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Pumps with Built-in VFD's

Pumps with Built-in VFD's

(OP)
Hi all

Does anyone have experience with pumps with built-in VFD's?
I suspect a few draw-backs in specifying these.
1. How does the pump react to pressure? does it have built-in PD or is PD provided by others near the pump? can it read multiple PD's far away? ASHRAE 90.1 require remote PD's, not close to pumps.
2. Upon VFD failure, does that mean you need to replace the entire pump?
3. do they have VFD by-pass?

Thanks

RE: Pumps with Built-in VFD's

Hi Cry,
1) I'm sure that any modern VFD today has built-in PI(D) control. Alternately, if you have a PLC or DCS, you can do the controls there if you wish. Is there a Plant standard?
2) Depends, do you have an installed spare? I would think that you should be able to swap out the VFD quicker than the entire pump & VFD.
3) Nobody offers VFDs with bypass in 2017.

GG

ps I can't think of a single situation where a pump c/w integral VFD would be a good idea.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Pumps with Built-in VFD's

GroovyGuy: most (if not all) external VFD offer bypass option that also acts as fused disconnect. Unless you were referring strictly to in-pump-VFD

For larger pumps like the Grundfos TP I found the cost with built in VFD is similar to just the pump and 3rd party VFD (I use Danfoss, so no crap). In that case i opt for external since i have better chance of repair. If i recall correctly the rep told me the entire unit needs to be replaced in case of failure. If that is true it would be costly.
For smaller pumps you have ECM pumps, so no VFD, but an inverter control (i.e. Grundfos Magna and Alpha). They are single-phase and you don't really find single speed pumps and VFDs (at least not that I know of good ones)

RE: Pumps with Built-in VFD's

(OP)
Wow, thanks for the pitch guys.

I run into a VA hospital situation (doing a facility assessment) and they had 17 domestic water pumps with built-in VFD (Grundfos brand). They had a power outage that resulted into 16 of their 17 VFD's failure.
They had to replace all 16 pumps. a by-pass would have done the trick.

What I don't see in these built-in VFD's (and here I agree with groovy guy) - I do not see a true pay-pack for something that is so much subject to failure, especially for domestic water use (where they tend to be implemented), not much pressure build-up in system to justify the VFD to begin with.

Thanks all

RE: Pumps with Built-in VFD's

2
Constant speed pumps have not been commonly used for domestic water systems for awhile - since VFD became affordable.

The problem with domestic water is the wide variability of flow rate. There is the chance you could have a lavatory be used at 0.35 gpm or you could flush a few toilets and need 75 gpm - all at the same pressure requirement. But at lower flows, the pressure from a constant speed pump could be excessive.

So a PRV could be used. But this is like driving your car with your foot on the accelerator and the brake at the same time.

A pneumatic tank could be used, but it would have to be fairly large to minimize pump cycling.

Using a VFD saves energy. In fact it is one of two items which plumbing can contribute to meeting ASHRAE 90.1.

If a power outage took out 16 of the 17 VFD - it sounds like there was something other than the VFD at fault.

RE: Pumps with Built-in VFD's

Howdy EnergyProf.: My comment was concerned with the fact that VFDs are so reliable these days, nobody bothers with a bypass anymore. (unless you are using Grundfos VFDs.) lol

The other issue I have with an integral VFD is "Who made the VFD?" I'd much rather spec a reliable make of VFD (ie (Mitsubishi, Toshiba, Siemens? etc) than end up with who knows what, made who knows where.
GG
ps Yeah I know... I'm a drive snob.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Pumps with Built-in VFD's

(OP)
Pedarrin2

Speaking of ASHRAE 90.1, 5 HP or less motors are not required to have VFD's, the savings being questionable. Most domestic water pumps have a much lower motor HP. Say a 2HP pump will hardly save you any money at all. i.e. no pay back can be achieved.

The other thing is: Booster pumps needs to maintain a certain water pressure anyway, pump cannot be slowed down to less than minimum water pressure, which makes the VFD fairly useless.

Then again, ASHRAE 90.1 calls for Pressure differentials to be remotely installed, at the end of the run of each major zone, and it is proven that delta P across the pumps main headers do not work, which is what built-in VFD's use.

So, in my opinion, this energy savings is all hype, built-in VFD's save pennies, add maintenance and less reliability (no by-pass).

What do you guys see as pay-back, are they really worth the money on anything 3 HP motor or less?

RE: Pumps with Built-in VFD's

Cry24

I do not see any minimum in the Service Water Booster Pump section 10.4.2. Granted, the lower the horse power, the less savings that would be seen.

I agree that the pumps need to maintain a pressure, but pump curves decrease pressure as flow increases. So a pump with a design point of say 100 gpm and 30 psi TDH, if only flowing 5 gpm, could be at 45 psi, which could exceed the 80 psi maximum for plumbing fixtures allowed by plumbing code. That is why constant speed pumps typically had a PRV to combat this situation.

