4/2 ELEMENTS OF ELECTRICAL ENGINEERING. 



109. A certain short-shunt compound generator delivers 10 

 kilowatts of power at a terminal voltage of 100 volts at full-load. 

 The resistance of armature, series field coils and shunt field coils 

 are 0.04 ohm, 0.03 ohm and 20 ohms respectively ; and the stray 

 power loss is found by test to be 430 watts. Find at full-load : 



(a) voltage between brushes ; (b) armature current ; (c) total 

 electromotive power induced in the armature windings ; (d) total 

 electrical power developed in the armature ; (e) electrical effi- 

 ciency ; (/) commercial efficiency. Ans. (a) 103 volts; (b) 

 105.15 amperes; (c) 107.21 volts; (d) 11,300 watts; (e) 88.5 

 per cent. ; (/) 85.4 per cent. 



110. A certain shunt motor is tested for efficiency at full-load 

 by the Prony brake method and the following observations are 

 obtained : speed, 1,200 revolutions per minute ; reading of spring 

 balance, 18.75 pounds; perpendicular distance from center of 

 pulley to the center line of the spring balance, 28 inches ; elec- 

 tromotive force applied, 1 10 volts ; total current intake, 83.6 am- 

 peres. Find : (a) output in horse-power and in watts ; (b) torque 

 in pound-feet ; (c) efficiency (true) of the motor ; and (d) tan- 

 gential pull in pounds at the rim of the motor pulley which is 10 

 inches in diameter. Ans. (a) 9.98 horse-power or 7,455 watts; 



(b) 43.75 pound-feet; (c) 81.1 per cent. ; (d) 105 pounds. 



111. A certain 6-pole, 25o-kilowatt, 55O-volt generator runs 

 at a speed of 320 revolutions per minute. The commutator is 

 37.4 inches in diameter. The machine has six sets of carbon 

 brushes, each set has four brushes, each brush has 1.09 square 

 inches of contact surface with the commutator, the brush pres- 

 sure is 1.25 pounds per square inch, and the coefficient of friction 

 of the carbon brushes upon the copper commutator is 0.3. Find : 

 (a) the watts lost in brush friction ; (b) the watts lost in contact 

 resistance between the brushes and the commutator at full load, 

 neglecting the shunt current. Ans. (d) 695 watts; (b) 911 

 watts. 



NOTE. Under ordinary conditions as to peripheral velocity and brush pressure 

 the contact resistance of carbon brushes in ohms per square inch of contact surface is 



