DIRECT-CURRENT DYNAMOS AND MOTORS: 9 
PRELIMINARY CALCULATION OF ARMATOURE.: 
13. Preliminary Determination of Dimensions of 
the Armature.—There is no absolute rule for deter- 
mining the size of an armature, since it depends upon 
many conditions which cannot be included in a formula 
or covered in any general way; nevertheless there are 
several facts that will aid us in assuming an approximate 
size, which can be modified later if found incorrect. In 
point of fact, as already stated in the introduction, it is 
almost an advantage if several different sizes are tried 
before arriving at the right one, since it gives a broader 
view of the question and enables one to choose finally the 
best size, al! things considered. 
The chief points to consider in arriving at the size of an 
armature are: 1. Peripheral speed. 2. Cooling sur- 
face. 3. Space for inductors. 4. Surface covered by 
pole-pieces. 
14. Peripheral Speed of Armature.—One guide in ob- 
taining the diameter of an armature is the fact that the 
peripheral velocity, v, must be kept within the limits 
established by practice, that is, between 7,500 and 6,000 
feet per minute, 3,000 being a common value. Since the 
peripheral velocity of an armature may be expressed as 
the product of the armature circumference, in feet, by the 
number of revolutions per minute, or 
Dx 
12) X Miy seeeeeeerecece. (5) 
the approximate diameter, D,, of the armature, in inches, 
can be found by the following equation, when ,, the 
number of revolutions per minute, is known: 
Do = 22 a ease% Lesh PRO a age ae (6) 
Nn, 4 
The value of v in (6) should be taken within the limits 
given in Table 2, according to the kind of machine. 
