DIRECT-CURRENT DYNAMOS AND MOTORS. 125 
electrical energy at the machine terminals, for compari- 
sons between direct and belt-driven dynamos, the loss in 
belting should also be included in the formula for the 
commercial efficiency of the belt-driven generator. The 
following Table 30 contains averages of these losses for 
various arrangements of belts: 
TABLE 30. 
LOSSES IN DYNAMO BELTING. 
Arrangement of Belts. arr rhe a Ee al ges 
BEOIORIGRL TIOIG wo wee een ese dito 10 per cent. 
CE Se eee Se ep 4 
Countershaft and Horizontal Belt. . 10: 35 “ 
Countershaft and Vertical Belt .... 23° ** 96 & 
Main and Countershaft with Belts. . 20 ** 30 Ke 
92. Gross Efficiency. or the effi- 
ciency of conversion, in a generator is the ratio between 
the total electrical power generated and the gross me- 
chanical power delivered to the shaft; and in a motor it 
is the ratio of the mechanical output to the total elec- 
trical power in the armature. 
The gross efficiency is, therefore, the quotient of the com- 
mercial and electrical efficiencies, or 
Gr. Eff. —— El Ef” eee e eee eee eeeeereee @eeeece (75) 
ond consequently varies between a = .825, or 82} per 
cent., for smail dynamos, and == = .965, or 964 per 
cent., for large machines. 
93. Example of Gross Efficiency.—Find the gross effi- 
ciency of the 50 K. W. compound-wound generator 
of Example-(3), Pars. 83 and 8%. 
