Synchroscopes are electrodynamic instruments, which rely on
the interaction of magnetic fields to rotate a pointer. In most types,there is
no restoring spring torque for the magnetically produced torques to overcome
therefore pointer system is free to
rotate continually. Synchroscopes have a damping vane to smooth out vibration
of the moving system.
A polarized-vane synchroscope has a field winding with a
phase-shifting network arranged to produce a rotating magnetic field. The field
windings are connected to the incoming machine. A single phase polarizing
winding is connected to the running system. It is mounted perpendicular to the
field winding and produces a magnetic flux that passes through the moving
vanes. The moving vanes turn a shaft that carries a pointer moving over a
scale. If the frequency of the source connected to the polarizing winding is
different from the source connected to the field winding, the pointer rotates
continually at a speed proportional to the difference in system frequencies.
The scale is marked to show the direction of rotation
corresponding to the incoming machine running faster than the running system.
When the frequencies match, the moving vanes will rotate to a position
corresponding to the phase difference between the two sources. The incoming
machine can then be adjusted in speed and than phase sequence is checked.