74 COMMERCIAL ELECTRICAL TESTING 



commutating field readjusted. With the best shunt 

 adjustment possible, the fibre brush should be used on 

 each stud, and readings made of the current shunted 

 and the shift of the brushes from the no-load neutral. 



If the full-load electrical neutral of one or more 

 studs is found to differ appreciably from that of the 

 others, the commutating pole spacing, brush spacing, 

 and air gaps of those poles and studs which affect the 

 neutral in question should be carefully checked. 



When a final adjustment has been obtained on any 

 commutating pole machine of 200 kw. or greater, the 

 fibre brush should be used on each stud and the results 

 with full load recorded. 



In general, shunting current from the commutating 

 field will shift the load neutral of all studs away from 

 the no-load neutral by the same distance. Shunting 

 less current will shift all neutrals toward the no-load 

 neutral. Where possible, all adjustments should be 

 made with the brushes on the no-load neutral, and the 

 brushes should be left permanently in that position. 

 The rocker arm of all commutating pole machines 

 should be plainly chisel marked, when the final adjust- 

 ment has been made. When satisfactory commutation 

 has been obtained, a heavy load should be thrown on 

 and off suddenly and a record made of the resultant 

 commutation and general behavior of the machine. If 

 the machine has an inductive shunt, and flashing of 

 violent sparking is produced by throwing a heavy load 

 on and off quickly, readjusting the air gap of the 

 inductive shunt should be tried. 



With a given winding on the core, the inductance 

 of the shunt may be varied by changing the gap, and 

 the relative inductance of the shunt and commutating 

 field winding be thus adjusted. If the current in the 

 shunt circuit quickly falls to zero when a heavy load 

 is thrown off by tripping the breaker, and the brushes 

 show sparking, there is too little inductance in the 

 inductive shunt and its air gap should be decreased. 

 The air gap should be adjusted to give the minimum 

 sparking when the machine is operating with a highly 

 fluctuating load. 



