462 ELEMENTS OF ELECTRICAL ENGINEERING. 



resistance of 4 ohms. Find : (a) the terminal voltage of the 

 battery when it is delivering a current of 1.2 amperes ; ($) the re- 

 sistance of the external circuit. Ans. (a) 6 volts ; (ft) 5 ohms. 



72. The electromotive force of a shunt generator rises from 

 no volts at full-load to 130 volts when the receiving circuit is 

 disconnected. What is the precentage regulation of the machine ? 

 Ans. 1 8. 2 per cent. 



73. The electromotive force of a shunt generator driven at 

 constant speed decreases from 115 volts when the brushes are 

 in the neutral axis to 90 volts when the brushes have 20 for- 

 ward lead. When the field of this generator is separately excited 

 the electromotive force decreases from 1 1 5 volts when the brushes 

 are in the neutral axis to 100 volts when the brushes have 20 

 forward lead. One million lines of armature flux are required to 

 give 1 1 5 volts when the brushes are in the neutral axis. What 

 is the value of the armature flux when the machine is giving, 

 self-excited, an electromotive force of 90 volts with 20 forward 

 brush lead ? The armature current in all the above is assumed 

 to be negligibly small. Ans. 900,000 maxwells. 



74. A separately excited generator with brushes in the neutral 

 axis gives an electromotive force of 1 1 5 volts. The electromotive 

 force betwen one of the main brushes of this generator and an 

 auxiliary brush 15 from it in the direction of rotation is 8 volts. 

 What would the electromotive force between the main brushes be 

 if they were given a forward lead of 15? The armature current 

 is assumed to be negligible. Ans. 99 volts. 



75. A certain bipolar shunt generator like Fig. 35, page 52, is 

 rated to give its full-load current of 50 amperes at a terminal 

 voltage of 100 volts. To give 100 volts at zero-load a field 

 excitation of 8, 100 ampere -turns is required. At full-load a field 

 excitation of 10,500 ampere-turns is required. Find the number 

 of turns required in the series field coil to give flat compounding. 

 Ans. 48 turns. 



NOTE. The ampere-turn is a unit of magnetomotive force and it is fully explained 

 in Appendix A. 



