Induction Motor Protection (Part2)
The three phase supply is given to the motor through various elements such as fuse, switch, contactor and thermal overload relay. The control circuit of contactor consists of energizing coil, start and stop buttons. The start (ON) push button is normally open green switch while the stop (OFF) push button is normally closed red switch.
When the start button is pressed then the conductor coil is energized as it gets supply voltage. The coil attracts the plunger when excited and the main contacts are closed along with the auxiliary contact. Even if the ON push button is released, the contactor coil remains energized as it gets supply through auxiliary contacts. Thus motor starts running.
The OFF push button which is normally closed when passed cuts the supply of the contactor coil and hence the main as well as auxiliary contacts are open so motor eventually stops. If supply voltage fails, control coil is de-energized which opens the contactor and motor stops.
During overload condition, the thermal overload relay operates. Thermal overload relay consists of bimetallic strips. Because of bending of one or more bimetallic strip results in operation of common lever which operates the trip contacts to de-energize the coil and disconnects the supply to the motor.
The bimetallic strips are either heated directly by flow of current or with the help of special heater coil through which motor current flows. For large motors, these relays are connected in secondary of current transformers. The bimetallic strips can be of self setting type or hand resetting type in which mechanical reset is required as the trip mechanism locks itself in operated condition. It should be observed that the rating of thermal relay should be such that it should not operate during normal starting conditions. A setting range is provided for adjustment for various load conditions. Protection against short circuit is provided with the help of HRC fuses.
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