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I would like to know, during a reverse power trip, what type of motor does the alternator become?

An electric motor.

Seriously, it depends on the reason for the trip. If you're talking about a lack of sufficient power from the prime mover to keep a synchronous generator (alternator) running at synchronous speed when connected to a grid in parallel with other alternators when there is no loss of field excitation, it will become a synchronous motor.

If there is a loss of field excitation, then it will be an induction motor.

For the record, while reverse power protection detects reverse power flow into the generator, its primary function is to protect the prime mover! Thus, operation requires that excitation be available so that the generator becomes a motor.

But, as CSA suggested, if reverse power and excitation loss occurred simultaneously (e.g., worst-case resulting in zero field-current) there is sufficient field-flux, although decaying, to provide torque to drive the prime mover!

Regards, Phil Corso (cepsicon@aol.com)

I would think think a reverse power trip would open the generator breaker and the end result would be a generator rolling to a stop.