AC motor turn into a generator.. 3

[THooper]

We have four 250hp ac motors running in tamdom on a belt drive, controlled from VFD's If one of these motors was unwired and left "coupled up" and would still rotate at 1800rpm driven from the other three motors, would this unpowered motor possibly produce any voltage? I have read that it takes reactive power to boost excitation connected to motor leads to make voltage, but would there be any residual magnetism to establish self excitation, that would produce some voltage and possibly become a safety hazard?

Thanks in advance for your help

 

[dpc]

From a safety point of view, you would have to assume voltage could exist at the motor terminals if it was rotating.

 

[jraef]

If you had capacitors connected to it, residual magnetism may be capable of creating a little bit of energy, which can charge the caps which can then excite the motor enough to be dangerous. That's how some people turn AC motors into wind generators in island mode. But without the caps it can't generate much.

That said, it would still be better to take dpc's advice and assume it might possibly happen.

 

[aolalde]

Only (as synchronous machine) the residual magnetism (phi)in the rotor core could generate very reduced voltage since e= -N d(phi)/dt, and phi---> 0