40 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
380. 
381. 
382. 
385. 
384. 
385. 
386. 
387%. 
388. 
389. 
State the weights per cubic inch of wrought iron, cast 
iron, copper, and brass. 
Give the average percentage by which the weight of the 
_principal parts (magnet frame, armature windings, 
bearings and bed, or brackets) of a high-voltage ma- 
chine must be increased to allow for accessory parts. 
By what average percentage must the weight of the prin- 
cipal parts of a low-voltage machine be increased to 
obtain its total weight? 
Name an approximate method for computing the weight 
of a toothed armature. 
Describe the accurate method of calculating the weight of 
a toothed armature core. 
Compute the electrical efficiency of a series-wound 25 arc- 
light generator (1,250 volts, 10 amperes), whose arma- 
ture resistance (warm) is 5 ohms, and whose field re- 
sistance is 6 ohms. 
A shunt-wound 150-K.W. 250-volt generator has an ar- 
mature resistance of .01 ohm, a field resistance of 42 — 
ohms, and a regulator resistance, at normal output, of 
8 ohms; determine its electrical efficiency. — 
Find the electrical efficiency of a compound-wound, 125- 
volt, 500-watt generator, if its armature resistance is 3 
ohms, its series field resistance 1.5 ohms, and its shunt 
resistance 312 ohms. 
What is the electrical efficiency of a 4} H.P. series motor, 
taking 4 amperes at 125 volts, if its armature has a re- 
sistance of 4.3 ohms and its field a resistance of 1.2 
ohms? 
Calculate the electrical efficiency of a shunt-wound 500- 
volt motor which takes 12 amperes and has an arma- 
ture resistance of 2.65 ohms, and a magnet resistance 
of 943 ohms. 
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