2/0 ELEMENTS OF ELECTRICAL ENGINEERING. 



When, however, n and n' are very different, that is when 

 the relative speed n n' of stator magnetism and rotor is large, 

 then the rotor electromotive forces are large, and if the rotor 

 windings are brought out to collecting rings as shown in Fig. 

 225 the rotor may deliver a very considerable amount of elec- 

 trical power to external receiving circuits at any desired frequency. 

 When an induction motor is used in this way it is called a fre- 

 quency changer. As usually employed the frequency changer is 

 used to change a low frequency alternating-current supply, say 

 25 cycles per second, to a higher frequency, say 60 cycles per 

 second for operating electric lamps. When so used the rotor 

 of the frequency changer is driven backwards by a synchronous 

 motor or by an induction motor taking power from the low fre- 

 quency supply mains. 



When the rotor runs backwards the relative speed of stator 

 magnetism and rotor is ;/ -f ;/ and the frequency of the rotor 

 electromotive forces is ( -j- n f )/n times the primary frequency. 



In order to determine the relation between the electrical power 

 delivered to the stator of the frequency changer and the mechani- 

 cal power required to drive its rotor backwards at speed n' , let us 

 consider the torque action T between stator and rotor, and let us 

 for a moment think of the stator as a mechanically revolved field 

 magnet. Then, ignoring losses, 2irnT is the power supplied 

 electrically to the stator, 2irn' T is the power supplied mechan- 

 ically to the rotor in driving it backwards at speed n 1 , and 2ir 

 (n -f n f )T is the electrical power developed in the rotor windings. 



Example. A four-pole, three-phase induction motor has a 

 A-connected stator winding supplied from 25 cycle, 22O-volt, 

 three-phase mains. The rotor winding is also three-phase, it is 

 A-connected to the collector rings, and it has the same number 

 of conductors as the stator winding, so that when the rotor is 

 standing still the voltage between the collector rings is 220 volts. 

 The speed of the stator magnetism is 12^ revolutions per sec- 

 ond, the rotor is driven backwards at a speed of 1 7 ^ revolutions 

 per second ; and, therefore, the rotor frequency is 60 cycles per 



