flux path for core loop test to find rotor bar problems

[electricpete]

A time-proven way to detect/confirm/localize rotor bar problems (*) is to excite the rotor core with a power supply or core tester sending current through wire looped through the center of the rotor.

Then using either thermography or green paper, the faulty bar/connection will be shown by having no current or reduced current in the associated bar and possible hotspot at the joint.

My question is: what is the flux path that induces current in the bars during this test. One would imagine that the primary flux induced will be circulating tangentially around the backiron... there is no obvious (to me) reason for flux to travel radially through the core teeth and into the airgap which is the path that produces bar currents during normal operation. Is there perhaps a homopolar current in all bars flowing opposite the exciting current... and what would be the return path? Can anyone explain?

* - this is time-proven at least to detect defects that remain when the motor is secured. We all know the on-line tests are the best for initial detection.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

replace "into the airgap" with "outside the rotor"

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[aolalde]

Hi Electricpete.

You are wright, the generated flux goes mainly into the rotor lamination back iron and the induction in the rotor bars is minimum. Although I have read technical recomendations like by EASA to test that way.

I have tried to test rotors following that procedure with negative results. To induce current in the rotor bars the flux should be radial to the bars. Large motors will demand a huge current to show any deffect.

When the stator is available excitation of a single phase will produce an alternating, fix, radial flux, if tne rotor is tested stand still in different geometrical positions the change of linked bars conductivity, will be reflected as current variations on the primary winding, in reality it is equivalent to a "growler" test but fitted to the motor size.