PROBLEMS. 471 



full-load torque developed m the generator; and (/) full-load 

 torque developed in the motor. Ans. (a) 105 volts ;- (fr) 729 

 revolutions per minute; (c) 8.63; (d) 950 watts; (e) 1,240 

 pound-inches; 1,218 pound-inches. 



NOTE. The armature current is so nearly the same for motor and generator in 

 the specifications of this problem that the armature flux may be assumed to be the 

 same. See note to problem 91. 



106. A certain 6. 5 -kilowatt shunt generator, giving 1 1 5 volts be- 

 tween its terminals, has an armature resistance of 0.29 ohm, a field 

 resistance of 57.5 ohms, and a stray power loss of 495 watts. Find 

 the armature current for which the efficiency is a maximum, and 

 find the value of this maximum efficiency. Ans. 50 amperes, 

 79.2 per cent. 



NOTE. This problem and the next are based on the condition that maximum 

 efficiency occurs when the variable loss is equal to the constant loss. 



107. A certain 5 horse-power I lO-volt shunt motor with a 

 speed of 1,200 revolutions per minute has a field loss of 1 10 watts 

 and a stray power loss of 250 watts, both assumed to be inde- 

 pendent of load. The armature resistance is 0.4 ohm. Find the 

 value of the armature current intake for which the efficiency of 

 the motor is a maximum, and find the value of this maximum 

 efficiency. Ans. 30 amperes ; 78.9 per cent. 



108. A lo-kilowatt roo-volt long-shunt flat-compound gener- 

 ator has a stray power loss of 600 watts, the resistance of its 

 armature including brushes and brush contacts is 0.04 ohm, the 

 resistance of its series field winding is o.oi ohm, and the resistance 

 of its shunt field winding is 25 ohms, (a) Calculate the watts 

 lost in armature winding and series field coil for various armature 

 currents up to 104 amperes and plot a curve showing these losses 

 as ordinates and the corresponding external currents as abscissas. 

 What is the equation to this curve ? What is the name of this 

 curve ? (fr) Plot a curve showing total losses as ordinates and 

 values of external current /. as abscissas ; and (c) plot a curve 

 showing efficiencies as ordinates and external currents as ab- 

 scissas. 



