108 THE MAGNETIC CIRCUIT [ART. 40 



8 ventilating ducts 9 mm. wide each. What is the maximum m.m.f. 

 required for the stator core per pole if the flux per pole is 0.15 weber? 



Ans. 190 using Arnold's method. 



Prob. 6. Draw a curve between the average H and maximum B 

 in the core, assuming a sinusoidal distribution of the flux density in the 

 tangential direction, for the carbon steel laminations in Fig. 3. 



Ans. H ave = 26.5 for B max = 18. 



Prob. 7. The cross-section of the cast-iron field yoke of a direct- 

 current machine is 370 sq.cm.; the mean length of path in it between 

 two consecutive poles is 85 cm. The length of the lines of force in each 

 pole-waist is 21 cm.; its cross-section 420 sq.cm. The poles are made 

 of steel laminations 4 mm. thick, so that the space lost between the 

 laminations is negligible. The reluctance of the joint between a bolted 

 pole and the yoke is estimated to be equivalent to 0.1 mm. of air. What 

 is the required number of ampere-turns for the pole-piece and the yoke, 

 per pole, when the useful flux of the machine is 5 megalines per pole? 

 The leakage factor is estimated to be equal to 1.20. 



Ans. About 930. 



40. Magnetic Leakage between Field Poles. It is of impor- 

 tance in modern highly saturated machines to know accurately the 

 leakage flux between the poles, in order to estimate correctly the 

 ampere-turns required for the field poles and the frame of the 

 machine. Moreover, the design of the poles 'can be improved, 

 knowing exactly where the principal leakage occurs and how it 

 depends upon the proportions of the machine. The value of the 

 leakage factor also affects the voltage regulation of the machine, 

 because at full load the m.m.f. between the pole-tips has to be 

 larger than at no-load, on account of the armature reaction. 



For new machines of usual proportions the value of the leakage 

 factor can be estimated from tests made upon similar machines. 

 But in new machines of unusual proportions the designer has to 

 rely upon his judgment, assisted if necessary by crude compara- 

 tive computations of the permeance between adjacent poles. In 

 this and in the next article some examples of such computations 

 are given, not so much in order to give a definite method to be fol- 

 lowed in all cases, as to show the student a possible procedure and 

 to train his judgment in estimating the permeance of an irregular 

 path. 



Four principal paths of leakage can be distinguished between 

 two adjacent poles (Fig. 29) : (a) between the sides of the pole 

 shoes which face each other; (6) between the sides of the pole 

 cores (waists) parallel to the shaft of the machine; (c) between the 



