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Wedoca (Electrical) (OP)
11 May 09 17:02
Hi people ,

I recently have seen a Culter Hammer VFD been used as a phase convertor get gain 3 phase at an area where only single phase is avaliable. now ....the problem is that when every we run the pump the output frequency would only go up to around 50 hz.. and it gets worst when I try to run both pumps (its a dual pump station) at the same time (Frequency drops to around 45 hz) ..I have checked the VFD setting and its all correct .. I have consult with the VFD Manufactor tech. and he said its because the utility is not supplying enough power to this local station......can someone tell me if thats true ?? or is there any other possible explaination??? is there a solution???
 
Helpful Member!(3)  jraef (Electrical)
11 May 09 18:16
A couple issues.
ANY VFD is inherently capable of "converting" 1 phase power to 3 phase. they rectify the AC to DC, then invert the DC back into pseudo AC. So to the VFD, the AC source power is just a "raw material" and it doesn't matter if it's 1 phase or 3 phase. But in converting from a 1 phase source, the amount of current it draws is going to be 1.732 times the amount of current that the motor consumes in 3 phase. In addition, the added ripple in the DC from converting 1 phase is higher and needs more capacitance to smooth it out. For those reasons, the VFD that you chose when you do this must be twice the size of the AC FLA of the motor. Some people either are unaware of this or think it's OK to ignore it because they think that if you have a VFD, it's because you WANT to run slower and therefore ill not need all of the power.

Issue #2 is that VFD manufacturers play some games in the sizing of VFDs for pump loads as well, stating that a drive that is rated for one size motor is capable of running a larger motor if it is a centrifugal pump. While not inherently incorrect, that rule does NOT apply if you are using a 1 phase supply, yet many VFD sellers are again either unaware or uncaring.

tying that together for your situation is the likelihood that you have one or the other of the above working against you, possibly even both, leaving you with a VFD that is incapable of supplying enough power to your motor if running full speed. How that translates to your not being able to get past 50Hz with one pump or 45Hz with 2 is that most VFDs can be programmed to respond to an overload by automatically reducing the speed in an attempt to keep the output current below the drive's capacity. In other words your speed is dropping because the drive it trying to keep from frying itself. the drive tech is probably incorrectly assigning that to the supply when what he means is that someone sold you a VFD that is too small for what you want to do, which he may he loath to tell you.

The only way I can think of that an inadequate supply could cause this is if the full loading of the pump or pumps at full speed is causing a line voltage drop, and that is in effect increasing the current that the motor(s) will draw, so the VFD responds in the same way. If that's the case, only your utility can help by installing a larger service transformer, something that is not going to come cheaply. You may need to re-think your capacity.


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sibeen (Electrical)
12 May 09 7:14
A gold star for jraef. A succint and helpful answer.
Helpful Member!  LionelHutz (Electrical)
12 May 09 8:39
The ripple voltage on the DC buss increases as the VFD load increases. I'd guess what is happening is a combination of line voltage dip and ripple voltage creates a lower effective DC (or average) buss voltage.

The VFD will be programmed to ouput the maximum AC voltage possible by taking the DC buss voltage and dividing by sqrt(2). So, with the effective DC buss voltage lower, the maximum AC output voltage is also lower. Since the drive follows a V/Hz curve, the lower maximum AC output voltage corresponds to a lower maximum frequency.

The end effect is that you need more capacitors in you VFD, obtained by buying a larger VFD and/or you need to increase the input voltage.

If the incoming voltage is 208VAC then don't try to set the motor voltage to 240VAC. You can't get out more than you put in. If the drive has DC buss terminals then you could build up an additional DC capacitor bank and add it to the drive.
 
Helpful Member!  DickDV (Electrical)
12 May 09 8:42
Golly, jraef, that answer was a work of art.  It belongs in the archives.  Well done!
sibeen (Electrical)
12 May 09 10:20
If the incoming voltage is 208VAC then don't try to set the motor voltage to 240VAC. You can't get out more than you put in.

I'm going to throw in a question here. I worked in the power electonics industry for about 20 years and yet never worked on a drive.

