CHAPTER VIII 

 COMMUTATION 



44. Introductory. A continuous-current dynamo is pro- 

 vided with a commutator in order that unidirectional currents 

 may be drawn from armature windings in which the current 

 actually alternates in direction as the conductors pass successively 

 under poles of opposite kind. 



As each coil in turn passes through the zone of commutation, 

 it is short-circuited by the brush, and during the short lapse of 

 time between the closing and the opening of this short-circuit 

 the current in the coil must change from a steady value of -\-I c 

 to a steady value of 7 C . 



Let W = surface width of brush (brush arc) in centimeters. 

 M = thickness of insulating mica in centimeters. 

 V c surface velocity of commutator in centimeters per 

 second. 



The time of commutation, in seconds, may then be written, 



W - M 



tc ~~ 



V c 



Since M is usually small with reference to IF. it is generally 



W 

 possible to express the time of commutation as t c = -y- ; that is 



to say, the time taken by any point on the commutator surface 

 to pass under the brush is approximately the same as the dura- 

 tion of the short-circuit. It is during this time, t c , that the 

 current in the commutated coil must pass through zero value 

 in changing from the full armature current of value +/ c to the 

 full armature current of value I c . If R is the resistance of 

 the short-circuited coil, and if any possible disturbing effect of 

 brush-contact resistance be neglected, it is evident that the 

 e.m.f. in the coil should be e = I c X R at the commencement of 

 commutation. At the instant of time when the current is 

 changing its direction i.e., when no current is flowing in the 

 coil the e.m.f. is e = X R = 0. At the end of the time 



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