364 



ELECTRICAL MACHINERY 



coil, AB, is equivalent to an armature having its brushes 

 in the plane XY, parallel to the field. Now such a short- 

 circuited turn as AB (Fig. 237) would have a large short- 

 circuit current flowing in it as soon as the field of the 

 motor was excited, but this large current could produce 

 no turning effort because the conductors A and B, in which 

 the current is supposed to be circulating, lie in a zero 

 magnetic field. 



Suppose, now, that the short-circuited brushes of the 

 repulsion motor are advanced 90 so that the one armature 



FIG. 237. To Show how Torque Depends upon Brush Position. 



coil, representing all the armature winding, is in the 

 position A'B', Fig. 237. Evidently the coil could produce 

 no turning effort because no current would circulate in it. 

 The plane of the coil is parallel to that of the flux and 

 hence there is no e.m.f. induced in it by the alternating 

 field flux. 



If, now, the brushes are placed in such a position that 

 the equivalent armature coil is shown by A"B" in Fig. 237, 

 it is evident that there will be a current flowing in the 

 conductors of the short-circuited armature and that these 



