuated diaphragm valve.  The pilot pressure for the diaphragm valve is on the discharge of the diaphragm valve and senses the pressure on the discharge of the valve.  This pilot signal can be adjusted with the small pressure regulator located behind the red valve which has a black top and a small red tag on it.  This regulator has been adjusted so that the discharge pressure on the valve remains at a constant 30 psig.  The water then goes through a wye strainer, has a pressure gauge to indicate pressure, then goes to a distribution header which distributes the water to the eight nozzles.  The nozzle solenoids are separately controlled by eight switches so that anywhere from 0 to 8 nozzles can be open.  The enclosure with the red switch is a four way switch for "off", "FVNR", "VFD", and "TEST".  To the left of the enclosure is an ammeter display for the FVNR circuit, to the right of the enclosure is the VFD with a HIM display showing rpm, pressure, and amps.  There is also a pressure transducer located near the pressure tank which provides input to the VFD for speed control.

The test to show that the valve uses less energy than the VFD is reading amps as displayed by the two devices above and NOT power.  The first test done showed that the VFD used more amps than the valve.  I then realized that the red diaphragm valve was still controlling in the circuit so I made a third run while disabling the pilot signal (by shutting of a valve in the pilot line) which is also shown:

Nozzles   1      2      3      4      5      6      7       8
1 VFD     6.3A 6.7A 7.2A 7.8A 8.3A 8.9A 9.5A  10.1A
2 Valve   3.8A 4.8A 5.9A 7.0A 8.0A 9.2A 10.0A 10.8A
3 VFD     5.6A 5.9A 6.3A 6.8A 7.2A 7.9A 8.5A  9.3A

Somewhat unsatisfied with these results I went to my car and retrieved my Amprobe 41PQ which measures power (one measurement at a time so to get total power I needed two measurements) and harmonics.  The VFD run below is done with the diaphragm valve disabled so that it cannot modulate.  Both measurements were taken at the same place in the circuit at the power input to the enclosure.  There was no flowmeter to check flow rates.

Nozzles   1                  4                  8
4 VFD     1354W          1945W          2760W
5 Valve   1710W          2230W          2927W

I was surprised that the VFD used less power with all of the nozzles open but I then realized that the diaphragm valve was restricting the flow even with all nozzles open.  The rpm display for the VFD was 3147 for 1 nozzle, 3180 rpm for 4 nozzles, and 3278 rpm for 8 nozzles.  Full speed on the motor is 3450 rpm.  

We were unable to run a test at the SCE AgTAC test center since they had a water problem with their cistern that stores the water for pump tests.






RE: Harmonics and VFDs

Thank You Gepman!!  I appreciate you taking the time to run this test.  I am glad you were able to do testing on our system since you could not get your system working.  I imagine you were as surprised as I was the first time I realized how little difference there is in power consumption between Valve and Drive control.

Eight nozzles is all our system has for output.  These do not let out quite enough flow to get to BEP as you can see from the Drive never getting the motor to 3450 RPM.  Of course we all know that at BEP, the power consumption would be less with Across The Line than with the Drive, as there would be no parasitic losses.  Therefore, we know the Drive would not be as efficient at BEP, and I did not think it important to get the pump to full output or BEP for this test.

Reducing the flow down to 8 nozzles, 4 nozzles, and 1 nozzle, you can see that there is only a couple hundred watts difference between valve control and VFD.  However, I purposely did not use any filters to keep the Drive looking as good as possible.  There is no line filter as this is trade show equipment, and I do not care if harmonics interfere with other equipment in the area.  I also do not care if the harmonics produced with my VFD cause other equipment in the area to be less efficient but, that power loss should be taken into account.

I did not use a load filter on this system as it is for trade shows only.  This motor only runs about 10 days a year at 8 hours a day so, it should be able to handle the voltage spikes for a while with no more hours of use than it is made to run.  It also has a very very short wire from Drive to motor, which reduces the reflective wave.  

However, you did not mention the measure of harmonics we were producing with the VFD.  As with the title to this thread we all know harmonics are a problem.  So in a real life situation I would need to add a line filter and/or a load filter as well.  When you add back in the heat losses from the filters that are needed, the couple of hundred watts you show giving the advantage to the VFD, will no longer be there.

Just adding the filters needed would about equalize the power consumption between Valve control and Drive control.  Then if you add in the parasitic losses from the Drive using power even when the pump is not running, and the losses of efficiency to other equipment in the area from "dirty power", Valve control looks even better.

All that being said, we can go back to the actual numbers of this test without adding parasitic losses, filter losses, and losses to other equipment in the area.  Even with the couple hundred watts difference in the best case scenario for the Drive, the payoff of the Drive itself is almost non existent.  If you consider the cost of the Drive, and shorter life of the Drive and equipment compared to running Across The Line, payoff would take many years, if possible at all.

I should mention all the other problems that exist as side effects of Drive control but, this is already a long thread.  I will say that you do not have to worry about these other side effects, when you realize that the Drive is not really saving energy, and therefore should not be used in these type applications.

Back to the topic of this thread, to eliminate harmonics, the best way is to not cause them in the first place, in systems where a Drive has no power savings benefits.

