THE OPERATION OF THE DYNAMO AS A MOTOR. 119 



connections of the motor to trolley wire and rail are essentially 

 the same as the connections shown in Fig. 87, except that pro- 

 vision is made for quickly reversing the armature connections (or 

 the field connections) in order to reverse the direction of running 

 of the motor. 



In street car service, where stopping and starting is frequent, 

 the motor runs for a great portion of the time at low speed with 

 a considerable resistance in series with it. Under these con- 

 ditions most of the energy supplied to the motor circuit is lost as 

 heat in the rheostat, and the efficiency of the motor is very low. 



trolley wire 



starting rheostat 



trolley wire 



r "7 



Starting rheostat 



motor No. i ~< 



motor No. t 



motor 



No. 2 - 



I--, 



rail 



motor No. a 



Fig. 88. Fig. 89. 



Some idea of the efficiency may be obtained with the help of 

 Siemens' law. (See Art. 54.) The approximate efficiency 

 E a j E x is small at starting, inasmuch as the counter electro- 

 motive force, E a ( = QZ'ii), is small when the speed is small. 

 By using two motors on a car, and arranging to have them con- 

 nected in series at starting, the counter electromotive force is 

 doubled, and the efficiency is nearly doubled. 



