186 PRINCIPLES OF ELECTRICAL DESIGN 



teeth; but the amount of excitation required to send the flux 

 through air gap, teeth, and slots, has already been calculated, 

 and may be read off the curve a of Fig. 49, page 133. The air- 

 gap density to be used in obtaining this component of the total 

 field ampere-turns will be the maximum value of the air-gap 

 density as obtained from the flux curve A of Fig. 51 (for open- 

 circuit conditions). 



The necessary cross-section of iron in the various parts of the 

 magnetic circuit is readily calculated if the leakage factor can 

 be estimated; but the length of the pole core (the dimension c 

 in Fig. 71) will depend upon the number of ampere-turns re- 

 quired, and therefore on the length of the air gap, which must be 

 decided upon at an early stage in the design (see end of Art. 36, 

 page 119). 



Let (SI)gt be the ampere-turns required at full load for the 

 air gap, teeth, and slots; then, if we assume a depth of winding 

 on the field coils of 1^4 i n -> a winding space factor of 0.5, and a 

 current density of 1,000 amp. per square inch of copper cross- 

 section, the length of the winding space (which is approximately 

 equal to the length of c of Fig. 71) would be: 



If, now, we make the further assumptions that (SI) g t is 50 

 per cent, greater than the ampere-turns necessary to overcome 

 the reluctance of the actual air clearance of length 5 in., and 

 that the air-gap density B g = 8,000 gausses, we may write: 



s = 5 X 2.54 X 8,000 

 JL.o 



and, putting 875c in place of (SI) gt , we get the relation 



2.54 X 8,000 X 1.5 



0.4rr X 875 

 = 285 (approximately) (92) 



For a preliminary calculation of the ampere-turns required 

 for the complete magnetic circuit, a length of winding space 

 rather greater than as calculated by formula (91) or (92) may 

 be selected. A length equal to 306 for flat compounded machines, 

 or 356 for over-compounded machines, is suggested. This 

 dimension will be subject to modification if it is afterward found 

 that the cooling surface of the field windings is insufficient to 

 prevent an excessive rise of temperature. 



56. Leakage Factor in Multipolar Dynamos. Apart from the 

 useful flux entering the armature core, there will be, in every 



