EXAMPLE OF ALTERNATOR DESIGN 339 



With an equivalent air gap d e = 1.25 in. and an assumed 

 sinusoidal flux distribution, the field ampere-turns on open cir- 

 cuit to overcome the reluctance of air gap only will be 



62.2X10* XT 1.25X2.54 



~. (51 X 31.42 X 6.45) X2 X 0.4* = 24jOC 



24 000 

 which makes the ampere-conductors per slot ^ = 8,000. 



o 



This does not include the ampere-turns to overcome the reluc- 

 tance of the teeth and the remainder of the magnetic circuit, 

 neither does it take into account the considerable increase of ex- 

 citation with full-load current on a power factor less than unity. 

 The current density in the copper may, however, be carried up 

 to 2,500 or even 3,000 amp. per square inch of copper cross-sec- 

 tion, and it is probable that a slot 5 in. deep will provide sufficient 

 space for the field winding. 



The numerical value of the symbol 8 in formula (62) should be 

 something greater than the actual clearance of % in. between 

 the tops of the teeth on armature and field magnet, and since the 

 difference between the equivalent and actual air gap is % in., 

 we may suppose the effect of slotting the surfaces to be equivalent 

 to removing ^f Q in. from both stator and rotor. Thus the numer- 

 ical value of d for use in formula (62) will be % in. -f- %Q in. = 

 l^f 6 i n - K we take the tooth width at a point halfway down 

 the tooth, the symbols in formulas (62) and (63) have the follow- 

 ing values: 



d = 1.0625 in. 



t = 1.89 in. 



d = 4. 125 in. 



t = 1.64 in. 



d = 5.0 in. 



Stator 

 Rotor 



Since we are considering the air-gap density over the surface of 

 the stator, the slot pitch for the rotor has been taken as X = 3.93, 

 or one and one-half times the stator slot pitch. As theve are no 



radial vent ducts in the rotor, the ratio j in formula (62) may be 



taken as 



Axial length of rotor _ 49.5 _ ^ 

 Gross length of armature 51 



Item (48). With equally spaced slots around the rotor 

 including the unwound portions of the pole face only one satura- 



