COMMUTATION 



141 



t c , when the coil is just about to be thrown in series with the 

 other coils of the armature winding carrying a current of I c 

 amp., the e.m.f. in the coil should be e = I C R. It is when the 

 e.m.f. in the coil has some value other than this theoretical value 

 that sparking is liable to occur. 



The theoretical investigation of commutation phenomena is 

 admittedly difficult, because it is almost impossible to take 

 account of the many causes which lead to sparking at the brushes. 

 Some of the problems to be dealt with are of a purely mechanical 

 nature, and it is necessary to make certain assumptions and to 

 disregard certain influencing factors in order that the essential 

 features of the problem of commutation may be studied. The 

 writer has deliberately departed from the usual method of 

 treating this subject because he believes that it is possible to 

 put the fundamental principles involved into a somewhat simpler 

 form than they are likely to assume when clothed in mathe- 

 matical symbolism. An attempt will be made to obtain a 

 clear conception of the physical phenomena involved in the 

 theory of commutation. 



Before the publication of MR. LAMME'S paper 1 the methods 

 of DR. STEiNMETz 2 andDR. E. ARNOLD S formed the nucleus around 

 which the bulk of our commutation literature clung. MR. 

 LAMME'S paper has the great merit of putting the more or less 

 familiar problems of commutation in a new light. The end he 

 attains is approximately the same as that attained by any 

 other reasonably accurate method of analysis, provided all 

 factors of importance are included, and the difficulties he en- 

 counters are of the same order and magnitude as those en- 

 countered by other investigators; but, by getting nearer to the 

 true physical conditions in the zone of commutation, he saves 

 us from drifting, sometimes aimlessly, on a sea of abstract specu- 

 lation. Although the presentation of the subject as given in 

 this chapter has undoubtedly been suggested by the reading of 

 MR. LAMME'S paper, yet its aim is not so much to furnish addi- 

 tional material for the designer as to give the student a clear 

 conception of the phenomena of commutation. The writer's 

 end is simplicity or clearness, even if the less important factors 



1 B. G. LAMME: "A Theory of Commutation and Its Application to 

 Interpole Machines," Trans. A. I. E. E., vol. XXX, pp. 2359-2404. 



2 "Theoretical Elements of Electrical Engineering." 



3 "Die Gleichstrom-Maschine." 