I thought the drive industry had mostly shifted over to active (force commutated) rectifiers. Is this the case or has some sales engineer been pissing in my pocket? The question was inspired by the answer by LionelHutz. If using an active rectifier, then you're basically using a boost circuit on the front end and the DC link voltage is then limitied by the front end inductors and the switching frequency.

If some have shifted to force commutated, are these rectifiers designed to operate in four quadrant mode?
Helpful Member!  ScottyUK (Electrical)
12 May 09 13:43
I think most of the commodity drives are still dumb six-pulse diode rectifiers unless you specify otherwise.
  

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

jraef (Electrical)
12 May 09 13:50

Quote (sibeen):

I thought the drive industry had mostly shifted over to active (force commutated) rectifiers. Is this the case or has some sales engineer been pissing in my pocket? The question was inspired by the answer by LionelHutz. If using an active rectifier, then you're basically using a boost circuit on the front end and the DC link voltage is then limitied by the front end inductors and the switching frequency.

If some have shifted to force commutated, are these rectifiers designed to operate in four quadrant mode?
Probably deserving of a new thread (hijacking is strongly discouraged in this forum), but we'll let this slide this one time. wink

Certainly there are a few VFD manufacturers who OFFER an active front-end inverter option, but they are essentially twice as expensive as a passive 6 pulse inverter, so they are by no means "mostly shifted over"; the market couldn't stand the price increase! So the salesman who told you that most likely had an agenda; i.e. convince you that you were buying the latest and greatest over priced drive because that's what he wanted to sell you. I know of no mfr that is offering that exclusively. Now certainly, if you have a high harmonic mitigation issue or a severe braking (4 quadrant) application, then it may be a BETTER choice than other options, but again, there is no universal movement in that direction.


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itsmoked (Electrical)
12 May 09 15:32
And "active (force commutated) rectifiers" does not infer "boost" of any kind.

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

ScottyUK (Electrical)
12 May 09 16:27
The active PFC types do boost the DC link slightly above the peak of the AC waveform, but force-commutated (thyristor) types won't: is terminology is getting mixed up?
  

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

sibeen (Electrical)
12 May 09 19:43
is terminology is getting mixed up?

Could be, Scotty. I'm way down under and we may use different terminology here :)

When I talk force commutated I mean the actve IGBT front end style. In this topology the DC needs to be boosted, otherwise it will just operate as a 6 pulse diode bridge.
jraef (Electrical)
12 May 09 19:44
Yeah, maybe I mixed them a little too much, but does anyone really use force-commutated thyristor front ends any longer? I just ASS-u-me-d...

Keith,
A true active front-end drive can boost the output voltage to about 20% above line voltage, or in other words, continue at full output when the line drops to 80% of normal.


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itsmoked (Electrical)
13 May 09 6:13
Hmmm interesting.  So they use the IGBTs as voltage doublers?  Anyone got a schematic lying around?

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

ScottyUK (Electrical)
13 May 09 7:13
This App Note from Vicor explains the principles - basically a boost converter is running continuously from the rectified unsmoothed line input.

A word of warning about active PFC circuits - our IT dept had an expensive power supply front end blow up when trying to follow a heavily distorted voltage waveform with a multiple zero crossing. The distorted waveform is another story but the active PFC certainly did not like it.
  

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

sibeen (Electrical)
13 May 09 9:00
Hopefully the pdf I've just done up helps. It is a fairly simplistic expalnation but hopefully it gets the idea across.

It's one of those things that given 5 minutes and a whiteboard it's fairly easy to explain :)  
Helpful Member!  Wedoca (Electrical) (OP)
27 May 09 15:34
I have checked the pump and it is 10.1HP, and the VFD is 20HP. so I would assume that the VFD is sized properly, and that would elimated the possibly of low DC link voltage caused by not having enough capacitance to smooth out the DC ripple. so the next possibly would be because the utility transfomrer is too small ...but how do I verify that???    
itsmoked (Electrical)
27 May 09 15:37
Observe the input voltage to the VFD.  Does it drop a lot?

