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rotor core loop test to find broken rotor bar - theory question

rotor core loop test to find broken rotor bar - theory question

rotor core loop test to find broken rotor bar - theory question

(OP)
I have been told by a respected motor expert that a broken rotor bar can be found during a core loop test of the rotor.

Several loops of coils looped thru center of rotor and around outside and back thru center etc. Then pass high current using a ductor to excite the core.

Then supposedly we can detect a cracked/broken rotor bar by infrared inspection or clamping onto to rotor bar extension if possible, or some similar method to look for current deviation in that bar.

Here is my question: At a given instant in time during this test, the flux is all flowing in the same direction uniformly around the core. This means that an identical voltage is induced in all rotor bars.... attempting to induce current in the same direction in all bars. Where would the return path for current flow be if current is flowing the same direction in all bars?  Is it through the shaft? Or maybe this test won't work to detect broken rotor bars?

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