DIRECT-CURRENT DYNAMOS AND MOTORS. 43 
36. Radial Depth of Armature Core.—The cross-sec- 
tion of the armature core must be made of sufficient area 
to carry the flux @ given by (21) without over saturation 
of the iron and without causing excessive power losses 
due to hysteresis and eddy currents. The length, L,, of 
the core having been determined by (20), the radial 
thickness of the armature core, B,, is obtained thus: 
3s ® 
=n x LeXBe 
where n, is the number of pairs of magnet poles, and B, 
the flux density in the armature. The latter should be 
chosen within the limits given in Table 1%. 
B, 
TABLE 17. 
FLUX DENSITIES IN ARMATURE CORE. 
Kind of Machine. Flux Density in Armature. 
Ordinary Bipolar Dynamos 
and Motors.............- 60,000 to 90,000 lines p. sq. in. 
JOrdinary Multipolar Dyna- 
mosand Motors.......... BO.D00; ** Oo 46 SF Se 
Are Lighting Machines....|100,000 ** 130,000 “* “* § 
Dynamos for Plating, Ac- 
cumulator-Charging, etc., 
and Battery Motors ...... 30,000 ** 50,000 % “ 
PRTUOTORIOTS. cic nus! v0.0. »oes}| 20,000 ** 40,000 << 
3%. Example.—Find the radial depth of a smooth-core ring 
armature of 45 inches diameter, having 1,200 in- 
ductors and generating 550 volts when revolving at 
a speed of 200 revolutions per minute in a 6-pole 
field having a density of 40,000 lines per square inch. 
Solution.—From formula (5) we have 
v= ox = 2,360 ft. p. min.; 
