274 



ELEMENTS OF ELECTRICAL ENGINEERING. 



(b) Split-phase starting. Small single-phase induction motors 

 are sometimes started as polyphase induction motors as follows : 

 The stator of the motor is provided with a two-phase winding, 

 and these two phases are connected in parallel to the single-phase 

 supply mains with a resistance or a condenser in series with one 

 of the phases. Under these conditions the currents in the two 

 stator windings are out of phase with each other sufficiently to 

 produce the desired starting torque, and when the machine 

 reaches moderate speed, one of the stator windings is discon- 

 nected, and the machine con- 

 tinues to operate as a single- 

 phase motor. The split-phase 

 method of starting is incorpo- 

 rated in many single-phase 

 alternating-current fan-motors 

 by means of a device called a 

 shading' coil. Figure 240 is 

 an end view of such a single- 

 phase fan-motor. The polar 

 projections of the stator iron 

 are wound with coils of wire 

 which are connected in series 



and receive current from the single-phase supply mains, and one 

 tip of each pole piece is surrounded by a short-circuited coil of 

 wire or frame of copper, as indicated in the figure. This coil, or 

 copper frame, is called a shading coil and it causes a phase differ- 

 ence between the pulsating flux that emanates from the main 

 portion of each polar projection and the pulsating flux which 

 emanates from the pole tip, thus introducing a two-phase action 

 on the rotor which is sufficiently pronounced to start the motor. 



(c) Repulsion motor starting. The best method of starting 

 single-phase induction motors is to design the motor so that it can 

 be operated at starting as a repulsion motor and be quickly con- 

 verted into an induction motor after it reaches approximately full 

 speed. The single-phase induction motor of the Wagner Electric 

 Company operates in this way. It is described in Chapter XIV. 



Fig. 240. 



