The role and principle of thermal relay

In the actual operation of the motor, such as dragging the production machine to work, if the machine is abnormal or the circuit is abnormal, the motor encounters overload, the motor speed decreases, the current in the winding will increase, and the winding temperature of the motor will be increased. Raise. If the overload current is not large and the overload time is short, the motor winding does not exceed the allowable temperature rise. This overload is allowed. However, if the overload time is long and the overload current is large, the temperature rise of the motor winding will exceed the allowable value, which will cause the motor winding to age, shorten the service life of the motor, and even burn the motor winding in severe cases. Therefore, this overload is unacceptable to the motor. The thermal relay is a protection device that uses the principle of the thermal effect of the current to cut off the motor circuit in the event of an overload that the motor cannot withstand, providing overload protection for the motor.

When using a thermal relay to protect the motor from overload, connect the thermal element to the stator winding of the motor, connect the normally closed contact of the thermal relay in series with the control circuit of the electromagnetic coil of the AC contactor, and adjust the setting current adjustment knob to make the person The font lever is at an appropriate distance from the push rod. When the motor works normally, the current passing through the heat element is the rated current of the motor, the heat element heats up, and the bimetal is bent after being heated, so that the push rod just touches the herringbone lever, and cannot push the chevron lever. The normally closed contact is in a closed state, the AC contactor remains closed, and the motor operates normally.

If the motor is overloaded, the current in the winding increases, the current in the thermal relay element increases, the temperature of the bimetal rises higher, the bending degree increases, the herringbone lever is pushed, and the herringbone lever pushes the normally closed contact. The contact is disconnected and the AC contactor coil circuit is disconnected, the contactor is released, the power of the motor is cut off, and the motor is stopped to be protected.

It can be seen that the thermal relay usually disconnects the control circuit of the contactor directly to disconnect the main circuit.

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