REGULATION OF ALTERNATORS 301 



High values; to be selected when pole pitch is small, 

 and radial length of pole core great in proportion to 

 width. 



Average values; for pole pitch 8 to 12 in., and length 



, .,,, r , 1.22 to 1.32 



of winding space 'about equal to width of pole core. 



Low values; for large pole pitch and small radial 

 length of pole core. 



1.32 to 1.42 



1 lo to 1 . 





These leakage coefficients apply to the case of alternators with 

 field excitation to give approximately normal voltage at terminals 

 on open circuit. 



Provided a reasonably high leakage factor has been used, the 

 cross-section of the poles and yoke of good dynamo steel may be 

 calculated for a flux density up to 15,500 gausses. Although 

 the flux density in the pole core (of uniform cross-section) will 

 fall off in value as the distance from the yoke ring increases, the 

 effect of the distributed leakage may be taken care of by calcu- 

 lating the ampere-turns for the pole core on the assumption that 

 the total leakage flux is carried by the pole core, but that the 

 length of the pole is reduced to half its actual value. This approx- 

 imation, although used by some designers, has no scientific basis. 



Bearing in mind the above-mentioned points, the open-circuit 

 saturation curve connecting ampere-turns per pole and resulting 

 terminal voltage can be calculated and plotted exactly as in the 

 case of a continuous-current dynamo with rotating armature and 

 stationary poles (see Art. 57, Chap. IX, and item 128 of Art. 63, 

 Chap. X). 



104. Regulation. Reference has already been made in Art. 77 

 of Chap. XI to the regulation of alternating-current generators, 

 and it was pointed out that the designer does not always aim at 

 producing a machine with a high percentage inherent regulation, 

 because it is recognized that automatic field regulation or some 

 equivalent means of varying the field ampere-turns is necessary 

 in order to maintain the proper terminal voltage under varying 

 conditions of load and power factor. The large modern units 

 driven at high speeds by steam turbines are, indeed, purposely 

 designed to have large armature reactance in order to limit the 

 short-circuit current, the maximum value of which at the 

 instant the short-circuit occurs depends rather upon the arma- 

 ture reactance than upon the demagnetizing or distortional effect 

 of the armature current. These considerations tend to empha- 

 size the importance of correctly predetermining the inherent 

 regulation of machines, and it is important to know exactly 

 what the term "armature reactance' 7 should include, in order 