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

Wedoca (Electrical) (OP)
27 May 09 15:42
I don't know have to check....
Wedoca (Electrical) (OP)
27 May 09 15:43
what percentage of line voltage drop is acceptable and whats not ?  
itsmoked (Electrical)
27 May 09 15:49
5% is fine.  More, may be fine, but should be assessed as that can relate to supply hardware heating up, (or not).

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

Wedoca (Electrical) (OP)
15 Jul 09 11:18
guys here is the latest ...

the DC bus voltage on the VFD drop from 336 to 287
as I turn on the pump, input voltage drops about 5 volts from 248 to 244.

base on that can I conclude that the cause of all these madness is because the utility incalbable of supplying power to the site?? and possiblely need a bigger transformer ?  
jraef (Electrical)
15 Jul 09 14:17
How fast are you trying to start the pump? It's possible that you are attempting to accelerate as if it were starting DOL, which may indeed be too much for the supply. But by having the VFD, you may also be able to stretch out the acceleration time enough to reduce the strain on the supply system. Usually, it is possible to start pumps with no more than 100% FLA using a VFD, as long as there is no mechanical issue with long slow acceleration (which can sometimes be a problem with self-lubricated pump designs).


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Wedoca (Electrical) (OP)
15 Jul 09 14:21
I tried with 3 second ramp up time and 7 seconds ...and its makes no difference
itsmoked (Electrical)
15 Jul 09 14:29
Tell us more about your setup.

Motor size?

Measured line voltage to the VFD?

VFD model?

Pump type?

Pumping what?

Expected approximate pressure desired?

Estimated head?

What is the power source? (Like a utility pole transformer)

How long is the wire run from the breaker to the VFD?

What size wire is it?
 

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

LionelHutz (Electrical)
15 Jul 09 14:31
No, it appears there is not enough DC buss capacitance.

A 4V drop in the line voltage should not cause a problem. Your line voltage is also high side to begin with allowing a higher voltage drop before it becomes an issue.
 
Wedoca (Electrical) (OP)
15 Jul 09 14:40
Motor Size: 10HP FLA 25A

Line Voltage:  Phase to Phase: 248.6 volts
               L1 to Ground: 123.3 volts
               L2 to Ground: 122.6 volts

VFD model: Culter Hammer SVX9000 25HP

Pump model: submiseable

Pumping Media: Sewage (station is tested using water)

Estimated head: 108ft

The wire run from the breaker to the VFD: 3~5' Max its all inside of a panel

power source: see attached picture

Wire size: unknown
 
Wedoca (Electrical) (OP)
15 Jul 09 14:56
power source: utility pole transformer
 
jraef (Electrical)
15 Jul 09 16:29
No attached picture.
 


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markcraig (Electrical)
15 Jul 09 16:35
What about the cable from the VFD to the Pumps? Is it VFD grade cable or is standard submersible cable being used for the run to the pumps?
Not sure exactly how these effects work, in fact no idea at all, but I have seen 100KW supply fans with VFD's not reach speed due to problems with cable from the VFD to the motor. We replaced the cable and the fans worked. Should have taken more interest in the theory but it was in the middle of a big project and not my part of it.
Wedoca (Electrical) (OP)
15 Jul 09 17:32
jraef (Electrical)
15 Jul 09 19:19
Wow, someone needs to trim that plant!

Appears to be a 15kVA 1 phase transformer. If so, it's probably marginally sized for ONE 10HP 3 phase load when you factor in throughput efficiencies of the VFD conversion, let alone two! Did the PoCo authorize this much load on that transformer?

Most likely your VD situation is happening too fast for your meter to pick up, but is triggering your VFDs Current Limit / Stall Prevention feature and automatically limiting output frequency.


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Helpful Member!  burnt2x (Electrical)
15 Jul 09 19:33
Service transformer is too small I believe. Since you have said you have two submesible pumps, that's too much for the 15 kVA supply. Ask for a bigger transformer from power company.
DickDV (Electrical)
15 Jul 09 19:34
Let's see if I've got this straight.  You've got a 230V 25hp VFD on a 10hp 230V motor on a pump.  The power supply is 240V single phase and runs from 244-248V over load changes.

