THE MAGNETIC CIRCUIT 



187 



design of dynamo, some leakage flux between pole shoes and 

 between pole cores which, in a plane normal to the axis of 

 rotation, will follow paths somewhat as indicated in Fig. 72. 

 The amount of this leakage flux is not easily calculated; but it 

 can be approximately predetermined by applying the conven- 

 tional formulas of Art. 5, Chap. II, or by the graphical method 

 as outlined in Art. 39, Chap. VII. Other approximate graphical 

 methods are used by designers 1 and, in the case of radical de- 

 partures from standard types, some such method of estimating 

 the leakage flux must be adopted. It will, however, be found 



FIG. 72. Leakage flux in multipolar dynamo. 



that the leakage factor does not vary appreciably in modern 

 designs of multipolar dynamos, and the following values may be 

 adopted for the purpose of determining the necessary cross- 

 sections of the pole cores and frame. 



Kw. output of dynamo 

 20 to 50 

 50 to 150 

 150 to 250 

 250 to 400 

 500 and larger 



Leakage coefficient 



1.15 to 1.3 

 1.14 to 1.26 

 1.13 to 1.23 

 1.11 to 1.19 

 1 . 10 to 1 . 16 



67. Calculation of Total Ampere-turns Required on Field 

 Magnets. The maximum values of the m.m.f. curves obtained 

 by the method followed in Chap. VII (see Arts. 40 and 43) 

 give the ampere-turns per pole required to overcome the re- 

 luctance of the air gap, teeth and slots. The balance of the 

 total ampere-turns is easily calculated since the lengths and 

 cross-sections of the various parts of the magnetic circuit are 



1 See p. 326 of WALKER'S "Specification and Design of Dynamo- 

 electric Machinery." 



