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ELECTRICAL MACHINERY 



called the Regulation of a generator. Thus a machine hav- 

 ing no-load voltage of 121 and full-load voltage of 110 

 would have a regulation of 10%. 



26. The Three-wire Generator. A great many electric 

 light installations are equipped with a three-wire distribu- 

 tion system. In such a system three wires are used for 

 carrying the current from the generator to the lamps. A 

 diagram of the scheme of connections for such an installa- 

 tion is given in Fig. 87; the voltages given here are those 

 generally used for incandescent lamp circuits. 



The generator which supplies such a three-wire system 

 must evidently have three wires connected to it; the extra 

 wire is called the neutral. Between each outside wire and 



FIG. 87. A Three-wire Distribution System. 



the neutral the lamps are connected so that there is on the 

 lamps an e.m.f. just one-half that between the outside wires. 



Connections for the Third Wire. The ordinary generator 

 has only two sets of brushes; the special feature in the 

 three-wire generator is the arrangement for connecting 

 the third wire (the neutral) to the armature windings. 

 This is always done by connecting an inductance coil 

 between two diametrical taps on the armature winding 

 and connecting the neutral wire to the center of this coil. 



The inductance coil consists merely of several turns 

 of insulated wire wound on a laminated iron core. Some- 

 times this coil is built right into the armature spider; in 

 other cases it is not in the armature at all but located behind 

 the switchboard. 



Fig. 88 illustrates these two schemes. In (a) the coil is 

 mounted in the armature spider and its center point is 



