ALTERNATING-CURRENT COMMUTATOR MOTORS 361 



turns on the armature since, for a given impressed voltage, that 

 would increase the speed of the motor and, further, since the field 

 is made comparatively weak the armature must be made corre- 

 spondingly strong in ampere turns in order to produce the required 

 torque. 



The armature m.m.f. as in direct-current machines is cross 

 magnetizing and distorts the main field and so weakens it and 

 interferes with commutation. The flux produced by it is alter- 

 nating and induces in the armature a back e.m.f. of armature 

 inductance. Two methods of reducing this flux are used, both of 

 which correspond to the use of interpoles on direct-current ma- 

 chines. A winding, called a compensating winding, is placed in slots 

 in the pole faces as shown in Fig. 339. It is distributed over the 



FIG. 339. Four-pole, single-phase, series motor 

 with compensating winding. 



whole periphery 01 the armature and exerts a m.m.f. opposing the 

 armature m.m.f. and so limiting the cross flux to a very small value 

 and reducing the armature inductance and reactance in the same 

 proportion. 



The m.m.f. of the compensating winding can be produced in 

 two ways illustrated in Fig. 340 and Fig. 341. The first is called 

 inductive compensation and the second conductive compensation. 



(1) In the inductively compensated series motor the compensat- 

 ing m.m.f. is produced by short circuiting the compensating coil. 



