322 ELEMENTS OF ELECTRICAL ENGINEERING. 



generators which are to be operated in parallel, the main conduc- 

 tors, to which the generators are to be connected, run back of all 

 the switchboard panels and these main conductors are called the 

 main bus-bars. 



An auxiliary set of bus-bars, called the synchronizing buses y 

 connects all of the generator panels, and is so arranged that a 

 single synchronizing device may be used when any one of the 

 generators is to be put into operation and connected in parallel 

 with the others. 



Whenever it is desirable to have a ground connection at each 

 switchboard, a single wire is installed back of all the switch- 

 boards and connected to ground. This wire is called the 

 ground bus. 



Generator Panels. Figure 275 shows the complete connec- 

 tions of an alternating-current three-phase generator panel 

 (A. T. G. panel) including the connections of the exciter. The 

 generator is represented as a revolving field machine, the exciting 

 current being delivered to the field windings through collector 

 rings. The field switch is provided with what is called a dis- 

 charge resistance to prevent excessive sparking and possible 

 damage to the insulation of the field windings when the field cir- 

 cuit is opened. A rheostat is shown in the field circuit of the 

 generator, and another rheostat is shown in the field circuit of the 

 exciter. The three armature terminals of the three-phase generator 

 are directly connected through the three ammeters and through 

 a three-phase single-throw oil switch to the main buses.* Two 

 " potential transformers " are shown connected to the three-phase 

 mains (the use of two transformers for three-phase transformation 

 is explained on page 222), and the low voltage coils of the poten- 

 tial transformers are connected to what is called the "potential re- 

 ceptacle " and the " synchronizing receptacle," as indicated. The 

 potential receptacle is a form of plug switch into which the "poten- 

 tial plug" may be inserted to connect the voltmeter so as to 



* In some cases, two separate sets of main buses are installed, and each of the main 

 switches is double-throw so that any generator may be connected to either set of buses. 



