DIRECT-CURRENT MACHINERY 151 



commutation since the two coils in one slot are not commutated 

 at the same time and thus the inductive flux is that due to half 

 the ampere turns acting in the case of a full-pitch winding. 



96. Equalizer Rings. In multiple-wound machines, if there 

 is any irregularity in spacing the brushes or if the air gaps under 

 all the poles are not of the same depth, the e.m.f.'s generated in the 

 different sections of the winding will not be equal and the un- 

 balanced e.m.f. will tend to cause current to circulate through the 

 windings even when the machine is not carrying any load. To 

 reduce the circulating currents similar points under the different 

 pairs of poles which should normally be at the same potential are 

 joined together by heavy copper connections called equalizer 

 rings (Fig. 114) and these prevent the current from circulating 

 through the windings. 



97. Series-drum Windings. In the series winding the ter- 

 minals of a coil are connected to two commutator bars approxi- 

 mately twice the pole pitch apart. Fig. 116 represents a series or 

 two-circuit winding for a six-pole machine with 44 conductors and 

 22 slots. One side of a coil is placed in the top of slot 1 and the 

 other side in the bottom of slot 5 and the terminals of the coil are 

 connected to commutator bars 1 and 8. 



Tracing out the winding from the positive brush BI to the neg- 

 ative brush BZ one half of the armature conductors are taken in. 

 There are therefore but two circuits in multiple between terminals 

 independent of the number of poles and the winding is called a 

 two-circuit or series winding. 



Only two sets of brushes are required to collect the current but 

 when the current is large it is usual to employ other sets of brushes 

 as shown at B 3 , B 4 , B 5 and B 6 and as many sets of brushes as there 

 are poles may be used. 



Series windings are used in small high-voltage machines or where 

 it is desirable to use only two sets of brushes, as in small railway 

 motors; but in large multipolar machines with many sets of 

 brushes the current does not divide equally between brushes of 

 the same polarity and commutation is unsatisfactory. 



The number of coils in a series winding must be one more or one 

 less than a multiple of the number of pairs of poles, or 



