90 PRINCIPLES OF ELECTRICAL DESIGN 



number of commutator bars to the number of slots, as the total 

 number of inductors in each slot may be subdivided, and a 

 correspondingly greater number of commutator bars can be 

 used. This point will be again referred to when treating of the 

 slot insulation. 



With a series-wound armature, the number of commutator 

 bars cannot be a multiple of the number of poles, because this 

 would lead to a closed winding after stepping once around the 

 armature periphery. The winding must advance or retrogress 

 by one commutator bar when it has been once around the arma- 

 ture, and this leads to the rule that a series-wound machine must 

 have a number of commutator segments such as to fulfil the 

 condition : 



Number of commutator segments 1 _ , p 



in wave-wound machine j 2 ~ 



where k is any whole number. 1 



24. Equalizing Connections for Multiple -wound Armatures. 

 If the magnetic circuits of the various parallel paths in the lap- 

 wound dynamo are not of equal reluctance, there will be an un- 

 balancing of the generated e.m.fs. producing circulating currents 

 through the brushes. The inequality of the magnetic perme- 

 ances is usually due to excentricity of the armature relatively 

 to the bore of the poles, and even when the unbalancing effect is 

 small in a new machine, it is liable to increase owing to wear of 

 the bearings. 



Fig. 31 is a developed view of a four-pole winding. Sup- 

 pose that the section A of the armature winding is nearer to the 

 pole face than the section C. The voltage generated in the con- 

 ductors occupying the latter position will be less than in the 



1 Although an armature may be provided with an odd number of slots, it 

 does not follow that it will accommodate a wave winding suitable for all 

 voltages, without modification. Thus, if a six-pole machine has 73 armature 

 slots, it may be necessary to have two coils (or four coil-sides) per slot in 

 order to obtain the necessary voltage and avoid too great a difference of 

 potential between adjacent commutator bars. This means that the number 

 of coils and of commutator segments would be 146, which would not be 

 suitable for a wave winding. By having one dummy or "dead" coil, the 

 total number of coils (and commutator segments) will be 145, which being 

 equal to (48 X %) +1 will give a wave winding. The "dead" coil is put 

 in for appearance and to balance the armature, but it is not connected up. 

 The use of dead coils should be avoided as it adds to the difficulties of 

 commutation. 



