6-A] 



GENERAL STUDY. 



199 



Note also the connections on various pieces of polyphase 

 apparatus (as generators, motors, etc.) which may be available, 

 and note for what kind of polyphase system the apparatus is 

 intended. 



PART II. 



5. Two-phase Measurement. Take two transformers* with 

 the same ratio of transformation (say 1:1). Connect the 

 primary of one transformer to phase A of a 2-phase circuit,f 

 and the primary of the other transformer to phase B. Measure 

 the secondary voltages when the secondary circuits are inde- 

 pendent, thus forming a 4-wire system with independent phases, 

 Fig. i (a). 



6. Addition of Electromotive Forces. Connect the two sec- 

 ondaries as a 3-wire system, Fig. I (&), and measure the voltage 

 of each phase (A and EB) and the voltage E between outside 

 wires. Lay off these voltages as a triangle and note how nearly 

 A and EB are at right angles, so making a true 2-phase system. 

 This triangle may be drawn as in Fig. 3, 4 or 5. 



FIG. 3. Topographic 

 method. 



FIG. 4. Addition 

 method. 



FIG. 5. Subtraction 

 method. 



7. If we use the topographic method of Steinmetz and omit 

 arrows, we can represent the electromotive forces of the 2-phase 

 3-wire system by Fig. 3 (see Appendix I.). This electromotive 



* It is preferable that each secondary consists of two equal coils : thus, 

 we might have primary no volts; secondaries 55 volts each, giving in 

 series no volts with a middle or neutral point. Note the various possible 

 voltage transformations for each transformer. 



f It matters not whether the supply circuit is 3-wire or 4-wire, or how 

 connected. If several kinds of supply circuits are available, use each one 

 in turn. Compare Fig. 6. 



