70 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
56. Length of Magnet Cores.—The length of the magnet 
5%. 
cores must be chosen so as to provide. sufficient space for 
the field winding, without, on the other hand, making the 
length of the magnetic path excessive; or, since the latter 
consideration usually ‘predominates over the former, the 
cores should be made as short as possible without increas- 
ing the thickness of the winding to an undue amount. 
In b7polar machines with toothed armatures the length of 
each field core is usually about J to 7} times the core diam- 
eter for small sizes, in which the diameter of the magnets is 
from 2 to 5 inches. For larger sizes, up to 18 or 20 inches 
diameter, the length of the magnet cores is ordinarily from 
3 to J1 times the core diameter: and for diameters above 
20 inches, the length is frcm 0 to 33 per cent. less than 
the diameter. In the case of a bipolar smooth-core arma- 
ture, the length of the field magnet is about 40 per cent. 
greater than for a corresponding bipolar toothed arma- 
ture, to give more space for the additional winding 
required by the larger air gap. 
In multipolar smooth-armature machines, the usual prac- 
tice is to make the length of the magnet cores from 4 to 
1} times their diameter; for large machines the length is 
chosen between } and } of the diameter; for medium-sized 
machines between 3 and J; and for small machines be- 
tween J and /1 times the diameter. For toothed-arma- 
ture machines the length of the cores is made from % to ? 
that of a corresponding smooth-armature dynamo. 
Length of Air Gap.—The air gap is made small with 
toothed armatures, being determined by the following 
conditions: 
1. Suffic'ent mechanical clearance for free rotation; 
2. Avoidance of eddy currents ‘in the pole faces, explained 
in connection with Figs. 2 and 3, page 8; 
8. Reduction of side pull on armature, epereey. in multi- 
polar fields (cee Par. 52); 
4. Sufficient distance between armature core and bare sur- 
face to overcome armature reaction. 
63). 
