EXAMPLE OF ALTERNATOR DESIGN 325 



and if we select the star connection of phases, this is also the 

 current per phase winding (item (7)). 



Referring to Art. 75 for values of the specific loading (page 250) 

 we find that the average value there suggested is q = 650; but 

 the peripheral loading is greater in turbo-alternators than in 

 slow-speed salient-pole machines, because it is desired to keep 

 the axial length as short as possible, and the greater losses per 

 unit area of cooling surface can be dealt with by a suitable system 

 of forced ventilation. We may therefore select a value for q 

 equal to, or even greater than, the proposed maximum for self- 

 cooled machines. Let us try q = 650 X 1.25 = 812. 



The pole pitch (item (11)) is 



and the approximate armature ampere-turns per pole (item (8)) 

 will be 



(SI). - = 12,750 



Referring to Arts. 76 and 77, Chap. XI, we can get a prelimi- 

 nary idea of the required length of air gap as explained in Art. 77. 

 We shall, in this design, deliberately select a high value for the 

 air-gap flux density, and if necessary saturate the teeth of the 

 rotor while keeping the density in the armature teeth within rea- 

 sonable limits to prevent excessive hysteresis and eddy-current 

 loss. Let us try B g = 6,000 gausses, which is higher than the 

 upper limit of the range suggested in Art. 76. The principal 

 advantage of using high flux densities is that the axial length of 

 the rotor can thus be reduced ; but if it is found later that the 

 selected value of B a leads to unduly high flux density in the 

 teeth, it will have to be modified. 



The probable maximum value of the air-gap density on open 



circuit is ^ X 6,000 = 9,450 gausses; and, assuming the ratio of 

 2i 



m.m.fs. to be 1.25, we have 



9,450 X d X 2.54 = 1.25 X 0.47T X 12,750 



whence 5 = 0.835 or (say) % in. 



In the case of medium-speed salient-pole designs, the periph- 

 eral velocity would not be decided upon by merely selecting 

 the upper limit of 8,000 ft. per minute as given in Art. 66 (page 



