86 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
145. Example 15.—The large alternator mentioned in Par. 
145 has a pole are a of 8 inches; a slot width s of 1 inch; 
a total armature length | of 20 inches; a tooth pitch o of 
2.4 inches; an air gap 8 of 0.35 inch; a depth taken up by 
the fixing wedge and insulation material w of 0.3 inch; and 
a width of tooth r of 1.4 inches. 
With this data and application of Equation (20), there is 
obtained an air gap reluctance value 
0.313 See: 
8X TA 20 20: %8 Rae dO 
2.4 X 0.3 i 2.4 X 0.65 
This value is about 5 times larger than the reluctance of the 
field system for the saturation assumed in Par. 143. 
It will be found in course of the calculations of practical 
examples, that the results will be very close to the pro- 
portion of reluctances given herein. 
Ans. 
T= 
146. Reluctance of Armature System.—The magnetic 
reluctance of the teeth and armature core is in most cases 
a small value, when compared with that of the field 
system and air path. 
The following equation expresses the reluctance of the 
armature system: 
=( aX oO aie Vv 
> NaxrxlxmexXg 2kX1UX pa Xp 
For the definition of the various symbols, reference is 
made to Fig. 42. 
Further », — the permeability of the iron in the teeth; 
)0.313. .. (22) 
Pa —= the permeability of the. iron in the arma- 
ture core, and 
j is a constant depending on the amount of 
insulation in the armature core and in this 
case may be taken as 0.85 to 0.9. 
147. Example 16. Par. 145, 
the dimensions a, 0 r, and | are already given. The depth 
a Pee weet ee eee 
