362 



ALTERNATING CURRENTS 



In the split-pole converter, the form of the alternating electro- 

 motive force wave is varied by means of auxiliary poles adjacent 

 to the main poles. The main poles are excited by the main-field 

 winding, and the auxiliary poles by a separate winding. By 

 changing the auxiliary excitation in conjunction with the excita- 

 tion of the main winding, the wave form of the alternating elec- 

 tromotive force may be changed, thus varying the ratio of the 

 alternating-current to the direct-current electromotive force. 





FIG. 330. Westinghouse synchronous converter with booster generator. 



The brushes in a generator must be moved forward, in order 

 that the machine may commutate in the fringe of a leading pole- 

 tip (see Vol. I, page 285, Par. 197). To balance the electromotive 

 force of self-induction, the brushes of the split-pole converter 

 must be moved forward, in order to commutate in the fringe of a 

 leading pole-tip, Fig. 331. This fringe must come from the main 

 poles, for their flux is nearly constant in strength, whereas the 

 flux of the auxiliary pole is varied over a wide range and may be 

 reversed even. Therefore, in a direct synchronous converter, the 

 armature must rotate from main to auxiliary pole, Fig. 331, 



