Wednesday, September 14, 2011

Phase Reversal Protection

Induction Motor Protection (Part4)
Abnormal Operating Conditions from Supply Side (Part4)
       The direction of induction motor depends on the direction of rotating magnetic field produced by the stator windings. For a particular phase sequence RYB the motor rotates in a particular direction due to corresponding direction of rotating magnetic field. But if any two lines are interchanged after repairs the phase sequence reverses such as YRB. Then the direction of rotating magnetic field also reverses and induction motor starts rotating in opposite direction. Such a change of direction is dangerous if the induction motor is used for cranes, lifts or in threading mills etc.
       Thus to disconnect induction motor from supply if there is phase reversal, phase reversal protection is provided.
        This protection is provided using motor driven disc working on electromagnetic principle. The secondaries of two current transformers connected in two lines drive the motor to operate the disc. The arrangement in such that for a normal direction of motor, disc rotates in a particular direction which keeps the auxiliary contacts closed. But if there phase reversal then the torque produced reverses to rotate the disc in opposite direction. Due to this auxiliary contacts get opened. This inturn either operates the circuit breaker or de-energises starter coil to disconnect the motor from the supply. Thus phase reversal protection for the induction motor is achieved. Now a days solid state phase reversal relay sensing the phase reversal is used.

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