56 ELECTRICAL MACHINERY 



has been placed in its proper position. The micanite rings 

 are shown at d. If the commutator has been properly 

 assembled the two surfaces marked e receive all the pressure 

 as the lock nut c is tightened; in this way the lock nut 

 serves not only to tighten the commutator bars endwise, 

 but also to squeeze them tightly together sidewise by 

 forcing them into a cylinder of smaller radius. 



Number of Bars. The number of bars to be used in 

 a commutator depends principally upon the voltage for 

 which the machine is designed. The function of the com- 

 mutator is to change the alternating e.m.f . of the armature 

 coils to a unidirectional e.m.f. on the external circuit. If 

 too few bars are used in the commutator the line e.m.f. 

 will be unidirectional but will not be constant in value 

 i.e., it will be a pulsating e.m.f. If as many as 12-16 bars 

 are used between brushes, these pulsations are so small 

 as to be negligible. 



If a two pole machine had only 8 bars in its commu- 

 tator, the amount of variation in the line e.m.f. would 

 be about 4%, hence this number of bars would be too 

 small for ordinary use. For an electroplating generator, 

 however, the pulsation would not be disadvantageous and 

 plating machines generally have very few commutator 

 bars. The number of bars to be used on railway genera- 

 tors or lighting generators etc. is fixed by the rule that 

 the voltage between adjacent bars should not be greater than 

 10 volts. Now as the full voltage of the machine exists 

 between adjacent sets of brushes this means that the mini- 

 voltage of machine 

 mum number of bars = X number of sets 



of brushes. 



This rule is only approximate for many reasons. The 

 voltage between adjacent brushes is not distributed uni- 

 formly among the commutator bars between the same 

 brushes. Between a pair of adjacent bars on the com- 

 mutator midway between two sets of brushes the e.m.f. 



