CHAPTER XIII. 

 GENERAL THEORY OF THE INDUCTION MOTOR. 



135. The induction motor considered as a transformer. When 

 the rotor of an induction motor stands still, the stator and rotor 

 windings constitute the primary and secondary coils of a trans- 

 former, differing in no essential particular from the ordinary trans- 

 former ; 'when the rotor of an induction motor is running, how- 

 ever, the action of the motor is not like the action of an ordinary 

 transformer, but all the details of action of an induction motor in 

 operation can be described and calculated in terms of ordinary 

 transformer action. 



Magnetizing current. When the rotor of an induction motor 

 is running at synchronous speed (it 1 = n) there is no relative mo- 

 tion of stator magnetism and rotor, no electromotive forces are 

 induced in the rotor windings, no rotor currents exist, and the 

 currents in the stator windings are sufficient merely to produce that 

 value of stator flux <I> which suffices to induce in the stator wind- 

 ings enough electromotive force to balance the supply voltage, exactly 

 as in a transformer with its secondary coil on open circuit. The 

 stator currents at synchronous rotor speed are therefore called 

 the magnetizing currents of the machine ; the magnetizing current 

 in each phase of the stator winding is represented in the following 

 discussion by the letter M. 



The magnetizing current of the transformer supplies the core 

 losses of the transformer as explained in Art. 115. In the induc- 

 tion motor the eddy current and hysteresis losses in the rotor iron 

 are negligible because the frequency of magnetic reversals in the 

 rotor iron is very low. The power components of the magnetiz- 

 ing currents of an induction motor supply the eddy-current and 

 hysteresis losses in the stator iron. 



Load current. When an induction motor is loaded, the rotor 

 runs at less than synchronous speed, the relative speed of stator 



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