362 ELECTRICAL ENGINEERING 



It then acts as the closed secondary of a transformer of which the 

 armature is the primary. The m.m.f . of the compensating winding 

 is almost equal to the m.m.f. of the armature but can never be 

 greater than it and, therefore, overcompensation is rot possible. 



FIG. 340. Inductively compensated FIG. 341. Conductively compen- 



series motor. sated series motor. 



The combined reactance of the armature and compensating wind- 

 ing corresponds to the reactance of a transformer on short circuit. 

 The mutual flux is almost destroyed but the leakage fluxes remain. 



(2) In the conductively compensated motor the compensating 

 coil is connected in series with the field and armature and the 

 amount of compensation can be varied. When the m.m.fs. of 

 the two windings are equal there is no mutual flux and the com- 

 bined reactance is a minimum. When the m.m.f. of the compen- 

 sating winding is stronger than that of the armature the armature 

 reaction flux is reversed but the reactance of the compensating 

 winding is increased and so part of the advantage is lost, but the 

 flux due to overcompensation assists commutation of the load 

 current in the same way that interpoles do and is thus a great 

 advantage. 



A conductively compensated motor can be operated on direct 

 current but an inductively compensated motor cannot since the 

 compensating winding would not be effective and sparking would 

 occur. 



230. Commutation. Satisfactory commutation is very much 

 more difficult to obtain in the alternating-current series motor than 

 in the direct-current motor because, as may be seen in Fig. 342, the 

 short-circuited coil is in the position of the short-circuited secondary 

 of a transformer with the main field as primary and tends to have 

 as many ampere turns induced in it as there are on a pair of field 

 poles. This large short-circuit current interferes with commuta- 

 tion and must be reduced as far as possible. For this purpose high- 

 resistance leads, called preventive leads, are connected between the 

 coils and the commutator bars, as shown in Fig. 343, and narrow 



