The positions are usually compared with the markings on the clock. Thus a 3 'O'clock position would indicate that the voltages are apart by an angle of 30 degrees. The 6 'O'clock position would indicate that the sources are apart by 180 degrees. When the pointer is at the 12'O'clock position, it indicates that the difference in phase angle between the two sources is zero. The breaker connecting the two sources can now be closed.
The dial of the synchroscope is marked with two arrows indicating the direction of rotation of the pointer. These arrows indicate the clockwise and the anti-clockwise direction. The clockwise indicating arrow is marked "Too Fast" while the anti-clockwise indicating arrow is marked "Too Slow".
These arrows indicate the speed of the incoming source as compared to the bus bar. If the incoming generator's frequency is more than that of the bus bar, the pointer rotates in the "Too fast" clockwise direction. The machine then needs to be slowed down. If the frequency of the incoming machine is less that that of the bus bar, the rotation of the pointer is in the opposite "Too Slow" direction.
During forward synchronization when the incomer is intended to supply power to the grid, the pointer of the synchroscope is allowed to rotate in the clockwise direction before it stabilizes at the 12'O'clock position after which the breaker can be closed. This is essential to prevent the machine from tripping on reverse power should power flow from the bus bar to the grid.
In the case of reverse synchronization, the direction of the rotation depends on whether power needs to be exported from the bus bar to the grid or imported from the grid to the bus bar. In the former case, the direction has to be clockwise in the latter case it is to be anticlockwise.
In newer models of the synchroscope, the pointer is replaced by LEDS which blink depending on the phase angle and give the appearance of "running lights".
Synchronization of AC Sources
Instruments used in Synchronization
Forward and Reverse Synchronization