MV reduced voltage starter autoxformer failure

[dithomas]

A 2300v 1500HP motor driving a screw compressor is started by a GE Limit-amp vacuum contactor reduced voltage starter with 3 coil auto-transformer. The 65% tap is used. This starter has been in service for over 10 years.

On two occasions in the last year during a lighting storm the utility voltage sagged to less that 50% on B phase for about 7 cycles. Both times the A phase winding on the GE starter failed.

There are three more 1500HP motors in the plant that are started with Eaton ampgards that rode through the sag due to the coil hold-in feature.

I am thinking that the GE starter contactor is dropping out on the sag and them at the end of the 7 cycles is picking back up since the run command signal is still applied. This quick drop out and pick up is catching the motor coasting damaging the auto transformer.

I am looking for some comments or correction on my thinking.

Thank you all in advance.

Dan

 

[dpc]

Seems plausible. The contactors can start dropping out below about 70% voltage in a standard NEMA starter.

Just as an aside, you should consider replacing the autotransformer RV starter with a solid-start reduced voltage starter. The autotransformer RV starters are fast becoming dinosaurs.


David Castor

http://www.cvoes.com

 

[jraef]

It's possible. Although 7 cycles (117ms) seems fairly short for a drop-out, the GE Vacuum Contactor has a drop-out time of just 50ms and a minimum hold-in of 65% voltage. So if your capture (assuming you have one since you know it's 7 cycles) shows you were below 65% voltage for 50ms or more, then that's a legitimate concern.

Then the pull-in time is another 350ms, so that means your motor is potentially off-line for long enough that the field would begin to collapse, but maybe not long enough to completely collapse. So when reconnected, it is like closing on an out of synch generator. That certainly could create a spike that could damage the AT coil, if not other windings such as the motor!

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