ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. T 
If necessary, the field magnet wheel can essentially con- 
tribute to the flywheel power of the combined engine and 
alternator set in connection with steam engines, gas 
engines or other prime movers. The revolving field type 
machine is almost universally adopted in modern practice. 
12. Characteristics and Efficiencies.—Reasonable work- 
ing conditions of an alternator only will enable the manu- 
facturer to turn out a good commercial machine, which 
will satisfy the station engineer. These conditions have 
been arrived at through recent experience, and for 
standard alternators they can be termed as follows. 
138. Commercial Efficiencies.—Table 1 indicates the com- 
- mercial efficiencies of different sizes of polyphase gen- 
erators for full and half loads. 
. TABLE 1.—EFFICIENCIES OF POLYPHASE 
ALTERNATORS. 
Average Commercial Efficiency. 
Output of Alternator. 
At Full Load, At Half Load. 
5 KVA, .85 .80 
| .86 .82 
Sy .88 , 84 
oo .90 .85 
1p0% =‘ 91 .86 
200.5% .92 87 
S007 .** .93 .88 
500 =* .94 .90 
Toe <<" .95 91 
1,000 ‘* .96 .92 
The efficiencies of single phase machines given in Table 2, 
are slightly lower than those of the corresponding poly- 
phase generators. 
