266 ELEMENTS OF ELECTRICAL ENGINEERING. 



a specified frequency and a specified voltage per phase, and in 

 addition, either the speed or the number of poles is given. Thus, 

 one would speak of a ten horse-power, two-phase, four-pole, 60 

 cycle, 22O-volt induction motor. 



To overload an induction motor, frequency and voltage of 

 supply being normal, causes the motor to take excessive current 

 at a reduced power factor, and the motor runs at a slightly re- 

 duced speed. Excessive overload causes the motor to break 

 down as described in Art. 125. 



An induction motor may be operated quite satisfactorily at a 

 frequency or voltage considerably different from normal. A 

 change of frequency produces a proportional change of speed ; 

 and a change either of voltage or frequency causes a change in 

 the value of the magnetic flux <J> which emanates from each polar 

 region of the stator iron, a change in the magnetizing (zero-load) 

 current, and a change in the eddy current and hysteresis losses 

 which losses occur chiefly in the stator iron.* 



The behavior of an induction motor when it is operated by an 

 abnormally high or low voltage is especially important. In fact 

 the torque that is developed by an induction motor at a given 

 relative speed, n n f , of stator magnetism and rotor is approx- 

 imately proportional to the square of the voltage. This is evi- 

 dent when we consider that to increase the voltage in the ratio 

 of i to k causes the same ratio of increase of flux <E>, of rotor 

 electromotive forces, and of rotor currents (for given value of 

 ;/ n f ) ; but, since the torque is proportional to the product of 

 flux and rotor currents, it is evident that the torque is increased 

 in the ratio of I to k z . 



128. The starting of the induction motor. (a) Small induction 

 motors with squirrel -cage rotors are nearly always started by con- 

 necting their stator windings directly to the polyphase supply 

 mains. 



(fr) Large induction motors with wire wound rotors are usually 



* See G. B. Werner, On the effect of voltage and frequency variations on induction 

 motor performance, Electric Journal, Vol. Ill, page 400, July, 1906. 



