62 
ALTERNATING GENERATORS AND. SYNCHRONOUS MOTORS. 
107. 
108. 
Advantages of Circular Poles. 
| out, that the mean length of a turn for a circular pole core 
i section is the most advantageous, as with a given sectional 
area, the periphery of a circular core is smaller than for the - 
-Assuming on the average a peripheral speed of 100 feet per 
second, the usual current density in the field copper for 
the maximum excitation current should not exceed 1,800 
amperes per square inch, under the condition that the 
winding depth is less than 2 inches and that the permissi- 
ble temperature rise is about 75 degrees Fahrenheit. Any 
departure from the above conditions, in one or the other 
way, allows an alteration in the value of the current den- 
sity. 
Length of Mean Turn.—Assuming that the pole core 
with its maget coil is already calculated and dimensioned, 
the determination of the mean length of a turn is prefer- 
ably made from a drawing of actual measurements. 
It may be pointed 
oval or rectangular forms. The saving effected in the 
copper by using round pole cores is at times an appreciable 
amount. For instance, the periphery of a circular 8- 
square-inch section is 10 inches as against 12 inches ecir- 
cumference of a rectangular form with 2 x 4 inch sides. 
It should be noted, however, that it is impracticable to make 
laminated pole pieces of cireular section. 
109. Advantages of Oval and Rectangular Poles.—If 
laminated poles are to be used, the rectangular shape is a 
necessity. In most cases of alternating current design it 
is difficult to comply with the requirements of circular 
pole sections, except for low speed machines, where the 
length of the armature core becomes approximately the 
same as the width of the pole piece. 
- In standard, medium or high speed generators the armature 
length is considerakly larger than the width of the pole, 