RE: Harmonics and VFDs

I'm just going to add a few comments;

It looks to me that you had about a 3hp, 208V motor there. This motor showed 167W saving running with all 8 nozzles open.

OK, the system had 8 nozzles to simulate a sprinkler or irrigation system. I would believe that in the real world you will very likely not have the same type of system perfectly designed so that you can run the motor at full speed with the sprinklers open and get the desired operating pressure. You would still either have to always valve throttle the pump or speed control it to get the system to work at the desired pressure.

So, now, scale this test system up to a real world application and you would maybe use a 30hp motor. So, the VFD would save 1670W of power compared to a valve when irrigating with the system running all 8 nozzles. Say the system runs 4 hours a day. That is 6680Wh of energy savings a day. Now assume that hydro costs about 16 cents per kWh this is about $1.07 saving per day.

Of course, the above numbers will change depending on how many nozzles are used when the system is operating.

So, looking at this example it is seems the VFD uses less energy but it really doesn't amount to much and the potential savings really can not be used to justify the added cost of the VFD. It appears the valve is probably the better solution for this particular system.

However, I don't really take the data measured using a test setup built by someone who is promoting valves over VFD's as  proof that applying a VFD on a pump is always wrong.

RE: Harmonics and VFDs

"However, I don't really take the data measured using a test setup built by someone who is promoting valves over VFD's as proof that applying a VFD on a pump is always wrong."

Except for the above statement, thank you LionelHutz for your comments.  Your scale up to 30 HP is a good idea and appreciated.  However, it will be even more important for a 30 HP to have line and/or load filters.  The heat losses of the filters will take even more away from the $1.07 per day the VFD is "saving".

I also disagree that a pump in the real would never run at BEP.  Most of the systems I work with run at BEP most of the time, and only need varied flow rates part of the time.  This makes Valve control look even better still.

As for the test being set up to favor Valve control, it is not.  Actually it is set up to favor the Drive, as I added no filters and never run it at BEP.  Gepman went all through the test set up and describes it very accurately.  If there had been a "man behind the curtain pulling strings", he would have easily seen it.  It is a very straight forward pump setup.

All of this test data is not really needed.  I go back to the fact that all this information is in the pump curve.  If you know how to read a pump curve and pick a pump with good horse power characteristics at low flow, actual kilowatt per gallon testing is not needed to see that Valve control can be just as efficient as Drive control.

See the attached pump curve again.  Thanks to all!!

RE: Harmonics and VFDs

(OP)
Asa matter of interest, what was the power consumption without the valve or the VSD?

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

With or without a valve, the power consumption will follow the horse power curve.  Regulating the pressure with a valve allows you to always have the same pressure on the sprinklers or nozzles no matter if you are running just one, or many at a time. Without the valve, the sprinklers or nozzles would have more pressure than needed, which would overshoot and mist when you were running too few at a time.  However, the power consumption is still dependant on the flow, and follows the curve.  It is "counter intuitive" that power consumption decreases as the flow is restricted but, it really happens no matter how you restrict the flow.

RE: Harmonics and VFDs

Yes Valvecrazy, we understand that the power consumption will go down as the flow goes down. Understanding that the power consumption is related to the work being done is not a difficult concept.

gepman provided some good info showing cases where a VFD can save energy and is worthwhile. This example is not proof that a VFD is always the best solution. Similarily, you give an example showing that a valve is a very good solution but that is not proof that a valve is always the best solution. So, as like most things in life, each application has to be evaluated to determine the best system to install.

What I did find disappointing was that your test jig used ammeters to show the power used in the different configurations. For me, this alone is a pretty big red flag. Also, see my above paragraph. Just because you built a system that can demonstrate that a valve is a good solution does not mean a valve is always a good solution.

RE: Harmonics and VFDs

Hello Valvecrazy;
We don't always get to design a system from the start. We sometimes have to modify existing equipment and do the best we can with equipment chosen by someone else.
Can you or someone else give us a guideline as to what to look for in a pump curve? What curve profile will work equally well with valves or VFDs and are there any pump curve profiles that will benefit from VFDs?
If throttling the pump output develops a fairly large pressure drop across the control valve is this an indication that the savings with a VFD may be more than shown with your test rig?
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

GREETINGS: HARMONICS... WELL ALLEN BRADLY has a new VFD drive out, it has 6, 12, or even more pulses on the input side to balance out the input SPIKES...We see these problems on oil well pumps ... down -hole submersiables A tough app. LAST VFDS are now sweet, I started out 34 years ago with only DC drives.... to work with.. A vfd must always be considered a narrow range controller say 35HZ to yepp 70HZ.. if the motor is sized correctly you can run it over base speed of 60 hz  ,,, BUT USUALLY a range of 35?hz to 60HZ was considered a good  range... I am courious though .... about the fall off of EFF. in large motors under 50 hz... HOPE this helps... When I lose a drive I can usually run in Bypass mode...

RE: Harmonics and VFDs

"Understanding that the power consumption is related to the work being done is not a difficult
concept."

