The systems discussed in Secs. 9-5.4 to 9-5.7 use an a-c generator driven by an internal-combustion engine in combination with other components to provide a sustained power supply. Once operation is established with the engine as the prime mover, the time during which the alternator will operate is limited only by the available fuel supply.

The simple system shown in Fig. 9 8 is the basic configuration of a typical large standby auxiliary-power supply used in nuclear power plants. For a two-unit nuclear power station, a number of such large standby power sources sufficient to deliver as much as 20 MW when the normal auxiliary power is lost are required to maintain safe shutdown conditions or to provide power for the engi­neered safeguard auxiliaries during a loss-of-coolant accident.

The principles involved in the design and application of such power supplies are well known and are defined for nuclear power generating stations by standards [19] These principles or criteria are also applicable to the much smaller engine—generator units discussed elsewhere in this chapter.

9- 4.5 A-C and D-C Drive Motors

The normal dnve motor for a nonmterruptible rotating power supply can be either induction or synchronous The choice is usually determined by the frequency requirement of the output generator. The standard "a-c squirrel-cage induction motor is considered more reliable and can operate for a longer period with minimum maintenance. The slip rings and secondary excitation required by a synchronous motor require maintenance. However, brush­less synchronous motors are available and approach the reliability of induction motors, although at a greater cost.

The a-c induction motor can accelerate a greater load from rest than a synchronous motor can accelerate. The induction motor can be designed to have a very low slip value and thus can provide an output speed as little as 1% below that of a synchronous motor.

Synchronous motors provide a constant output fre­quency with constant input frequency. If a brushless synchronous motor is used, it is necessary to furnish an external exciter with voltage control and an external out-of-step protective relay, these are not required by an induction motor. Usually a large synchronous motor, when used with a stored-energy flywheel, is required to accelerate the flywheel into step. Thus, when large flywheels are used for stored energy, a synchronous motor that has several times the horsepower needed to drive the load may be required. In addition to the higher initial cost of the larger motor, the motor would be running normally at light load with decreased efficiency