COMMUTATING ALTERNATING CURRENT MOTORS 357 



used for a similar c-c. motor, and the self-induction of the 

 armature winding is reduced to practically zero by the 

 use of a compensating winding. 



This winding consists of a set of coils, wound in slots 

 in the field frame in such a manner that their magneto- 

 motive force is just equal and opposite to that of the arma- 

 ture coils. Hence the armature cannot build up a magnetic 

 field because of these compensating coils and so it has no 

 self-induction. The compensating winding is connected 

 in series with the armature so that the magnetic effects 

 of the armature coils are neutralized at all loads. 



Low Frequency Power Necessary. Even though the 

 self-induction of the armature is reduced to zero by the 

 compensating winding, the motor cannot be successfully 

 used on 60-cycle circuits. The self-induction of the field 

 coils is so large that the power factor of the motor is small 

 if used on a circuit of greater frequency than 25 cycles. 

 The advocates of this type of motor have agitated for 

 15-cycle power for railway operation, but this frequency 

 is not likely to become standard in America. Practically 

 all alternating-current series motors are designed for 25- 

 cycle power. 



Low Flux Density. The flux density in an a-c. motor 

 must be kept low in order to prevent heavy hysteresis and 

 eddy current losses in the field frame; thus for a given rat- 

 ing the a-c. motor is larger than the c-c. motor. Also, to 

 produce the required torque, the armature must have a 

 larger number of conductors in the a-c. motor because of 

 the low flux density used; this larger number of armature 

 turns does not increase the self-induction of the motor 

 because of the action of the compensating winding. 



In Fig. 231 is shown a cross-section of an a-c. series 

 motor showing the relative positions of the main field, com- 

 pensating field, and brushes. As the compensating coils 

 carry twice as much current as the armature coils, it is 

 necessary to have only half as many turns in the com- 



