162 



PRINCIPLES OF ELECTRICAL DESIGN 



but this flux links with only 2T X -3 conductors, and the equiva- 

 lent slot flux, which would generate the same e.m.f. if cut by all 

 the conductors in the slot, is therefore 



(equivalent) ~ 



and 



(equivalent) 



Q.&irTI c C d 

 = ~^s~~l *' 



Jo 



= ^ TIc 



dx 



(76) 



FIG. 62. Illustrating slot-flux calculations. 



This is the equivalent flux in maxwells per centimeter of axial 

 length of armature slot. If l a = axial length of armature core 

 expressed in centimeters, 1 the total slot flux cut by each coil-side 

 during commutation, being twice the flux per slot, as shown in 

 Fig. 57, is 



1 _x7 



(77) 



3s 



The voltage component due to the cutting by both coil-sides 2 

 of the slot flux considered separately from other fluxes would be 



where the time of commutation 



brush arc in centimeters of armature periphery 

 peripheral velocity in centimeters per second 



Wa 



V 

 Thus, 



t c = 



"' ~ 3 X 10W (78) 



The factor W a in the denominator of this formula indicates that 

 the slot flux is of less importance with a wide than with a narrow 



1 It is well to let l a stand for the gross length of armature core, although 

 in slot flux calculations the total width of vent ducts is sometimes deducted. 



2 The 2T conductors in the one slot are here considered as equivalent to 

 the two coil-sides, each of T wires, in separate slots one pole pitch apart. 



