ea) 
DIRECT-CURRENT DYNAMCS AND MOTORS. 
12. 
Disadvantages of Toothed Armatures.—The dis- 
advantages of the toothed armature core are:— 
1. It is somewhat more expensive to make. 
2. The teeth tend to generate eddy currents in the pole- 
pieces. 
3. The self-induction of the armature coils is increased. 
The second difficulty can be practically overcome by mak- 
ing the distance between the teeth at their ends not more. 
than 2 to 3 times the air-gap, so that the lines can spread 
from the corners of the teeth, and become nearly uniform 
on the pole-faces, as represented in Fig. 2. If the slots 
near the periphery are wider than three times the length 
_ of the air-gap, as shown in Fig. 3, there will be spaces, 
M, on the pole-pieces, where there are practically no lines. 
LUA’ Wi “WP: 
WUT 
; 
LJ 
p} 
Fias. 2 AND 3.—DISTRIBUTION OF Fic. 4.—PERFORATED 
MAGNETIC LINES ON POLE-FACE ARMATURE. 
WITH TOOTHED ARMATURE, UN- 
DER VARYING CONDITIONS. 
These spaces will move across the pole-faces, and the shift- 
ing field thus produced tends to induce eddy currents in 
the pole-pieces, which should in such cases be laminated 
or grooved in the direction of motion of the armature, in 
order to reduce the flow of these wasteful currents. In 
some cases the entire armature has been covered with a 
thin layer of iron wire, producing a continuous maguetic 
surface. The same effect is obtained by perforating the 
edge of the armature core, as shown in Fig. 4, instead 
of having open slots. The objections to both of these 
latter forms are their increased self-induction and the 
fact that a certain magnetic leakage occurs through the 
iron outside of the inductors. 