I completely disagree with the above statement.  If it were not a difficult concept to understand, a simple pump curve would be all that is needed to make the point.  In this thread we have batted the math back and forth, studied pump curve after pump curve, and even demonstrated the idea with a working pump system.  Apparently we are still arguing the point.  Again, if you can read a pump curve, the test rig is simply proving the obvious.   I don't agree that an amp meter to measure power on our test rig should cause a "big red flag".  167 watts or 1.2 amps difference is not much difference anyway you look at it.  I guess if I am using a compass instead of a GPS, I cannot prove the sun rises in the east either, even though it is obvious.


I agree that each application should be evaluated to determine the best system to install.  However, I believe that each application should be evaluated to see if it can be controlled efficiently by any other means, before a Drive is even considered.  Adding a Drive to applications where the system could be easily controlled by other means, adds a multitude of other complications that must also be dealt with.  Harmonics, as is the title to this thread, along with reflective waves, voltage spikes, resonance frequencies, bearing currents, and many other problems are never present, if you don't throw a Drive into an application that could easily be controlled by other means.

"Just because you built a system that can demonstrate that a valve is a good solution does not mean a valve is always a good solution."

Just because you can build a system that can demonstrate that a Drive is a good solution, does not mean that a Drive is always a good solution.  However, most people have been brain washed to falsely believe that a Drive can always save energy.  Therefore, they think that a Drive is the best solution, and that all the side effects that go with a Drive are worth it, because you are supposedly saving energy.

Hello Waross
"We sometimes have to modify existing equipment and do the best we can with equipment chosen by someone else."

This is what I deal with everyday.  Someone who didn't know what they were doing, chose the wrong pump.  Now the volume is too high, or the pump is cycling itself to death.  As long as the pump has some drop in horse power, a valve can still be the best alternative.  The existing and badly chosen pump may only drop from 30 HP to 22 HP when throttled with a valve.  If the correct pump were chosen or a Drive were used, the 30 HP could drop to 15 HP at low flow rates.  This small difference in power consumption still may not be worth changing out the pump, or the problems of added side effects when using a Drive.

"If throttling the pump output develops a fairly large pressure drop across the control valve is this an indication that the savings with a VFD may be more than shown with your test rig?"This is the concept that is hard to understand.  It doesn't matter how much pressure differential you have across the valve, what matters is the Brake Horse Power characteristics of the pump.  This is why I say that back pressure is a free byproduct of horse power.  Even though the back pressure increases, the horse power decreases.  This is also how you choose the best pump.  Don't just look at horse power for maximum flow, also look at horse power required for smaller flow rates.  The only reason back pressure is even considered, is to make sure the control valve and associated piping can handle the pressure, it has nothing to do with how much energy is supposedly  "burned across a valve".

As I have explained many times, 1200 GPM at 100 HP is using much less energy per gallon, than when a Drive is used on the same pump to produce 100 GPM with 38 HP.   So in this respect, you are also  "burning energy with the Drive".

Now for picking a pump that works best with a Drive, you need a performance curve that is as steep as possible.  Because head is reduced by the square of the speed, a steep performance curve gives you more possible reduction in speed while still producing the head required.  You should temper this steep performance curve, with the Brake Horse Power curve as well.  Because once you have reached the minimum speed that will produce the head required, further reduction in power come only from the pumps natural Brake Horse Power characteristics, the same as with valve control.  You simply don't start using the pumps natural Brake Horse Power characteristics, until the Drive has slowed the pump down to the minimum possible speed, that will still produce the head required.

Greetings Crazytexacan,  which I am also one of.  Since you started using Drives 34 years ago, there have been new Drives introduced every few months.   New Drives are supposed to solve the problems of the last Drives, which are only a few months old themselves.  They do seem to be coming up with a few band aids for some problems but, unless you can change the laws of nature, most of the problems associated with Drives CANNOT be fixed.  What I understand about the 18 pulse Drives, is that they do counteract much of the harmonics but, at the expense of decreased efficiency which causes increased power consumption.  Kind of defeats the original purpose of the Drive.  Also, the more pulses, the faster the rate of voltage rise, the more damage is done to the windings.

Down hole oil well submersibles, because of their very steep performance curve, can be reduced in speed more than other pumps, and still produce the head required.  Also, oil well submersibles do not usually have to produce a constant outlet pressure as needed with such things as irrigation  sprinklers.  Maintaining a level in the well, instead of a constant outlet pressure, a Drive is able to further reduce the speed and horse power required.  Of course efficiency is not as much of a concern as precise level control with these systems, as the fluid you are pumping is now worth more than $100 per barrel.  I would think that the extra long wires to the motor could cause a problem.  I am sure you must be using a really high voltage cable with extra shielding?  You say a narrow range is 35 to 70 hertz, I say for most pump applications the range is even more narrow than that.  50 to 60 hertz is usually the range limit for a correctly sized pump in a constant pressure application.

"When I lose a drive I can usually run in Bypass mode..."

All the drives I installed ran in Bypass mode more than they ran in the Drive mode.  Of course we used valves to control the flow when running on Bypass.  This is how I discovered that the power consumption was the same with Valve control as with Drive control.  This is also how I figured out that Valve control was much more reliable than Drive control.  Then I should also mention that all the problems with harmonics, vibration, voltage spikes, electrical fluting of ball bearings, and other problems DO NOT even exist when running in Bypass with Valve control.

