206 PRINCIPLES OF ELECTRICAL DESIGN 



Item (16): Number of Inductors. Refer Art. 18, page 72, 

 and Art. 19, page 74. 



- irDq TT X 19.5 X 475 . 



Z = 5- 1 = - ^5-7 -- = 350 (approximately) 



(17) : Full-load Armature Ampere-turns. Refer Art. 20, 

 page 79. 



(IS) '.Length of Air Gap. Refer Art. 36, page 119. 

 3 650 



s = - - 228 ; or 



Item (19): Maxwells per Pole. Refer formula (38), Chap. IV, 

 page 72. 



The flux per pole on open circuit, if Z has the value as calculated 

 for item (16), is 



220 X 60 X 4 X 10 8 

 * = 4 X 600 X 350 = 6,290,000 maxwells. 



Item (20): Pole Pitch. Refer Art. 19, page 74 and Art. 20, 

 page 78. 



TTX 19.5 . 



r = - - = 15.34 in. 



(21) : Pole Arc. Refer Art. 19, page 74. 



T X r = 15.34 X 0.72 = 11.05, or (say) 11 in. 

 Items (22), (23) and (24): Dimensions of Air Gap. 

 * 6,290,000 



0, 8,OC = 122 sq. in. 



whence Z a = 12 Ki = 11.1, and the axial length of pole face will 

 be something less, or (say) 10.6, to avoid the large amount of 

 flux which would otherwise curve round into the flat surface of 

 the core discs, where it would cause eddy currents. These axial 

 dimensions, both of armature and pole shoe, are, however, sub- 

 ject to correction after the actual winding details have been 

 settled; because the practical considerations may lead to a 

 change in the number of inductors (Z), and a corresponding 

 change in the amount of flux entering the armature. 