We surely can't fault the drive sizing.  It is more than twice the size of the motor.

We can't fault the power supply.  At motor maximum speed it holds up to 244V.

So, it seems to me that the system is either running up against a current limit setting somewhere or the drive is simply not programmed properly.

I'd give improper programming about a 90% lead in this case.

Has the correct motor data been entered into the drive?

What is the current limit setting in the software?

When the motor hits the speed limit, what is the drive output current at that point?
Wedoca (Electrical) (OP)
15 Jul 09 20:21
Sorry Guys ,, its actually a 20HP drive my mistake ..all motor data has been input in the VFD and but the current limit is set at the FLA which is 25A ... the drive never reach the speed limit the limit is speed set at 60hz 3480 RPM  
burnt2x (Electrical)
15 Jul 09 21:06
Wedoca measured the DC bus link dropped from 336 to 287V (see post 15 Jul 09 11:18). I believe the supply voltage at that 287VDC instant couldn't be higher than 200VAC. Am I correct to say that the measurement done by Wedoca are off by some percent?
jraef (Electrical)
16 Jul 09 0:43
I think that's a distinct possibility, I'm surprised that none of us picked up on it earlier. PLS for you.

Wedoca,
It's entirely possible that your voltage readings are skewed by the harmonics on the line as a result of the VFD.

Be that as it may, the fact that you have the current limit set for 25A on a VFD capable of 61A, it's entirely possible that the VFD is artificially limiting frequency to maintain the current limit setting. It may also be related to the following:

Quote (SVX9000 Manual):

620 Load drooping
Setting a proper value to the Load Drooping parameter enables the load drooping.
This function causes the speed to decrease as the load torque increases. At rapid load
changes this gives a smoother torque response, when a part of the required power is
taken from the system inertia. This operation causes a constant, torque-dependent
steady-state error in speed response.
So check to see what parameter 620 is set at.


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Helpful Member!  ozmosis (Electrical)
16 Jul 09 2:34
I think your current limit is set too low.
For the SVX9000 to operate on a 1ph 240V supply at such high current ratings you will have high instability due to the ripple on the DC link. I have seen this with other 'brands'(vacon) same as the SVX9000 where the current limit PI gains are all wrong and the current limit is not set up for a 1Ph supply.
It is a common issue for drives that simply disable the input phase loss detection, oversize the drive (as you have done) but then do not further work on commissioning the drive correctly to cater for the instabilities working on a 1ph supply.
I have no idea what the settings should be but suggest you maybe leave the default motor settings in the drive and adjust the current limit up from a point where it stops going into current limit. Not too scientific but unless Eaton provide you with actual settings running on a 1ph supply, it may be all you get.
Make sure you have motor protection (e.g. thermistor) if doing this to prevent motor burnout.
 
ScottyUK (Electrical)
16 Jul 09 9:29
If the smoothing caps are desperately undersized - which they probably are because a 3-phase rectifier has comparatively little ripple at 300Hz compared to large amount of ripple at 100Hz for a 1-phase rectifier - then an average DC bus voltage of 287V could quite possibly be a correct reading from a 240V incoming supply. The DC would be very 'ripply' to say the least. Lionel already hinted at this in a previous post.
  

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

burnt2x (Electrical)
16 Jul 09 10:55
ScottyUK,
I cannot reconcile 287VDC coming out of the DC link supplied with a 240VAC, 1-phase!
Unless a DC boost is employed in this SVX9000 unit or better still voltage doublers!
A full-wave rectifier (resulting from non-supply of the 3rd leg) will roughly produce:

CODE

Vaverage = VDC = 2 X sqrt(2) X VRMS-INPUT/PI.
My hand calculator tells me I can only have around 216VDC. Or did i miss a lot here?
ScottyUK (Electrical)
16 Jul 09 12:40
Well, if you had a massive DC link capacitor you would find the DC link was running about 340V with a 240V single phase input.

If you have a bit of DC link capacitance but not enough to smooth it properly then you will strike somewhere in the middle ground between having no capacitance (your equation) and having a very large capacitor which would charge to the peak of the rectified sine at about 340V, so the 287V and the fact that the voltage collapses rapidly under load both support the idea that the DC link capacitor is inadequate.

