EXAMPLE OF ALTERNATOR DESIGN 



349 



This is the diameter of the equivalent circle in Fig. 144, and the 

 angle a, obtained as explained in Art. 102, is found to be 11 

 degrees. 



It is obvious that the wave shape under no-load conditions 

 will be very nearly a true sine-wave, and the form factor will 

 therefore be approximately 1.11. 



Item (59). No reference has been made to item (58) because 

 this applies mainly to the salient-pole type of machine; the pro- 

 cedure in proportioning the field poles and yoke ring being then 

 similar to that followed in D.G. design. The depth of iron below 

 the slots in the rotor of a turbo-alternator is usually more than 

 sufficient to carry the flux, including the leakage lines. In the 

 particular design under consideration we shall be able to provide 

 air ducts at the bottom of the rotor slots as shown in Figs. 135 

 and 145, and still leave enough section of iron to carry the 

 flux. 



The amount of the leakage flux at the two ends of the rotor 

 is not easily estimated; but when expressed as a percentage of 

 the useful flux it is never large in extra high-speed machines with 

 wide pole pitch; the greater the axial length of rotor in respect 

 to the diameter, the smaller will be the percentage of the flux 

 leaking from pole to pole at the ends. The rotor leakage which 

 occurs from tooth to tooth, and in the air gap, over the whole 

 length of the machine is shown diagrammatically in Fig. 145. 

 This sketch shows a total of four lines of leakage flux which pass 

 through the body of the rotor, but do not enter the armature. 

 This leakage flux will not appreciably affect the flux density in 

 the rotor teeth near the neutral zone, because it will follow the 

 path of least reluctance and be distributed between several 

 teeth. 



The calculation of the rotor slot flux may be carried out as for 

 the stator windings. Thus, at full load, with 12,300 ampere- 

 conductors per slot, the flux passing from tooth to tooth below 

 the wedge is 



0.4ir X 12,300 (3.5 X 49.5) X 2.54 



X 



1.625 



2,100,000 maxwells. 



Average m.m.f. 



Permeance 



The flux in the space occupied by the wedge and insulation 

 above the copper, including an allowance for the spreading of the 



