What causes a 3 phase motor rotor bar to fail and how do you test for it?

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wolram
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i had a problem with a 3 phase motor last week that
had me stumped, it would rapidly overheat even under
no load, resistance readings of the windings cheked
out ok as did short circuit tests, a report from
the local rewind shop said that one of the BARS on
the rotor was found to be the fault, can someone
please explain what they mean, and how to test for
this fault?
 
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An 3-phase induction motor works a bit like a transformer. Current is induced (hence the name of the motor) in the secondary (the squirrel cage on the rotor) by the primary (stator) windings.

If there is a short on the rotor, you get the same problem as a transformer with a shorted turn in the secondary - bug induced currents and overheating.

The secondary (rotor) windings are very simple - often just a single turn of copper per winding, and if you could remove the secondary winding from the rotor intact, it would look (in some motors) like a hamster's wheel, or 'squirrel cage'.

The short was (I guess) between the cage and the magnetic (iron) path through the rotor. You can check for that by just belling between the cage and the iron stampings - there should be no current path.
 
many thanks CEPTIMUS, its obvious isn't it, thick old me.