EXAMPLE OF ALTERNATOR DESIGN 335 



Solving for the numerical values, we get: 



OE g = 3,875 volts (item (39)) 

 and 



OE' g = 4,000 volts. 



The full-load flux per pole (item (40)) is therefore, 



Item (17) X OE' g = 62.2 X 10 6 X 4,000 

 OEt 3,810 



= 65.2 X 10 6 maxwells. 



Item 41. Since we have decided upon a cylindrical rotor, the 

 variation of flux density over the pole pitch must be obtained by 

 distributing the field winding in slots on the rotor surface; but 

 in the design of salient-pole machines, the pole face should be 

 shaped as explained in Art. 90 (page 269), and the approximate 

 dimensions of the pole core should be decided upon with a view 

 to providing sufficient space for the exciting coils. The cross- 

 section of the pole cores would be determined as in the case of 

 continuous-current dynamos by calculating or assuming a leakage 

 factor (see Art. 103, Chap. XIV) and deciding upon a flux density 

 in the iron (about 14,000 or 15,500 gausses). 



Items (42) to (44) . Let us try a rotor as shown in Fig. 103 (page 

 270), with eight slots per pole, only six of which are wound, leav- 

 ing two slots without winding at the center of each pole. The slot 



pitch* (item (43)) is therefore ^~ '- = 3.76 in. This dimen- 



oZ 



\ X 12 

 sion expressed in terms of the stator diameter is ^ = 3.93 in. 



o 



The slot width may be decided upon by arranging for a fairly 

 high density in the rotor teeth. Thus, the open-circuit flux 

 (item (17)) which passes through a total of eight teeth is 62.2 X 10 6 

 maxwells. If t r is the average width of rotor tooth, in inches, the 

 average tooth density in maxwells per square inch is B" = 



62 2 X 10 6 Tr 



Q \/ * ^ AC\ K> which must be multiplied by ~ to obtain the ap- 



o s\ tr j\ <y.O Zi 



proximate maximum density in the teeth near the center of the 

 pole. Neglecting leakage flux, and assuming B" max , = 120,000, 

 the tooth width t r will be 2.06 in., which indicates that a slot 1% 

 in. wide will probably be suitable. 



Before deciding upon the depth of rotor slot, it will be advisable 

 to calculate the equivalent air gap in order that the field ampere- 

 turns and necessary cross-section of copper may be determined. 



