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Examoogle • User • info@examoogle.com • EE-2024-8821
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In Phase Sequence Indicator three coils are
Star connected
Delta connected
Not connected
Shorted
Star connected
Quick Summary: A Phase Sequence Indicator is an electromechanical instrument designed to determine the order in which the three phases (R, Y, B) of a three-phase supply reach their maximum positive value. It typically employs a small induction motor mechanism where the three input terminals are connected to a star-connected winding arrangement to create a rotating magnetic field.
A Phase Sequence Indicator is an electromechanical instrument designed to determine the order in which the three phases (R, Y, B) of a three-phase supply reach their maximum positive value. It typically employs a small induction motor mechanism where the three input terminals are connected to a star-connected winding arrangement to create a rotating magnetic field.
Td∝Φ1Φ2sinα — Torque developed in the induction-type element where α is the phase shift between fluxes.
The indicator consists of three coils arranged symmetrically in space, which are star-connected to the three-phase supply. When energized, the phase currents produce a rotating magnetic field, the direction of which depends on the sequence of the phases (R-Y-B or R-B-Y). This rotating field induces eddy currents in a metallic disc (rotor), creating a torque that causes the disc to rotate in the direction of the phase sequence; by observing the rotation direction, the phase sequence is identified.
Used to identify the sequence of phases in a 3-phase AC system.
Operates on the principle of a miniature induction motor.
The star point remains at neutral potential if the system is balanced.
Rotation direction confirms the phase sequence.
Simple construction
Direct visual indication
Requires manual observation of disc rotation
Mechanical wear over time
Verification of phase sequence before connecting 3-phase induction motors
Testing of power distribution panels
If the sequence is reversed (e.g., R-B-Y instead of R-Y-B), the disc will rotate in the opposite direction.
Option B (Delta) is incorrect because delta connection would cause a short circuit if the coils were designed for line-to-neutral voltage or would not produce the required rotating magnetic field correctly for the induction mechanism.
A is correct — The three coils in a phase sequence indicator are star-connected to provide a common neutral point and facilitate the creation of a rotating magnetic field.
Always ensure the neutral point in a star-connected indicator is not grounded if the source is an isolated 3-phase supply to avoid potential loop currents.