ASHRAE 90.1 also states "logic shall be employed that adjusts the setpoint to simulate operation of remote sensors.

And since horse power is proportional to the third power of the speed which is directly proportional to the flow, if I can lower the flowrate at lower than peak demand, I decrease horse power and thus energy consumption by a third root. There are published studies that indicate there definitely is a cost savings with using VFD on domestic booster pump systems.

RE: Pumps with Built-in VFD's

The logic of arguing the third power relationship is not really correct because that assumes there is no flow control valve. The alternative to a vfd is not nothing, it is a throttling valve. While throttling valves do consume some energy it is often far less than made out to be, because a small increase in pressure drop can have a large effect on flow rate when a centrifugal pump has a flat pump curve.
"And since horse power is proportional to the third power of the speed which is directly proportional to the flow" is simply not correct. Power is equal to flow times differential pressure across the pump.

RE: Pumps with Built-in VFD's

Cry24; You appear to be completely ignoring the major PITA of water logged pressure tanks and galloping pressure cycles granted by said systems. Most people could care less about a dollars difference in a month's bill to have steady instant quiet water supplied. Around here no one bothers with a bunch of space consuming eye-sore pressure tanks in new installs. No one does this based on energy savings or even cost.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Pumps with Built-in VFD's

On bypass: they still are needed (nothing is 100% reliable) and don't really cost more since when you have a VFD-manufacturer disconnect you also get the required disconnect and fuses. If you opted for not having that bypass you would need to field-install a fused disconnect. Certainly costly too and clutters up the installation.
also for testing (if you just need to know if the motor works...) a bypass is valuable. i don't know other manufacturers since we use Danfoss only, but all the VFDs offer bypass.

Probably moot for this discussion since the built-in VFDs don't have bypass. but if you use external VFD, I'd opt for them.

for smaller pumps (where you can't get good 3-phase pumps) the Magna, Ecocirc with ECM and built-in control probably are OK. but for larger pumps I think having a pump and separate VFD is more substantial. not sure where the cutoff is. smallest 3-phase pumps we use are 0.5 or 0.75 hp. so anything below is a candidate for built-in inverter (it is not a VFD for ECM)

RE: Pumps with Built-in VFD's

Howdy EnergyProf; For the last several years I have installed hundreds of VFDs, with a TCL of > 50,000hp, and have never once installed a drive with a bypass. Where downtime cannot be tolerated, you always have an installed spare. This includes not only the drive / motor, but the mechanical / process equipment as well.
This is typical for the industry I am currently in, as well as every industry that I have worked in for the last 25 years. However, I do remember a time when VFDs were first introduced to the market, when reliability was not what it is today, that every drive would be equipped with a bypass, but that was many years ago. I would suggest that if you are experiencing lotsa drive failures today, you need to change your drive OEM, or look at your installation methods / standards.

BTW, a bypass requires more than a fused-switch; it requires a complete starter.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Pumps with Built-in VFD's

True, the starter is big part of the cost. But VFD manufacturer buy thousands of starters at low price and has it installed by robots or efficient or cheap labor. A field-installed disconnect doesn't need the starter part. But the EC buys only one at a time, and uses very expensive labor to install, add conduit, fasten to a wall etc. At least for the small drives we use (typically up to 3 hp, sometimes up to 25 hp) it really doesn't make much difference in cost. I deal with HVAC pumps and fans. For fans we typically don't have redundancy. For pumps we mostly do have lead/lag.
You make a good point, though.

RE: Pumps with Built-in VFD's

Compositepro,

I was trying to state the Affinity Laws which state S1/S2 = Q1/Q2 = H1^.5/H2^.5 and BHP1/BHP2 = S1^3/S2^3 where S is the impeller speed, Q is the flow rate, H is the pressure head, and BHP is the brake horsepower. If I miswrote this in my text, I apologize.

The way I understand it, the lower pump speed required for less than design flows will reduce the BHP of the pump at that point, thus reducing the energy being used.

Using a flow control valve, or a PRV is, like I said, analogous to having your foot on the accelerator and the brake at the same time. which is a waste of energy. In this analogy, slowing the RPM of the engine to go 20 mph is better than holding the brake while the engine wants to go 60 mph. That might be a simplistic analogy, but that is how I understand it.

RE: Pumps with Built-in VFD's

Pump laws can't be applied like that. They assume no added throttling and equal efficiency at any point of the pump curve and speed. You can use them in a loop that increases or decreases flow and pressure accordingly. In a throttled system (valves) pressure often doesn't decrease as much. It gets a bit better when you use static pressure reset, though.

For domestic water I'd think another advantage of VFD is less cycling, which also increases efficiency and reduces wear.

