10 - DIRECT-CURRENT DYNAMOS AND MOTORS. 
TABLE 2. 
ARMATURE SPEEDS FOR VARIOUS KINDS OF DYNAMOS. 
Peripheral Velocity, 
Kind of Machine. elation! pri pcnese 
Usual Limits. | Average. 
High-Speed Ring Machine......... 3,000 to 6,000} 4,500 
Mediui-Speed Ring Machine...... 2,000 ** 4,000] 3 000 
Drum-Armature Machine.......... 2,000 ** 4,000] 3,000 
Low-Speed Direct-Driven Machine ./1,500 ‘‘ 3,000} 2,250 
The lower limit in all cases refers to small dynamos, while 
the upper limit refers to large machines, the average in 
each instance corresponding to a size of about 100 K. W. 
For machines below that size, therefore, use a value 
smaller than the average, and for machines above 100 
K. W. take a value between the average and the upper 
limit. The general average of 3,000 feet per minute is a 
good practical value for belt-driven drum armatures of 
from 10 to 300 K. W. capacity, and for direct-driven 
machines of from 500 K. W. output up; while for high- 
speed ring armatures it applies only to sizes smaller than 
4K. W. 
15. Approximate Cooling Surface of Armature.— 
Another method of predetermining the size of an arma- 
ture is based upon the surface required to dissipate the 
heat generated within the armature. There is some un- 
certainty as to what surface we are to consider the cool- 
ing surface of an armature. 
For a drum armature we will here take it to be the sum of 
the cylindrical surface of the armature core and of the 
area of a circle the diameter of which is the external 
diameter of the core, or 
8. = DizL, + Diz Sena rece (TY 
where D, is the diameter, and L, me Lesiestl of the arma- 
ture core. Ina drum armature, then, only one of the 
