COMMUTATION 



157 



Let W a be (as previously defined) the brush arc expressed in 

 centimeters of armature periphery; then the space moved through 

 by a conductor during the time of commutation is W a sin a, as 

 indicated by the shaded strip in Fig. 61, which includes merely 

 the portion ADBC of Fig. 60. It is desired to develop a formula 

 giving the total flux entering the shaded area of length AB and 

 width W a sin a. 



E 



W n sin er 



FIG. 61. 



The conductors, each carrying a current of I c amperes, are 

 moving relatively to the space under consideration, and any ele- 

 ment parallel to A B of width dy centimeters and length EF centi- 

 meters (see Fig. 61), may be considered as containing ampere 

 conductors of value: 



where n = No. of slots per pole (i.e., in space AC = T) 

 and T = No. of turns per coil (t.e., one-half the number of 



inductors per slot). 



The length BC is equal to T sin a or, since a = 45 degrees, to 



1= and it will be convenient to substitute this value in the above 

 V2 



expression, whence: 



ampere conductors \ r= ^ (dy\ 



f .,,1 , f = v ^m i c [' 

 in space of width dy I \r / 



The average field intensity (or flux density in maxwells per 