I know that many of you make your living by mitigating problems of Drive controls.  I know you don't want to hear that Drives do not save energy, cause many side effects, and even cause health problems with animals and humans.  

You can tell me that "Drives are hear to stay", and "I should just get used to it".   I submit that I replace Drives with Valves everyday.  You may not want to hear it but, the best way to solve the problems of Drive control, is to not cause the problems in the first place.  Making me feel like I have stuck my head in a bee hive every time I bring it up, is not going to change this fact.  People who use Drives on every application, and then try to mitigate the problems with the Drive, are just a few years behind the learning curve. There are good applications for Drives but, Drives are not good for many applications.  Sorry for the long post but, it seems I am still having to prove the obvious.

RE: Harmonics and VFDs

Thank you Valvecrazy.
Thank you also gepman.
Yours

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

Valvecrazy,

Many of us have listened to you. And some of us (me, for instance) have considered what you are saying and made our own decisions how to evaluate the pros and cons of VFDs. I have come to the conclusion that we need to be more careful when selecting technology. Especially when taking things like EMI, short bearing life, cost for VFD specialists and also for replacing a VFD because there are no spare parts or anyone that can repair it any more. We have gained from such insights. And I think we owe you for making us think.

But, praise stops here.

You do not at all have the same attitude. You are not prepared to listen to us. You march blindly with your VALVES FOR PRESIDENT banner held high and you are so busy shouting your message that you miss a whole lot of facts that we try to tell you.

One particular point that you may need to understand is that current (amps) do not at all represent load of a motor. Only power meters (showing watts) do that. There are actually motors where amps go down when the shaft is loaded. It is a result of changing power factor. So, please, hold your horses and try to understand what we are saying. You may also benefit from it. If you continue like this, someone (I would say many) may get so tired with your one-eyed view on this matter that you are not welcome here any more. And I would not like that.

Please.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

(OP)
Hello Valvecrazy

I, and I am sure that many others, totally understand that there are alternatives to all technologies and I totally agree that some VFD sales people get far to overzealous when it comes to making promises, however I do believe that there is a place for all equipment and all have their strengths and weaknesses. Valves also do have problems. Valves can be damaged by freezing, by solids such as lime in the water etc.

If we worked in the ideal world, then the correct engineering would be done and overall efficiency would be much improved. This argument applies to all forms of engineering, not just valves vers VFDs, but the practical reality is that this is not the case.
I recently visited an installation where the pump was in the ground and a VFD as installed and running and in this case, the VFD was achieving results. If the engineering was done properly at the beginning, the cost would have been much less, but it was not.
The pump was a 75KW and it runs at almost constant flow. When I looked at the installation, the pump was drawing 42KW, so the pump was well oversized. If then engineering was done correctly, a 45KW pump would have been used along with a soft starter. Instead, the irrigation specialist sold a 75KW pump and VFD. - The farmer paid far more than he should have, but it was cheaper to leave it there than to pull it out and replace it with a smaller pump and soft starter. So often, by the time I get involved, it is too late. The capital cost is higher but the energy used is reduced because a VFD is fitted. Granted, a flat curve pump of the appropriate size would do even better, but that is beyond our control. The selection of VFD or valve is also well beyond our control.

One issue that I do have through all this discussion is this :

Quote:

This is why I say that back pressure is a free byproduct of horse power.  Even though the back pressure increases, the horse power decreases.
If we consider the pump output, the hydraulic power is a function of flow and volume, just like electrical power is the product of voltage and current. The pressure is the hydraulic voltage and the flow is the hydraulic current.As we reduce the flow from the pump at a constant head, the power reduces. If we add a resistive component in series with the flow in order to control the pressure across the load, there is a pressure drop across this resistive element and so there is a a power loss. It is not a free byproduct.
If we have variable flow requirements and we have a constant pressure requirement across the load, then we can either select a source (pump) that has a very low impedance and provides the correct pressure output across the flow range, (Flat curve pump), and it we do so correctly there is no need for a series element, or we can use a source with a higher output and reduce it by applying a series element, or we can regulate the source and use no series element.
The VFD is a means of regulating the source with out the losses associated with a series element and therefore reduces the losses associated with the series element.
There is no doubt that the different pumps can have different efficiencies at different parts of their curves, and there is also no doubt that there are losses in the addition of a valve or series element in the line.
All approaches have merit, but too often the correct engineering is not done. I could argue that the use of the valve is due to bad pump selection also. If the pump was selected to have a constant pressure output at the right pressure, then the valve would not be required.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

I was going to respond in more detail but Gunner and Marke gave very good responses. I wouldn't be so kind and you wouldn't listen anyways so what's the point?

Actually, I will say this. Anyone who comes to me and tries to prove that their AC device saves power by showing me that the amps have dropped pretty much gets discredited right on the spot. Sorry, but if you don't understand basic AC theory then don't bother trying to BS an electrical engineer pretending you do.

Also, the number of pulses in the front end rectifier has nothing to do with the dV/Dt on the motor.

