THE SINGLE-PHASE SERIES MOTOR. 31$ 



of the electrical equipment high, but the synchronous converters 

 must operate a large portion of the time at light load so that the 

 power losses must be great and the efficiency low. If the single- 

 phase series motor is used, the trolley wire can itself be the high 

 voltage transmission line, and a step-down transformer on the 

 car or locomotive can deliver current at any desired voltage to 

 the motors. 



The most serious difficulty involved in the use of the single- 

 phase series motor is its large size as compared with a direct- 

 current motor of the same power rating. Thus, the electric 

 locomotives of the New York Central Railway Company have 

 direct-current motors of sufficient capacity to slip the wheels, 

 whereas the electric locomotives of the New York, New Haven 

 and Hartford Railway Company, which are about the same 

 weight as the New York Central locomotives and which are 

 equipped with single-phase series motors, can exert a tractive 

 effort not to exceed 50 per cent, of that which would correspond 

 to slipping of wheels. 



The series motor requires a very low voltage at starting, and 

 the voltage across the motor terminals must be increased as the 

 speed increases. When direct current is used, this variation of 

 motor voltage can be accomplished only by the use of rheostat 

 control, as explained in Art. 61 of the first volume of this text, 

 but when alternating current is used, a number of taps can be 

 brought out from the secondary coil of the step-down transformer 

 so that the low voltage required at starting can be taken directly 

 from a few secondary turns, and the voltage can be increased at 

 will by bringing a larger and larger number of secondary turns 

 into action as the speed increases. Rheostat control is therefore 

 not necessary when alternating current is used and the loss of 

 power in rheostats is obviated. Furthermore, the series -parallel 

 system of direct-current motor control, the object of which is to 

 reduce the range of rheostat control of voltage, is unnecessary 

 when alternating current is used. 



145, The repulsion motor. The repulsion motor consists of a 







