160 ELECTRICAL ENGINEERING 



103. Generation of Electromotive Force. The electromotive 

 force generated in the armature of a direct-current generator or 

 motor is 



= Zn3> 2- 10- 8 volts, 



where Z = number of conductors on the armature, 

 n = speed of armature in r.p.s., 

 < = flux crossing the air gap from one pole, 

 p = number of poles, 



and pi = number of paths in multiple between terminals. 



In one second each conductor cuts n$p lines of force and thus 

 the average e.m.f. generated in each of the Z conductors is 



e = n$p 10~ 8 volts. 



^ 



Between the terminals there are conductors connected in series 



Pi 

 and therefore the e.m.f. between terminals is 



& = Zn$ 2 10- 8 volts. . (200) 



pi 



This is the electromotive force equation of a direct-current gener- 

 ator. In a motor it is called a back electromotive force since it 

 opposes the impressed e.m.f. and therefore the current in the 

 motor armature. 



This equation may be written 



g = Kr&, ............ (201) 



where K = Z 10~ 8 is a constant of the machine. 



Pi 



The electromotive force is therefore directly proportional to the 

 speed and to the flux crossing the air gap. 



104. Effect of Moving the Brushes. The equation 



6= 



Pi 



only holds if the brushes are on the no-load neutral points. 

 When the brushes are moved ahead of the neutral points or 



