326 PRINCIPLES OF ELECTRICAL DESIGN 



238) . This has to be considered in connection with the pole pitch 

 (see Art. 74), a preliminary diameter of rotor being selected in 

 keeping with what seems to be a reasonable pole pitch. A few 

 rough calculations will very soon show whether or not the tenta- 

 tive value of T will lead to a suitable axial length of armature 

 core. 



Items (13) to (16). On the basis of q = 812, the number of 

 conductors per phase would be 



1 (irDq\ 

 -3V7T/ 



48.7 



With four slots per pole per phase, and three conductors in 

 each slot, we have Z = 3X4X4=48. 



For item (15) we have X = j^ = 2.62 in., giving a cor- 



700 V^ S 

 rected value for peripheral loading of q = ~ ^ = 800 approx. 



Items (17) to (20). For the purpose of calculating the flux 

 required on open circuit, we may use formula (94) of Art. 70, 



/> 



where E per phase = ~~t and fc = 0.958. 



The required flux per pole is therefore 



6,600 X 10 8 



= 



V3 X 2.22 X 0.958 X 60 X 48 

 = 62.2 X 10 6 maxwells. 



With the assumed value of 6,000 gausses for B , the axial 

 length of armature core will be 



_ 62.2 X 10 6 __ . 



la ~ 6,000 X 6.45 X 31.42 



This is a short armature for a machine with a rotor 38.25 in. 

 in diameter; but it is what we are aiming at, and if the field 

 winding can be accommodated in the space available, the design 

 should be satisfactory. 



We shall attempt to ventilate this generator by means of axial 

 air ducts only. If, then, there are no radial air spaces, the net 

 length of iron in the armature core will be approximately 

 l n = 0.92Z = 46.8 in. (Art. 84); but these dimensions cannot 

 be finally decided upon until the slot proportions and tooth 

 densities have been settled. 



Whirling Speed of Rotor. A matter of considerable impor- 



