ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
43. 
74. 
75. 
46. 
speeds now obtainable, it is very common to find alter- 
nators of large capacity, which are designed as two pole 
machines, also generators with a large number of poles, 
as many as 80, for comparatively small outputs. 
Electrical Characteristics.—The electrical character- 
istics of alternators, must be dealt with on the basis of 
experimental data, before entering into the discussion: 
of their design. 
These characteristics are: the efficiency and the inherent 
regulation. 
Commercial Efficiency.—The commercial efficiency of 
a generator is defined as the ratio of its useful output to 
the input, where the output is equal to the electrical energy 
measured at the terminals of the alternator and the input 
is the sum of the useful output plus the variable and. 
constant losses. 
LOSSES. 
Losses.—tThe variable and constant losses are as follows: . 
1. The friction losses (1) between the revolving part and 
the bearing, and (2) the rotating wheel and the air. 
2. The hysteresis and eddy current losses in the armature 
and field iron. 
3. The excitation losses in the field coils. 
4. The energy consumption in the armature copper. 
Losses 3 and 4 are usually termed the Copper Losses, and 
2, the Iron Losses. 
Classification of Losses.—The classification of the above 
into constant and variable losses, will greatly facilitate 
the reading of the test data as well as the fixing of the 
different items of loss in the design. For instance, very 
often it is specified, that the efficiencies expressed in per- 
« * Seay 
