induction motor

If the insulation is in poor condition, such as from small cracks or moisture absorption, imposing a suitably high voltage will cause leakage current from the conductor to the frame. This can be detected with a megohmmeter (Megger^®) test or a "Hi-pot" (high potential) test.

Usually a megger instrument is used for this purpose. The higher the insulation resistance the better the motor and the lower the insulation resistance the worst spoilt the motor.

With Ur megger at a 500V range, a 0.5 and above Meg ohm reading with all the phases bunched together and earth, is regarded as a good result. Reading lower than than 0.5Meg ohms is regarded as bad or spoilt motor.

Actually most companies don't allow or accept their insulation resistance reading to fall to 0,5 Mohms. A reliable and realistic accepted value is 1 and above Mohms

Note: Insulation resistance has to do with between pole test to determine if there is a shot between conductors, since it is possible to have a shot among the conductors that is not bonded to earth.

Dickson

This is a good little tester, will test any kind of coil. [http://www.mainelectronics.com/flybacktest.htm]

if the link doesn't work, it's called a shorted turns tester.

I think you can differentiate if an induction motor is internally damaged or not just by having a look at the copper wires inside it, so if damaged, they will apparently be burnt-out.this means there's a total breakdown in the motor's insulation... if a motor is partially damaged, then you can use " Megger" to check it.

Thanks.