I amusing a 112kw slip ring induction motot on a crane hoist. The motor failed and upon investigation, we found a damaged cable and the rotor had developed a short circuit between phases.

The dilemma is, we have a short circuit on the rotor, and the supply is tripping on earth fault, instead of overload as I have expected. I cant explain that!

Hello emms

Welcome to the forum.

It sounds to me as though you have tow faults, one on the stator and one on the rotor. I would not expect a rotor problem to bring up an earth fault unless there is an earth connection in the starter also and the earth fault is derived by monitoring the current flowing in the earth conductor.

Best regards,

Dear all

We may consider motor as a rotating transformer , with air gap as the flux tranfer medium

Now if a secondary earthfault in a transformer can reflect in the primary , why not in a motor+++ provided the stator winding is star connected ?????

Kamath