The rotor should have negligible effect on the DC step voltage test.  The stator coils in the slot are already surrounded on three sides with ground.  Adding the rotor just adds ground at the last side with a gap.

I would look for some other variable that may have changed since last test.  Was motor stored in humid area.  Humid conditions at time of the test.  Stator winding damage during reassembly.   Maybe some kind of contamination or debris found it's way to the winding.  Different tester or different test equipment.

Thanks @electricplte for the reply.

No.nothing has been changed since last test.

Had there been some contamaination or debris found or some particles stuck in way of winding than when we test without rotor at that time also it should have been shorted to ground right?

subsequently after test failed with installation of rotor and complete asembly, it was dimantled again and DC step voltage was carried out once again but test passed without any short to ground and leakage current found to be less.

> Had there been some contamaination or debris found or some particles stuck in way of winding than when we test without rotor at that time also it should have been shorted to ground right?

In my defense, I didn't know you checked again after disassembly, so I thought it could be slot section contamination or damage introduced during resassembly.

..But that scenario is ruled out since you found it good after removing rotor again (and stator slot section ground plane is  relatively unaffected by the rotor)

So I would say it cannot be a problem in the stator slot section (although maybe you could double check rotor centering as a very remote longshot).

then the only scenario  left seems to be a problem in the stator end windings.

most likely when you reassemble,  the endbracket / endbell is touching the end windings.  there may be a metallic air baffle attached to  end bell (I have seen that on vertical motors). inspect end windings closely for signs of contact, damage, or discharge. inspect parts (possible during assembly) to figure out where the closest possible points of contact to stator winding might be.

Thank you for the feedback!

we will check the rotor centering and the clearance between the stator, also try to analyse or look at the closest possible points of contact to stator winding.