126 ELECTRICAL MACHINERY 



Effect of Armature Reaction on Commutating Poles. 

 The commutating poles must be designed with turns enough 

 so that the armature cross-m.m.f. is neutralized under the 

 face of the commutating pole and, in addition, the proper 

 amount of flux required for sparkless commutation is 

 forced into the armature core around the coil being corn- 

 mutated. The magnetic circuit of a two-pole generator 

 equipped with commutating poles is shown in Fig. 80. 

 It may be seen that the commutating poles do not prevent 

 the flux from crowding into the tips of the main field poles. 

 But this does practically no harm; the principal thing to 



FIG. 80. Magnetic Flux in a Commutating Pole Machine. 



obtain is the proper field for e.m.f. commutation at the 

 two points A and B. 



23. Characteristic Curves of Generators. If the load of 

 a generator is increased (speed, etc., being kept constant) 

 the terminal voltage of the generator decreases because of 

 the effects of armature resistance and armature reaction. 

 The curve showing :how the terminal voltage changes with 

 increase in load is called the external characteristic of the 

 generator. If the terminal voltage is kept constant by 

 increasing the field current (thus increasing the generated 

 e.m.f.) as the load increases and a curve is plotted to show 

 how the field current varies with the load, the curve is 