RE: Harmonics and VFDs

“To measure the real power or reactive power, a wattmeter designed to properly work with non-sinusoidal currents must be used.”
Yeah, OK.  I have always used amps as an easy indicator of power because with these type systems, the voltage and power factor change very little.  Apparently the amps ARE a direct indicator of power on my demonstrator as was shown by the reading of the watt meter.  This was the same thing I discovered 15 years ago, when I stopped worrying about having a watt meter.  However, with the harmonic component of a Drive, I can see where measuring watts would be the only correct way, and could make power consumption of a Drive look even worse.

I forgot most of this stuff as my electronics classes were over 30 years ago.  That far back, and before AC Drives, I honestly don’t even remember discussing harmonics as a problem.  Maybe that is because there were no AC Drives back then to cause the problems?

This discussion is not about Valves.  I have never once mentioned a particular kind of valve.  I am simply comparing Drives to Valves because they BOTH burn about the same amount of energy.  This discussion is about Drives being given credit for saving energy, when it is simply not true.  Every application I have seen, or every article I have read, that gives credit for saving energy to a Drive, has been incorrect.  If you read these articles carefully, you will see that a dump valve was discontinued, a smaller pump was installed, or larger pipe lines were installed to reduce the  pressure required.  A Drive may have made it easier for these steps to be implemented but, it was these steps that decreased the energy consumption, not the Drive itself.  Slowing the pump with a Drive always increases the energy used per gallon pumped, especially in a constant pressure application.  So Drives also “burn energy”!

As I have explained many times, a 100 HP pump moving 1200 GPM is using .083 HP per gallon.  The same pump slowed down with a Drive to produce 100 GPM while using 38 HP is using .38 HP per gallon.  This means the Drive is “burning” 4.5 times more power per gallon than a properly sized pump, or when compared to the same pump with pressure or level switches, feeding a pressure or elevated tank.  This does not even take into consideration the parasitic losses, short life span, and other problems of Drive control.

I have learned a lot from many people on this forum.  Now as Gunner would say, the praise stops here.  I don’t believe it is MY attitude that is the problem.  It is MY message that no one wants to hear.  Almost everyone here is marching blindly with their DRIVES FOR PRESIDENT banner held high, and you are still missing the point.  You should just admit to yourselves and to everyone else that as a Drive reduces the RPM of a pump, the power consumed per gallon increases.   

Apparently everyone needs to see an actual demonstration as gepman did.  I thought gepmans test would prove this once and for all.  I hope the brainwashing by the VFD companies, along with the ever increasing “stray voltage” problem, has not already caused permanent and irreversible brain damage.  The Romans  thought lead pipes were the greatest technology.  However, the lead poisoning that followed caused mental problems that kept them from seeing clearly, and ultimately lead to the destruction of their entire society.

BTW, gepman should have taken readings on the harmonics with and without the Drive.  This would have proven that doing away with the Drive is the best way to eliminate the harmonics, as is the point of this thread.

I appreciate Marke bringing up that traditional valves can have problems themselves.  I would love to discuss this and other REAL issues on a different thread, if you guys can ever get past the allusion that “Drives save energy”.

I am not the one who is full of BS, and I am not the one with the attitude problem.  I am wide open to learning something here but, no one has yet proved me wrong on this point. You keep saying Valves burn energy, and Drives save energy.  I keep saying only when “ifs and buts are candy and nuts”.

Anyone who mentions something negative about a VFD in this forum, even though it is the truth,  can expect a tongue lashing or a keyboard whipping.  I am beginning to wonder if you want real answers, or if this site is owned and controlled by a VFD company?

RE: Harmonics and VFDs

No use answering any more. This guy is beyond education.  Power factor seems to be a mythical being to him - an elf, perhaps? How could he ever understand?

Ever read Don Quixote?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

This thread has been most interesting and entertaining.

I must admit that initially the reduced current had me thinking maybe Valvecrazy is onto something, until I realized that all that happened is that the load had been reduced and less work was being done.  Naturally the amps will drop.

The current on the drive cannot be compared to that of a throttled valve because the frequency and voltage are varied, so the current may actually be higher.

The true test will be to measure the output power of the pump and compare it to the power drawn from the supply.
efficiency = output/input.

How much different is throttling a valve to closing dampers on a fan.  When dampers are closed the fan housing heats up. Surely that is energy lost.  Maybe it is not as apparent where the fluid being pumped absorbs this energy and it is carried away.

Regards
Trevor Morrison

RE: Harmonics and VFDs

Valvecrazy made some good points about valves versus drives and I thank him for the insights I have gained,
BUT, sadly, when he displays starts talking about power factor, he loses all credibility to talk about the efficiency of drives and motors.
This is becoming as nonproductive as an argument with someone who has a perpetual motion machine.
End

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

“This is becoming as nonproductive as an argument with someone who has a perpetual motion machine.”

That is exactly what this is.  However, I am not the one claiming to have a VFD perpetual motion machine.  Please, I give up.  Nothing left but to start talking about my mother.

RE: Harmonics and VFDs

Hi, all
A couple of points have confused me in this fascinating thread:
1. ValveCrazy said of the demonstrator results:
"Reducing the flow down to 8 nozzles, 4 nozzles, and 1 nozzle, you can see that there is only a couple hundred watts difference between valve control and VFD"

This "couple of hundred watts" needs to be expressed as a percentage, or the numbers are meaningless for any other scale of installation. The VFD is using between 94.2% of the power that the valve uses (8 nozzles) and 79.2% with 1 nozzle. Up to 20% saving on power consumption seems fairly significant to me.

