EXAMPLE OF ALTERNATOR DESIGN 333 



The average projection beyond ends of slots measured along 

 the side of the coil, whether straight or bent (see page 264) is 



l e - 2r 275 - (63 X 2.54) 



i -, - A - = zo.o cm. 



4 4 



The reactive voltage due to cutting of end flux, with full-load 

 current per phase, is then 



(24\ 

 y) logio (12 X 4 X 28.8) 



X 700 X 10~ 8 = 116 volts. 



The loss of pressure due to the slot flux can be calculated as 

 explained in Art. 97 using formula (106) on page 288, wherein 

 the quantities /, p, T s , n s , and I c have the same numerical values 

 as in the formula for end-flux reactive voltage. The remaining 

 quantities are: the gross core length l a = 51 X 2.54 = 129.5 cm., 

 di = 2> in., and s = 1 in. The permeances P 2 and P 3 of the 

 flux paths in the air spaces above the conductors can be calcu- 

 lated as follows. Neglecting the widening of the slot to accom- 

 modate the wedge, the permeance of the slot above the winding 



2X1 



(see Fig. 135) is P% = ^ = 2, per centimeter length of arma- 

 ture core measured parallel to the shaft. The permeance of the 

 path from tooth top to tooth top may be calculated by assum- 

 ing the m.m.f. of one armature slot to set up the flux in an air 

 space of radial depth d = % in., and of length X = 2.62 in. 



7X1 



Thus P 3 = Q g = 0.334. This last quantity cannot have 

 o X ^bZ 



a numerical value smaller than as calculated by this method. 

 If the rotor teeth were built up of thin plates like the stator, the 

 numerical value of Pa would be greater than 0.334; but we are 

 assuming a solid steel rotor (which is customary), and for this 

 reason it will be best to neglect the flux paths through the iron 

 of the rotor teeth. We are concerned mainly in providing enough 

 armature reactance to keep the short-circuit current within 

 reasonable limits, and as a sudden growth of leakage flux is im- 

 possible in solid iron owing to the demagnetizing effect of the 

 eddy currents produced, the value for P 3 as here calculated will 

 be about right. 



By inserting all these numerical values in formula (106), we 

 get for the volts lost by slot leakage when the armature conduc- 

 tors are carrying full-load current, E 9 = 189, or (say) 190 volts. 



