470 ELEMENTS OF ELECTRICAL ENGINEERING. 



amperes, and the voltage across the brushes is 5 1 2 volts. Find 

 power delivered to the press. Ans. Stray power during test 430 

 watts ; power to drive press 9,696 watts. 



103. A certain shunt motor when connected to loo-volt supply 

 mains takes an armature current of 4 amperes and runs at a 

 speed of 1,063 revolutions per minute at zero load. The armature 

 resistance is o. I ohm. Find the stray power loss in watts and 

 the corresponding torque in pound-inches. What opposition to 

 rotation is overcome by this torque? Ans. 398.4 watts; 31.7 

 pound-inches. 



104. An efficiency test of a shunt motor (rated at 5 horse- 

 power, 220 volts, 1,200 revolutions per minute) by the stray 

 power method gave the following results : Volts between brushes 

 220, and current delivered to armature 2.0 amperes. The 

 machine was then shut down, a current of 10 amperes was 

 passed through the armature, and a voltmeter across brushes 

 read 5.0 volts. Find : (a) stray power loss in watts ; (ft) full-load 

 current in the armature ; (<r) full-load power intake ; (d ) true 

 efficiency at full load ; and (e) true efficiency at half load. The 

 resistance (hot) of the shunt field winding is 733 ohms. Ans. 

 (a) 438 watts; (b) 19.85 amperes; (c) 4,433 watts; (d) 84.1 per 

 cent.; (e) 76.7 per cent. 



NOTE. For a given power output, P, in watts, the armature current of the motor 

 is determined by the quadratic equation : 



P=E,(I U + f a }-FS- a I* 



in which F(E X I S ] is the shunt field loss and S is the stray power loss, both of which 

 are assumed to be independent of load. 



105. A shunt dynamo when run as a generator at a speed of 

 800 revolutions per minute delivers 1 1 kilowatts at a terminal 

 voltage of 1 10 volts, with a field current of 2 amperes. The re- 

 sistance of the armature (hot) is 0.05 ohm. When this machine 

 is run as a motor taking 102 amperes from 1 1 o- volt mains, it de- 

 livers 12.8 horse-power at its pulley. Find : (a) counter electro- 

 motive force of the motor ; (b) speed of the motor ; (c) induction 

 factor, <E>Z' of the motor ; (d) stray power loss in the motor ; (e) 



