168 PRINCIPLES OF ELECTRICAL DESIGN 



thus conflicting with condition (a) and presenting one of the 

 difficulties of commutating-pole design. 



(c) the minimum width of pole face must be such that the equi- 

 valent pole arc (which must include an allowance for fringing) 

 shall cover the commutating zone of width W a . 



(d) The equivalent pole arc should, if possible, be an exact 

 multiple of the slot pitch (either once or twice the slot pitch) 

 as this tends to reduce the magnitude of the flux pulsations in 

 the interpole. The effects of these flux pulsations, caused by 

 variations in the reluctance of the interpole air gap, are, however, 

 usually of no great practical importance, but the width of brush 

 should not be determined independently of the interpole design. 



(e) In order to keep down the PR losses in the series turns on 

 the interpole (usually amounting to less than 1 per cent, of the 

 total output), the ampere-turns and the length per turn should 

 be as small as possible. The gain resulting from a small air 

 gap is, however, not great, because the ampere-turns required 

 to overcome the air-gap reluctance rarely exceed 25 per cent, 

 of the total, the balance being required to oppose the armature 

 m.m.f. A reasonably large air gap has the advantage of re- 

 ducing the flux pulsations referred to under (d). 



(/) The effect of the interpole being to increase the flux in 

 that portion of the yoke which lies between the interpole and 

 the main pole of opposite polarity, it is important to see that 

 the resulting flux density in this part of the magnetic circuit is 

 not excessive. A similar condition exists in the armature core, 

 but this does not usually determine the limits of the allowable 

 average flux density below the teeth. 



(g) Series- or wave-wound armatures are to be preferred on 

 machines with commutating poles, especially when the air gap 

 under the main poles is made smaller than it would have to be 

 if interpoles were not used. 



(h) The total line current should, if possible, pass through 

 all the interpole windings in series; that is to say, parallel 

 circuits should be avoided because of the possibility that the 

 current may not be equally divided. If the total current is 

 too great, a portion may be shunted through a diverter. 1 The 

 diverter should be partly inductive, the resistance being wound 



1 The use of a resistance as a shunt to the series field winding usually 

 known as a diverter will be referred to again later when considering the 

 design of the field magnets. 



