THE CONTINUOUS CURRENT GENERATOR 119 



commutating pole and are left fixed in this position; the 

 brushes on a commutating pole machine must never be 

 shifted after correct adjustment has once been made. 



As these poles are equipped with a series winding (shown 

 in Fig. 73) the strength of field produced by them (and hence 

 the magnitude of the e.m.f. induced in the short circuited 

 coil) is proportional to the load; the magnitude of the short 

 circuit current will then be proportional to the load and it 



C, D, Main Poles 



A, B., Commutating Poies 



FIG. 73. Position and Connection of Commutating Poles. 



was shown in the discussion of Fig. 72 that this was the 

 necessary condition for maintaining ''black" commutation 

 as t he load varies. The use of commutating poles (sometimes 

 called interpoles) has become almost universal on c-c. 

 motors and generators, especially on variable speed motors 

 in which the speed variation is obtained by field weakening. 

 Without the use of commutating poles the variable speed 

 motors could not be built to operate satisfactorily. 



The field frame of a machine equipped with com- 

 mutating poles is shown in Fig. 13. The armature has been 



