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In Fig. 439A is shown a portion of a 6-pole 500-kw, 250-volt d-c generator. Two complete magnetic circuits are shown, as well as portions of two others. The outside diameter of the armature is 30 in., the radial thickness of the armature iron including the teeth is 4.5 in, and the gross axial length of the armature is 14 in. The slots are 1.5 in deep. The pole cores are 7 by 13.5 in, and the pole faces are 10 by 13.5 in. The radial thickness of the yoke is 3 in., and the axial length is 16 in. The pole cores and the armature are made of dynamo sheet-steel laminations, the magnetization curve for which is given in Fig. 202 (p. 258). The ratio of net to gross iron in the armature and the field cores is 0.92. The yoke is of rolled steel, the magnetization curve of which is essentially that for cast steel in Fig. 202.


The general + path of the useful flux and the leakage flux 1 is indicated. The leakage factor, the ratio ( + 1)/ , is 1.20. In computing the ampere-turns for the pole cores, the ampere-turns for the pole pieces may be neglected, a length of 7 in., being used. The air-gap has an equivalent length of 0.3 in. When the air-gap flux is 5.6 x 106 maxwells, determine:

a) Average flux density in air-gap
b) Flux density in armature laminations
c) Pole cores
d) Yoke
e) Ampere-turns for air-gap

Using the magnetization curves, Fig. 202, determine: f) Ampere-turns for each pole core g) Ampere-turns for yoke h) Total ampere-turns per pole