100 



ELEMENTS OF ELECTRICAL ENGINEERING. 



unequal in value and more or less than 90 apart in phase, and 

 (&) that a two-phase (4-ring) rotary converter cannot be connected 



to three-wire mains. 



i 

 50. Two-phase electromotive forces and currents. The two 



ring i 



A 



ring 2 



main 2 



ring 3 



^ o 



main 3 



Fig. 9 1 . 



lines A and B in the clock diagram, Fig. 92, represent the elec- 

 tromotive forces of the A and B windings respectively of a two- 

 phase alternator. If the circuit which receives current from the 

 A winding has the same resistance and 

 reactance as the circuit which receives 

 current from the B winding, the system 

 is said to be balanced. In this case the 

 currents delivered by A and B are equal 

 to each other in value, and they lag in 

 phase equally behind A and B respec- 

 ft > tively, as shown by the two lines a and b. 



A 



J 



It is worthy of note that it is not possible to say 

 whether A is 90 behind B, or B is 90 behind A, 

 Fig. 92. even when the physical facts concerning a two-phase 



alternator are completely established. Complete 



definiteness of phase relation depends upon an arbitrary choice of the direction in 

 which the electromotive force and current of each phase is to be considered as posi- 

 tive. Conversely, a clock diagram like Fig. 92 cannot completely represent the 

 physical facts concerning a two-phase alternator until positive directions are chosen. 

 Let the arrows in Fig. 91 represent the arbitrarily chosen positive directions in the 

 windings, in the middle main, and in the receiving circuits, and suppose that, on the 



