VOLTAGE REGULATION 367 



field rheostats is divided between the relays and a change in posi- 

 tion of the movable arm would unbalance the load on the different 

 contacts. An external resistance is, therefore, provided called 

 the equalizing rheostat and inserted in the field circuit of the more 

 active exciter (usually the higher speed), as shown in the dia- 

 gram. Equalizing rheostats are required for all but one of 

 several exciters in parallel. Compound wound exciters in par- 

 allel are also provided with equalizer connections in the same way 

 as other D.C. generators. 



It is also possible to operate a common regulator in connection 

 with two or more exciters when these are not operated in parallel 

 on the exciter bus, although such an arrangement is not recom- 

 mended as the best operating conditions. With certain modifi- 

 cations, the connections are just the same as if the exciters were 

 in parallel. If the exciters to be thus operated have similar char- 

 acteristics, very satisfactory regulation will probably be obtained 

 over the whole saturation range, but if the exciters have different 

 characteristics, it may happen that if satisfactory parallel opera- 

 tion of the alternators is obtained at one point of the saturation 

 curve of the exciters, successful operation will probably not be 

 obtained at a different point. Under various load conditions it 

 will, therefore, be necessary for the operator to either adjust the 

 generator field rheostats or the equalizing rheostats, which should 

 be provided as with the previous arrangement. 



A third arrangement is that of individual regulator operation. 

 In large central stations, where there are installed a large number 

 of A.C. generators and exciters, and it is desired to operate the 

 generators in parallel but not on the exciters, each exciter being 

 arranged to excite its own individual generator 'see Fig. 214, page 

 359), it is possible to operate a voltage regulator on each combina- 

 tion of generator and exciter. The generator, exciter and regu- 

 lator then form an operating unit in itself, and can be operated 

 without affecting the operation of the other units. This is accom- 

 plished by simply placing a current transformer in the opposite 

 phase from that to which the potential transformer for each regu- 

 lator is connected (Fig. 220). 



At unity power-factor the phase angle between the current and 

 the potential transformer acting on the regulator magnet core is 

 90 and the current winding of the regulator has no effect upon 

 the voltage of the regulator. However, should the voltage of 



