Capacitive Coupled EDM

Capacitive Coupling in an induction motor.

Within the induction motor, there are a number of capacitive elements that can and do pass high frequency currents.

  • Stator Winding to Stator frame.
  • Stator Winding to Rotor.


Stator Winding to Stator Frame.

The obvious capacitance, is between the stator windings and the stator frame. This can be quite substantial and result in very high current pulses in response to high the dv/dt of the applied PWM waveform from the VFD.
The high currents flow through the stator iron and frame towards the earth return to the drive frame. (typically the motor terminal box.)


Stator Winding to Rotor.

There is a much lower level of capacitance between the stator winding and the actual rotor itself. Any steep edges on the voltage waveforms will result in current impulses flowing into or out of the rotor. This current will find it's way back to the return path to the VFD frame and this is usually via the motor terminal box and screened cable after passing through the bearings, unless there is a shaft grounding brush fitted.
The magnitude of the current flowing is a function of the dv/dt of the applied waveform and the magnitude of the capacitance in series with any limiting impedance.