2. Valvecrazy made this point:
"Every application I have seen, or every article I have read, that gives credit for saving energy to a Drive, has been incorrect.  If you read these articles carefully, you will see that a dump valve was discontinued, a smaller pump was installed, or larger pipe lines were installed to reduce the  pressure required.  A Drive may have made it easier for these steps to be implemented but, it was these steps that decreased the energy consumption, not the Drive itself."

Surely this is precisely the point? My experience of pumping systems is very limited, but from the customers viewpoint, they don't care which part of the system is saving them money, they look at lower installation costs, running costs, maintenance costs, etc.
I'm also very confused about this "free by-product" of back pressure. My mother always told me you didn't get anything for free in this world....
Regards,
Mort (still trying to find the quote button - looks so much nicer smile)

RE: Harmonics and VFDs

Average with gepmans test was 14% better for the VFD, without considering any runs at maximum flow, or any load or line filters.  Adding the needed filters and considering different possible flow rates, the Drive can do anything from costing you 5% to saving you maybe 10% on average.  Not usually enough to justify the added expense or the trouble.  See this link;

http://www.powerqualityanddrives.com/payback_analysis_vfd/

“”A Drive may have made it easier for these steps to be implemented but, it was these steps that decreased the energy consumption, not the Drive itself.”“

“Surely this is precisely the point?”

92 pages and this is the only point I am trying to make!!


Also I realize that there is no such thing as free energy.  I just use the “free byproduct” thing to help explain that as the back pressure of a centrifugal pump goes up, the horse power required goes down.  It is just a hard thing to explain.  Even people who say they understand it, loose credibility when they do not realize that throttling can reduce energy consumption within a few percent of variable speed.

RE: Harmonics and VFDs

(OP)

Quote:

as the back pressure of a centrifugal pump goes up, the horse power required goes down
If you consider constant flow, then as the back pressure goes up for the same flow, hydraulic power also goes up. (flow x pressure) The fall in power is due to the reduction in flow, not the increase in pressure. If you reduced the flow, and kept the pressure constant, then the hydraulic power would reduce.The restriction of the valve is reducing the flow and increasing the pressure at the pump. The reduction in power due to the reduction in flow is greater than the increase in power due to the increase in pressure.
Reduce the flow without an increase in pressure and you get a better reduction in power, hence the flat curve pumps yield a better efficiency when throttled than the steeper curve.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Harmonics and VFDs

Valvecrazy, I have seen VFD pumping installations running at a constant pressure with a low flow. These installation are a waste of money because, just like you have argued, the $/gallon pumped goes up. The pump may be running slower and using less kW but the required kWh to pump X amount of liquid is higher.

However, your valve installed in the same application would still be the same waste of money, maybe even slightly more so depending on the system. At any rate, the valve is not going to save any money compared to the VFD in the above mentioned applications.

I was recently in a pumping station with 500hp-1000hp pumps where they used valve control to adjust the flow. As the pump valve was closed to lower the flow the pump or valve would begin to make a fair bit of noise. In fact, it was quite loud in the station when running at minimum flow. So, there has to be some negative effect caused by this turbulence in the pipes.

I have seen other VFD installations on down hole screw pumps where, besides controlling the pump speed, the VFD also helps to keep from twisting off the rod. These applications are outside in harsh environments but the much higher priced VFD (compared to a full-voltage starter) is still purchased because the well owners have tried a few solutions and feel it is the best one.

So, as I and others have already said before, every application needs to be evaluated for the best solution. Remember, some people here don't just work in one field (irrigation?) but rather see every application you could think of.

One thing not discussed yet is Marke's application. It may be an application where different solutions have already be tried and the VFD has already been proven to be the best solution. Maybe a valve can not work or does not work nearly as well.

RE: Harmonics and VFDs

"At any rate, the valve is not going to save any money compared to the VFD in the above mentioned applications."

I never said a Valve would save money over a VFD.  What I said was that in constant pressure applications, a Drive can rarely save energy over a Valve.


"I have seen VFD pumping installations running at a constant pressure with a low flow. These installation are a waste of money because, just like you have argued, the $/gallon pumped goes up. The pump may be running slower and using less kW but the required kWh to pump X amount of liquid is higher."

Thank you for this, as this is really the only point I am trying to make here.


"As the pump valve was closed to lower the flow the pump or valve would begin to make a fair bit of noise. In fact, it was quite loud in the station when running at minimum flow. So, there has to be some negative effect caused by this turbulence in the pipes."

The noise you were hearing was from the water squirting through a small opening as the valve was throttling the flow.  This turbulence does not hurt the pipe but, causes the seat in the valve to wear, known as "wire draw".  This wear causes the valve and seat to leak when closed.  The valves I use are designed to never completely close so, wear on the seat that would make other style valves fail, has little effect on these type valves.


There are many good uses for Drives.  I would only use valves to control pumps that produce fairly clean, fairly cool, water only, produced at a constant pressure or head.  This is a very small niche in a very big market for pump systems.  However, there is a very large number of pump systems in this particular niche.  This niche includes irrigation but, also includes domestic housing, municipal systems, booster systems, feed lots, chicken farms, pork producers, dairies, car washes, concrete plants, heat pump systems, snow making machines, and thousands of other types of systems.

