148 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



through voltage dropping resistors which are normally in the circuit 

 to hold the load voltages below a maximum of 135 volts. These resistors 

 are automatically shorted to maintain a minimum load voltage of 125 

 volts when the battery voltage drops. A voltage detecting relay also 

 steps the fixed field adjustment of each dc motor when the battery 

 nears its final discharge voltage so that dc motor speed is kept within 

 about ±3 per cent of normal ac drive speed during battery operation. 

 The 66-cell battery is floated at 143 volts by means of voltage regulated 

 rectifiers and, after a discharge, is recharged by automatic operation of 

 a regulated 100-ampere motor-generator set. As indicated in Fig. 3, the 

 rectifiers for this plant are connected so that loss of one service cabinet 

 will still leave sufficient charging capacity to float the load from the 

 other cabinet. 



Rectifiers and Associated Controls 



As mentioned earlier, each cable is supplied at all times by two regu- 

 lating bays in parallel, each operating from a separate ac source and each 

 capable of taking over the cable load should the other fail. Thus failure 

 of an alternator supply or of a regulating bay itself will not interrupt the 

 cable power. A spare regulating bay for each cable is arranged for re- 

 placing either of the two regular bays and may be connected in parallel 

 with the other two without overloading the associated 2.5-kva alternator. 



As shown schematically in Fig. 2, the ac supply to the rectifiers is con- 

 trolled by a continuously-tapped variable autotransformer, operated 

 normally by a two-phase low -inertia reversible motor, with provision 

 for manual adjustments also. The autotransformer output is stepped up 

 and rectified by a series arrangement of five rectifiers, each to give a 

 maximum of 550 volts for a total of 2,750 volts when the autotrans- 

 former is at the upper limit. The high voltage rectifier output is filtered 

 and supplied to the cable through the current regulating unit, cable con- 

 necting switch, and common cable control circuit. The cable current is 

 regulated by controlling the plate to cathode drop across the electron 

 tubes through which this current flows. A dc amplifier, which derives its 

 signal from a resistance in series with the total cable current, varies the 

 grid bias of the series regulating tubes. The series tubes have a control- 

 drop range from about 150 volts to the full supply voltage, the dc am- 

 plifier being capable of driving the regulating tubes to cut-off". However, 

 to protect the series tubes and to keep them operating at practically a 

 constant plate ^'oltage of 300 volts, a ser^'o system automatically raises 

 and lowers the output from the autotransformer by means of the motor- 



