ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 85 
143. Example 14.—Assume that the pole system of an alter- 
nator has the following dimensions: 
d= 10inches;b = 6 inches; f = 17 inches; 
e= 2 inches; g = 24inches;h = 5Sinches. Substituting 
these values in Formula (19) and taking “, = 350 for 
the pole core and #; = 150 for the wheel rim, we obtain 
_a field system reluctance value of 
2 SSNs Se SO | | Bes 
ei Frateet +r 2X24xX5x15ut—— 1U* ae. 
144. Reluctance of Air Gap.—The air path between the 
rotating and stationary parts, termed the clearance, con- 
tributes the greatest item to the apparent magnetic 
reluctance of the alternator and its calculation requires 
special attention, because the correct dimensioning of the 
air gap infiuences not only the electrical characteristics, 
but also the price of the machine. 
The following expression has been found to give good prac- 
tical results. The symbols referring to the demensions 
indicated in Fig. 42: 
1 
txX'@ ( - us) 
o(u-+ 0) 
The above tsrmiala refers to armatures with open slots. 
For closed, or slightly open, holes the whole pole shoe surface 
which is opposite to the toothed body has to be taken into 
consideration, so making the expression in this ease: 
x 0.313...(20) 
To = 
axrxl 
oxo 
-4- 
i 
It may be pointed out, that as a general rule the width of 
the pole is 1 to 1% inches less than that of the armature, 
in order to effect the cutting of the armature core- and 
winding by part of the stray lines of the field. 