Every system does need to be evaluated.  First evaluation is to see if the pump systems fits into the niche described above.  If it does, then further evaluation needs to be done to see if a pump with good brake horse power characteristics can be used.  If this is possible, then a valve will make a good control system without adding the side effects of Drive control.  

If you are referring to Markes system where a 75 KW pump was used when a 45 KW would have done the job, a valve would have probably restricted the pump to about the same 45 KW as the Drive.  If the pump had good brake horse power characteristics, a valve would be so close to drawing the same 45 KW as the Drive, that the cost of the Drive would never see a payoff.

I would never chime in on a thread unless the pump application fit the particular niche described above.  Because Drives work so well on many applications not within this niche, many people make the wrong assumption that a Drive is also the best fit for pump systems within this niche.  I personally would have to first rule out valves, level switches, pressure tanks, and any other possibility before I would even consider a Drive.  Anytime you can successfully control a pump system with anything other than a Drive, you do not have the problems or side effects that are injected into a system when a Drive is added to the equation.

RE: Harmonics and VFDs

Oh my god, I've re-read some of this thread and apparently VFD's cause brain damage through the "stray voltage" problem?
I've googled it and come up with phrases such as "Dirty Power", "Electric pollution", etc.
Check this out:
"But, similar to fluoride, tobacco, alcohol, drugs, dioxin and certain chlorinated hydrocarbons, when electricity is not properly controlled, or when biological systems (people and all other living creatures) are over-exposed, electricity destroys certain functions of the brain and the immune system and increases proliferation of cancer. This is an unseen and unheard killer."

I'm terrified!! Valvecrazy, thank you for warning me of this "Unseen Killer". I'm off to find a job that doesn't involve VFD's....maybe something with valves?
Cheers,
Mort.

RE: Harmonics and VFDs

Personally I thank Valvecrazy for his insight, which at the very minimum gives me pause for thought.  

As for the previous post.  The phrase "sarcasm is the lowest form of wit" comes to mind.

RE: Harmonics and VFDs

This has been a most informative thread.  I have been viewing its development from the original post,  and the numerous diversions from there.  Declaration,  I'm a mechanical engineer,  and this is definitely an electrical engineering forum,  and I admire those of you who understand electrons.  
I would never presume to intrude on the electrical engineer's domain,  but let us reflect that while we mechanical people bow to the wisdom of the electrical people regarding drives, and the behaviour, and yes losses inherent in the drives, we also have some knowledge regarding the loads to which these drives are connected,  and hopefully something constructive to contribute to the greater good.
Let's not get upset, let's not get overexcited, and above all let's not get personal.  There's a lot of value in the contributions here,  and I would be disappointed if it gets lost in people taking 'positions'.
Every application has to evaluated on its own merits.  As professionals,  it's important to remember that.  Typically,  we rely on our own experience to evaluate  initially the comments made,  which may have been made on a different set of parameters,  and that may influence our perception.
And another point is that the $$$ involved in the project should be considered on a lifetime basis,  including capital and operating costs , including energy and maintenance and downtime and perhaps even frustration.
mac

RE: Harmonics and VFDs

Hi macmil;
Thanks for the comments. Well said and appropriate.
A little analogy to help understand things from the electrical side.
Consider a plant that must from time to time move rail cars.
It takes about 1000 lbs. of force to start the rail cars moving. The crew uses a truck on a road parallel to the track and a long rope with an ultimate strength of about 3000 lbs and have never had a problem. One day for some reason someone tries to pull the cars with a very short rope and instead of the angle between the tracks and the rope being almost zero degrees, the angle is about 85 degrees. The rope breaks before the cars move. Why? The rope is good for three times the force needed. The answer, as you well know, is that pulling at an angle to one side the force in the rope may be several times the force actually applied parallel to the tracks. At an angle of 85 degrees, the tension in the rope will be about 11 times the effective force moving the load.
That is power factor, the cosine of the angle between the volts and the amps. Also the power factor of a motor varies as the load changes. When we measure amperage, we know that we are not measuring power. We must make an adjustment for the power factor or the angle between the volts and the amps before we know the true power. Those of us who work with power are making these adjustments frequently. Some equipment is rated in watts and some is rated in amps. If we are concerned with energy usage we must convert the amps times volts (VA) to real power. If we are concerned with panel, conductor or transformer loading we must convert the watts to amps times volts (VA). Always using the cosine of the angle between the amps and the volts. When the electrical power professionals were told that the difference between amps (apparent power) and real power was not important a lot of them followed Elvis and left the building. Too bad. Someone who does know a lot about pumps shot himself in the foot with that one.
Thanks again for your comments, macmil. I felt that you deserved an explanation for the sudden chill that fell on this thread.
(Volts times Amps = Volt Amps (VA)
Volts times Amps times the cosine of the angle of displacement = Watts
There is a difference)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

Volts times Amps times Power Factor equals watts.  I didn't mean to make light of the power factor but,

"At low power levels, the differences between VA and watts are often slight."

