DIRECT-CURRENT MACHINERY 



203 



m.m.f . and provide a commutating field, the speed can be increased 

 to four times normal by field weakening without injurious sparking. 

 The loss in power in the resistance controlling the field current 

 is very small since the power used for field excitation is only 1 or 2 

 per cent of the rated output. 



Current I/ 



FIG. 174. Variation of speed with field current. 



(3) At no load with the brushes on the neutral line, all the con- 

 ductors on each half of the armature are effective in generating 

 the back e.m.f. and this is therefore the position of minimum 

 speed. (Fig. 126 (a).) 



When the brushes are moved back against the direction of ro- 

 tation, Fig. 126 (6), only the belts of conductors under the poles 

 are effective in generating the back e.m.f. and the result is the 

 same as though the flux had been decreased. The speed is there- 

 fore increased. 



Under load, when current is flowing in the armature, the con- 

 ductors between the poles exert a demagnetizing m.m.f. and cause 

 a decrease in the flux; the speed therefore rises more than at no 

 load. 



This method of speed variation is not used to any extent since 

 it interferes with commutation and causes injurious sparking. 



125. Speed Characteristics of Motors. The speed charac- 

 teristic is the curve showing the variation of speed with armature 



