THE INDUCTION MOTOR 339 



resistance of the rotor circuit can be controlled by varying 

 the resistance of this external circuit. This is done through 

 a controller switch similar in appearance and construction 

 to the railway controller. 



The resistance may sometimes be located inside of the 

 rotor itself, on the spider. In such a rotor no slip rings 

 are necessary; the ends of the rotor windings are con- 

 nected to sliding fingers which make contact with the resist- 

 ance. The position of these fingers controls the amount 

 of resistance in the rotor circuit and the position of the 

 fingers is regulated by a lever which operates a sliding 

 sleeve mounted on the rotor shaft. 



The three types of rotor are shown in Fig. 221; the 

 brush rigging for the slip ring rotor is also shown. 



For induction motors of less than five horsepower, the 

 squirrel-cage rotor is nearly always used; it may be used in 

 sizes as large as 150 or 200 h.p. For larger sized motors the 

 wound rotor with external resistance, is generally used for 

 reasons given in the succeeding paragraphs. 



93. Development of Torque. Consider a squirrel-cage 

 rotor in a revolving magnetic field. A cross-section of the 

 magnetic field and rotor conductors is shown in Fig. 222. 

 A two-pole motor is represented and it is supposed that the 

 field turns counterclockwise. If the rotor is stationary 

 (or turning at any speed less than that of the field) the con- 

 ductors on the top of the rotor will have a positive e.m.f. 

 induced in them (into the paper in the figure) and those 

 on the bottom will have a negative e.m.f. (out of the paper 

 in the figure). And as the rotor conductors are all connected 

 together by the end rings, currents will flow in the rotor in 

 the same direction as these e.m.fs. so that the top con- 

 ductors carry current toward the reader and the bottom 

 conductors carry current away from the reader. 



All Conductors give Torque in the Same Direction. But 

 we know that a conductor carrying current in a magnetic 

 field is acted on by a force which tends to move the con- 



