To protect the electric motor from unacceptable long-term current overloads that can occur when the load on the shaft increases or one of the phases is lost, a thermal protective relay is used. Also, the protective relay will protect the windings from further destruction if an interturn short circuit occurs.

This relay (abbreviated as TR) is called a thermal relay because of its operating principle, which is similar to the operation of a circuit breaker, in which bimetallic plates that bend when heated by electric current break the electrical circuit, pressing on the trigger mechanism.

Features of a thermal relay

But, unlike an automatic protective switch, TP does not open the power supply circuits, but breaks self-retaining chain magnetic starter. The normally closed contact of the protective device acts similarly to the Stop button and is connected in series with it.

Tandem contactor and thermal relay

Since the thermal relay is connected immediately after the magnetic starter, there is no need to duplicate the functions of the contactor in case of emergency opening of the circuits. With this choice of protection implementation, significant savings in material for contact power groups are achieved - it is much easier to switch a small current in one control circuit than to break three contacts under a large current load.

The thermal relay does not directly break the power circuits, but only issues a control signal if the load is exceeded - this feature should be remembered when connecting the device.

As a rule, a thermal relay has two contacts - normally closed and normally open. When the device is triggered, these contacts simultaneously change their state.

Characteristics of thermal relay

The choice of TP should be made by comparing the typical characteristics of this protective device according to the existing load and operating conditions of the electric motor:

• Rated protection current;
• Adjustment limit for the operating current setting;
• Power circuit voltage;
• Number and type of auxiliary control contacts;
• Switching power of control contacts;
• Operation threshold (ratio to rated current)
• Sensitivity to phase asymmetry;
• Trip class;

Connection diagram

In most schemes, when connecting a thermal relay to a magnetic starter, a normally closed contact is used, which is connected sequentially with the “Stop” button on the control panel. The designation of this contact is a combination of the letters NC (normal connected) or NC (normally closed).

With this connection diagram, a normally open contact (NO) can be used to signal that the thermal protection of the electric motor has tripped. In more complex automatic control schemes, it can be used to initiate an emergency algorithm for stopping the equipment conveyor chain.

To independently connect a thermal relay to protect an electric motor, without having experience working with such equipment, it would be correct to first familiarize yourself with this site.

Regardless of the type of connection of the electric motor and the number of contactors of the magnetic starter (direct and reverse starting), the implementation of a thermal relay in the circuit is quite simple. It is installed after the contactors in front of the electric motor, and the opening (normally closed) contact is connected in series with the “Stop” button.

Elements of connection, control and configuration of TR

According to GOST, the control contact terminals are designated 95-96 (normally closed) and 97-98 (normally open).

This figure shows a diagram of a thermal relay with the designation of terminals and control elements. The “Test” button is used to check the functionality of the mechanism.

The “Stop” button is used to manually turn off the protection device.

The “Re-arming” function allows you to restart the electric motor after the protection has tripped. Many TRs support two options - automatic (return to the original state occurs after the bimetallic plates cool down) and manual cocking, which requires direct action by the operator to press the corresponding button.

The operating current setting allows you to select a value overload, in which the relay will turn off the contactor coil, which will de-energize the electric motor.

When choosing a protection device, you need to remember that, by analogy with a circuit breaker, thermal relays also have a time-current characteristic. That is, if the set current is exceeded by a certain value, the shutdown will not occur immediately, but after a certain time. The speed of operation will depend on the multiplicity of exceeding the set current.

Different graphs correspond to the nature of the load, the number of phases and temperature conditions.

As can be seen from the graphs, if the load is doubled, more than a minute may pass before the protection is triggered. If you choose a TP that is not powerful enough, then the engine may not have time to accelerate when the starting overload current setting is exceeded multiple times.

Also, some thermal relays have a protection activation flag.

The protective closing glass serves both for marking and protection of settings by means of sealing,

Connection and installation of TP

As a rule, modern thermal relays have protection for all three phases, in contrast to the thermal relays common in Soviet times, designated TRN, where current control was carried out only in two wires going to the electric motor.

Based on the type of connection, thermal relays can be divided into two types:

The input conductive terminals in modern models simultaneously serve as part of the fastening of the thermal relay to the contactor of the magnetic starter. They are inserted into the output terminals of the contactor.

