286 



ALTERNATING CURRENTS 



may be efficiently controlled by taps on a transformer. This 

 efficient speed control is not possible with direct current. 



The single-phase series motor operates satisfactorily in railway 

 work, notably on the New York, New Haven & Hartford Railroad. 

 From New Haven to Harlem the locomotives take power at 

 11,000 volts, 25 cycles, from an overhead trolley wire, by means 

 of a pantograph trolley. An auto-transformer on the locomotive 

 reduces this voltage to 250 volts, the rated voltage of the series 

 motors. The electric locomotives run from Harlem into the 

 Grand Central Station, New York City, over the New York 



100 150 200 250 300 350 400 450 500 550 600 

 Amperes 



FIG. 263. Characteristic curves of 430-amp., 235-volt, 25-cycle, single-phase 

 Westinghouse railway motor. Continuous rating, 200 amp., 235 volts. 



Central 600-volt, direct-current system. The same motors are 

 used for both direct-current and alternating-current service, the 

 control devices switching over automatically when transition is 

 made from one to the other. The motors which operate at 250 

 volts each on alternating current are connected two in series for 

 direct-current operation. 



115. The Repulsion Motor. If an ordinary direct-current 

 armature be placed in a single-phase magnetic field and the 

 brushes be short-circuited, a simple repulsion motor is obtained. 

 In order to develop torque, however, the brush axis must be 

 displaced from the axis of the main field by about 18 or 20 elec- 

 trical space-degrees, as will be shown. 



