CHAPTER X 



DIRECT CURRENT GENERATORS-COM- 

 MUTATING POLES AND THE LOCA- 

 TION OF THE ELECTRICAL 

 NEUTRAL 



Commutating Poles 



The commutating pole produces the necessary flux 

 for neutralizing the effect of armature reaction, and 

 prevents that shifting of the electrical neutral point 

 between no load and full load which occurs in direct 

 current machines not equipped with commutating 

 poles; and, in addition, aids the current reversals in 

 the armature coils at commutation. To obtain the 

 reversal without sparking, with normal load current 

 flowing, a definite number of ampere turns is required. 

 In many cases, fractional turns are necessary in the 

 commutating field winding; but as only whole turn 

 or half turns are possible for mechanical reasons, a 

 shunt is connected across the terminals of the com- 

 mutating field winding and adjusted in test to shunt 

 the current in excess of that required. As the electrical 

 neutral does not shift, the brushes are set on the no-load 

 electrical neutral, the adjustments made, and the rocker 

 arm chisel-marked for that setting. Because of this 

 position of the brushes, the machine is sensitive to 

 conditions that under-excite the commutating poles, 

 or make them inactive. Such conditions may cause 

 the neutral to shift, resulting in bad sparking at the 

 brushes or even a flash over, particularly in the case 

 of machines of 500 volts or over. 



Consider, for instance, a 300 kw., 500 volt generator, 

 with a heavy german silver shunt across the terminals 

 of the commutating field winding. If the machine is 

 short circuited, the inductance of the corr mutating 



[71] 



