276 INDUCTION MOTORS. [Exp. 



56. Operation of a Single-Phase Motor. At standstill a single- 

 phase induction motor has no torque. Currents are induced in the 

 secondary conductors, but from symmetry one half the conductors 

 tend to give rotation in one direction and the other half, in the oppo- 

 site direction. In a single-phase motor there is no rotating field at 

 standstill. When running, however, there is a rotating field due to the 

 combination of the main or field flux (which, referring to a 2-pole 

 model, we will call vertical) and the cross or rotor flux (which we will 

 call horizontal). On account of this rotating field the rotor is 

 dragged around in the same manner as in a polyphase motor. The 

 running of a single-phase motor becomes clear, therefore, when the 

 production of the rotating field is understood. 



57. The main or vertical flux in a single-phase motor is set up 

 directly by the field. When the motor is running, the conductors 

 on the rotor under the field poles cut this field flux so that an electro- 

 motive force is generated in the rotor and this is a maximum when 

 the field flux is a maximum. The rotor current which is caused to 

 flow by this electromotive force sets up a flux in a horizontal direction. 



Since this rotor flux must cause a counter-electromotive force pro- 

 portional to its time rate of change to balance the impressed or 

 generated electromotive force, it must come to its maximum a quarter 

 of a period after the electromotive force generated in the rotor and 

 therefore a quarter of a period after the field or vertical flux. The 

 vertical flux and horizontal flux are out of phase with each other, as 

 though set up by a 2-phase current ; one is a maximum when the other 

 is zero. They therefore combine to set up a rotating field. 



58. At synchronous speed the vertical and horizontal fluxes are 

 equal,* thus producing a uniform or circular magnetic field. As the 

 slip increases, the generated electromotive force and the horizontal 

 flux become less and the field is elliptical; at zero speed the ellipse 

 becomes a straight line. 



59. Variable Speed and Multispeed Motors. It has been pointed 

 out that the induction motor is nearly a constant speed motor and that 

 the falling off of speed with load or slip is due to the secondary resist- 



*(58a). The generated electromotive force is proportional to speed 

 times main flux; the counter electromotive force that balances this is pro- 

 portional to frequency times rotor flux. When speed equals frequency, the 

 two fluxes are equal, various constants being equal. 