RE: Pumps with Built-in VFD's

EnergyProfessional,

I understand the affinity laws are fairly constrained to "perfect" cases, but even with efficiency changes on the pump curve, a 1%-2% or so loss of efficiency is not going to counteract a third power relationship much.

The amount of static head requirements on the piping system does play a part, but i don't know how significant a part it plays.

Also, the use of throttling valves is a necessary evil that had to be used to minimize the pressure from the constant speed pump being utilized at lower flows than the design point, which is a good part of the argument to use VFD.

Another way they used to get around this was to use a large expansion tank which allowed the constant speed pump to cycle less and maintain system pressure. But these take up a lot of real estate. When the VFD controls became more economical and available about 10-15 years ago, their use proliferated on the domestic system design.

RE: Pumps with Built-in VFD's

I agree with the points EnergyProfessional has made about VFD's bypasses. It all depends on how much redundancy you have and how long you can tolerate a disruption.

I would caution against using VFD's without good reason. Harmonics can be a big issue, especially if you do not really understand what harmonics is.

RE: Pumps with Built-in VFD's

Pedarrin2, have we not had this conversation before? The analogy between an electric motor on a pump and the speed of a car is completely inappropriate and explains your confusion.

By design electric motors run at nearly constant speed, but the speed has no bearing on how much energy the motor uses. If there is no load on the motor it will use almost no energy running at full speed. The important and unchanging relationship is that pump power is equal to flow rate (Q) times pump differential pressure (DP). The way that motor speed affects Q and DP can be complicated and depends on the pump curve and the system curve, which is variable when there is throttling valve. In a well designed system the throttling valve will increase the DP a little bit and result in a relatively large drop in Q. This causes a large drop in the energy that the motor must provide and thus a large energy savings even though there is some energy loss at the valve. This loss may even be less than the constant energy loss in a VFD (about 4% of full load).

So, please abandon the car analogy because it is wrong and will lead to wrong decisions about where using a vfd is appropriate.

RE: Pumps with Built-in VFD's

It is incorrect to say no one offers bypasses on VFD's in 2017. It is just not true. Also as you can see from the discussion above there is a lot of misunderstanding of VFD's as being an energy saver.

In the example above in a VA hospital you know that something is not quite right with the design concept if you need to have 17 domestic water pumps on 17 VFD's. In this case, 16 VFD's had to be replaced in one shot. This is the perfect example of people throwing in VFD's willy nilly without understanding what they are doing and what the implications are. I hope there was no lives lost because a lot of pacemakers and medical equipment could have malfunctioned because of the harmonics that was created

When there is a problem with a VFD a maintenance person does not know what to do. He must bring in a specialist and most often the VFD will have to be replaced lock stock and barrel and then it has to be reprogrammed. Good luck on reprogramming the VFD if there is no past documentation on how it was supposed to be programmed. I again say this....do you really need a VFD or is it a lazy man's way to try to quickly solve a problem you have not attempted to even try to understand? It is my belief that a lot of engineers are weak in the basics of engineering if they blindly try to explain that VFD's are being used because they save energy

RE: Pumps with Built-in VFD's

QualityTime,

Wow, where to start on the myths?

Basic programming of a VFD is within the capability of many maintenance techs, especially reloading of a previously-defined parameter set into a replacement drive. Record keeping and backups of settings are just prudent management of the system. Most techs are willing to learn, so invest in some basic training.

Harmonics should not be a problem on a properly-designed system. There are standards which define limits for harmonic pollution, and when equipment is installed the system should be designed to meet these standards. Incompetent design doesn't mean a technology is bad, just badly applied.

Sixteen failures from a single event suggests either a very poor quality drive, or something very wrong with the installation, or that the electrical system has experienced a major event which has likely affected a lot of other power electronics devices.


I do completely agree that VFDs are often mis-applied, and that their benefits are almost always over-sold by salesmen trying to hit their targets. The most obvious solution to save energy - switch the motor off - is often overlooked. Energy saving is a possible benefit if the VFD is a suitable device to control the process variable, but in many cases there are other options for control elements which give a lower overall cost.

RE: Pumps with Built-in VFD's

Hi Scotty:

...Incompetent design doesn't mean a technology is bad, just badly applied...
...Sixteen failures from a single event suggests either a very poor quality drive, or something very wrong with the installation, or that the electrical system has experienced a major event which has likely affected a lot of other power electronics devices....
...I do completely agree that VFDs are often mis-applied, and that their benefits are almost always over-sold by salesmen trying to hit their targets...


I think you just echoed my point about engineers who use VFD's willy nilly smile .

