ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 123 
keep the ratio of slot depth to slot width within the 
limit of 3 to 7. 
202. Height of Pole Cores.—The next investigation, is to 
see if the field winding can be conveniently placed in the 
available space, especially on the bottom periphery. It 
is well to consider that very high pole cores are mechan- 
ically weak, besides possessing magnetic disadvantages due 
to the inerease of the leakage factor. On the other hand, 
in order to obtain a low pole core, it is necessary to use a 
magnet coil of large winding depth, which should, under 
any circumstances, not exceed 2 inches. The latter condi- 
tion is based on the experience that coils with a great 
winding depth are lable to break down, because the 
cooling of the inner turns is imperfect, producing a car- 
bonizing of the insulation material. 
20S. Temperature Rise.—The temperature rise presents 
itself as an important item with regard to the reliable 
working of the generator. In a good design it is easy 
to arrange the electrical details, so that the temperature 
will not exceed the permissible limit. 
The temperature rise in any part of a machine depends (1) 
on the amount of energy dissipated in the form of heat, 
(2) on the specific radiating constant of the material, (3) 
on the effective cooling surface, and (4) on the velocity of 
the air which passes by and around any particular part. 
204. Cooling Surface.—Practice has shown that for station- 
ary armatures, where the losses are composed of those in 
the iron and the armature copper, a cooling surface of 
about 1 to 1.5 square inches per watt loss is a fair figure. 
The following cooling surfaces are to be considered: 
The inner cylindrical surface, equal tol X D xX 3.14; 
the external cylinder, equal to Lx D, X 3.14; 
(D2 — D’) x 3.14. 
4 5 
the two side faces which are equal to 2X 
