COMMUTATING ALTERNATING CURRENT MOTORS 365 



conductors lie in a magnetic field. Hence there will be a 

 force exerted between the field and the armature con- 

 ductors tending to make the armature turn. This torque 

 will continue to be in the same direction as the flux alter- 

 nates, because the current in the armature and the field flux 

 both change together, so that the force between them is 

 constantly in the same direction. 



To discuss this point more completely it would be 

 necessary to consider the relative phases of the field flux 

 and armature current and this would complicate the ques- 

 tion too much. We have given enough analysis to show 



"-90 +90 



Brush position Values of a 



FIG. 238. Torque and Current of the Repulsion Motor Vary Greatly 

 with the Position of the Brushes, as Shown Here. 



that the torque of such a motor (like that of the series 

 a-c. motor) is uni-directional and pulsating. 



Direction of Rotation. When the armature conductors 

 are represented by the equivalent coil A"B" the brushes 

 must be at right angles to the plane of this soil, as shown 

 by CC' in Fig. 237. When the brushes are placed back 

 of the center line of the magnetic field, the armature will 

 rotate in a counter-clockwise direction and vice versa. 

 Hence to vary the torque of a repulsion motor in magnitude 

 and direction it is only necessary to change the position of 

 the brushes on the commutator. The variation of the 

 tcrque and current with the position of the brushes is 



