350 THE INDUCTION MOTOR. 



If the armature is stationary, the lines of the rotating 

 field will cut each of the conductors 2n times per second 

 (where n = frequency of current supplied to the motor), 

 since the lines each cross the layer of conductors twice 

 in opposite directions. If, however, the armature rotates 

 in the same direction as the field n l times per second, the 

 field will only be cut 2 (n - n 1 ) times per second by each 

 conductor. 



Consequently, with a constant rotating field the 

 electromotive force induced in the rotor conductors is 

 proportional to the difference in speed of rotating field and 

 rotor. 



The difference between the speed of the conductors of 

 the rotor and of the rotating field is called the slip of the 

 motor, and is measured either in terms of revolutions per 

 second or as a percentage of the speed of the rotating 

 field. 



Thus in a 2-pole motor : slip* = n - n 1 

 Where n = periodicity of current. 



= revs, per second of rotating field. 

 n 1 = revs, per second of rotor. 



1 i 



The slip of a multipolar motor is - where p = 



number of pairs of poles, and the synchronous speed of 



such a motor is - . 

 P 



Thus the electromotive force induced in the rotor 

 conductors by a constant rotating field would be a 

 maximum when the rotor was stationary, i.e., with a slip 

 equal to n, and would be zero with the rotor revolving 

 synchronously, i.e., at the same rate as the field and slip 

 = 0. A curve representing the dependence of induced 

 rotor voltage upon speed would be a straight line, as in 

 the case of an alternator or direct-current dynamo. 



The wave-form of the voltage thus induced in the 

 conductors of the rotor must depend on the distribution 

 of the magnetic field in the air-gap between stator and rotor 

 in which the conductors rotate. Usually, the field is so 

 distributed that no serious error is introduced by 

 assuming that the electromotive force produced is 



i 



* Sometimes the slip is defined as being equal to -JLlL , i.e., the ratio of the 



n 



difference of speeds to the frequency. This would be better defined as the 

 " fractional slip," or, when multiplied by 100, as the "percentage slip." 



