120 THE DIRECT-CURRENT MOTOR CH. V 



motion ; this current will be either larger or smaller than 

 that required to balance the load of the other motor. 

 In the first case, B would accelerate, the induced tension 

 on B would increase, and the current would decrease. A 

 would then be retarded, and finally stopped if the difference 

 in the loads was sufficiently great. In the second case, B 

 would be retarded and stopped. 



If two similar motors driving a car are connected in 

 series, the two induction factors are always the same. If 

 one of them slips, the load on that motor is reduced, at 

 the same time the mechanical connection previously 

 secured by the adhesion of the rail is inoperative ; the 

 speed of the slipping motor will therefore increase, and 

 with it the induced tension, hence the current will be 

 decreased, and the torque assisting the motion of the non- 

 slipping wheel, and therefore of the car, also decreased ; 

 if slipping continues, the car must stop. The use of 

 series- connected motors for railway work is 

 therefore objectionable, unless the driving 

 wheels can be coupled by connecting rods. 



If any number of motors are connected electrically in 

 series, being independent mechanically, the condition of 

 uniform running for all the motors is that the induction 

 factor for each motor shall be proportional to its load. We 

 have here the conditions under which it is possible to 

 operate a number of motors in series with a constant 

 current, each motor being mechanically independent of 

 all the rest. 



Let us suppose that we have several motors connected 

 in series and a constant current passing in the circuit. 

 Assume that one of the motors is at rest, the current 

 being cut out by a short circuiting switch. When this 



