EXAMPLE OF ALTERNATOR DESIGN 341 



make no allowance for the flux lines which pass from the sides 

 of the tooth into the iron at the bottom of the slot, thus causing 

 the actual density at the narrowest part of the tooth to be some- 

 thing less than the calculated value. 



The previously estimated depth of 5 in. for the rotor slot seems 

 rather large, as it leaves hardly sufficient section of iron at the 

 root of the tooth. We shall therefore reduce this depth to 4% 

 in. as dimensioned in Fig. 135. The width of the tooth at the 



bottom is therefore ^~ - 1.625 = 1.2 in. 



040 



If a larger section of iron should be found necessary, it can 

 be obtained by reducing the size of the slots at the center of the 

 pole face (i.e., those which carry no field coils) ; but this question 

 can be settled later. 



The curve marked "air gap, teeth, and slots" in Fig. 139, 

 shows what excitation is required to produce a particular density 

 in the air gap. The departure from the air-gap line (the dotted 

 straight line) is due almost entirely to saturation of the rotor 

 teeth, the reluctance of the stator teeth being negligible as com- 

 pared with that of the IJ-^-in. air path. 



Items (50) and (51). The upper curve of Fig. 140 shows the 

 ideal flux-distribution curve for open-circuit conditions. It is 

 a sine curve of which the average ordinate is 



62.2 X10 6 



6.45 X 31.416 X 51 



and of which the maximum value is therefore ~ X 6,010 = 9,450 



A 



gausses. The area of this curve is a measure of the total air- 

 gap flux on open circuit (item (17)). The pole pitch represented 

 by 180 electrical degrees has been divided into eight parts, 

 and the height of the vertical lines is a measure of the flux density 

 in the air gap over the center of a rotor tooth. 



By providing a datum line and vertical scale of ampere-turns 

 immediately below the no-load flux curve, it becomes a simple 

 matter to plot an ideal curve of m.m.f. distribution over the 

 pole pitch, the shape of this curve being such as to produce the 

 desired flux distribution (see Art. 93, Chap. XIII). It is merely 

 necessary to read off the curve of Fig. 139 the ampere-turns 

 corresponding to the required air-gap density and to plot this 

 over the center of the corresponding tooth. In this manner 

 the lower curve of Fig. 140 is obtained. The practical approxi- 