As you can see from the photo below, within certain limits you can change the distance between the terminals to adapt to different types of contactors.

For additional fixation of the TP, corresponding protrusions are provided on the device itself and on the contactor.

Mechanics of thermal relay

There are many varieties of TR, but their operating principle is the same - when increased current flows through bimetallic plates they bend and act through a system of levers on the trigger mechanism of the contact groups.

Consider, for example, the LR2 D1314 thermal relay device from Schneider Electric.

Conventionally, this device can be divided into two parts: a block of bimetallic plates and a system of levers with contact groups. Bimetallic plates consist of two strips of different alloys, connected into one structure, having different thermal expansion coefficients.

Due to uneven expansion at high current values, this structure expands unevenly, which causes it to bend. In this case, one end of the plate is fixed motionless, and the moving part acts on the lever system.

If you remove the levers, the contact groups of the thermal relay will be visible.

It is not recommended to immediately turn on the thermal relay after tripping and restart the electric motor - the plates need time to cool down and return to their original state. Besides, it would be wiser to first find the reason protection activation.

Equipment equipped with engines needs protection. For these purposes, a forced cooling system is installed in it so that the windings do not exceed the permissible temperature. Sometimes it is not enough, so a thermal relay can be additionally mounted. In homemade products you have to install it yourself. Therefore, it is important to know the connection diagram of the thermal relay.

Operating principle of a thermal relay

In some cases, a thermal relay may be built into the motor windings. But most often it is used in conjunction with a magnetic starter. This makes it possible to extend the service life of the thermal relay. The entire starting load falls on the contactor. In this case, the thermal module has copper contacts that are connected directly to the power inputs of the starter. The conductors from the motor are connected to the thermal relay. To put it simply, it is an intermediate link that analyzes the current passing through it from the starter to the motor.

The thermal module is based on bimetallic plates. This means that they are made from two different metals. Each of them has its own coefficient of expansion when exposed to temperature. The plates, through the adapter, act on the movable mechanism, which is connected to the contacts going to the electric motor. In this case, the contacts can be in two positions:

• normally closed;
• normally open.

The first type is suitable for controlling a motor starter, and the second is used for alarm systems. The thermal relay is built on the principle of thermal deformation of bimetallic plates. As soon as current begins to flow through them, their temperature begins to rise. The more current flows, the higher the temperature of the thermal module plates rises. In this case, the plates of the thermal module shift towards the metal with a lower coefficient of thermal expansion. In this case, the contacts close or open and the engine stops.

It is important to understand that thermal relay plates are designed for a specific current rating. This means that heating to a certain temperature will not cause deformation of the plates. If, due to an increase in load on the engine, the thermal module is triggered and shuts down, then after a certain period of time, the plates return to their natural position and the contacts close or open again, sending a signal to the starter or other device. In some types of relays, it is possible to adjust the amount of current that should flow through it. To do this, a separate lever is provided with which you can select a value on the scale.

In addition to the current regulator, there may also be a button on the surface labeled Test. It allows you to check the thermal relay for functionality. It must be pressed while the engine is running. If this stops, then everything is connected and functioning correctly. Under a small plexiglass plate there is a thermal relay status indicator. If this is a mechanical option, then you can see a strip of two colors depending on the processes taking place. On the case next to the current regulator there is a Stop button. Unlike the Test button, it turns off the magnetic starter, but contacts 97 and 98 remain open, which means the alarm does not work.

Note! The description is given for thermal relay LR2 D1314. Other options have a similar structure and connection diagram.

The thermal relay can operate in manual and automatic mode. The second one is installed from the factory, which is important to consider when connecting. To switch to manual control, you must use the Reset button. It needs to be turned counterclockwise so that it rises above the body. The difference between the modes is that in automatic mode, after the protection is triggered, the relay will return to normal after the contacts have completely cooled. In manual mode, this can be done using the Reset key. It almost instantly returns the contact pads to their normal position.

