The double voltmeter is a voltmeter with two dials which can simultaneously display two voltage values of the sources to be synchronized.
Double Frequency Meter
The double frequency Meter shows two frequencies, that of the two sources
The synchroscope is a device that shows the difference in phase angle between two sources. It consists of a dial with a rotating pointer. The pointer takes up a position depending on the difference in the phase angle between the two sources. The When the pointer is at the vertical position i.e 12 'O' clock position, the difference in the phase angles is zero. The two sources are now said to be "in phase" with each other.
The Phase Sequence Indicator
The phase sequence indicator is similar in functioning to a miniaturized three phase induction motor. The indicator has a rotating disc which rotates in the clockwise sequence when the phase sequence is positive i.e. R-Y-B. When the phase sequence is negative i.e R-B-Y, the disc rotates in the opposite direction.
Synchronization of AC Sources
Forward and Reverse Synchronization