You didn't miss a lot, but maybe a little bit. wink
  

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

burnt2x (Electrical)
17 Jul 09 10:31

Quote (ScottyUK):

"Well, if you had a massive DC link capacitor you would find the DC link was running about 340V with a 240V single phase input.
Absolutely correct. That is because the average DC voltage out of the rectifier can only be as high as the peak voltage of the input voltage, i.e. 240 X 1.414 = 340VDC, roughly(assuming no voltage booster used).That is the reason why I posted that if the DC link voltage measured was 287VDC, the input should be around 287/1.1414 = 203 VAC.
LionelHutz (Electrical)
17 Jul 09 11:12
Yes, that's what I've postulated is happening all along. A lot of DC ripple voltage and the VFD will not handle it.

In the time under question, the DC buss discharge rate will be fairly limear. So, if you have changed from 6 pulses per cycle to 2 pulses per cycle then it should be fairly obvious that doubling the size of the VFD might not give you enough extra capacitance.

It really depends on what capacitor the manufacturer decided to buy in bulk to build that particular VFD line and just how conservative they were when they calculated the minimum capacitance required for each model.
ScottyUK (Electrical)
17 Jul 09 12:28
Burnt2x,

Your logic is flawed: when you divide 287/√2 you are assuming the capacitor is large enough to hold the bus at AC peak voltage (287V) when you are calculating the input of 203V. That isn't the case, and you won't have anything remotely like a constant DC level: more likely there will be roughly a 200V DC level with a rather distorted half-sine 100Hz ripple peaking at 340V superimposed on it. The equation of that waveform is considerably more complex than what you are considering which is the two extremes: no capacitance or infinite capacitance. This is somewhere in between.
  

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

Wedoca (Electrical) (OP)
17 Jul 09 17:06
Just a thou. ... is adding external caps. to the VFD a perm. fix?? another words ..with greater capacitance which holds more charge therefore able to sustain the charge for "longer" period of time (please correct me if I am wrong)since 1 time constant is R.C therefore since R is fixed so bigger C = longer time right? on that note, back to the oringal problem ( assuming insufficient capacitance is the cause) what happended was because the capacitance was too small therefore the caps. discharge too quicky....dsicharging faster than it can be recharged ???
is that correct???

I am sorry for my lack of electrical knowlege if this question is too stupid..

   
jraef (Electrical)
17 Jul 09 19:18
It would probably be cheaper and easier to buy a different VFD.


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ScottyUK (Electrical)
17 Jul 09 19:32
Yeah, that's pretty much the principle. Bigger caps take more energy from the supply near the crest of the voltage, and release it during the 'trough' between peaks of the rectified AC voltage.

It's definitely not a stupid question. Have a look at this document for a diagram of what is going on (look for the sixth one down) and the design equations to size capacitance for a given level of ripple. Maybe just the diagram will be enough. smile
  

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

jraef (Electrical)
17 Jul 09 19:45
I have to say, I have learned a LOT more about ripple and capacitor design in this thread. Bravo people.


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ozmosis (Electrical)
17 Jul 09 20:40
wedoca
adding more caps in theory would be a potential solution but in practice not. Running a high load on a single phase supply requires more thought than just this.

However, it appears you have a number of issues and one of the main ones is the low capacity of your supply transformer that is simply not big enough for the load. It is possible the distortion (very high triplen harmonic distortion) on the 1ph supply is causing problems with your supply voltage distortion. This in turn is causing your drive to be unstable and there could be  a possiblity of resonance causing increased ripple on the DC link.
It's a bit of a catch-22 really.
Have you tried setting the VFD back to default (disabling the phase loss detection) and simply running the pumps without making any settings on the drive?
Actually, I just read your posts again and you mention pumps (in the plural). How many pumps are connected to each VFD? If it is more than 1 and each pump is 10Hp on a 20Hp VFD, then your problem is there. If it is 1xpump to each VFD and there are more than 1 pumps, then it is the capacity of supply.
Sorry it's a bit of a rambling thread but it's saturday morning...
 
