288 



ALTERNATING CURRENTS 



In Fig. 264, the brushes are shown as being in the geometrical 

 neutral and short-circuited. Each brush is at the mid-point of 

 its transformer winding. As the total emfs. in each winding are 

 the same and the windings are connected in parallel, each mid- 

 point must be at the same potential. Therefore, the brushes 

 short-circuit two points at the same potential and no current 

 flows between brushes. 



It is clear that without brushes there is no armature current, 

 and even with brushes there is no armature current, provided the 

 brush axis is at right angles to the pole axis. Therefore, under 

 both these conditions there is no armature current and, hence, no 

 torque. 



FIG. 265. Currents in the windings of a repulsion motor, brushes along pole 



axis. 



Figure 265 (a) shows the same condition existing in the field 

 and armature as was shown in Fig. 264 (a), except that the 

 brushes now lie along the pole axis. As the general direction 

 of the induced emfs, has not changed, the brushes are now short- 

 circuiting the points of the armature winding across which the 

 maximum potential difference exists. Therefore, current flows 

 between the brushes from both sides of the armature, and in this 

 brush position, the current in the armature is a maximum. 

 But the motor develops no torque with the brushes in this posi- 

 tion for the following reason: Two conditions are necessary for 

 the development of torque. The angle between the space-position 

 of the flux axis and the brush axis must be greater than zero. 

 For maximum torque this angle should be 90. For example, in 

 a direct-current motor with fixed flux and armature current, 



