590 ELECTRICAL EQUIPMENT 



be necessary to shut it down for inspection or repairs, and under 

 normal conditions it carries the steady load due to the signal lamps, 

 and supplies a small amount of charging current to the battery 

 in order to keep it fully charged at all times and ready for service. 

 This direct-current machine is of the shunt-wound type having 

 a decidedly drooping characteristic, so that when a heavy demand 

 occurs, due to the opening or closing of oil switches, etc., the load 

 is divided between the machine and the battery, and the machine 

 itself is thus protected against excessive momentary overload. 



The normal voltage of the control circuit is approximately 

 125 volts, but the B.C. generator is designed for the maximum 

 charging voltage of the battery, which may rise to about 2.80 

 volts per cell. The ampere capacity of the generator should be 

 equal to the normal charging rate of the battery plus the current 

 required for the signal lamps. 



It will be noted from the above that under ordinary conditions 

 of operation the battery does very little work, and the maximum 

 demand upon it occurs only when it is necessary to open or close 

 a number of switches simultaneously at a time when the motor 

 generator set is inoperative. 



The ampere capacity of the battery is determined by ascer- 

 taining the maximum possible demand due to the simultaneous 

 operation of as many of the remote-control devices as are liable 

 to be operated at once, and, selecting a battery of sufficient size 

 to supply this current for the period of time necessary without 

 dropping in voltage below a certain permissible minimum. The 

 number of cells is usually fixed at 60, and for this number a 

 floating voltage of about 127 volts is suitable. 



Standard remote-control apparatus is usually designed to 

 operate over a comparatively wide range of voltage variation, 

 owing to the fact that such apparatus' is in some cases operated 

 from an exciter circuit whose voltage is varied by automatic 

 regulators. In order to provide ample margin of safety, a mini- 

 mum voltage of 90 is usually fixed for the battery when carrying 

 its maximum load. This is equivalent to 1.5 volts per cell. A 

 properly designed storage battery equipped with low-resistance 

 intercell connections and provided with conductors of ample 

 capacity for connecting to the switchboard may be discharged at 

 five times the one-hour rate (twenty times the eight-hour rate) 

 for a period of one minute without dropping below the limiting 



