a De 8s? Oy 
Pa ~ - 
DIRECT-CURRENT DYNAMOS AND MOTORS. 121 
To obtain the approximate power lost by friction for any 
intermediate size not given in Table 29, take the near- 
est, or the proportionate, percentage from the last column, 
and multiply the number so obtained by 10 times the out- 
put of the machine in kilowatts. The result will be the 
required power loss, in watts. Thus, for a 15 K. W. 
machine, the percentage of power loss by friction is about 
6% per cent.; hence the approximate amount of the power 
loss: 
62 X 15 X 10 = 1000 watts. 
If the nearest percentage given in the table is taken, 
- the power loss for a 15 K. W. machine is found 
7X 15 X 10 = 1050 watts. 
87. Examples of Calculating Commercial Effi- 
ciency.—To show the use of formulas (66) to (71), 
and of Table 29, let us determine the commercial effi- 
ciencies of the machines considered in Par. 83. 
(1) The 2500-volt 10-ampere series generator referred 
to in Example (1) of Par. 83 has a hysteresis loss of 
400 watis, and an eddy current loss of 100 watts; 
determine its commercial efficiency. 
Solution.—According to Table 29, the friction loss of a 
25 K. W. machine is about 1500 watts; therefore, by 
formula (66): 
| 25,000 
Com. Eff. = 25,000 + 2000 + 400 + 100 + 1500 
25,000 _ 
= 57000 = 8625, or 8614. 
(2) Find the commercial efficiency of the 3 K. W. shunt- 
wound generator of Example (2), Par. 83, tf tts hys- 
teresis loss is 25 watts and its eddy current loss 10 
watts. ’ 
‘. Solution.—From Table 29 it is found that the percent- 
age of friction loss for a 3 K. W. machine is about 9%, 
