110 



ELECTRICAL MACHINERY 



but few turns per coil and by placing the coil in an open or 

 semi-closed slot. An armature wound in closed slots 

 would not commutate well at all because of the high self- 

 induction of its coils. 



Rate of Current Change During Commutation. The 

 time during which the coil is short circuited is easily 

 calculated. Suppose a commutator having 90 segments 

 is making 1800 r.p.m. and that the brush has a width 

 equal to two commutator bars. Any two adjacent bars 

 have a coil connected between them so that the coil is 



FIG. 66. C Represents Position of Coil being Commutated. 



short circuited during the time required for the mica insula- 

 tion between the two bars to move the width of the brush 

 contact. 



In the' above case this is equal to 2/90 X 60/1800 = 

 1/1350 of a second. If the armature is carrying 10 amperes 

 per path the rate of change of the current during commu- 

 tation is about 27000 amperes/sec. The e.m.f. of self- 

 induction is equal to the coefficient of self-induction for 

 the coil, L, multiplied by the rate of change of current, and 

 as this latter term is so high, L must necessarily be kept 



