THE INDUCTION MOTOR. 367 



In special cases, where motors are required to run at 

 two different speeds, the number of poles of the motor is 

 changed by altering the connections of the windings in 

 the stator by means of a special switch. This is possible, 

 since the number and position of the poles depend only 

 on the connections between the windings, and not on the 

 structure of the stator. 



A resistance in the primary circuit is not an admis- 

 sible method of regulation. Such a resistance produces 

 no appreciable effect upon the speed unless it is sufficiently 

 high to produce very great effect upon the primary voltage. 

 This would then be not only very wasteful, but would 

 seriously diminish the torque which the motor could 

 exert. 



By inserting resistances in series with the rotor 

 windings, and thus practically increasing the rotor 

 resistance, it is possible to produce a certain amount of 

 variation in the speed. This is owing to the fact that 

 the torque exerted by the motor is proportional to the 

 product of the rotating field strength and the rotor 

 current. For a given load to be overcome, this product 

 must, therefore, remain constant. Suppose the motor to 

 be running at a certain speed and to be driving a certain 

 load, and that an increase in the resistance of the rotor is 

 suddenly produced, the rotor current will at once become 

 weaker, and the torque exerted by the motor will diminish 

 and the speed will drop : that is, the slip will increase. 

 This at once produces the result of an increased electro- 

 motive force in the rotor circuit and a consequent rise in 

 rotor current. The loss in speed of the rotor will, 

 consequently, continue until the rotor current has attained 

 its previous value, enabling the motor to overcome the 

 load. The possible speed variation attainable by this 

 method may be seen from the curves given later. 



Speed variation by insertion of resistance in the rotor 

 circuit is necessarily accompanied by loss of efficiency, 

 since the losses C 2 R will increase in the same proportion 

 as the increase in the resistance of the rotor circuit. A 

 further disadvantage is that the current in the rotor is 

 different for every value of the load. Consequently, for 

 a given variation 'in speed, the amount of resistance 

 inserted is also different for each load. This method of 

 regulation is, consequently, analogous to and suffers from 