The thermal relay also has additional functionality that protects the motor not only from current overloads, but also when the supply network or phase is disconnected or broken. This is especially true for three-phase motors. It happens that one phase burns out or other problems occur with it. In this case, the metal plates of the relay, which receive the other two phases, begin to pass more current through themselves, which leads to overheating and shutdown. This is necessary to protect the two remaining phases as well as the motor. In the worst case scenario, such a scenario can lead to failure of the engine, as well as the supply wires.

Note! The thermal relay is not intended to protect the motor from short circuits. This is due to the high breakdown rate. The plates simply do not have time to react. For these purposes, it is necessary to provide special circuit breakers, which are also included in the power circuit.

Relay characteristics

When choosing a TR, you need to be guided by its characteristics. Those declared may include:

• rated current;
• operating current adjustment spread;
• mains voltage;
• type and number of contacts;
• calculated power of the connected device;
• minimum response threshold;
• device class;
• reaction to phase imbalance.

The rated current of the TP must correspond to that indicated on the motor to which the connection will be made. You can find out the value for the engine on the nameplate, which is located on the cover or on the housing. The network voltage must strictly correspond to the one where it will be used. It can be 220 or 380/400 volts. The number and type of contacts also matter since different contactors have different connections. The TR must be able to withstand the engine power so that false triggering does not occur. For three-phase motors, it is better to take TP, which provide additional protection in case of phase imbalance.

Connection process

Below is a TP connection diagram with symbols. On it you can find the abbreviation KK1.1. It denotes a contact that is normally closed. The power contacts through which current flows to the motor are designated by the abbreviation KK1. The circuit breaker located in the TP is designated as QF1. When it is activated, power is supplied in phases. Phase 1 is controlled by a separate key, which is marked SB1. It performs an emergency manual stop in case of an unexpected situation. From it the contact goes to the key, which provides start-up and is designated by the abbreviation SB2. The additional contact, which extends from the start key, is in standby condition. When starting is performed, then the current from the phase through the contact is supplied to the magnetic starter through the coil, which is designated KM1. The starter is triggered. In this case, those contacts that are normally open are closed and vice versa.

When the contacts, which are abbreviated KM1 in the diagram, are closed, then three phases are switched on, which send current through the thermal relay to the windings of the motor, which is put into operation. If the current increases, then due to the influence of the TP contact pads under the abbreviation KK1, three phases will open and the starter will be de-energized, and accordingly the motor will stop. The usual stop of the consumer in forced mode occurs by pressing the SB1 key. It breaks the first phase, which will stop supplying voltage to the starter and its contacts will open. Below in the photo you can see an improvised connection diagram.

There is another possible connection diagram for this TR. The difference is that the relay contact, which is normally closed, when activated, does not break the phase, but the zero, which goes to the starter. It is used most often due to its cost-effectiveness when performing installation work. In the process, the zero contact is connected to the TP, and a jumper is mounted from the other contact onto the coil, which starts the contactor. When the protection is triggered, the neutral wire opens, which leads to the switching off of the contactor and the motor.

The relay can be mounted in a circuit where reverse movement of the motor is provided. The difference from the diagram above is that there is a NC contact in the relay, which is designated KK1.1.

If the relay is triggered, then the neutral wire is broken by contacts designated KK1.1. The starter is de-energized and stops powering the motor. In an emergency, the SB1 button will help you quickly break the power circuit to stop the engine. A video about connecting the TR can be seen below.

Summary

Diagrams that depict the principle of connecting a relay to a contactor may have other letter or digital designations. Most often, their decoding is given below, but the principle always remains the same. You can practice a little by assembling the entire circuit with a consumer in the form of a light bulb or a small motor. Using the test key, you can work out a non-standard situation. The start and stop keys will allow you to check the functionality of the entire circuit. In this case, it is necessary to take into account the type of starter and the normal state of its contacts. If there are any doubts, then it is better to consult an electrician who has experience in assembling such circuits.

Magnetic starter (contactor) is a device designed for switching power electrical circuits. Most often used to start/stop electric motors, but can also be used to control lighting and other power loads.

What is the difference between a contactor and a magnetic starter?

Many readers may have been offended by our definition, in which we (deliberately) mixed the concepts of “magnetic starter” and “contactor”, because in this article we will try to emphasize practice rather than strict theory. But in practice, these two concepts usually merge into one. Few engineers will be able to give a clear answer as to how they really differ. The answers of various experts may agree on some points and contradict each other on others. We present to your attention our version of the answer to this question.

