In normal operation of the system (Fig 9.11), an induction motor drives a flywheel at a higher speed than the speed of the generator shaft. The induction motor — flywheel system is coupled to the generator shaft through an eddy-current coupling. The variable slip provided by the coupling allows the generator to be maintained at syn­chronous speed under all normal load conditions (see Sec. 9-4.3). When the frequency or voltage of the plant auxiliary-power system deviates from preset limits, it is interrupted. Through the use of speed sensing and slip control of the eddy-current coupling, the generator is maintained at synchronous speed as the flywheel speed drops to that approaching the generator synchronous speed. Simultaneously, the incoming plant auxiliary-power system voltage — and frequency-sensing relays determine whether a momentary transient or a complete loss of power has occurred. At a predetermined flywheel speed, the standby engine is started and brought up to speed, but it is not connected to the system at this time. If the speed of the flywheel continues to drop and reaches a preset minimum value prior to the return of the plant auxiliary-power system, the engine is connected to the system by energizing the magnetic clutch. The energy storage capability of this system varies between 10 sec and a maximum of approxi­mately 2 min. If the allowable outage time for which protection is desired is less than 2 min, the engine standby unit can be eliminated.

This system gives excellent results in speed control, output voltage, and frequency regulation, and, when combined with the engine backup, it is able to operate for extended periods of time. However, eddy-current-coupling

systems are inefficient and generate considerable heat that has to be dissipated. Other disadvantages include high maintenance cost and, when combined with an engine backup, the requirement for fuel storage, exhaust ventila­tion, and scheduled exercising