XFMR Differential Picks Up on Motor Start

Sorry for being blunt but had I been your boss, I would have berated you for not testing or verifying your suspicions before asking for help. If you had doubts about the motor, why haven't you checked it?

The motor was meggered prior to starting. I meggered again after problem but can't see any problems with insulation.

I can't find a short but can't think of another reason for this to happen, which is why I asked the question in this forum.

Don't bother posting to this thread if all you have to comment on is what my boss would do. If you have nothing constructive to add you are providing nothing useful to me. If I was your boss I would berate you for wasting time.

I am thinking it is a short in the motor winding. Any suggestions?

Your words. You didn't mention that you've already checked it. You just said you think it's shorted. Sorry for the mistake but you didn't make yourself clear.

If the megger says it's okay, try checking the winding resistances. Are they balanced?

Check if your voltages are balanced.

Try replacing the differential relay.

Check the wiring of the diff. relay (is it correct) as well as the peripherals.

Please clarify (motor bump) why did you have to swap the phase over? Motor reversed? Is this a single phase motor brushed or inductive? Small define in Watts. I know I ask a lot of questions but to give good advice one needs to know the details.

I believe I have gotten to the bottom of the problem.

The instantaneous differential setting based on percentage current is picking up causing this nusance trip. I believe the problem is related to very low loading on the transformer (3% secondary loading). I have to wait until later tonight to continue testing but believe differential setting update will do the trick. Thanks all for input.