148 



ALTERNATING CURRENTS 



the vector difference of OA and OA' increases, the lamp LI beginning 

 to glow. At the same time, the vector difference of OB and O'C' 

 decreases, owing to the increase of the angle BOG' in diagram (c) ; 

 the lamp L 2 therefore grows duller. Lastly, the vector difference of 

 00 and O'B' increases, owing to the decrease in the angle COB' in 

 diagram (d), hence the lamp L 3 grows brighter. Thus LI begins to 

 glow, La approaches maximum brightness, while L 3 grows less bright. 



FIG. 110. To explain Principle of Three-phase Synchronizer. 



This goes on -until the vector O'B' in (6) conies into phase opposition 

 with OC in (), corresponding to a lag of 60 ; at this instant, L 2 is at 



its maximum brightness, while LI and L 3 

 are both equally bright, but much below 

 maximum brightness. After another in- 

 crease of 60 in the lag, O'C' comes into 

 phase coincidence with OB, and L 2 is 

 dark. A further increase of 120 in the 

 lag will bring O'B' into phase coincidence 

 with OC, and L 3 will become dark. 



We thus see that when the machine is 

 running too slowly, the lamps darken and 

 brighten in the order LI, L 2 , L 3 . Similarly, 

 it may be shown that when it is running 

 too fast, the order of brightening or extinction is LI, L 3 , L 2 . 



Hence, by noticing the order in which the lamps go out we can 



FIG. 111. Showing Arrange- 

 ment of Lamps of Three- 

 phase Synchronizer. 



