154 TREATISE ON ALTERNATING CURRENTS. 



is at rest, so that at the start the currents induced in the rotor 

 coils will have the same frequency as those in the stator. The 

 mutual reaction of the rotor currents and the rotating field pro- 

 duces a torque which, if sufficiently great to overcome the load 

 and the friction of the rotor bearings, will make the rotor follow 

 the magnetic field. 



It will then speed up until the torque produced electrically 

 balances that due to the friction and the load. As the speed 

 increases, the rate at which the conductors on the rotor are cut 

 by the rotating magnetic field diminishes, because the rate of 

 cutting is directly proportional to the angular velocity of the field 

 relative to the rotor. In consequence of this, both the magnitude 

 and the frequency of the rotor currents diminish as its speed 

 increases, if the load on the motor is kept constant. If there is 

 no load on the rotor, and if friction is negligibly small, the rotor 

 will keep increasing its speed until it runs synchronously with 

 the rotating field. The rotor currents would then be zero, since 

 the rate of cutting lines of force is zero. As soon as the load is 

 put on the speed is diminished, and again currents are induced 

 in the rotor coils. 



From this it is evident that rotary field motors must run at 

 some speed less than that corresponding to synchronism. 



The ratio of the frequency of the rotor currents to that of the 

 stator currents is called the Slip, and is denoted by the Greek 

 letter K . 



It will now be seen that an induction motor may be considered 

 as a transformer with a short-circuited secondary circuit free to 

 rotate. The points of difference between the two are, (1) that 

 while the energy given to the secondary of a stationary trans- 

 former is expended chiefly in an external circuit, that given to 

 the rotor of an induction motor produces mechanical rotation 

 against an opposing couple ; and (2) whereas in a stationary trans- 

 former the frequency of the secondary currents is the same as that 

 of the primary currents, the frequency of the rotor currents is, 

 except at the instant of starting, always less than that of the 

 stator currents. 



Stationary transformers and induction motors may, in fact, be 

 brought under one theoretical treatment, as has been done by 

 Steinmetz. 1 



1 ' Theory of the General Alternating-current Transformer," by C. P. Fteinmetz, 



