ALTERNATOR REGULATION AND OPERATION 



163 



short-circuit the exciter field rheostat and the voltage for both the 

 exciter and the alternator rises. When the voltage has reached 

 the value for which the regulator has been adjusted, the control 

 magnets open the main contacts, the relay contacts open and the 

 ge of the exciter and the alternator both drop. The cycle 

 is then repeated. When in operation, the main contacts and the 

 relay contacts vibrate continuously so that voltage fluctuations 

 are scarcely noticeable. 



A compensating winding, supplied by the secondary of a 

 current transformer in series with the line, compensates for line 

 drop. It increases the pull of the alternating-current control 

 mairnet so that the main contacts are drawn closer together and 

 the duration of short-circuit of the h'eld rheostat is increased. 



73. Parallel Operation of Alternators. The same reasons 

 which make it necessary to operate direct-current generators in 



No.2 





No.l 



b Load- 



(o) (b) 



162. Alt4Tii:it.rs in p:ir:i!lcl mid speed-load chara<'t-ri 



parallel (see Vol. I, page 372, Par. 235) apply to alternators. 

 Alternators, however, are made in units of very much greater 

 capacity than it is possible to make direct-current machines, 

 since there are no commutation ditlicult ies. Tin- largest single 

 alternating-current unit at the present lime is of 50,000 kv-a. 



In order to operate sat ily in parallel, direct-cm-rent 



genei .'ist have dmnpmg voltage characteristics, lii 



order that alleniaior> may op- lorily in parallel, their 



OMrt mu>t have //////;///;/ spe.-d-load eharact critics. 



()tln-r\si.-e ill' 'ion will be un>at Mart or\ . 'Hie reason 



MI- is as foil* 



