116 THE DIRECT-CURRENT MOTOR CH. V 



Generally, if there are x motors in series, the speed of 

 each may be written 



where n )t is the speed of each when placed alone on the 

 full tension of the line, and t is the torque on each motor. 



It may not perhaps at first sight be obvious why the 

 induction factor remains the same when the motors are 

 thrown in series. The reason is that when the torque 

 on the motor is constant, the product of the current 

 into the induction factor is constant, and independent of the 

 tension on the motor terminals. Hence the current per 

 motor remains the same, whatever the arrange- 

 ment of the motors may be. 



For instance, if the motors in the case quoted above 

 were each taking 30 amperes when connected in parallel, 

 they would each take 30 amperes when connected in series, 

 the only difference would be a reduction in the speed and 

 in the current drawn from the line. When the motors 

 are in parallel the current from the line is 60 amperes, 

 but when they are in series the current from the line is 

 30 amperes, since the current that passes through one 

 motor also goes through the second motor. There is thus 

 a saving of current by connecting the motors in series, 

 but there is a corresponding reduction in the speed, and 

 we see that the amount of this reduction is largely 

 determined by the resistance of the motors. If the resist- 

 ance were nothing the speed would be directly proportional 

 to the tension on the motor terminals, and the speed in 

 series would be exactly one half of that in parallel. With 

 four motors in series the speed would then be one fourth 

 of that of each motor in parallel, and so on. 



