Phase-Sequence Indicator

an electromagnetic or induction instrument used to indicate the phase sequence in three-phase electric circuits. The phase sequence determines the direction of rotation of three-phase electric motors; the correct operation of some measuring instruments and automatic control devices also depends on the phase sequence. Phase-sequence indicators are usually constructed as miniature asynchronous electric motors in which an aluminum disk serves as the rotor. The disk rotates when acted upon by a magnetic field excited by currents in three windings when the windings are connected to the conductors of the circuit being tested. A certain order of phase sequence is marked on the terminals of the phase-sequence indicator. If the disk rotates in the direction of an arrow marked on the disk, the phase sequence corresponds to the marks on the terminals of the instrument. General-purpose phase-sequence indicators can be used to determine the power factor and the phase shift between voltage and current.