VOLTAGE REGULATION 363 



as by T.A.-regulator control, the battery cannot be connected 

 directly across the bus. In such a case two different methods of 

 handling the battery have been used. The first consists in pro- 

 viding a constant potential exciter bus to which the battery is 

 normally connected, and introducing between this bus and the 

 common excitation circuit a booster whose voltage is automatically 

 controlled by the T.A. regulator. This system is described in 

 detail in the section on " Voltage Regulation," page 369. 



The second method consists in connecting the two outer ter- 

 minals of the battery to the corresponding sides of the exciter bus 

 and opening the battery circuit in the middle, with an automatic 

 switch at this point for connecting the battery in one series in case 

 of failure of the normal source of exciting current. The two halves 

 of the battery are provided with a trickling charge to keep the cells 

 in a healthy and fully charged condition, by connecting through 

 high resistance to the opposite side of the bus, as shown in dia- 

 gram, Fig. 217. 



Where there is no common excitation circuit, but each alter- 

 nator is provided with its own independent exciter, a different 

 arrangement is adopted. In such a case an emergency exciter 

 bus is used to which the battery is normally connected and to 

 which a spare exciter may be connected when required. Should 

 the source of excitation for any one of the alternators fail, its field 

 circuit is automatically connected to the emergency exciter bus 

 and the spare exciter may then be started up, if it is not already 

 in service, to relieve the battery as soon as this can conveniently 

 be done. 



6. VOLTAGE REGULATION 



Hand Regulation. The simplest system of regulation is by 

 means of hand-operated rheostats connected in the field circuits of 

 each generator. The pressure of the exciter bus is then generally 

 kept constant at the rated exciter voltage and all the regulation 

 is done by manipulating the generator rheostats. In order to 

 regulate the exciter voltage it is, of course, also necessary to pro- 

 vide rheostats in the exciter fields. 



T.A. Regulator. Of the various schemes proposed for auto- 

 matic voltage regulation, the T.A. regulator is now most widely 

 used. With this system the desired voltage is maintained by 

 rapidly opening and closing a shunt circuit across the exciter field 



