CHAP. VI) EXCITING AMPERE-TURNS 107 



of the useful flux, from one's experience with previously built 

 machines, or it can be calculated by the methods explained in the 

 next article. Thus, a leakage factor of 1.20 means that the flux in 

 the field poles is 20 per cent higher than that in the armature, the 

 leakage flux constituting 20 per cent of the useful flux. The usual 

 values of the leakage factor vary between 1.10 and 1.25, depending 

 upon the proximity of the adjacent poles, the degree of saturation 

 of the circuit, and the proportions of the machine. 



The ampere-turns required for the pole-pieces are calculated in 

 a similar way, assuming the whole leakage to take place between 

 the pole-tips, so that the flux density in the pole-waist corresponds 

 to the total flux, including the leakage flux. In exceptional cases 

 of highly saturated pole-cores this method may be inadmissible, on 

 account of too large a margin which it would give as compared to 

 the ampere-turns actually required. In such cases part of the 

 leakage may be assumed to be concentrated between some two 

 corresponding points on the waists of two adjacent poles, or it may 

 be assumed to be actually distributed between the two pole-waists. 

 See probs. 9 and 10 in chapter II. 



In some machines the joint between the pole and the frame 

 offers a perceptible reluctance, like the joints in the transformer 

 cores discussed in Art. 33. Some designers allow a certain 

 fraction of a millimeter of air-gap to account for this reluctance, 

 and add the number of ampere-turns required to maintain the 

 flux in this air-gap to those for the pole-piece. The length of 

 this equivalent air-gap is found by checking back no-load 

 saturation curves obtained from experiment. As a usual rule, 

 it is advisable to increase the total calculated ampere-turns of the 

 magnetic circuit by about 5 to 10 per cent. This increase covers 

 such minor points as the reluctance of the joints, omitted in 

 calculations, as well as certain inaccurate assumptions; it also 

 covers a possible discrepancy between the assumed :md the 

 actual permeability of the iron. With a liberally proportional 

 field winding and a proper regulating rheostat a designer can 

 rest assured that the required voltage will be obtained, though 

 possibly at a somewhat different value of the field current than 

 the estimated one. 



Prob. 6. The stator core of a six-polo induction motor baa the 

 following dinirn-iMiiH ; bore 112 cm.; outside di:iin< : I r. i : 

 length 55cm.; the slots are 2 cm. X 4.5 cm.; the machine is pmvi.lr.l \\!!. 



