INDUCTION MOTOR 



353 



223. Horizontal Field at Slip s. When the rotor runs at a 

 speed S = (1 s) X synchronous speed, the e.m.f. generated in 

 it due to cutting the stator flux is less than at synchronous speed 

 in the ratio 1 s : 1 and the horizontal flux and the rotor mag- 

 netizing current are less in the same ratio. 



The stator current is /# + (1 s) I M + I' and the rotor current 

 is (1 s) I M + 7 2 . The frequency of the rotor magnetizing 

 current is (2 s)f and the frequency of the rotor load current is 

 s/, where / is the frequency of the e.m.f. impressed on the stator. 



The revolving field at slip s is not constant in value but has the 

 horizontal axis shorter than the vertical in the ratio 1 s : 1, 

 Fig. 333. The field follows an elliptical instead of a circular locus. 



FIG. 332. Revolving field of a 

 single-phase induction motor at 

 synchronous speed. 



FIG. 333. Revolving field of a 

 single-phase induction motor at 

 slip s. 



The torque which is proportional to the product of the rotor 

 load current and the horizontal field is less than that produced in 

 the polyphase motor in the ratio 1 s : 1. 



224. Starting Single-phase Induction Motors. In order to 

 obtain the torque required to start a single-phase induction motor 

 a component of flux in quadrature in time and in space with the 

 stator flux must be produced at standstill. It has been shown 

 that when once the motor is started the rotor produces the 

 required quadrature flux and thus the torque to carry the load. 



Two principal methods are employed to produce the quadrature 

 flux at standstill, (1) phase splitting and (2) shading coils. 



(1) If the two stator windings of a two-phase induction motor 

 are connected to a single-phase supply, phase 1 directly and phase 

 2 through a suitable resistance or condensive reactance, the flux 

 produced by phase 2 will have a component in quadrature in time 



