94 ELEMENTS OF ELECTRICAL ENGINEERING. 



When the brushes are shifted from the neutral axis to the 

 points a' and b 1 , Fig. 72, for instance, the available electromotive 

 force between them is reduced in value to (E a 2e), where e is 

 the electromotive force induced in the portion of the armature 

 winding between a and a 1 ', or in the portion between b and b' . 



The angle between the neutral axis and the axis of commuta- 

 tion is called the angle of lead of the brushes. When the shift 

 of the brushes from the neutral axis is in the direction of rotation 

 of the armature, the brushes are said to have a fonvard lead, and 

 when the shift of the brushes is in the opposite direction, the 

 brushes are said to have a backward lead. Thus the angle a, 

 Fig. 72, is the angle of forward lead of the brushes when they 

 are at the points a' and b' . 



The sparking at the brushes of a generator is a minimum when 

 the brushes have a slight forward lead, the amount of lead de- 

 pending upon the current output. The sparking at the brushes 

 of a motor is a minimum when the brushes have a slight back- 

 ward lead, the amount of lead depending upon the current intake 

 of the motor, that is to say, upon the motor load. A poorly 

 designed dynamo usually requires continual adjustment of the 

 brushes forwards or backwards to keep them at the minimum 

 sparking point with changes of load, otherwise the sparking will 

 be excessive. A well-designed dynamo, however, will operate 

 satisfactorily from zero load to full load with fixed brushes. 



The fundamental equation of the dynamo, equation (21), was 

 derived on the assumption that the angle of lead of the brushes 

 was zero. 



