294 



ALTERNATING CURRENTS 



When the rotor operates near synchronous speed and rotates 

 in the direction of T\, its slip is nearly two as regards TV There- 

 fore, the rotating field which produces T 2 induces double 

 frequency currents in the rotor at this speed. These double- 

 frequency currents, however, produce but little torque because 

 of their high frequency. This frequency is double the stator 

 frequency. Therefore, the rotor reactance is many times its 

 value at slip frequency. Consequently, these currents are small 

 in magnitude and make a considerable space-angle with the air- 

 gap flux, developing little counter-torque (see Par. 103, page 237). 



It is obvious that the single-phase induction motor rotates 

 in the direction in which it is started. 



117. Reactions in a Single-phase Induction Motor. Although 

 the foregoing treatment of the single-phase induction motor 



FIG. 273. Transformer currents in 

 the rotor of a single-phase induction 

 motor. 



FIG. 274. Speed current and result- 

 ing flux in the rotor of a single-phase 

 induction motor. 



gives some idea of its method of operation, it is not a rigorous 

 analysis nor does it give a physical conception of what actually 

 occurs in the motor. 



The reactions occurring in the rotor of a single-phase induction 

 motor are not simple and several factors must be considered if 

 an exact analysis is to be made. In Fig. 273 the main flux <f> M due 

 to the stator winding passes, at the instant shown, down into the 

 armature from the north pole N. In so doing it links the rotor 

 conductors and due to transformer action, currents are induced 

 in these rotor conductors. These induced currents in the rotor 



