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???

 

[jraef]

Generator? I don't see a generator mentioned earlier...

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.


"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

 

[LionelHutz]

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]

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]

for some reason the pictrue didn't show ...so here is

 

[Wedoca]

here, lets try this again

 

[Wedoca]

here is the next part

 

[LionelHutz]

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]

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]

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]

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]

Any chance the VFD can show the power?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Wedoca]

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]

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]

Amotor,


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

 

[Amotor]

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]

Amotor,

How could the motor be producing 14HP????? please explain

 

[burnt2x]

10kW motor shaft output is possible with a 36 amp draw!
P_in =1.732 x 36 x 230V x 0.8 = 11.47 kW, less mechl losses - could be near 10kW

 

[LionelHutz]

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]

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]

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.