TRANSATLANTIC CABLE POWER SYSTEM 147 



the ac drive when the battery voltage is at the mean discharge value. 

 During ac drive, the EMF generated in the dc motor armature is a few 

 volts below that of the battery. Accordingly, when the fast acting con- 

 tactor AR shorts the 75-ohm resistance in the armature circuit, the 

 motor finds itself essentially at the desired operating flux condition and a 

 smooth pickup of drive occurs. Oscillograms indicate that the interval 

 between failure of ac power and operation of contactor AR is less than 

 0.1 second. During the transfer from ac to dc drive, the change in the 

 nominally 230- volt output is less than 5 volts. 



Return to ac drive is delayed approximately 20 seconds after the 

 ac supply voltage has returned to normal to allow time for the ac to sta- 

 bilize. Fixed field settings for both dc motor and alternator fields provide 

 simple control arrangements without the overspeed or overvoltage 

 hazards which automatic regulators might add. Alternator output is 

 monitored, however, to give alarms for voltage changes exceeding ±5 

 per cent and to control transfer to dc drive if the output should drop 

 more than 10 per cent for any reason. When the latter occurs, the ma- 

 chine locks on dc drive. This feature guards against ac motor failure 

 or low ac drive speed because of low supply frequency without low 

 supply voltage. 



Failure of the alternator output after transfer to dc drive causes the 

 set to stop and automatically transfer the load to its emergency alter- 

 nator. This transfer causes a break in the alternator supply to its bus, 

 but cable power is maintained constant by the parallel dc regulating bay 

 fed from the other alternator bus. 



Battery Plants and Distribution 



Battery power for dc motor drive is supplied from a 66-cell, 1,680- 

 ampere-hour battery at Clarenville and from two 68-cell, 1,680-ampere- 

 hour batteries at Oban. The latter station has double capacity to pro- 

 vide stand-by power for the ac supplies to the inland transmission 

 equipment. 



At both cable terminals, control battery for the small alternator plants 

 and the cable dc regulating equipment provides 24 volts and is split 

 so that a fuse or a battery failure on either supply will not interrupt cable 

 power. To guard against so remote a hazard as loss of a common battery 

 for this vital control, two separate 24-volt power plants have been pro- 

 vided with one half of the critical control circuits furnished from each 

 plant. 



In addition to supplying dc motor power, the 130-volt battery at 

 Clarenville supplies current to the carrier terminal and test equipment 



