76 



PRINCIPLES OF ELECTRICAL DESIGN 



B g corresponding to the smaller outputs are required because 

 the increased taper of the teeth with the smaller armature 

 diameters would lead to abnormally high densities at the root 

 of the teeth if the air-gap density were not reduced. The figures 

 given in the table are applicable to machines with pole shoes of 

 steel or wrought iron. If the pole shoes are of cast iron, these 

 values should be reduced about 20 per cent. Cast-iron pole 

 shoes are rarely used except in very small machines. 



The quantity q in formula (43) is determined in the first place 

 by the heating limits; but armature reaction and sparkless 

 commutation have some bearing upon its value. Suitable 

 values of specific loading for use in formula (43) may be taken 

 from the accompanying table. 



APPROXIMATE VALUES OP q 

 (Ampere Conductors per Inch of Armature Periphery) 



must 



The quantity r in formula (43) usually has a value between 0.60 

 and 0.80, a common value being 0.70. When the machine is 



T*WM 



provided with commutating interpoles the ratio po i e 



be small in order to make room for the interpole. In this 

 case the lower figure of 0.60 would probably be selected as a 

 suitable value for r. 



Approximate Constants for Use in Output Formula. For a 

 first approximation, the output formula (43) may be simplified 

 by substituting average values for the quantities B g , q, and r. 

 Thus, if B = 7,500; q = 500; and r = 0.7, the output formula 

 becomes 



* 

 kw. output 





