SYNCHRONOUS MACHINERY 



289 



The current in F is in phase with the line voltage, the current 

 in A is in phase with the machine voltage and the current in B is 

 in quadrature behind the machine voltage. 



When the incoming machine is exactly in synchronism the 

 coil A takes the position shown in the figure, since the current in 

 A is in phase with the current in F. When the machine is 90 

 degrees behind or ahead of the position of synchronism, the cur- 

 rent in B is in phase with the current in F and the armature 

 turns through 90 degrees and brings the coil B in line with the 

 poles. 



For intermediate phase relations the armature takes inter- 

 mediate positions, such that the revolving field produced by the 



Load 



A.C.BUS 



Bars 



123 



4 Incoming Machine 



FIG. 264. Synchroscope. 



armature winding is in line with the field poles when the current 

 in F is maximum. The phase relation is indicated by a pointer 

 on the dial of the synchroscope. 



When the frequency of the incoming machine is lower than that 

 of the line, the phase of the current in A continually falls be- 

 hind that of F and the pointer rotates in the direction marked 

 "Slow." When the incoming machine is running too fast, the 

 rotation of the pointer is in the opposite direction marked "Fast." 



When the machine is running exactly at synchronous speed and 

 is exactly in phase, the pointer is vertical and stationary. The 

 switch can then be closed and the excitation adjusted until the 

 machine takes its proper share of the load. 



The synchroscope described is for a single-phase circuit. It 

 can be used for a two-phase machine by connecting the two coils 

 A and B to the two phases of the machine and the coil F across 

 one phase of the line. 



For three-phase systems the armature is wound with a three- 

 phase winding connected to the three phases of the machine. 



