176 



ELECTRICAL ENGINEERING 



touches bar 2 and must be completed when the brush breaks con- 

 tact with bar 1. If the width of the brush is d ins., the thickness 

 of insulation between bars is 5 ins., and the peripheral speed of the 

 commutator is V ins. per sec., the time of commutation is 



rr 

 T - 



sec. 



Since d is very small, the time of commutation varies directly as 

 the width of the brush and inversely as the speed of the 

 machine. 



The resistance of the brush contact plays a very important part 

 in commutation; it tends to reduce the current in the short-cir- 

 cuited coil to zero and then to build it up in the opposite direction. 

 It would produce complete commutation if it were not opposed 

 by the effects of the resistance and self-induction of the coil. Its 

 function is illustrated in Fig. 143. If the resistance of the total 



lc 





Bi + 



21. 



FIG. 143. 



brush contact is r c , then in Fig. 142 the drop of potential between 

 the brush BI and bar 1 is 2/ c r c . As soon as the brush touches bar 

 2 commutation begins and the brush-contact resistance must be 

 separated into two parts, r\ the resistance from the brush to bar 1 

 and r 2 the resistance from the brush to bar 2. If at the instant 

 represented in Fig. 143 the current in the coil is i, then the current 



