DC motor earthing (3 Phase AC at earth potential)

[eleclearner]

Been having some big DC motors (1mW) blow up. Potential problem noted. On the AC 3 phase side, there was a surge protector that was shorted to earth. This was 610V secondary of a YYN0 transformer. The neutral was not used and is not earthed. So say phase A is at earth potential... the plant still runs fun for the most part, but could this be contributing to DC motors blowing up? DC motor is not referenced to earth so i can't work it out.

Any thoughts? Thanks

 

[davidbeach]

Didn’t know that a milliwatt was a big motor. Perhaps a MW (Megawatt)?

 

[waross]

Bad things happen on unearthed circuits.
The line to neutral voltage of your 690 Volt circuit is 400 Volts.
Your drive electronics expects to see 400 Volts to ground also.
With the neutral ungrounded and a shorted surge suppressor your drive will see 690 Volts to ground on two phases.
The end result may be a rise in the common mode voltage to ground at the motor by a factor of root three.
For those who argue that they must run ungrounded for reliability and reduced down time:
Contrast the time lost and the expense of replacing a surge suppressor with the time and expense of replacing/repairing a I MW motor.


Bill
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"Why not the best?"
Jimmy Carter