PROBLEMS. 467 



the full-load speed on the no-volt mains being 980 revolutions 

 per minute ; (b) the horse-power developed by the armature at 

 this reduced speed expressed as a fraction of the horse-power 

 developed by the armature when running under full-load on 1 10- 

 volt mains at a speed of 980 revolutions per minute. Ans. (a) 

 26.8 volts ; (ft) 20.4 per cent. 



91, The actual full-load speed of the motor of problem 78 is 

 980 revolutions per minute (armature current 50 amperes). This 

 machine is driven as a generator at a speed of 980 revolutions 

 per minute, and the field rheostat is adjusted to give the same 

 field current (2.6 amperes). Find : (a) the terminal voltage of the 

 generator when the armature current is 50 amperes ; (<) the total 

 resistance of the field circuit (field coils plus field rheostat) when 

 the machine is operated as a motor and as a generator respec- 

 tively ; (c) the resistance which has been cut out of the field 

 rheostat. Ans. (a) 99 volts; () 42.3 and 38.1 ohms; (c) 4.2 

 ohms. 



NOTE. It is assumed in this problem that the armature flux <f> is the same in 

 value whether the machine operates as motor or generator. This is true if the back- 

 ward lead of the brushes for the motor is equal to the forward lead of the brushes for 

 the generator, armature current being the same in each case. 



92, A given shunt generator is rated at 10 kilowatts at 100 

 volts between its terminals when driven at a speed of 1,200 revo- 

 lutions per minute. The resistance of the shunt field winding is 

 40 ohms (hot) and the resistance of the armature is 0.08 ohm 

 (hot). Find : (a) the armature current of the generator at full 

 load ; and (&) the speed of the machine when driven as a motor 

 from loo-volt mains with a load such that the armature current 

 is equal to that found under (a). Ans. (a) 102.5 amperes; (&) 

 1,018 revolutions per minute. 



93, Find the speed of a railway motor in revolutions per 

 minute corresponding to a car speed of fifteen miles per hour, 

 the gear ratio between the motor and the axle being 14 : 68, and 

 the diameter of the car wheels being 33 inches. Ans. 742 revo- 

 lutions per minute. 



