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Common Mode Voltage from a PWM drive question

Common Mode Voltage from a PWM drive question

Common Mode Voltage from a PWM drive question

(OP)
I read the comment in an article "common-mode voltage arising from the non-sinusoidal current produced by an inverter's power-switching circuitry.".

This doesn't make sense to me.  It's the current that produces the common mode voltage?  I thought it was the inverter switching the voltage on and off that produced the common mode voltage.

RE: Common Mode Voltage from a PWM drive question

You are right, common mode voltage is not related to the current waveform.

RE: Common Mode Voltage from a PWM drive question

High frequency common mode voltage can appear due to current flowing back to inverter through non standard returns (i.e. parasistic earth connections that will have an impedance and then will cause a common mode voltage).
 
Usually motor providers ask to use a filter to limit this.

Also using suitable special ferrite-kind rings for the 3-phase output limit this problem.  

RE: Common Mode Voltage from a PWM drive question

xj25, common mode voltage will be there even if there are no parasitic capacitances. It's just caused by the fact that in an inverter there are three-phase legs that may be either switched to DC+ or DC-. And as soon as the switching state changes there will be a step in common mode voltage.

But common mode voltage wouldn't be a problem on it's own. Unfortunately you can never avoid the parasitic capacitances and hence you will also see common mode currents flowing.

A sinewave filter intended to get sinusoidal currents will also adress this topic, but at a big penalty in cost and efficiency. Common mode issues can be adressed by much more simple filters.

RE: Common Mode Voltage from a PWM drive question

Gunnar,

That's a very nice explanation - thanks for sharing.  

dpc

David Castor
www.cvoes.com

RE: Common Mode Voltage from a PWM drive question

(OP)
So it should actually read "common-mode voltage arising from the non-sinusoidal VOLTAGE produced by an inverter's power-switching circuitry"

Thanks for all the help!!!

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