ARMATURE WINDINGS. 433 



is in no way altered by the addition or removal of the second 

 pair of brush sets. 



When, however, two brushes only are used for a /-pole wave- 

 wound armature then //2 winding elements, all in series, are 

 short-circuited under a brush when the brush touches two com- 

 mutator bars (in a simplex winding). Therefore the wave wind- 

 ing using two brushes has a greater tendency to spark at the 

 brushes than the lap winding with p brushes. This disadvan- 

 tage of the wave winding is obviated by using / brushes instead 

 of two.* 



51. Conditions which must be satisfied by a closed coil drum 

 winding. Lap and wave windings. No drum winding can 

 have an odd number of inductors. Both front and back pitches 

 must be odd in a simplex winding. This is due to the fact that 

 odd numbered inductors are to be thought of as returns for 

 the even numbered inductors, and therefore even numbered in- 

 ductors must be connected at both ends to odd numbered induc- 

 tors. This requires an odd pitch at both ends. 



Both front and back pitches must be approximately equal to 

 Z\p in order that the two inductors which constitute a winding 

 element may move simultaneously under field poles of opposite 

 polarity. Under these conditions electromotive forces induced 

 in the two inductors will be additive. The smallest value of 

 front or back pitch which meets this requirement is a pitch which 

 stretches completely across a pole face, and the largest value of 

 front or back pitch which meets this requirement is a pitch which 

 barely stretches across from a given pole tip to the nearest pole 

 tip of like polarity. When front and back pitches differ consider- 

 ably from Z\p the winding is called a chord winding. 



The advantage of the chord winding is that the demagnetizing action of the arma- 

 ture current is to a great extent eliminated thereby, and its disadvantage is that it gives 

 a very narrow neutral zone on the commutator. The elimination of the demagnetizing 

 action is due to the fact that the armature current flows in opposite directions in alter- 

 nate inductors in each of the two bands of inductors shown in Fig. 101, Chapter VI. 



* See Hobart's Electric Motors, page 48, Whitaker & Co., London, 1904. 

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