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ELECTRICAL MACHINERY 



which no e.m.f. is being generated, they must be shifted 

 forward in a generator but backward in a motor. 



Effect of Shifting Brushes. When the brushes are 

 shifted backward the armature m.m.f. of the motor may 

 be split up into two components, one of which acts across 

 the main field and the other tends directly to demagnetize 



\ Proper position for brushes 



Motor 



Generator, 



FIG. 102. Field Shift in Motor and Generator, Caused by Armature 



Reaction. 



the main field. In a motor with brushes shifted backward 

 (i.e., against rotation) there is, therefore, an armature reac- 

 tion which weakens the main field as the load increases. 

 But a weakening of the field tends to make a motor speed 

 up because, if the field is weakened, the counter e.m.f. 

 deci eases and so more current flows through the armature, 

 tending to increase the speed. 



