PROBLEMS. 469 



98. (a) Calculate the efficiency of the motor specified in prob- 

 lem 78, driven as stated from I lo-volt supply mains, when the 

 current intake of its armature has each of the following values : 

 5, 10, 15, 25, 35 and 50 amperes, (ft) For what armature cur- 

 rent is the efficiency a maximum ? (c) Calculate the maximum 

 value. Ans. (a) 26.1, 54 8, 66 9, 77.5, 82.0 and 84 6 per cent ; 

 (&) 74.8 amperes , (c) 85.6 per cent. 



99. The motor referred to in problem 98 is run ten hours each 

 day as follows : I hour with 5 amperes armature intake, 2 hours 

 with 15 amperes armature intake, 2 hours with 25 amperes arm- 

 ature intake, 4 hours with 35 amperes armature intake, and I 

 hour with 50 amperes armature intake. What is the mean all- 

 day efficiency of the motor under these conditions of service ? 

 Ans. 78.4 per cent. 



NOTE. The all day efficiency of a dynamo is the total output in watt-hours 

 divided by the total input in watt- hours during the day. Thus in this problem find 

 the total input in watt-hours and the total output in watt-hours during the lO-hour run 

 and divide the output by the input. 



Evidently a high all -day efficiency, when a motor is run for a large portion of the 

 time at light load, depends upon high efficiency at light loads, and this in turn de- 

 pends upon small constant losses (field loss plus stray power loss). 



100. What is the efficiency of conversion and the mechanical 

 efficiency of the above motor . (a) when its armature current in- 

 take is 1 5 amperes , and (fr) when its armature current intake is 50 

 amperes. Ans. (a) 83.9 and 79.8 percent; (fr) 90.3 and 93.7 

 per cent. 



101. From the results of problem 98 calculate the output of 

 the motor in horse-power corresponding to each value of current 

 intake of the armature, and plot a curve showing efficiencies as 

 ordinates and outputs of power in horse-power as abscissas. 



102 A certain shunt motor is used as a dynamometer to 

 measure the power absorbed by a printing press. The stray 

 power loss corresponding to E a = 525 volts is found by previous 

 test to be 450 watts, and the armature resistance is 0.53 ohm 

 including resistance of brushes and of brush contacts. During 

 the test the current delivered to the armature of the motor is 20.2 



