<P> 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 . </P> <P> 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 . </P> <P> 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 beat frequency). 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 so that the two systems are in phase agreement . </P> <P> In the moving iron instrument, an iron vane is mounted on a shaft along with the pointer . The field winding is a three - phase winding, with the phases connected to both the running and incoming sources through a phase - shifting "impedor" network containing resistors, capacitors, and inductors . In this instrument, conceptually the field winding produces two rotating magnetic fields due to the running and incoming sources . The iron vane moves in response to the resultant sum of the two fields . </P>

Describe the working principle of synchroscope and phase sequence indicator
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