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Pfc Capacitors On Vfd Driven Motors

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#1 edison


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Posted 24 July 2010 - 06:38 AM

Hi Marke

I plan to run MV motors (3.3 & 6.6 KV) in no-load with a 415 V VFD/step-up transformer. To reduce the KVA and the supply current, I plan to use appropriate MV capacitors (3.3 & 6.6 KV) across the motor.

Also, to reduce the in-rush current during starting, I plan to use a 415 V VFD to the transformer primary.

When being fed by the VFD/Trafo set up, will the PFC MV capacitors across the motor, create any problems ? One reputed VFD supplier claims that the capacitor will not be problem, as long as they are not switched on or switched during the VFD operation. He says to pre-determine the capacitor to be added (which I can do) and then connect it across the motor and then energize the motor+capacitor.

Will this work ?

#2 marke


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Posted 24 July 2010 - 07:02 AM

Hello Edison

Welcome to the forum.

Power factor correction capacitors are a definite no no on the output of a VFD.
The PWM waveform would damage the capacitors and the capacitance would damage the VFD.

If you apply a sinusoidal filter and a delta to star transformer on the output of the VFD, then the output stage will not see the capacitors and the capacitors will not see the PWM waveform and there will not be the same issues.

Best regards,

#3 edison


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Posted 24 July 2010 - 07:25 AM

Thanks Mark for the quick reply.

Just so I understand this correctly, if I use a VFD with a sinus filter on the output side and then apply the VFD output to a trafo with Delta on LV winding and Wye on HV winding, then there is no problem using the MV capacitors in paralell with the MV motors ? In such a case, the VFD output will only be loaded to net (inductive - capacitive current) + minor windage loss current, right ?

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