My understanding of AC motor windings is that They should all be open to ground or the case?

Question

and have a low resistance from one winding to another. Is this not the case?

Answer 1

it is the case.

Answer 2

To generalize, a "Healthy" motor should have a high insulation resistance from the windings to frame (>1M Ohm @ 500V). A low resistance (dependent on the test voltage of course) is an indication of voltage tracking through the insulation, due to moisture, dirt, carbon from a fault etc. A short circuit to the frame indicates that the winding is shorted to the core, rotor, or other part of the frame.



The winding resistance itself is dependent on many things; windings design, connection (star or delta) etc. Considering whether the resistance is lower or higher than it should be, you must have data to compare to such as a similar motor or type test data from the factory.



Remember that a single phase motor will have a start winding with a higher winding resistance than the run winding.

Answer 3

An AC motor should have an insulation of at least 1 megohm when taken with a 500V megger tester.If the insulation resistance is lower than this value, the motor may be burnt. Also, resistance from one phase winding to another should still be at least 1 meg-ohm, otherwise, the motor is not safe to run.

Answer 4

you want resistance between windings if it shows open u got a blown winding.....if u dont get infinity from winding to ground its grounded.....the windings r connected inside the motor