Troubleshooting Strange Noise from 250kw 3 Phase Motor

[lottis]

Hi guys

I have been testing a brand new 250kw 3 motor wired in delta. I have it connected to a 170 amp vfd with no load for testing. It draws 105 amps at 50hz and is balanced.

Intermittently the motor makes a strange that i have not heard before. It sounds like a spoon taping another spoon, a metal on metal sound tapping at no more than 1 hz off and on. When power is removed and the motor free spins to a stop the sound is gone. I have tried different carrier frequency but it does not make any difference.

Sometimes while making the noise the vfd trips on over current but not on output short circuit. The drive does not show a raise in current before tripping.

The motor has been check for mechanical issues, bearing/loose parts etc and none have been found.

I have tried the motor on another drive (250 amp) and it does not make the noise. I have tried other motors (55kw and smaller) on the vfd and they do not make the noise.

Thanks

 

[the_emi_guy]

Does it sound like it could be a spark?

 

[lottis]

No not like a spark, a clang but not loud, spoon on spoon is the best description i can think of. When i heard it first i would have sore there was something loose in the motor, like a balance or something, but nothing loose was found.
If these was a short to Earth or across a winding the vsd would either fault "output short" or "earth fault" i would assume.

 

[Windadct]

You are saying the motor is balanced? How did this get checked? At 250KW it is worth the investment to get a professional to assess it. ( there are a lot of tests to consider: http://www.motordiagnostics.com/papers/AC%20Motor%20Testing%20and%20Predictive%20Maintenance.pdf )

Also as a new motor - you can ask, or pay for the manufacturer to come check it out, and then have them certify the warranty is intact, it may cost a few $ now - but to ensure the warranty is valid it may be well worth it.

The VFD could be the problem, again at this power level - I would lean on the manufacturer.

 

[lottis]

The current is balanced across each phase +/- 2 amps at 105amps 50hz. The vfd shows the current for each phase and i also checked with a clip on ammeter. I also checked winding resistance and insulation resistance which both check out fine.
The dc bus is stable and the vfd output voltages are also stable and balanced.