146 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



made in such a manner that loss of one cabinet alone will ha\'e minimum 

 effect on the cable power supplies or office loads. At Oban, where 50- 

 cycle commercial service is normally used, special distribution ar- 

 rangements have been provided to gi\'e maximum power supply relia- 

 bility, with three manually operated 50-cycle, 90-kw, diesel-alternator 

 sets arranged for standby service. The diesels at this terminal are larger 

 to care for greater local power loads. 



Continuous AC Power From Two-Motor Alternators 



At both cable terminals, two reliable ac buses supply power to the dc 

 cable regulating bays. Each of these buses is fed from a continuously 

 operated, self-excited, single-phase, 230-volt alternator normally driven 

 by a 3-phase induction motor on the same shaft with a 130-volt dc motor. 

 Each regular alternator is backed-up by a similar emergency alterna- 

 tor running at no load. A fifth motor-alternator is provided which can 

 be used whenever any other set is out of service for routine maintenance 

 or repair. 



As alternator loads are essentially constant, and since induction motor 

 speeds are fairly insensitive to power supply voltage variations, alter- 

 nator outputs are set by fixed adjustments of their field rheostats. Suppl}^ 

 voltages are monitored to control automatic transfer to dc motor drive 

 whenever the supply voltage drops below 80 per cent of the normal 

 value. Fig. 4 shows the normal running circuit for an alternator set, 

 with the dc motor connected to the battery through a resistance of 75 

 ohms inserted in the armature circuit. The field resistance FR is preset 

 so that when the battery is driving the set, the speed matches that of 



ALTERNATOR 



DC MOTOR 

 CONTROLLER 



ALTERNATOR 

 VOLTAGE 

 MONITOR 

 ALARM & 

 TRANSFER 

 CONTROL 



Fig. 4 — Two-motor alternator set. 



