_ DIRECT-CURRENT DYNAMOS AND MOTORS. 123 
(6) What ts the commercial efficiency of the 75 H.P. 
compound motor considered in Example (6) of Par. 
83, if its hysteresis and eddy current losses aggre- 
gate 1200 watts? 
Solution.—The capacity of this motor is about 60 K. W., 
hence the friction loss, by Table 29, is about we = 5 X 
60 X 10 = 3000 watts, and by (71): 
_ 61,610 — (1945 + 1200-+3000) 55 465 
Com. Eff. = 61,610 ~ 61,610 
= .90, or 902%. 
88. Magnitude of Commercial Efficiency.—The com- 
mercial efficiency of well-designed machines ranges from 
.7, or 70 per cent., for small dynamos, to .96, or 96 per 
cent., for large ones. The commercial efficiency of a 
dynamo is always smaller than its electrical efficiency, 
since the former, besides the electrical power dissipation, 
includes all mechanical and magnetic power losses, such 
as are due to journal friction, to hysteresis, to eddy cur- 
_rents, and to magnetic leakage. The commercial effi- 
ciency, therefore, depends upon the amount of the elec- 
trical efficiency, upon the shape of the armature, upon 
the design, workmanship, and alignment of the bearings, 
upon the pressure of the brushes, upon the quality of the 
iron employed in armature and field magnets, and upon 
the degree of lamination of the armature core; while the 
electrical efficiency is a function of the electrical resist- 
ances only. The mechanical and magnetic losses vary 
very nearly proportionally to the speed; the no load 
power consumption for any given speed, consequently, is 
approximately equal to the open-circuit loss at normal 
speed multiplied by the ratio of the given to the normal 
speed. ; 
89. Efficiency of Belt-Driven Machines.—In the case 
of belt-driving, the mechanical power at the dynamo 
shaft, in foot-pounds per second, can also be expressed 
