114 ELECTRICAL MACHINERY 



brush contact resistance is tending to stop the original cur- 

 rent through B it is tending to build up an equal current in 

 the opposite direction. 



Current During Commutation. If this resistance com- 

 mutation is to work successfully the current through B 

 must have reversed and built up to the same value of cur- 

 rent as that flowing in coil C, before segment g leaves the 

 brush. If this condition is not satisfied, there will be more 

 or less sparking. The variation of current in coil B, as 



I 



m 



Time during V 

 K which coil is- - 1 



t ^i,,. 



short circuited 



FIG. 69. Possible Variation of Current During Commutation. 



it is being commutated, may be well shown by a curve, 

 as in Fig. 69. If the current is 10 amperes in each side 

 of the armature and the time during which the coil is short 

 circuited is .001 second, and if the current in B completely 

 reverses during this time, the current curve has the form 

 shown by the full lines in Fig. 69. 



Now suppose that the effect of varying the brush con- 

 tact resistance is not great enough to reverse completely 

 the current in B during .001 second. The current curve 