ScottyUK (Electrical)
18 Jul 09 1:48
I honestly think you're over-complicating the problem. You're right that adding caps will increase the size of the pulse of current taken at the voltage crest, but the measurements Wedoca posted on 15 Jul 09 14:40

Quote:

the DC bus voltage on the VFD drop from 336 to 287
as I turn on the pump, input voltage drops about 5 volts from 248 to 244.
don't support there being a major supply capacity problem. Sure there's some supply impedance there but that includes the transmission & distribution system right through to the local cabling. 5V is well within the acceptable tolerance for a utility supply. The measurements do support there being a severe lack of bus capacitance.  

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

LionelHutz (Electrical)
18 Jul 09 7:37
I should have posted last time that to know for sure you need to scope the AC input and the DC buss.
 
burnt2x (Electrical)
18 Jul 09 11:29
We all agree that there is a problem with the available DC at the DC bus voltage. Now the question is whether it's because the input AC dropped much or the DC bus capacitance is low!
I am always wary of measurement errors, especially on "dirty" voltages. Unless Wedoca certifies his instruments are good and calibrations are up-todate, I will take the "336 to 287" and 248 to 244" thing, hook, line, and sinker, so to speak.
Wedoca (Electrical) (OP)
28 Jul 09 17:14
just for the record, all readings were taken from the VFD display panel, as I was questioned last time if my measurements were good..so I turn in my Fluke and take all readings are from the display panel ...direct read out from the VFD.

Okay guys, I have just recived 2 external cap kit for the drive ....and I will be installing them on the 3rd of Aug...and I shall let you all know how it turn out, if that solves the problem or not .....

   
Wedoca (Electrical) (OP)
11 Aug 09 8:45
1.    External capacitor kit did not change the condition of the drive. All electrical output was the same prior to the installation of the external capacitor kit; which it eliminates the possibility that the drive was undersized.  
2.    Current limit on the VFD was then increased to see if any change to the operating frequency and the input motor voltage. As result, the operating frequency and the input motor voltage increased proportionally to the raise of the current, and after a few adjustments, the VFD was then able to reach its maximum frequency of 60Hz and 230 volts at around 36 amperes.  Meaning that the electrical system is capable of delivering the demanding power consumption of the load.
3.     The pump was then taken out of the wet well and allowed to run under no load condition, data was taken. Under no load, pump was operating at 230 volts, 60Hz, full RPM, and it was pulling 8.7 amperes. Of which matches the factory test result of 8.5 amperes when the pump is running under no load, also which verified the integrity of the pumps.   

Based on the observation in the field and the test results, the pumps were appeared to be under greater load then they were originally designed for. Thus causing the pumps to draw more current than the VFD current limit
LionelHutz (Electrical)
11 Aug 09 11:11
Thanks for the feedback. Knowing the drive was reaching 25A or going into current limit at 50Hz earlier on in the thread could have helped. The colleagues who suggested the current limit was too low were correct.

Did you happen to check the DC buss voltage after adding the capacitors or did you disconnect the capacitors again? I'd be curious how much the DC buss measurement changed.  
Wedoca (Electrical) (OP)
11 Aug 09 12:35
I checked the DC buss voltage 10 mins after I installed the capacitor, and there were no changes. VFD reacted the same way it was without the cap. kit.




Thanks guys for all the helpful inputs.  
itsmoked (Electrical)
11 Aug 09 14:57
Thank you Wedoca. A star.

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

jraef (Electrical)
11 Aug 09 16:20
Thanks for providing "closure" to this, it's seldom that we find out the outcome.


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DickDV (Electrical)
15 Aug 09 21:24
Thanks to all for a rather exhaustive discussion on DC bus ripple and it's relationship to single phasing and three phasing the input.

While the thread took a rather torturous path, its comforting to know that the cause was simply drive programming rather than something more obscure.

Thanks again.
Wedoca (Electrical) (OP)
29 Oct 09 15:33
Update:   

Based on the previous discusion, we agree that the over current condition was due to excessive load to the motor thus causing motor over current.

