Answer / sunil kumar sinha

An alternator is AC generator. It will generate power only
when it is rotated by a prime mover and supplied electrical
field in its rotor circuit. The alternator when producing
electric power is connected with grid and supply electric
power. Here the prime mover supplies active power and field
maintains the voltage.
when primemover fails but excitor supplies rotor field, the
alternator will continue to rotate on grid frequency for
which it will draw power from grid. the reverse power will
flow, nothing wrong will hapen with alternator but if prime
mover is steam turbine there is every possibility of rotor
blade failure of L.P. turbine.
If prime mover with rotor field also fails, alternator will
continue to run but it will damage turbine and generator
rotor as well.In this case the alternator will take active
power and field from grid. The field current will flow
through the surface of rotor through moon slot which will
excessively heat rotor which may damage rotor winding.

Answer / amit pandey

When input to the prime-mover fails, the alternator starts running as a synchronous motor and draws some current from the supply system. This motoring condition is known as "inverted running".

Answer / gms

prime mover decides the operating mode of the machine.
if the prime mover of an alternator fails,motor will start running as a motor..

Answer / thennarasu

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