204 



POLYPHASE CURRENTS. 



[Exp. 



system, and may be applied to a 3-phase system by making the 

 triangles more or less equilateral. A 4-wire system or any other 

 system can be treated in a similar manner. 



Furthermore, the method just discussed for treating the 

 effect of resistance drop and reactance drop in line conductors 

 is not limited to non-inductive loads, but is applicable as well to 

 other loads, either with leading or lagging currents. (See 56, 

 Exp. 3 -B.) 



17. Conclusion. In the main it has been seen that 2-phase 

 circuits are essentially the same as two single-phase circuits and 

 can be so treated. Three-phase circuits are likewise essentially 

 three single-phase circuits and the conception of polyphase cir- 

 cuits is thus made simple. In any polyphase circuit the funda- 

 mental principles for the vector addition of currents and electro- 

 motive forces apply as in single-phase circuits. For 3-phase 

 circuits, however, there are modified forms of treatment that are 

 found practically convenient; these will now be considered. 



PART III. 



1 8. Three-phase Measurement. The most important 3-phase 

 connections (Fig. 2) are the star and delta connections, the elec- 

 trical relations of which will 

 first be studied. Other 3-phase 

 connections will then be stud- 

 ied with reference to various 

 arrangements of transformers 

 on 3-phase circuits. 



19. Star-connection. On 

 a 3-phase line, connect three 

 approximately equal resist- 

 ances* in star-connection ; see 

 Fig. ii. Measure the line 

 voltages XY, YZ and ZX; these are also called delta voltages 

 * Some measurements should also be made with unequal resistances. 



Line Voltage 



FIG. ii. Star- or y-connection of 

 load resistances. 



