PROBLEMS. 461 



no volts from supply mains and delivers 85 per cent, of its 

 power intake to the generator which it drives. This generator in 

 its turn delivers 84 per cent, of its power intake to a receiving 

 circuit at 20 volts. Find the current delivered by the generator. 

 Ans. 392.7 amperes. 



67. A homopolar dynamo like Fig. 62, Chapter II., is driven at 

 a speed of 3,000 revolutions per minute. The flux passing 

 through the cylindrical armature AA is 5 million lines. What is 

 the electromotive force between positive and negative brushes ? 

 Ans. 2.5 volts. 



CHAPTER III. THE DYNAMO AS A GENERATOR. 



68. A given shunt generator gives an induced electromotive 

 force of no volts when run at a speed of 1,200 revolutions per 

 minute with a total resistance of 56 ohms in its shunt field circuit. 

 If this machine is driven at a speed of 1,500 revolutions per 

 minute what must be the total resistance in the shunt field circuit 

 to cause the machine to generate an electromotive force of 137.5 

 volts ? Ans. 70 ohms. 



69. The electromotive force of a shunt generator decreases 

 from no volts to 93 volts when the speed is reduced from 1,000 

 to 900 revolutions per minute. The armature flux at the higher 

 speed is 1,000,000 lines, (a) What is the armature flux at the 

 lower speed ? (b} What would the electromotive force of the 

 generator be at the lower speed if its armature flux were kept 

 constant at a value of 1 ,000,000 lines ? (c) What causes the flux 

 to decrease with the speed ? Ans. (a) 939,390 lines ; (b) 99 volts. 



70. The resistance of a generator armature, including brushes 

 and brush contacts, is 0.14 ohm. The total electromotive force 

 induced in the armature is 1 20 volts. Find the value of the elec- 

 tromotive force between the brushes when the current in the 

 armature is 70 amperes. Ans. 110.2 volts. 



71. A battery consisting of ten Daniell cells has an electromo- 

 tive force of 10.8 volts (measured on open circuit) and an internal 