Since then, I and a team of city utility workers have not been able to determine the cause. However, we have found that the frequency on the generator flucates while the pumps are runing. so, take a step further. I took a power quality anyliazer to the site and check the harmonic distoration level on these drives. and the attachments are what I found out...
 
Wedoca (Electrical) (OP)
29 Oct 09 15:39
and here is the voltage and current waveform for that harmonics

I am hopeing someone have delt with this situation before and tell if or not this level of harmonics will or wouldn't casuing pump to go over current. and I have being trying to read up on these stuff and I can't find anything that talk extensivly in deapth about these stuff .... can anyone suggest a good reference material??  

Thanks  
jraef (Electrical)
29 Oct 09 17:15
Generator? I don't see a generator mentioned earlier...

Quote (Wedoca (Electrical)):

      
15 Jul 09 14:56
power source: utility pole transformer

So is the generator issue new? Generators and power electronic equipment is a whole different kettle of fish. In a nutshell (as if that's going to help now), harmonics from the VFD can affect the AVR on the generator and cause it to "go squirrely". Newer generator designs are taking that possibility into account, but older installed or rebuilt gensets often exhibit this problem.


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LionelHutz (Electrical)
30 Oct 09 7:24
The harmonics appear to be fairly typical of what is expected on a drive except for the "N" readings. The output of the drive basically depends on the DC buss, not the harmonics on the input.
Wedoca (Electrical) (OP)
30 Oct 09 10:46
LionelHutz,

"The output of the drive basically depends on the DC buss, not the harmonics on the input."


what if I have harmonics on the out put as well???


The current harmonics on the load side is roughly about 5%
and the voltage harmonics is in the 17% range, and by looking a the waveform, the output voltage is almost like a square wave, and its in very rough shape, is that normal ?? do you see a problem there???  
Wedoca (Electrical) (OP)
30 Oct 09 10:48
for some reason the pictrue didn't show ...so here is  
Wedoca (Electrical) (OP)
30 Oct 09 10:53
Wedoca (Electrical) (OP)
30 Oct 09 10:53
LionelHutz (Electrical)
2 Nov 09 8:16
The output is a higher frequency voltage PWM so, yes, I'd expect that you meausured something odd on the output if you used an analyser meant for a 60Hz system.
 
Amotor (Electrical)
2 Nov 09 11:02
Don't put too much stock in the 'harmonics' on the load side.  The VFD throws a mess of currents at the motor which, in a scope, resembles a dog's breakfast.  The motor itself looks like a big inductor and uses the dog's breakfast to make things spin.

The danger with VFDs, or any power supply, is that it can cause problems on the supply side and start damaging other components on the same system.  In your case I don't think you have a lot of equipment on this system.  It seems like it's just a pumphouse.  If everything is running acceptably with the new settings, I would let it run.

There seems to be an under size issue with the pumps, however.  Depending on what your pumping and at what pressure you wish to pump it there may be need to look at:
Lowering your pressure (ie less head)
or thinning out the 'sludge' (ie just add water).
Wedoca (Electrical) (OP)
2 Nov 09 11:22
Amotor ,

motor size seems to be fine, we are pumping with lower head pressure then spec. and there are no difference from pump water and sluge ( tested ). Got anymore ideas that can possiblely causing the over load condition???? anythign would help..

Thanks for your response by the way

 
Amotor (Electrical)
2 Nov 09 13:07
Perhaps we can put a wattmeter on the motor?  I don't know why I like watt meters but they seem to smooth things out and give a 'real' world number.  We can make assumptions based on current and voltage but the wattmeter takes this into accound and removes the power factor issue from the equation.

Then we can tell if the 36 amp draw is related to actual power usage (requiring almost 15 hp).  Or if it is a function of the DC bus voltage being pulled down.
itsmoked (Electrical)
2 Nov 09 14:19
Any chance the VFD can show the power?

