104 THE DIRECT-CURRENT MOTOR CH. V 



large currents can be obtained, the induced tension being 

 made up on the residual magnetism. This is the method 

 adopted in operating electric brakes for railway motors ; 

 in practice the induced tension seldom exceeds 10 volts. 

 For the purpose of braking, the armature is completely 

 disconnected from the line and from the magnets and is 

 then short-circuited through a rheostat. The dynamo 

 then acts as a generator with constant induction factor, 

 and the retarding torque is given by the product of the 

 induction factor and the current generated. 



We shall now consider the action of two series- 

 wound motors connected electrically in parallel, and 

 mechanically coupled so that they both run at the same 

 speed. The induction curves of the two motors are drawn 

 in Fig. 23, and we shall suppose that they are dissimilar, 

 but both pass through the origin, in other words, there is 

 no residual magnetism in either motor. Construct the 

 speed and torque curves as before. If the induction curve 

 of A lies above that of B, the torque curve of A will lie 

 above that of B, and the speed curve of A below that of B. 

 Construct the curve of combined speed, which shall 

 represent the current from the line horizontally, and the 

 speed of the combination vertically. Construct also the 

 curve of combined torque, which shall give the total torque 

 of the combination vertically, and the current drawn 

 from the line horizontally. The speed of the combination 

 is determined by the total load ; setting this off in inch- 

 pounds 011 the combined torque curve we find the total 

 current, and thus obtain the speed from the combined 

 speed curve ; this gives us the current in each motor. 

 Motor A, having the higher induction curve, will draw 

 the smaller current for all loads. Unlike two shunt- 



