352 ELECTRICAL ENGINEERING 



by rotation. This flux must, therefore, be of the same value as the 

 stator flux and it is in phase with the rotor current. 



The rotor current goes through two complete cycles during one 

 revolution. In Fig. 329 it is maximum and is opposed to the 

 stator current, but the e.m.f. impressed on the stator is constant 

 and the stator flux is constant, and, therefore, a current must flow 

 in the stator to balance the m.m.f . of the rotor current I M f . Since 

 the ratio of turns has been taken as 1 : 1 the increase in stator 

 current is I M f and the total stator current at synchronous speed is 

 I M + I M f > In the position shown the rotor flux is not produced 

 because the rotor m.m.f. is opposed by an equal and opposite m.m.f. 

 on the stator. 



Fig. 330 represents conditions after the rotor has turned through 

 one half a revolution and the stator current has passed through one 

 half cycle. The rotor current is in the same direction as before 

 and has completed one cycle. 



Fig. 331 represents conditions midway between Fig. 329 and 

 Fig. 330. The stator current is zero and the rotor current is 

 maximum and exerts a m.m.f. in the horizontal direction. There 

 is no stator m.m.f. opposing it and a flux is produced of the same 

 value as the stator flux in Fig. 329 or Fig. 330. Since the reluct- 

 ance of the path for the horizontal flux is the same as that for the 

 vertical stator flux, the rotor magnetizing current IM must bo 

 equal to the stator magnetizing current at standstill I M , and, 

 therefore, at synchronous speed the stator magnetizing current 

 is 2 I M and is double its value at standstill. 



Thus at synchronous speed there is a resultant m.m.f. of con- 

 stant value revolving at synchronous speed and the magnetic field 

 of the single-phase motor is identical with that of the polyphase 

 motor, Fig. 332. The m.m.f. to produce the vertical field is 

 supplied by the true stator magnetizing current, while the m.m.f. 

 to produce the horizontal field is provided by an equal stator 

 magnetizing current, in phase with the true stator magnetizing 

 current, which induces in the rotor the rotor magnetizing current. 



When the rotor runs at synchronous speed its conductors do not 

 cut this revolving flux and the only current in the rotor is the 

 double-frequency magnetizing current. 



When the rotor runs at a slip s below synchronous speed the 

 rotor conductors cut the flux and currents are produced in them 

 and torque is developed just as in the case of the polyphase motor. 



