Phase Reversal Protection | Induction Motor Protection:
Phase reversal protection is used to protect the induction motor, from running in reverse direction. Normally running a motor in reverse direction does not affect the system when the motor is empty (without mechanical coupling). But Running a induction motor in reverse direction with mechanical loads causes heavy load current flow on the motor’s stator winding and severe damages on the mechanical side when the direction should not have reversed.
Example cranes, hoists, lifts or in threading mills, belt conveyors. i.e The belt conveyor motor mechanical gears are having mechanical reverse direction protection arrangement. That arrangement blocks the motor rotation if its reverse, due to that the motor take high current (Motor goes to locked rotor condition). These high current damages the motor winding or increase temperature etc. But some of the application does not affect the motor example: pumps. Pumps can be rotated in any direction. But the wrong direction, the output of the pump will be affected. However, Motor over current protection will trip the motor. But it takes few seconds to operate the tripping circuit. So that we need high speed tripping when the phase sequence of the motor changed. Phase reversal protection generally do the job perfectly.
Phase Reversal Protection working Principle:
The direction of induction motor depends on the direction of rotating magnetic field produced by the stator windings. These directions can be controlled doing phase reversal of the motor. In motor terminals are powered by R-U, Y-V, and B-W always gives clockwise direction (Note: It may be reversed). However, the direction of the motor is purely depending on the direction of rotating magnetic field. The direction of the motor can be revised by changing the phase sequence of the motor. It can be done either in motor terminal or input three phase terminal. But you should not do in both the places. See the reverse sequence will be R-U, Y-W, and B-V.
Reason for Phase Reversal
- In home island generator, upon changing the phase sequence of the generator, the connected all motor’s phase sequence also changes.
- Drive with VFD we can changes the direction of the motor. VFD has option to change the output voltage to the motor
- Manual errors.
Mechanical phase reverse protection:
In old days, before microprocessor relays, this protection is provided using motor driven disc working on electromagnetic principle. The disc rotates parallel with the motor. The auxiliary contact for particular direction from the disc is given to the motor starting circuit. Under normal condition, the disc does not open the auxiliary contact and the motor starts rotating. If the direction of the motor is reversed, then simultaneously the direction of the disc also reversed. Hence the disc contact open the circuit and motor will be isolated.
Solid state phase reversal protection:
Solid state relay sense the phase sequence of the motor. To install these relay, first we should run the motor without relay. By that we should find out the correct direction of the motor. Only the direction or phase sequence finalized, then we have to install the phase reversal relay. The relay catches the existing sequence. If the sequence is reversed, then the relay de-energize the circuit.
Phase reversal relay circuit diagram:
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