COMMUTATING ALTERNATING CURRENT MOTORS 363 



over the series motor in that its armature is not connected 

 to the power line and so its power supply, which is fed into 

 the field circuit only, may be of much higher voltage than 

 can well be used on the series motor. 



Principle of Operation. The principle on which the 

 motor operates may be understood by reference to Fig. 236. 

 The field circuit is connected to the power supply and so 

 an alternating flux is set up in the armature. The arma- 

 ture brushes are short circuited and the plane of the 

 brushes, A-B, is at an angle with the direction of the magnetic 

 field. An e.m.f. is induced in the armature coils by the 

 alternating field flux, and this e.m.f. causes a current to 

 circulate through the armature coils and the path con- 



Power 

 Supply 



\ 



FIG. 236. Circuits of a Simple Repulsion Motor. 



necting the brushes together. The armature conductors 

 carrying current react on the magnetic field set up by the 

 field coil and so produce a torque. The magnitude of this 

 torque depends upon the value of the angle a, Fig. 236, 

 and the direction of the torque is always in the same 

 direction as that in which the brushes A-B have been 

 moved away from the center line of the field. 



Torque Varies with the Position of the Brushes. The 

 dependence of the torque upon the angle a may be seen 

 by reference to Fig. 237. Any armature having short-cir- 

 circuited brushes may be represented by one short-circuited 

 turn, this turn being in a plane 90 away from the plane 

 of the brushes. In Fig. 237, for example, the short-circuited 



