124 DIRECT-CURRENT DYNAMOS AND MOTORS. 
by the product of the belt-speed, in feet per second, and 
of the effective driving power of the belt, in pounds; or, 
converted into watts: 
Ww = 8% Lm ¢,)=1.9564x "x (Fr—fr), -...(92 
550°" 60 nshey a 4 b—Jo), «+ (42) 
where 746 = number of watts equal to 1 horse-power; 
550 = number of foot-pounds per second equal to 
1 horse-power; 
Vv,’ = belt velocity, in feet per minute; 
Vv,’ = belt velocity, in feet per second; 
F, = tension on tight side of belt, in pounds; 
f» = tension on slack side of belt, in pounds. 
The commercial efficiency of a generator, therefore, may 
be expressed by: 
Ww EC 
Com. Ef. a Ww" ~ 1.3564 xX Up’ X (Ff, — fr)’ : 
and the commercial efficiency of a motor, by: 
"7 ” Sad al 
Com. Ef. = Gr = ERX AMD) cua) 
.(73) 
90. Example of Belt-Driven Machine.—Compute the 
commercial efficiency of a 3 K. W. belt-driven ma- 
chine, tf its speed is 1800 revolutions per minute, 
its.pulley diameter 64 inches, and if the tension on 
the tight side of its belt is found to be 82_pounds and 
that on the slack side 30 pounds. 
Solution.—tThe belt velocity in this case is “axe x 1800 
= 3060 feet per minute, or 41 feet per second. Conse- 
quently, by (73): 
3000 3000 — 
Com. Ef. = T3564 x 51 x (82 = 80) ~ 3600 
= .833, or 83.34. 
91. Losses in Dynamo Belting.—Since in a direct-driven 
generator the commercial efficiency is the ratio of the 
mechanical power available at the engine shaft to the 
