Inductive Coupled EDM

Inductive coupling to the rotor.

The high frequency currents flowing in the frame of the induction motor, as a result of the applied PWM waveform, will create high frequency flux fields. If there is a perfect balance in these flux fields, then the resultant High Frequency magnetic flux in the rotor will cancel and equal zero. In practice, they do not fully cancel and they induce a current flow along the length of the rotor.
If there is not a return path (loop), then there will be a voltage induced from one end of the shaft to the other. If the flux levels are high, then the voltage levels can be high also.
There is a potential return loop path via the bearings and motor frame. If the differential voltage along the shaft is high enough, the insulation from the bearing lubrication can break down and allow current to flow. Some current will flow due to the capacitance of the bearing, and this will not cause major problems, however if a discharge occurs, then there can be micro pitting of the bearing race which can develop from surface frostin to full blown EDM fluting of the bearing surface.


Inductive coupling can and does occur in any size of motor, it is not restricted to large motors only.


Solutions :

Insulated Bearings.


Common Mode Filter


Output Filter


Switching Pattern