Current distortion from capacitors on star-delta motor

[testtech]

I was testing a chiller and noted substantial distortion in the current, but not the voltage. This chiller has a star delta starter and 60 kvar of capacitors connected at load side of the M1 contactor. The attached waveforms show the distortion level at the motor terminals, capacitor and source. The capacitors appear to be responsible for the high frequency current distortion. Does anyone know why this might be happening. Of the hundreds of chillers I have tested, this may be the first I have seen that used capacitors.

 

[electricpete]

Capacitor current tends to respond heavily to the higher frequency content of the voltage since Zc = 1/(2*pi*f*C).

Deja vu!

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Skogsgurra]

Yes. A capacitor is a high pass filter for current. So, even if the voltage looks OK, more or less, the higher order current harmonics will be amplified. And more so the higher the frequency.

Saying same thing as Pete. Only more words.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

Hello testtech

In addition to the comments by Pete and Gunnar, I would like to add the following:
1. I assume that the top two waveforms are current waveforms and the bottom waveform is the suppl voltage waveform phase to neutral or phase to earth?
2. The current in the capacitor is very much affected by the harmonics in the voltage waveform.
3. If the lower waveform is the voltage waveform, then I am not surprised at the capacitor current waveform as the lower waveform is quite distorted.
4. The current into the motor is not affected or greatly influenced by the capacitors.
5. The current waveform into the motor is quite distorted in a manner that strongly suggests that the iron in the motor is overfluxed. If this is a no load current then, it may not be too bad as this would be essentially magnetizing current. If the motor current is a loaded current, then I would expect the motor to run very hot due to excess iron loss.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[testtech]

Thanks for your comments. Mark-The bottom waveform is current and is acquired at line side of the main chiller breaker. It is essentially the sum of the capacitor and current waveforms. The magnitude difference between the motor and breaker currents occurs because this is a star-delta starter and the motor current was measured only at the M1 contactor. Concerning point 5, this motor is about 80% loaded. It is a hermetic chiller motor, cooled by the refrigerant.

 

[PQ101]

The waveform looks normal. You should be concerned if there is excessive voltage harmonics (above 5%). That's the cuprit that will cause current distortion. Current THD can be as high as 10-20% during normal operation. The wavewform looks OK though, but I recommend for you to retrieve or pull up the voltage-current THD and make your power quality evaluation based on factual data.