Synchroscope

In AC electrical power systems, a synchroscope is a device that indicates the degree to which two systems (generators or power networks) are synchronized with each other.

For two electrical systems to be synchronized, both systems must operate at the same frequency, and the phase angle between the systems must be zero (and two polyphase systems must have the same phase sequence). Synchroscopes measure and display the frequency difference and phase angle between two power systems. Only when these two quantities are zero is it safe to connect the two systems together. Connecting two unsynchronized AC power systems together is likely to cause high currents to flow, which will severely damage any equipment not protected by fuses or circuit breakers.

The simplest aid to synchronizing a generator to another system uses lamps wired between similar phases of the two systems; when the lamps stay dark, the voltage and frequency of the two systems are the same and the generator may be connected. However, the accuracy of this approach is low since it is difficult to discern slight phase differences, and the lamps do not show the relative speeds of the two systems. Synchroscopes are instruments that show the relative frequency (speed) difference and the phase angle between the machine to be synchronized and the system voltage.

Since most synchroscopes are connected only to a single phase of the two systems, they cannot assure that the phase sequence is correct. When generators are newly connected to a power system, or temporary connections are used, other means are required to assure both systems have the same phase sequence.

Synchroscopes are electrodynamic instruments, which rely on the interaction of magnetic fields to rotate a pointer. In most types, unlike voltmeters and wattmeters, there is no restoring spring torque for the magnetically produced torques to overcome; the pointer system is free to rotate continually. Synchroscopes have a damping vane to smooth out vibration of the moving system.

...
Wikipedia