DYNAMO DESIGN 



81 



By assuming values for the quantities in the right-hand side 

 of the equation, a reasonable figure for the number of poles can 

 be obtained. As an example, let the assumed values be as 

 follows, 



k = 1 

 q = 650 

 r = 0.7 

 v = 4,500 

 B g = 7,500 

 (SI) a = 7,500 



The number of poles will then be 



p = 0.0138 kw. 



If the machine is to have an output of 500 kw., the estimated 

 number of poles is 



and since an even number of poles is necessary, the required 

 figure is 6 or 8. This, of course, would mean a change in one 

 or more of the assumed quantities. 



The proper number of poles is determined partly by the amount 

 of the current to be collected from each brush set. This will 

 influence the selection of suitable values for k and (SI) a in 

 formula (50). Values as high as 1,000 amp. per brush arm are 

 used in connection with low- voltage machines; but, on machines 

 wound for 250 to 500 volts, the current collected per brush arm 

 usually lies between the limits of 700 and 300 amp. 



As a guide in selecting a suitable number of poles for a pre- 

 liminary design, the accompanying table may be of use. It is 

 based on the usual practice of manufacturers. 



NUMBER OP POLES AND USUAL SPEED LIMITS OF DYNAMOB 



