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Thermal Overload Relay Wiring Diagram and Reset Method

The main structure of the thermal overload relay consists of two parts, which consist of a thermal element and an output auxiliary contact.

Thermal overload relay wiring diagram

According to the wiring diagram below, we can clearly see that the left side is the terminal of the relay in the main circuit. There are 6 points in total for 3 channels, and the output auxiliary contact terminals are on the right. The thermal components of thermal overload relays are generally made of a metal material with a different expansion coefficient, so that it will bend when it is heated.

When working, the hot element and the main circuit in the circuit are wired in series (just like the motor power supply circuit in the motor starter). The specific wiring method of the thermal overload relay is as shown in the figure below. There are 3 contacts in each of the two rows, and each of the thermal components is connected between the upper and lower terminals, and a total of three thermal components are connected from the series to the three power input lines of the motor. The auxiliary contacts are usually a normally open (NO) and a normally closed (NC), suitable for use in the control loop of the motor starter.

When the motor is overloaded, the current input to the motor will exceed the rated current of the motor, so the thermal element in the thermal overload relay will generate heat and bend according to the excessive current flowing. The internal mechanical structure drives the output contact to perform the switching action, thereby achieving the purpose of outputting an “overload” signal. The NC contact set of the relay auxiliary contact can be connected in series to the power supply line of the control circuit (simultaneously connected with the stop button). Once the motor overloads the auxiliary contact NC contact, the control loop power can be cut off immediately to realize the motor stop. Operation plays a protective role. At the same time, the auxiliary contact NO contact group can be connected to the alarm accessory (such as the indicator light). When the motor stops running due to overload, the cause of the motor stop can be given the “overload” prompt.

Thermal overload relay and air switch, AC contactor wiring method

Want to wire first, we must have some understanding of the function of the equipment, of which the air switch is believed to be very clear to everyone, the more common electrical appliances in life can be seen by opening the household distribution box, if these three equipment configurations are in accordance with When wiring from top to bottom, it is an air switch, a contactor, and a thermal overload relay.

First of all, the contactor has three pairs of main contacts and one coil (because the voltage specifications of the coil are relatively large, it must be clearly seen when connecting, usually using AC AC380V, 220V or DC24). I believe this is not used here. Let me elaborate. The contactor also has a pair of normally open contacts, a pair of normally closed contacts, depending on the model, but it is also designed to be normally open and normally closed.

Generally, the normally open contact of the contactor is mainly from the lock, and the self-locking function is to lock the device’s own line. That is to say, the self-protection of the previous self-protection means that it can continue to maintain the continuous operation of its own control line. The normally closed contact of the contactor is generally used in the interlock mode, and the interlocking is said to prevent the contactor from moving at the same time. When the forward rotation occurs in the forward and reverse lines, the reverse rotation is not possible, which requires Interlocking with a normally closed point can cause a short circuit fault if it is operated at the same time.

Secondly, let’s talk about the thermal overload relay, which usually has a pair of normally open and a pair of normally closed points. From its function, it can mainly protect the line. This normally closed point wiring of the thermal overload relay is more commonly used in series to the line. Once the overload is normally closed, the line will be disconnected. It should be noted here that it is better to adjust the operating current of the thermal overload relay according to the size of the load. If the adjustment is too large, it will not be protected. If it is adjusted, it will not work properly.

Finally, according to the 380V contactor specification, the thermal overload relay and the air switch specific wiring method are connected from the three open live wires to the three pairs of main contacts of the contactor, and then the lower end of the contactor is connected to the thermal relay. The three main wires are connected to the main terminal, and the three terminals at the lower end of the thermal relay are connected to the 380V contactor. Connect a wire on the main contact of the AC contactor to the normally closed contact of the thermal relay, and then connect the other end of the normally closed contact to the A1 of the contactor. Then connect a line power from the main contact of the AC contactor to the coil A2 of the contactor. Here, the wiring work is basically completed, but remember that the main power supply from the contactor must not be the same line. Attach a physical wiring diagram of the thermal overload relay in three-phase power.

Thermal overload relay reset adjustment instructions

On the front of the housing of the device, the general red button is the stop button, and the blue button is the reset button. There is also a knob for adjusting the current. The thermal overload relay can be adjusted in two ways, including manual reset and automatic reset.

Relay manual reset adjustment: After the device performs the overload protection action, it must use the hand to press the reset button to return its normally closed contact to the closed state. It should wait 2 to 3 minutes when taking the manual reset. It will be carried out later.

Relay automatic reset adjustment: When the device performs the overload protection action, the normally closed contact can automatically complete the recovery to the closed state, and the automatic reset time will not exceed 5 minutes.

The reset method of the thermal overload relay can be selected by resetting the adjusting screw. Use a slotted screw to extend into the adjusting hole on the lower side of the device, and then tighten the reset adjusting screw in the clockwise direction (to the end), which is Automatic reset adjustment operation, if you want to return to manual, just loosen the adjustment screw counterclockwise to a certain distance.

Many thermal overload relays now have adjustment knobs on the top cover, as shown in the figure above. When this adjustment knob is aligned with the H position, it represents a manual reset, and when it is aligned with the A position, it represents an automatic reset.

The method of adjusting the current of the thermal overload relay is to adjust the current adjustment knob in the front of the device housing according to the actual rated current of the motor, because basically there will be a clear current scale indication.


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