As far as for basic programming of VFD's by instrument techs my point, in my head, was that I would think that the vast majority of commercial office buildings, smaller sized factories etc have no instrument techs. They would source that expertise to an outside contractor. That was most likely how it was installed in the first place. Unless you are programming EVERYDAY and that IS YOUR CORE BUSINESS there is no point to trying to KEEP UP with the technology because you will just forget it. The drives are changing all of the time and a replacement drive will likely be "another model" which will much likely have different programming subroutines. Then there is the issue of whether you understand what you are programming. An example would be that a programmer may be responsible for programming the operation of a filter in a municipal water treatment plant. He may have programmed the water level in the filter so that it bounces up and down and yet he would think that is normal because he does not understand water treatment. An experienced water treatment engineer would walk by the filter and would know that the water level should be constant all of the time for water quality reasons. He would then talk to the programmer for corrective action. My point is that, for your everyday maintenance and electrical department, it is easier to diagnose and replace a starter, motor and fan belt than a VFD full of electronics, resistors etc .....Apply the KISS principle smile

RE: Pumps with Built-in VFD's

Differences between industries I guess. In the power industry and O&G many maintenance techs would be capable of doing this type of work. Electricians wouldn't necessarily have the skills, but they're two different competence levels within the trade even though some organisations blur the lines.

Yes, VFD's are an excellent modulating device in the right application, and a very poor one in the wrong application. Good engineering distinguishes between the two and makes an appropriate choice.

RE: Pumps with Built-in VFD's

Compositepro

The car analogy may be simplistic, but it makes sense to me.

And please educate me where I am wrong. I want to learn more.

As I understand it, running at constant speed, the pump has a single curve it runs on (higher pressure at lower flow). This is dependent of course on other factors, but let's keep it simple. The system curve is based on piping, fittings, flow rate. Where they intersect is what determines the actual flow conditions which includes hydraulic efficiency. Choosing a pump where this point is maximizing the hydraulic efficiency is the ideal. Total efficiency includes both hydraulic and motor (electric) efficiencies.

So, if my design point is 100 gpm and 50 psig boost, but at times the actual flow required is 25 gpm, the system curve will change, but the pump curve will not. I know such a reduction in flow will likely be a big hit on the efficiency and the affinity laws don't like that, but it is real world (at least in my world). It will follow its curve to the new intersection, which will be a higher pressure. But, if i am required to stay below 80 psig (it is plumbing in my world) and my static head is around 30 psig, then I have to put in a PRV, throttling valve, etc. to keep the total pressure less than 80 psig. But I have now wasted that energy the constant speed motor is inputting to the impeller.

But, with the VFD, the pump slows, changing its curve and the intersection of the two curves is at a lower pressure and less energy being used.

I admit I am not knowledgeable about the electrical aspects of how the VFD works - I deal with the water in the pump. But if I put energy into pushing the water, but then throttle it back, how is that different than having your foot on the accelerator and brake at the same time. Wouldn't it be better to pull off the gas and run at a lower rpm?

Please tell me where I am wrong.

RE: Pumps with Built-in VFD's

Hi Pedarrin2:

I have three comments:
  1. When you speed up or slow down the pump, the intersection of the system head curve and the pump curve will be elsewhere. That means you are on a different part of the pump curve with respect to efficiency. That has to be taken into account
  2. A VFD will produce about 3-5% reject heat at the electronics. You just lost efficiency right there and that is on top of the motor efficiency loss.
  3. For large VFD's, HVAC systems MIGHT need to be installed to get rid of the reject heat. More energy issues and maintenance costs to consider
I guess all of the above has to be weighed off against putting in a modulating valve

RE: Pumps with Built-in VFD's

QualityTime,

1. Efficiency does play a part with using the affinity laws, but from what I have read, it is only a factor if you have a significant static lift which makes the system curve start higher so it is more "horizontal" than vertical and crosses more of the efficiency curves.
2. Efficiency for the pump includes both electrical and hydraulic. From my perspective (dealing with the water side only), I am focused more on the hydraulic portion. But it is interesting about the heat loss and effect on the electrical efficiency.
3. The pumps I deal with are typically 5 hp at most - so I typically don't have an issue with requiring extra HVAC for my VFD.

As an aside, my VFD are always on the pump, not remote like a lot of HVAC VFD. Not sure how that plays into the discussion. Maybe the plumbing pumps are VFD light.

RE: Pumps with Built-in VFD's

I agree with an earlier comment by @PEDARRIN2 - if a power failure took out a raft of pump inverters it sounds like they need to review their power infrastructure. I'd suggest they need to do a protection and short circuit study to ensure any fault is cleared by breakers closer to the source, protecting all the other system components.

I also agree with the sentiment of removing VFD bypasses - if it is critical have a spare fed of different power distribution. I have a large MCF client removing VFD bypasses from all their future buildings as they have sufficient redundant components.

RE: Pumps with Built-in VFD's

(OP)
Great discussion,
Thank you very much for your inputs

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