126 DIRECT-CURRENT DYNAMOS AND MOTORS. 
ee 
Solution.—The electrical efficiency of this generator was 
found to be .95, and its commercial efficiency .872; there- 
fore by (75): 
Gr. EF, = 22 = 917, or 91.74. 
9d 
94. Weight-Efficiency.—The weight-efficitency of a dyna- 
mo, that is, the output per unit weight, is obtained by 
dividing the output, in watts, by the weight, in pounds. 
The numerical value of this ratio shows at a glance 
whether or not the machine is designed economically, 
since the weight efficiency indicates how the cost of the 
prospective ‘machine is going to compare with that of . 
existing dynamos. 
The weight-efficiency of modern machines ranges between 
2 and 20 watts per pound, according to their speed and 
size. For low-speed dynamos, the limits are from 2 to 10 
watts per pound, the former figure referring to very 
small machines (less than 4 K. W. capacity), and the 
latter to very large ones (2,000 K. W. and over). In 
medium-speed machines the weight-efficiency varies 
from 3? to 15 watts per pound, according to the size; and 
high-speed dynamos have weight-efficiencies from 4 tu 20 
watts per pound. 
To determine the total weight of the machine, compute the 
weights of the magnet frame, armature, and windings 
from the dimensions; estimate the weights of the shaft, 
the bearings, and the bedplate or supporting brackets; 
and, according to the size and current output of the 
machine, allow from 4 to 25 per cent. of the total weight 
so found, for accessory parts, such as commutator, pul- 
ley, brushes, brush-rocker, switches, connectors, cables, 
etc. A small allowance for accessory parts, from 4 to 10 
per cent., is to be made in the case of large machines of 
high voltage in which all the current-carrying parts are 
- comparatively small; and a large allowance, from 15 to 
25 per cent., in the case of small, low-voltage machines 
having comparatively massive current-conveying parts. 
