354 



ELECTRICAL ENGINEERING 



with phase 1 and will thus give the required starting torque. 

 (Fig. 334.) When the motor has come up to half speed, the starting 

 winding is cut out and the motor runs as a single-phase motor on 



Line 



Phase 1 



Dis 



Operating Winding 



Phase 2 

 ^Starting 

 Winding 



connecting 

 Switch 



Resistance 



FIG. 334. Phase splitting. 



phase 1. This method of starting is called phase splitting. The 

 second winding need not have as many turns as the first but it 

 should be placed at 90 electrical degrees to it. 



(2) The shading coil, Fig. 335, is a short-circuited coil surround- 

 ing part of each pole of the 

 stator. Currents are induced 

 in it and oppose the increase 

 and decrease of the flux in the 

 part of the pole which it in- 

 closes. Thus the north pole 

 in section A will reach its max- 

 imum value before it is maxi- 

 mum in section B. When the 

 north pole has decreased to 

 zero in B it will be increasing 

 in section C and thus there is 



a rotation of the magnetic field and torque is produced. When 

 the motor is started the short-circuited coils are opened and they 

 are then idle. 



225. Comparison of Single-phase and Polyphase Motors. 

 Take the case of a two-phase motor operating on a single-phase 

 circuit using only one phase of the stator winding. 



FIG. 335. Shading coils. 



