126 ELEMENTS OF ELECTRICAL ENGINEERING. 



minal voltage at full load, its terminal voltage generally rises 

 with decrease of current output. The increase of terminal voltage 

 from full load to zero load expressed in per cent, of normal full- 

 load voltage, field excitation and speed both "being constant, is 

 called the voltage regulation of the machine. 



The voltage regulation of an alternator varies greatly with the 

 power factor of the receiving circuit. For example, a certain 

 alternator delivers its full-load current to a non-inductive receiv- 

 ing circuit (power factor equal to unity) at its normal full-load 

 voltage of noo volts, and the terminal voltage rises to 1188 

 volts when the load is reduced to zero by opening the main 

 switch. The same alternator has its field excitation adjusted so 

 that it gives its normal full-load voltage of iioo volts when it 

 delivers its full-load current to an inductive receiving circuit of 

 which the power factor is 0.80, and the terminal voltage rises to 

 1386 volts when the load is reduced to zero. That is, the given 

 alternator has a regulation of 8 per cent, when it delivers current 

 to a receiving circuit having unity power factor, and it has a reg- 

 ulation of 26 per cent, when it delivers current to an inductive 

 receiving circuit having a power factor equal to 0.80. When an 

 alternator delivers current to a receiving circuit like a condenser, 

 in which the current is ahead of the electromotive force in phase, 

 the regulation of the alternator is better (smaller) than when the 

 receiving circuit is non-inductive. Thus, the above alternator 

 has a regulation of zero per cent, when it delivers current to a 

 condenser circuit having a power factor of 0.97, and a regulation 

 of minus 1 4 per cent, when it delivers current to a condenser cir- 

 cuit having a power factor equal to 0.80. A zero per cent, regu- 

 lation means no rise of terminal voltage from full load to zero 

 load, and a negative regulation means a decrease of terminal 

 voltage when the current output is reduced from full load to zero 

 load. 



Example. The two curves in Fig. 112 show the regulation 

 of a 135 kilowatt, 2 -phase, 60 cycle inductor alternator of the 

 Stanley Electric Manufacturing Company. The abscissa of a 



