HomeDefinitionVariable frequency drive common mode voltage

Variable frequency drive common mode voltage

The definition of common mode voltage is as follows:

Vcm = (Vag+Vbg+Vcg)/3

Where Vag, Vbg and Vcg are the voltages measured at the respective terminal of the variable frequency drive with respect to ground.

It is difficult to make this measurement but not impossible. One way to do it is to make a Wye resistive network and then connect a resistor from the common neutral point to chassis ground. You would then measure the scaled common mode voltage from the neutral to ground.

More importantly, the result of common mode current is the common mode current flowing from the drive into the motor. This is a function of the motor internal capacitance to ground. It is easy to measure the common mode current. You simply find a rogowski coil that has sufficient high frequency bandwidth or (better yet) use a current transformer. Measure the VFD common mode current by routing all three output phase conductors through the sensor. This will give you an indication of the common mode voltage (and is what you care about anyway).

If you are trying to meet an EMI conducted emissions requirement, you should be more concerned about the common mode current going into the variable speed drive, not the motor. The vfd drive heat sink will be a part of the common mode impedance. You should then route the input cables to the motor drive through your current sensor.

If you want to reduce the conducted common mode current into the VFD (which is what is needed to meet EMI requirements) then the best approach is to insert a common mode inductor in series with the input terminals and the motor drive and then connect capacitors to ground (typically between 0.1uF and 1uF) from each motor drive input terminal (on the source side of the common mode inductor) to chassis ground. It is difficult to find a good common mode inductor vendor. Usually this is a custom item. However you can make your own by wrapping all three input cables (AC fed drive) or all two input cables (DC fed drive) around a ferrite core. If you wrap too many times around the ferrite you get more common mode inductance but you will likely saturate the core--so it won't do any good. I recommend one or two turns.

If you want to reduce conducted common mode current into the motor then connect the common mode inductor to the three output phases of the variable frequency drive and connect the capacitors to ground on the motor end of the common mode inductor.

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