156 



ELECTRICAL ENGINEERING 



the bars or by means of vertical connectors called commutator 

 risers. In order that the brushes, which collect the current from 

 the commutator, may run smoothly without vibrating or chat- 

 tering the commutator surface must be perfectly round and 

 smooth. 



The function of the commutator is illustrated in Fig. 120. The 

 current from the machine is I amperes and the current in each 



conductor is I c = amperes. During the tune taken for the brush 



to move across the insulation between bars 2 and 3 the current 

 hi coil c must change from I c in one direction to I c in the opposite 



FIG. 120. Commutation. 



direction. This reversal of the current is called commutation and to 

 be satisf actory it must be effected without sparking. The brushes 

 are shown placed on the neutral line and the coils short circuited 

 are not cutting any flux and therefore have no e.m.f. generated in 

 them due to rotation. If the short-circuited coil had no induct- 

 ance the current would reverse completely due to the contact re- 

 sistance between the brush and the commutator. 



In (1) the current in coil c is 7 C ; hi (2) it is zero since current I c 

 from bar 1 goes through one hah* of the brush-contact area and 

 current I c from bar 2 goes through the other half, and the drop of 

 voltage on both sides is the same and therefore there is no voltage 

 available to drive the current through the resistance of the coil. 

 Between (1) and (2) the resistance from bar 1 to the brush is greater 

 than the resistance from bar 2 to the brush and so part of the cur- 

 rent from 2 flows through the coil c. Between (2) and (3) the 

 resistance from bar 2 to the brush is less than from bar 1 and part 

 of the current from 1 flaws through coil c. In (3) the current in 

 c is I e but in the opposite direction from that in (1) and commu- 

 tation is complete. 



