Working principle of thermal relay

Thermal relays are protective devices used for overload protection of motors or other electrical equipment and electrical circuits.
In actual operation of the motor, such as when the production machinery is being dragged to work, if the machinery is abnormal or the circuit is abnormal and the motor is overloaded, the motor speed will decrease, the current in the winding will increase, and the winding temperature of the motor will rise. If the overload current is not large and the overload time is short, the motor winding does not exceed the allowable temperature rise, and 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, causing the motor winding to age and shorten the service life of the motor. In severe cases, the motor winding may even burn out. Therefore, this overload is unbearable for the motor. Thermal relays use the thermal effect principle of current to cut off the motor circuit when an overload that the motor cannot bear occurs, providing overload protection for the motor.
When using a thermal relay to protect the motor from overload, connect the thermal element in series with 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 set current adjustment knob so that the herringbone lever and the push rod are at an appropriate distance. When the motor is working normally, the current passing through the thermal element is the rated current of the motor, the thermal element heats up, and the bimetallic strip bends after being heated, so that the push rod just contacts the herringbone lever, but cannot push the herringbone lever. The normally closed contact is in a closed state, the AC contactor remains energized, and the motor operates normally.
If the motor is overloaded, the current in the winding increases, and the current in the thermal relay element increases, causing the temperature of the bimetallic strip to rise higher, the degree of bending increases, and the herringbone lever is pushed. The herringbone lever pushes the normally closed contact, causing the contact to disconnect and disconnect the AC contactor coil circuit, so that the contactor is released and the power supply of the motor is cut off, and the motor stops and is protected.
It can be seen that the thermal relay usually disconnects the main circuit by directly disconnecting the control circuit of the contactor.

Working principle and characteristics of thermal reed relay
The thermal reed relay is a new type of thermal switch that uses thermal magnetic materials to detect and control temperature. It consists of a temperature-sensitive magnetic ring, a constant magnetic ring, a reed switch, a heat-conducting mounting plate, a plastic substrate and some other accessories. The thermal reed relay does not use coil excitation, but the magnetic force generated by the constant magnetic ring drives the switch to operate. Whether the constant magnetic ring can provide magnetic force to the reed switch is determined by the temperature control characteristics of the temperature-sensitive magnetic ring.