PROCEDURE IN DESIGN OF D.C. GENERATOR 205 



Item (10): Type of Winding. Refer Art. 23, page 84. In 

 this case some doubt exists as to whether a wave or lap winding 

 should be adopted. If the pressure were higher say 500 volts 

 a series, or two-circuit, winding would be preferable. With 

 only 220 volts to be generated, we shall adopt a simplex multiple 

 winding which, with our four-pole machine, will give us four 

 armature circuits in parallel. 



Item (11): Apparent Air-gap Density. Refer to the table on 

 page 75 in Art. 19, and select B g = 8,000. 



Items (12) and (13): Line Current = 7500 M30 = 326 amp. 

 The current in each armature conductor will be one-quarter 

 of this (Art. 23, page 87) if we neglect the shunt exciting 

 current. The shunt excitation of a 75-kw. machine might 

 amount to 2.3 per cent, (see Art. 58, page 192), so that the 



326 

 full-load current per conductor will be -j- (1 + 0.023) = 83.4 



amp., approximately. 



Item (14): Armature Diameter. Using the output formula 

 (43) as developed in Art. 19 (page 74), we have: 



LD' 



75,000 X 60 X10 8 



6.45 X 7T 2 X 8,000 X 475 X 0.72 X 600 



4,290 



Referring now to page 77, we can use formula (46) to get l a 

 in terms of D. The ratio l a /T, for an economical design of 

 machine of this size and number of poles, will probably have a 

 value between 0.5 and 0.9. For a square pole face, 



! = Z = r , o.72 



r T 



and this seems a good proportion to aim at, especially as it will 

 allow of cylindrical pole cores being used. With the square 

 pole face, 



l a = = 0.567D 

 whence 



= 7 ' 560 



and 



D = 19.63 in. 



Let us therefore decide upon armature punchings of 19.5 in. 

 external diameter. 



