Exam Question

(a) When generators are running in parallel, if one of the prime mover fails or if there is a loss of excitation, the generator stops producing power and will start drawing power from other sources running in parallel. There will be reverse power flowing into the generator and it starts operating like a synchronous motor and the prime mover becomes the active load. This can damage the prime mover and is therefore undesirable.
(b) The upper electromagnet has a voltage coil connected through a transformer between one phase and an artificial neutral of the alternator output. The lower electromagnet has a current coil also supplied from the same face through a transformer.
The voltage coil is designed to have higher inductance so that the current in the coil lags voltage by approximately 90°. Both fields pass through the aluminum desk and cause Eddy currents. The effect of Eddy current is to provide torque in the desk. With normal power flow, trip contacts on the disc spindle are open, and the disc bears against a stop. When the power reverses the current energies before the voltage and the disk rotates in the other direction, away from the stop and the contacts are closed so operating the trip circuit.