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

Wedoca (Electrical) (OP)
2 Nov 09 14:48
Amotor,

even we prove the 36A draw was the result of the harmonic and power factor, and its not the true power consumption of the power, but still it does not change the fact that the motor has gotten over heated due to the high current draw.

so are you saying if we can prove the motor is drawing the right amount of power, then we can say that the over heating is causing by the harmonics and the power factor issue???  
Amotor (Electrical)
2 Nov 09 15:11
Not necessarily,  You are correct.  If the motor is delivering 10hp but the current is 36A we have a problem.

I guess what I mean to say is.  If the Pump requires a 10hp motor this motor could be operating at (around) 180v RMS and requiring 36A to produce the HP.  Or, if the pump is using 15 hp at 230v it could use a nominal 42A.

Is the heat caused by harmonics and power factor?  Yes, they contribute, but no, not any more than any other VFD application.  A 10hp motor rated for inverter use is just that.  It shouldn't overheat because it is on an inverter.

The 'real' 36A is creating the heat regardless of P.F. or harmonics.

If the motor is only receiving 180V at the end of the day it needs to be oversized.  The difference between a 600v 25hp motor and a 460v 20hp motor is... very little.  The insulation in the windings is the same low voltage insulation.

Likewise a 230v 15hp motor receiving 185v will put out 10hp at FLA (more or less).

Does this make sense to anyone but me?
Wedoca (Electrical) (OP)
2 Nov 09 16:54
Amotor,


are you talking about the high current condition is caused by the low input voltage ????

 
Amotor (Electrical)
2 Nov 09 18:20
It may be low input voltage OR low dc bus voltage due to the various issues which have been discussed in this thread.  My guess is, if the imput voltage seems to be fairly stable, the DC bus voltage is allowed to dip.  

The other possibility is that the motor really is producing something like 14hp. (around 10kw)

I don't think the drive will read the watts used.  However, perhaps you could put a cheap, single phase, meter on the input and get an idea what's going on.
Wedoca (Electrical) (OP)
4 Nov 09 16:45
Amotor,

How could the motor be producing 14HP????? please explain
burnt2x (Electrical)
5 Nov 09 10:54
10kW motor shaft output is possible with a 36 amp draw!
Pin =1.732 x 36 x 230V x 0.8 = 11.47 kW, less mechl losses - could be near 10kW
LionelHutz (Electrical)
5 Nov 09 14:44

Quote:

Got anymore ideas that can possiblely causing the over load condition????

The pump requires more power than 10hp to run at 60hz in this application....

If your pump has a discharge valve try closing it. I bet the current drops as you close the valve.
 
Wedoca (Electrical) (OP)
5 Nov 09 16:29
that is correct....I decrease the opening on one of the vavles and the current did dropped ....


but the question is ...according to the engineer and the pump manufactur ....10HP pump should be sufficent for this application. so....other than get a 15HP pump out there ....what can I do to prove that the pump is under sized ??   
LionelHutz (Electrical)
6 Nov 09 8:10
It's one of 2 things.

You've still got the drive set-up wrong. Try looking at the motor voltage and frequency settings or the V/Hz settings.

The engineer and pump manufacturer are both wrong.
 
burnt2x (Electrical)
6 Nov 09 19:59
If money is the main issue, (your principal doesn't want to spend more on this project) why don't you just set your VFD to an output frequency where your motor doesn't overheat? Keep the discharge valve fully-opened and watch the inverter amps while you up your frequency to a level where your motor is happy with!
Just a suggestion though, it's your call.
Amotor (Electrical)
9 Nov 09 10:58
I would suggest throttling the valve and allowing the motor to run at 60hz with a lighter load.  But burnt is on the right track for a cost saving system.

I have some experience with a pump which the mechanical guys stroked out somehow.  (it was a reciprocating pump).  The control was calling for 7 l/s or something and the vfd had to throttle back to hold this flow.  Running at low speed kept causing 'electrical' problems and we found the pump was now missmatched to the system.

Of course, its itching by brain to know why the 36A load.
 
rasevskii (Electrical)
24 Nov 09 10:24
For Wedoca:

Throttling the valve is probably not an option, as it is sewage and may clog. Also reducing the frequency as a permanent fix may cause too low velocity in the force main.

There are threads about force mains problems under the water and wastewater blog on eng-tips.

regards rasevskii

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