170 ELECTRICAL MACHINERY 



and N help in generating a counter e.m.f. in the armature 

 winding. Those inductors marked M evidently add their 

 e.m.f. to that generated in the inductors X and those 

 marked N add their e.m.f. to that generated by the 

 inductors F. 



The e.m.f. generated in the inductors M and N is pro- 

 portional to the load because the strength of field produced 

 by the commutating poles is proportional to the load. 

 The e.m.f. generated in the inductors X and Y is independent 

 of the load (so long as the speed remains constant) but the 

 counter e.m.f. of the armature winding as a whole tends to 

 increase with a load increase because of this effect of the 

 inductors M and N. In fact the effect of the conductors 

 M and N is just the same as though there were no com- 

 mutating poles and the main field increased in strength 

 with an increase in load. Hence these conductors and the 

 armature reaction (with a forward brush shift) tend to 

 make the motor act like a compound-wound motor which, 

 as we know, has a speed-load curve which drops quite 

 rapidly as the load is increased. 



A certain commutating-pole motor ran at a speed of 

 1000 r.p.m. with no load; with the brushes in the plane 

 AB (Fig. 103), the full-load speed was 900 r.p.m. and with 

 the brushes in the position A" E" the full-load speed was 

 670 r.p.m. This increase in speed decrease (670 r.p.m. 

 as compared to 900 r.p.m.) was caused by the combined 

 effect of the armature reaction and the commutating poles. 

 When the motor was carrying full load with the brushes 

 in the position A" B" , they sparked badly. Why? 



Effect of Backward Brush Shift. When the brushes 

 are moved backward as shown at A' B' (Fig. 103) the 

 speed-load curve tends to become more nearly flat. The 

 effects of the inductors M and N and of the armature 

 reaction both decrease the counter e.m.f. of the armature 

 as the load increases and thus tend to make the speed of 

 the motor increase with a load increase. The effect of the 



