INDUCTION MOTOR 



351 



Fig. 327 shows the relative directions of the currents in the two 

 windings at standstill. The stator carries a current I\ which 

 consists of two components, I' the load component and I M the 

 magnetizing current. The rotor carries a current Iz opposite in 

 phase to /' and equal to it in m.m.f. If the ratio of turns is 

 assumed to be HI : n 2 = 1 : 1 ; then 7 2 = I'- The motor at stand- 

 still is a transformer with a short-circuited secondary. 



Y 



FIG. 327. FIG. 328. FIG. 329. FIG. 330. FIG. 331. 

 Single-phase induction motor. 



The flux which crosses the air gap and links with both stator 

 and rotor is produced by the stator exciting current. It is always 

 directed along the line YOY. There is no component of flux in 

 the horizontal direction XOX and therefore no torque is exerted 

 tending to turn the rotor in either direction. Thus the rotating 

 field which is produced in the polyphase induction motor does 

 not exist in the single-phase motor at standstill. The single- 

 phase induction motor, therefore, has no starting torque. If, 

 however, it is started in either direction it will develop torque 

 and will accelerate and come up approximately to synchronous 

 speed at no load. 



Fig. 328 represents the motor with the rotor open circuited and, 

 therefore, without current in its windings. The stator carries only 

 the magnetizing current. 



Fig. 329 represents conditions at synchronous speed at the 

 instant when the stator magnetizing current is maximum. The 

 stator flux is then maximum downwards. 



The rotor conductors moving at synchronous speed cut the 

 stator flux and an e.m.f. is generated in them proportional to the 

 product of flux and speed. Since the flux is alternating the e.m.f. 

 generated is of double frequency and produces a current of double 

 frequency in the closed rotor winding. The current produces a 

 flux the rate of change of which through the rotor windings gene- 

 rates in them an e.m.f. equal and opposite to the e.m.f. generated 



