112 THE DIRECT-CURRENT MOTOR CH. V 



We now come to the case where two motors 

 are connected electrically in series and also 

 coupled mechanically so that they both run at 

 the same speed, each taking its own share of the total 

 load. 



The current is here the same in each motor, hence the 

 heat-drop is the same if the resistances are equal, and 

 since the speeds are the same the induced tensions will 

 depend on the induction factors. If the induction curves 

 do not coincide, the induced tensions will be proportional 

 to the vertical ordinates of the two curves, and the line 

 tension will be unequally divided between the two motors. 

 In some experiments made by Mr. H. S. Bering the 

 average difference in the terminal tensions of two street 

 railway motors under test was six volts, the tensions being 

 242 and 248 volts for 41*8 amperes in each. 



If the two induction curves coincide, the induced 

 tensions will be equal, and the tension of the line will be 

 equally divided between the two motors. Thus, when two 

 similar motors are connected in series on a 500-volt line, 

 the terminal tension will be 250 volts on each. If four 

 motors are connected in series the terminal tension on each 

 will be 125 volts. In railway practice two motors are 

 frequently used together in this way and switched over 

 from series to parallel, according to circumstances. It is 

 often convenient to be able to represent graphically the 

 result of this alteration in the connections. The change 

 consists generally in doubling the terminal tension of the 

 motors. 



Fig 28. shows the curves of torque and current, both 

 on a speed base, of the motor illustrated in Fig. 20. The 

 curves represent the following conditions. (1) When the 



