THE POLYPHASE CIRCUIT. 299 



Methods of Connection for three-phase System. In the preced- 

 ing paragraph it was assumed that each circuit was 

 connected to the other two, so that they should form the 

 return path for its current. The method of connection 

 between the circuits must evidently fulfil two conditions. 

 (1) It must be symmetrical; (2) the current must flow 

 through the resistances or machines forming the three load 

 circuits. 



There are two methods of connecting the load circuits 

 fulfilling these conditions, both in general use. They 

 are indicated in the diagrams, Figs. 147 and 148, and are 

 called respectively the star and the delta or mesh connections. 



In the star connection, Fig. 147, the three load circuits 

 are connected with one end to a common or neutral point, 

 the free ends being joined directly to the supply lines. 



In a mesh-connected system, Fig. 148, the load circuits 

 are all joined end to end in series, so as to form a closed 

 circuit, and the supply conductors are joined to each 

 junction point. 



Connection must be made in the alternator itself 

 between the three windings or phases with which it is 

 wound. This can either be done in the machine itself, so 

 that it has three terminals for connection to the three 

 transmission lines, or the alternator may have six termi- 

 nals, and these may be inter-connected, so as to supply 

 current of the right character to the line. In either case there 



A> 



B. B id C 



Fi. 147. STAR CONNECTION. FIG. 148. MESH CONNECTION. 



are available only the two alternative methods just described, 

 and the three windings of the armature must be connected 

 in one of the ways shown in the diagrams, Figs. 147 and 

 148, in star or mesh connection. The effect of this inter- 

 connection of the phase-windings upon the voltage of 

 transmission must now be discussed. 



