THE CONTINUOUS CURRENT GENERATOR 109 



ampere-turns in the series field (for any given load) may 

 be changed as desired. After this shunt has once been 

 adjusted it is not necessary to change it, and so it is not 

 made adjustable; in this respect, it is different from the 

 shunt-field rheostat. 



21. Commutation. The function of the commutator and 

 its construction have been taken up in previous paragraphs, 

 and mention has been made of the fact that there may be 

 sparking at the contact surface of the brush and com- 

 mutator. The reasons for this sparking will now be given, 

 the superiority of the carbon brush over the copper brush 

 will be shown, and the purpose of the commutating poles 

 will be discussed. 



The direction of the current through any coil is reversed 

 as the commutator bars, to which the coil ends are attached, 

 move under the brush. This must be so because before 

 the coil reaches the brush as at (A) Fig. 66 the current is 

 in one direction and when the coil is at (B) the current 

 is in the opposite direction in the coil. This reversal of 

 current takes place just as the coil is short-circuited by the 

 brush (C) Fig. 66. The reversing of the direction of cur- 

 rent in the coil as the coil moves under the brush is called 

 commutation. If this reversal takes place with no visible 

 sparking at the brush contact, the machine is said to have 

 sparkless commutation. 



Effect of Self-induction on Commutation. The time in 

 which this reversal of current has to take place is very short 

 so that the rate of change of current as the coil is commutated 

 is very high, especially when the machine is carrying much 

 load. As the coil possesses self-induction this rapid reversal 

 of current sets up an e.m.f. of self-induction which tends to 

 maintain the current in its original direction. When this 

 e.m.f. of self-induction is high it is almost impossible to 

 obtain sparkless commutation and because of this fact 

 armature coils must be made with low self-induction. 

 The low coefficient of self-induction is obtained by winding 



