ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 45 
% X 1.34 = 10.9%, which equals an efficiency of 100 — 10.9 
= 89.1% at two thirds load. Ans. 
84, Subdivision of Losses.—By specifying any two efficien- 
cies, the constant and variable losses are more or less fixed 
and the proper subdivision of the different items will be 
the next step. This work can be achieved on the basis of 
experimental data and empirical formule. 
85. Friction and Ventilation Losses.—As the friction 
and ventilation losses are uncertain to calculate, it is found 
advisable to keep on the safe side when using the data 
given in the Table 8. Fig. 25 indicates the usual average 
friction and ventilation losses as experimentally found in 
commercial machines. 
TABLE 8.—USUAL LIMITS OF FRICTION AND 
VENTILATION LOSSES, FOR DIFFERENT 
SIZES OF ALTERNATORS. 
Output of Alternator. Usual Limits in Per Cent. 
10 KVA. 400 to 7.00 
50 6 3 2.00 ** 4.00 
100 ss $:60°.5*>3:50 
200 * 1 10°"* 380 
300 oe at 3" 3.30 
400 - <80 **. 2:20 
500 Ae et tO 
750 ves STO7 >": 288 
1,000 de gh QS INS 
The sources of the above losses are, the friction between the 
bearings and the shaft; friction between the brushes and 
slip rings; and the energy consumed due to the friction 
of the magnet wheel rotating in the air. The friction 
losses largely depend on the weight, speed and construc- 
tion of the machine. 
These losses can be kept well within the limits given in 
Table 8, by the skilful design of the bearings and shaft 
