CHAP. IX] ARMATURE REACTION IN D.C. MACHINES 



173 



yoke of the machine is negligible, because the commutating flux is 

 small as compared with the main flux and is displaced with respect 

 to it by ninety electrical degrees. The winding on the interpoles 

 is connected in series with the main circuit of the machine, because 

 the armature m.m.f. is proportional to the armature current, and 

 also because the density of the reversing field must be proportional 

 to the current undergoing commutation. 



The flux density under the interpoles is determined from the 

 condition that the e.m.f. induced in the armature conductors by 

 the commutating flux be equal and opposite to the e.m.f. due to 

 the inductance of the armature coils undergoing commutation. 

 For practical purposes, the inductance can be only roughly esti- 

 mated (see Art. 68 below), but on the other hand an accurate cal- 



Compcnsuting Commutating 

 Main pole Winding /pole or interpole 



/ 





Generator 



Motor 



FIG. 44. Interpoles and a compensating winding in a direct-current machine. 



culation is not necessary, because the number of ampere-turns on 

 the interpole is easily adjusted by a shunt around its winding, as 

 in the case of a series winding on the main poles. Or else the 

 commutating flux can be increased by " shimming up " the 

 interpoles. The m.m.f. required for establishing a required 

 commutating flux is calculated in the same manner as in the case 

 of the main flux, viz., the saturation curves (Figs. 2 and 3) are 

 used for the pole body and the teeth, while the reluctance of the 

 air-gap is estimated as is explained in Arts. 36 and 37. The 

 commutating poles must be of such a width that all the coils 

 undergoing commutation are under their influence. 



A comparatively large leakage factor, say between 1.4 ami 1 ..">. 

 or over, is usually assumed for the comnmtat ing poles, on account 

 of the proximity of t he main poles. It is advisable to concentrate 

 the winding near the tip of the interpole, in order to reduce the 



