282 



ALTERNATING CURRENTS 



Compensating 

 Winding 



FIG. 258. Conductively-compensated 

 series motor. 



group of armature conductors directly under it, as is also indi- 

 cated in Fig. 257. 



The compensating winding may be connected in series with 

 the armature, Fig. 258, in which case the motor is said to be 

 conduciively compensated. When it is necessary to use the 

 motor on a direct-current system as well as on an alternating- 

 current system, conductive compensation is necessary. 



If the compensating wind- 

 ing be short-circuited on itself, 

 Fig. 259, the winding is linked 

 with the cross-magnetizing 

 flux of the armature and 

 therefore becomes the short- 

 circuited secondary of a trans- 

 former, the armature ampere- 

 turns being the primary. As 

 the secondary ampere-turns of a transformer are practically 

 opposite in phase and equal in magnitude to the primary ampere- 

 turns if the magnetizing current be small, the ampere-turns of 

 the compensating winding nearly neutralize the ampere-turns of 

 the armature. It is not possible to eliminate entirely the cross- 

 magnetizing flux by this method, any more than it is possible 

 to eliminate the mutual flux in a short-circuited transformer, but 

 it may be reduced to a very small value. 



The cross-magnetizing flux 

 links the armature turns, 

 causing the armature itself 

 to have large self-inductance. 

 Therefore, the reactance of the 

 armature alone (X a = 2irfL a ) 

 is large, which in turn would 

 produce a large reactance FlG - 259 '~ I ^ e c s ti ^tor 01 ^ 8 ^ 

 (IXa) drop and so lower the 



power and the power-factor of the motor. The compensating 

 winding, acting like the short-circuited secondary of a trans- 

 former, reduces this armature reactance to a small value. 



This is analogous to the ordinary transformer, which on open- 

 circuit is a very high impedance. When the secondary is short- 

 circuited, the impedance is reduced to a very low value. The 



ompensathig 

 Winding 



