EXAMPLE OF ALTERNATOR DESIGN 337 



merical values of the permeances per centimeter of air gap 

 measured axially are: 



Path A = 0.5725 

 Path B = 0.408 

 Path B = 0.408 

 PathC = 1.49 

 PathZ) = 0.2845 

 Pathtf = 0.245 



Total =3.408 



The permeance per square centimeter cross-section of air gap 

 is therefore ' Q = 0.3408, and the equivalent air gap is d e = 



Q QAQQ = 2.94.. cm., or 1.156 in. If great accuracy is required, 



a similar set of calculations should now be made with the relative 

 position of rotor and stator teeth slightly changed so as to bring 

 the center lines of two teeth to coincide, instead of the center 

 lines of two slots as shown in Fig. 135, and the mean of the two 

 calculated values will more nearly correspond with the average 

 air-gap permeance. Let us assume that this has been done and 

 that a reasonable value for the average "equivalent" air-gap is 

 d e = 1.25 in. 



Should any difficulty be experienced in calculating the per- 

 meance of a flux path such as E, with curved flux lines at both 

 ends, 'it is always permissible to divide it in two parts as indi- 

 cated by the letters EI and E 2 in Fig. 135, the permeance of each 

 part being calculated separately. Thus, in the example which 

 has just been worked out, the permeance of EI is 0.655, and the 

 permeance of E 2 is 0.39. The total of 0.245 is obtained by 

 taking the reciprocal of the sum of the reluctances. 



Item (47). The calculations for the curves of Fig. 138 have 

 been carried out in the same way as for Fig. 81 (item (70), Art. 

 63, Chap. X), using formula (62) of Art. 37 for obtaining the ap- 

 proximate relation between air-gap and tooth densities when the 

 iron is nearly saturated. A curve must be plotted for the rotor 

 teeth as well as for the stator teeth; indeed it is in the rotor teeth 

 that the difference between " apparent" and actual density in 

 the iron will be most marked, since it is there that the flux density 

 will attain the highest values. In practice it will rarely be neces- 



