Using VFD as a phase convertor to run 3 phase pumps 9

[Wedoca]

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???

 

[burnt2x]

"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]

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]

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.


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

 

[Wedoca]

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]

It would probably be cheaper and easier to buy a different VFD.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

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.


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

 

[jraef]

I have to say, I have learned a LOT more about ripple and capacitor design in this thread. Bravo people.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

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]

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

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.


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

 

[LionelHutz]

I should have posted last time that to know for sure you need to scope the AC input and the DC buss.

 

[burnt2x]

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]

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]

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]

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]

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]

Thank you Wedoca. A star.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Thanks for providing "closure" to this, it's seldom that we find out the outcome.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

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]

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]

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