Also Drive guys try to make a big deal out of the fact that Drives can keep the power factor up.   However, the Volts times Amps part is also very important.  You can say that a Drive maintains  .97 power factor.  Without a Drive the power factor will vary and the efficiency will vary.  However, what really matters is the electric bill at the end of the month.  No matter how you slice it or how you do the math, there is very little difference between the electric bill when using a Drive and when throttling a full speed pump, when constant pressure is the objective.  I am used to the sudden chill when discussing this with Drive people, but then again, I always feel a chill when I am around Drive people.

RE: Harmonics and VFDs

Could someone tell this guy (you know who I mean) that the difference between W and VA is at its maximum when the motor is lightly loaded. That is when cos(phi) is low, often below .5 sometimes as low as .3

It is interesting that he has found someone electrical that he trusts that has told him about the relationship between V, A and W. It is a pity that he didn't get - or didn't bother to remember - the correct formula. It is three phase motors we are discussing? Isn't it?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

Valvecrazy

While I found some interesting points of view about valves as a power saving device, your post "At low power levels, the differences between VA and watts are often slight." is wrong.

On the contrary, on the low power levels with rated voltage to the motor, the difference between VA and Watts is very high due to very low power factor. Also, electric bills, (which you rightly say is what matters), is based on power, which in turn is a product of voltage, current and power factor.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

RE: Harmonics and VFDs

Thanks edison.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

VC

Your link clearly defines the relation of power, voltage, current and p.f.  The ac example clearly shows the vast difference bewten VA and power when a low power factor is factored in.

I think you latched on to the last paragraph (which you posted above verbatim) and misunderstood it.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

RE: Harmonics and VFDs

Quote:

The difference between the 240 VA apparent power and the 156 watts of true power is the reactive power or 84 VAR or volt-amps-reactive.
The reactive power in this example is actually 182 VAR
I think that by using this inaccurate website to prove your point you may have just shot your self in the other foot.
The simple difference between 240 and 156 is 84. Not a very big number when talking about watts, VAs or VARs.
It is about 56%. (156/240) That is a very big number when large amounts of power are involved.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

edison123 T
Take the watts and VA in the example and work out the real VARs. Then have a good laugh. He almost snuck that one past us.
I saw it on the internet!!! It must be true!!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Harmonics and VFDs

Bill

You are right. I just looked at VA & Watts. Mebbe, this company does not believe in "complex" math.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

RE: Harmonics and VFDs

Waiting for next blamage. That guy, did he actually use single phase motors in his "demonstrator"? Or is he throwing them in now to get away with that incorrect formula?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Harmonics and VFDs

Please, guys, let's put this thread out of its misery!

Refer Mr. Valvecrazy to his nearest engineering university and arrange for an independent test of the two methods.

I'd settle for that!  All this back and forth reasoning is like nailing Jello to the wall.  You accomplish nothing, you make a mess, you spoil the Jello, and, for sure, frustration sets in about the same time logic floats away!

Please stop!

RE: Harmonics and VFDs

I would like to respect DickDV's wishes and put this thread out of existence but I feel like I have earned the privelege of making one last post on this.  After the World Ag Expo Show I went sailing up the coast of California from Mexico on a small (45') boat and have been out of internet contact.  I just got back and logged on and was surprised to see the number of replies.

I won't make any replies to Valvecrazy's statements in particular since using any engineering principles doesn't seem to go very far.

I would like to say that it is fairly obvious that using a valve does NOT create energy as has been intimated by Valvecrazy.  The system was also about the best setup one could have to test a constant head system without actually pumping up a hill.  Although the VFD was still better than the valve in the constant head system it probably wasn't better by enough in this circumstance to make a VFD worthwhile in a typical irrigation application.  Valvecrazy should realize that contrary to his experience, constant head systems are NOT the norm in the vast majority of applications, mostly the flow is of concern, not the pressure and therefore VFDs would have greater advantages over valves in these cases.  My Amprobe 41PQ indicated a 1% voltage THD and a 50% amperage THD.  A line filter would not produce more than a few watts loss on this system.  I don't think that I have seen any reputable evidence that VFDs cause birth defects like I believe I heard in an offhand comment at the booth with the test setup.

To answer Marke's question it was basically impossible to remove the valve without a lot of trouble so it was not done.  Obviously this would have provided more savings for the VFD since even at wide open the valve will have some pressure drop (apparently around 7 psig per Valvecrazy).

To answer Waross's question, a pump curve with a rising pressure curve to zero flow would be good on a VFD application with a lot of static head.  This way you could get higher pressure at lower flows than with a flat curve which would allow you to reduce the speed of the VFD more.  A flat pressure curve to zero flow would be good for a VFD application with no static head or for a valve with constant head.

I did not think of this thread for the whole week that I was sailing and I don't plan on thinking about it again.  If Valvecrazy makes a response then we should just let him have the last word.







RE: Harmonics and VFDs

Just an idea for the original post question....often the rural feeders such as this will have one or multiple sets of mid-line voltage regulators.  If there is an existing mid-line voltage regulators the connection could be changed such that it introduced a 30 degree phase shift in the line.  If this feeder has normally open ties to adjacent this may not be a possibility, for safety reasons.

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