ALTERNATOR Ji'I-dfL ATluX AND OPERATION 161 



too large at low power-factors since the coil is acting directly 

 upon the magnetic circuit of the generator, as has been shown 

 in Fig. 139, page 134. 



From the foregoing it is obvious that for a given current the 



regulation depends on the pou-tr-fuctor. The regulation has the 



values at low power-factors, lagging current. At unity 



powr-factor. the regulation is usually some nominal value, that 



:u ") to 10 per cent. With leading current, the voltage tends 

 to rise as load is applied and the regulation may be zero or even 



ive. Figure 160 shows three typical load curves of an 

 alternator, one being taken at unity power-factor, the second at 

 0.8 power-factor, lagging current, and the third at 0.8 power- 

 factor, leading current. The regulation in each case is as follows : 



ac ab 

 Regulation = ~^ 



hould be kept in mind that for a fixed kilowatt output the 

 regulation with lagging current is even poorer than the values 

 obtained for fixed current output. 



72. The Tirrill Regulator. An automatic voltage regulator 



of the Tirrill type for direct-current machines is described in 



Vol. I, Chap. XI, page 306. An automatic voltage regulator is 



much more essential in the smaller alternating-current stations 



than in direct-current stations The voltage changes in th< 



erator and throughout the system are greater with alternating 



current than with direct current because of the added reactance 



drop in the generator armature, transformers, feeders, etc. 



Alternating-current generators cannot readily be compounded 



to compensate for voltage drop as direct-current generators are. 



The Tirrill regulator is aK< designed to be used with alterna- 



urrent generators. As with large direct-current machines, 



the regulator acts through the field of an exciter. The under- 



prineiple of the regulator is the same whether used for 



alternating or for direct current, the voltage being controlled 



'h case by the rapid <hort -circuit ing of the exciter field 



.ure i;i showfl the connections of the alternating- 



current type as applied to a three-phase alternat- 



Th- "Html in m alternating-current control 



rid a direct-current control mannet . 



The alternating-current control mauiiet i> operated primarily 

 11 



