170 PRINCIPLES OF ELECTRICAL DESIGN 



is exactly in the zone corresponding to the brush position that 

 the armature m.m.f. has its maximum value. In the case of 

 the interpole machine, the windings necessary to compensate 

 for armature cross-magnetization are an objectionable feature, 

 and, except for the added cost and tendency to interfere with 

 ventilation, there is much to be said in favor of pole-face wind- 

 ings, the function of which is to neutralize the magnetizing 

 effect of the armature winding and maintain an approximately 

 constant flux density over the pole faces. . The writer has in 

 mind machines such as those which, for the last 20 years, have 

 been constructed under the THOMPSON-RYAN* patents. One 



Center Line of Pole 



Compensating Coi 

 Main Exciting Con 

 Interpole Winding 



FIG. 64. Compensating pole-face winding. 



of the attractive features of such designs is the fact that the 

 winding on the commutating poles need be no greater than that 

 required to overcome the reluctance of the air gap and send the 

 requisite flux into the armature teeth comprised in the commu- 

 tating zone. A pole-face compensating winding is shown in 

 Fig. 64. The balancing coils pass through slots in the pole 

 face and carry the full current of the machine; that is to say, 

 they are connected in series with the commutating-pole windings 

 and the compounding series turns (if any) on the main poles. 

 The connections between the pole-face compensating coils are 

 so made that the current in these will always tend to neutralize 

 the magnetic effect of the currents in the armature coils, and so 

 prevent distortion of the flux over the pole face. 



