DIRECT-CURRENT DYNAMOS AND MOTORS. 41 
390. Tho armature resistance of a compound-wound motor is 
.085 ohm, its series field resistance .025 ohm, and its 
shunt resistance 88 ohms; the motor consumes a current 
of 80 amperes on a 220-volt circuit; find its electrical 
efficiency. 
391. Find the commercial efficiency of the arc-light generator 
of Question 385, if it has a hysteresis loss of 275 
watts, and an eddy current loss of 50 watts. 
292. Required the commercial efficiency of the 150 K.W. shunt 
dynamo given in Question 386; its hysteresis loss is 
1,100 watts and its eddy current loss is 130 watts. 
393. The power absorbed by hysteresis and eddy currents in 
the 500-watt compound-wound generator referred to in 
Question 38% is 13 watts; what is its commercial effi- 
ciency? , 
394. The4H.P. series motor of Question 388 has a total iron 
loss of 12 watts; determine its commercial efficiency. 
3295. Compute the commercial efficiency of the shunt motor re- 
ferred to in Question 389, if its iron losses are known 
to be 60 watts. 
396. What is the commercial efficiency of the compound-wound 
motor described in Question 390, if its hysteresis loss 
is 200 watts and its eddy current loss is 50 watts? 
397. Find the commercial efficiency of a 75 H.P. motor from 
the following test: E. M. F. of supply circuit, 440 
volts; current taken by motor, 140 amperes; speed of 
motor, 750 revolutions per minute; pulley diameter, 21 
inches; belt tension, tight side, 950 pounds; slack side, 
350 pounds. 
398. Compute the commercial efficiency of the generator given 
in Questions 386 and 392, including the loss in belt- 
ing, if it is driven by a vertical belt. Use the average 
of the respective percentages given in Table 30, 
Instruction Book. 
