THE INDUCTION MOTOR. 265 



vision is made for having a large rotor resistance at starting, as 

 explained in Art. 125, it is very desirable to arrange for the cutting 

 out of this resistance when the rotor reaches nearly full speed, in 

 order to give high efficiency, and also to lessen the tendency for 

 the motor to change its speed with change of load. 



Relation between the mechanical power developed in turning the 

 rotor and the electrical power developed in the rotor windings. 

 The power 2'jrnT required to drive the " revolving field mag- 

 net " against the opposing torque 7 1 , as above explained, is all 

 delivered to the rotor, but the mechanical power developed in 

 the rotor is 2irn f T (less than 27rnT), and the remainder, 

 2irnT 2Trn f T, is the electrical power developed in the rotor 

 windings. Therefore, ignoring the losses above specified, the 

 power P delivered to the stator of an induction motor, the me- 

 chanical power P m developed in the rotor, and the electrical 

 power P e developed in the rotor windings are proportional to 

 n, n f , and n n' respectively, that is, 



P:P m :P e as n :n' :n n f 



Analogy between the induction motor and the friction clutch. 

 A friction clutch, the two parts of which continue to slip on each 

 other so that the driving part A runs continuously at speed n 

 and the driven part B runs continuously at a slower speed n* ', 

 is analogous in some respects to the induction motor. If we 

 ignore the friction losses in the two bearings at either side of the 

 clutch, the total power required to drive the part A is 2'jrnT, 

 and the mechanical power developed in part B is 2Trn r T, where 

 T is the torque with which the driving part A drags the 

 driven part B\ and the difference, 2irnT 2irn ! T is the 

 energy developed as heat where the slipping occurs. The 

 efficiency of transmission of such a clutch would evidently be 

 equal to n' In. 



127. Rating of induction motors. An induction motor is al- 

 ways rated to carry a specified load in horse-power when sup- 

 plied with a specified number of phases of alternating current at 



