298 



ALTERNATING (Tlt/tENTS 



Another method of splitting the phase is to use a three-phase 

 winding, as shown in Fig. 276, and to connect resistance nnd 

 inductance as shown. Resistance and capacitance may also be 

 used. Either a delta-connected stator, or a Y-connected stator 

 may be used. The resistance and inductance, when connected 

 as shown, displace the phase relations of the currents in the differ- 

 ent phases of the stator with respect to one another and so pro- 

 duce a sort of rotating field. All these phase-splitting devices 

 produce an elliptical rotating field. Because of the character- 

 istics of the field combined with the squirrel-cage characteristics 



of the rotor, the resulting torque is 

 barely sufficient to start the motor, 

 even without load. 



The shaded-pole method is shown in 

 Fig. 277. A short-circuited coil of low 

 resistance is connected around one 

 pole tip. When the flux is increasing 

 in the pole a portion of the flux at- 

 tempts to pass down through this 

 shaded tip. This flux induces a cur- 

 rent in the coil which by Lenz's law 

 is in such a direction as to oppose the flux entering the 

 coil. Hence, at first the .greater portion of the flux passes 

 down the right-hand side of the pole, as shown in Fig. 277. 

 Ultimately, however, the main flux reaches its maximum value, 

 where its rate of change is zero. The opposing emf. in the 

 shading coil then becomes zero, and later the opposing mmf. 

 of the short-circuited coil ceases, the current in this coil 

 lagging its emf. Considerable flux then penetrates the short- 

 circuited coil. After the main flux begins to decrease, the 

 induced current in the shading coil tends to prevent the flux 

 then existing in the shaded portion of the pole tip from decreas- 

 ing. Therefore, the flux first reaches its maximum value at the 

 right-hand or non-shaded side of the pole, and later reaches its 

 maximum at the left-hand or shaded side. The effect of the 

 shading coil is to retard in time-phase a portion of the flux, so 

 that there is a sweeping of the flux across the pole face from the 

 right-hand to the left-hand side in the direction of the shading 

 coil. This flux cutting the rotor conductors induces currents, 



Kotor 



FIG. 277. Shaded-pole 

 method of starting a single- 

 phase induction motor. 



