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Anatomical Thermal Relay

The thermal relay consists of a heating element, a bimetal, a contact, and a set of transmission and adjustment mechanisms. The heating element is a resistance wire with a small resistance value, which is connected in series in the main circuit of the protected motor. The bimetal is formed by rolling two metal sheets of different thermal expansion coefficients.

Thermal Relay Introduction

Thermal relays work on the principle of thermal effects of electrical energy. Bimetals, heating coils and current transformers are important components of thermal relays. The current transformer supplies current to the coil of the heater. The thermal energy of the heater coil heats the bimetallic strip. The bimetallic strip is made of an alloy of nickel and steel. Alloys of nickel and steel have high steel resistivity and are not thermally aged. The insulated liver arm is connected to the trip coil together with the spring and bimetal. The tension of the spring can be changed by means of a sector plate.

When the system is in normal operation, the spring will remain straight. When the system fails, the bimetallic spring heats up and bends. The tension of the spring is released, causing the relay contacts to trip. The contacts of the relay energize the trip circuit so the contacts of the circuit breaker are closed. Therefore, the system remains safe.

Thermal relays are mainly used for low-voltage squirrel cage induction motors and low output rated DC motors. Thermal relays have low overload capability. It is designed to operate at 6 to 7 times the full load current. This type of relay is not used in short circuit conditions. The short circuit current increases the temperature of the bimetal, so the contacts of the relay close. Thermal relays are used with short-circuit relays or time-limited fuses.

Thermal relay working principle

structure of thermal relay.
structure of thermal relay.

In the figure: 1 – current adjustment cam, 2 – leaf spring (2a, 2b), 3 – manual reset button, 4 – bow reed, 5 – main metal piece, 6 – outer guide, 7 – – inner guide, 8 – normally closed static contact, 9 – moving contact, 10 – lever, 11 – normally open static contact (reset adjustment screw), 12 – compensation bimetal, 13 – Push rod, 14 – connecting rod, 15 – compression spring

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 is working normally, the current passing through the heat element is the rated current of the motor, the heat element is heated, and the bimetal is bent after being heated, so that the push rod is just in contact with the herringbone lever, and the herringbone lever cannot be pushed. 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.

The other parts of the thermal relay function as follows: the left arm of the chevron lever is also made of bimetal. When the ambient temperature changes, the bimetal in the main circuit will have a certain deformation and bending. At this time, the chevron lever The left arm also undergoes deformation and bending in the same direction, so that the distance between the chevron lever and the push rod remains substantially unchanged, and the accuracy of the thermal relay operation is ensured. This effect is called temperature compensation.

The screw 8 is a normally closed contact reset mode adjusting screw. When the screw position is to the left, after the motor is overloaded, the normally closed contact is disconnected, and after the motor stops, the thermal relay bimetal is cooled and reset. The moving contact of the normally closed contact is automatically reset by the action of the spring. At this time, the thermal relay is in an automatic reset state. When the screw is rotated counterclockwise to the right to a certain position, if the motor is overloaded at this time, the normally closed contact of the thermal relay is disconnected. Its moving contact will swing to the right side of a new equilibrium position. After the motor is powered off, the moving contact cannot be reset. The movable contact must be pressed after pressing the reset button. At this time, the thermal relay is in the manual reset state. If the motor overload is faulty, in order to avoid starting the motor again easily, the thermal relay should be manually reset. To adjust the thermal relay from manual reset to automatic reset, simply turn the reset adjustment screw clockwise into position.

Some models of thermal relays also feature phase-break protection. Its structure is shown in Figure 3:

Schematic diagram of thermal relay with phase failure protection
Schematic diagram of thermal relay with phase failure protection

The phase-break protection function of the thermal relay is provided by a differential amplification mechanism consisting of inner and outer push rods. When the motor is working normally, the current through the thermal element of the thermal relay is normal, and both the inner and outer push rods are moved forward to the appropriate position. When there is a phase loss caused by the disconnection of the power supply, the phase current is zero, the bimetal of the phase is cooled and reset, and the inner push rod is moved to the right, and the bimetal of the other two phases is bent due to the increase of current. Increase, the external push rod is moved to the left. Due to the differential amplification, the normally closed contact is pushed off in a short time after the occurrence of the phase failure fault, the AC contactor is released, and the motor is powered off. And get protected.

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