The contactor is a complete device that does not require the installation of additional modules. The magnetic starter can be equipped with additional devices, such as a thermal relay and additional contact groups. A magnetic starter can be called a box with two buttons “Start” and “Stop”. Inside there may be one or two interconnected contactors (or starters) that implement mutual interlocking and reverse.

The magnetic starter is designed to control a three-phase motor, therefore it always has three contacts for switching power lines. In the general case, a contactor may have a different number of power contacts.

The devices in these figures are more correctly called magnetic starters. The device number one suggests the possibility of installing additional modules, for example a thermal relay (Figure 2). In the third figure, a “start-stop” block for controlling the engine with overheating protection and an automatic pick-up circuit. This block device is also called a magnetic starter.

But the devices in the following figures are more correctly called contactors:

They do not require installation of additional modules on them. The device numbered 1 has 4 power contacts, the second device has two power contacts, and the third has three.

In conclusion, we will say: all the above-mentioned differences between a contactor and a magnetic starter are useful to know for general development and to remember just in case, however, you will have to get used to the fact that in practice no one usually separates these devices.

Design and principle of operation of a magnetic starter

The contactor device is somewhat similar to — it also has a coil and a group of contacts. However, the contacts of the magnetic starter are different. Power contacts are designed to switch the load controlled by this contactor; they are always normally open. There are also additional contacts designed to implement starter control (this will be discussed below). Auxiliary contacts can be normally open (NO) or normally closed (NC).

In general, the magnetic starter device looks like this:

When control voltage is applied to the starter coil (usually the coil contacts are designated A1 and A2), the moving part of the armature is attracted to the stationary part and this leads to the closure of the power contacts. Additional contacts (if any) are mechanically connected to the power contacts, therefore, at the moment the contactor is triggered, they also change their state: normally open ones close, and normally closed ones, on the contrary, open.

Magnetic starter connection diagram

This is what the simplest diagram of connecting a motor through a starter looks like. The power contacts of the KM1 magnetic starter are connected to the electric motor terminals. A QF1 circuit breaker is installed in front of the contactor for overload protection. The relay coil (A1-A2) is energized through a normally open “Start” button and a normally closed “Stop” button. When you press the “Start” button, voltage comes to the coil, the contactor is activated, starting the electric motor. To stop the engine, you need to press “Stop” - the coil circuit will break and the contactor will “disconnect” the power lines.

This scheme will only work if the “start” and “stop” buttons are latched.

Instead of buttons, there may be a contact of another relay or a discrete output of the controller:

The contactor can be turned on and off using the PLC. One discrete output of the controller will replace the “start” and “stop” buttons - they will be implemented by the controller logic.

Scheme of “self-recovery” magnetic starter

As already mentioned, the previous scheme with two buttons only works if the buttons are latched. In real life it is not used because of its inconvenience and unsafety. Instead, they use a circuit with automatic pickup (self-pickup).

This circuit uses an additional normally open contact of the starter. When you press the “start” button and the magnetic starter is triggered, the additional contact KM1.1 closes simultaneously with the power contacts. Now the “start” button can be released - it will be “picked up” by contact KM1.1.

Pressing the “stop” button will break the coil circuit and at the same time the additional circuit will open. contact KM1.1.

Connecting the motor via a starter with a thermal relay

The figure shows a magnetic starter with a thermal relay installed on it. When heated, the electric motor begins to consume more current - this is detected by a thermal relay. On the body of the thermal relay, you can set the current value, the excess of which will cause the relay to operate and close its contacts.

The normally closed contact of the thermal relay uses the starter coil in the power circuit and breaks it when the thermal relay is activated, providing an emergency shutdown of the engine. The normally open contact of a thermal relay can be used in a signal circuit, for example, to light an “emergency” lamp when the electric motor is turned off due to overheating.

A reversible magnetic starter is a device with which you can start rotating a motor in forward and reverse directions. This is achieved by changing the phase sequence at the motor terminals. The device consists of two interlocking contactors. One of the contactors switches phases in the order A-B-C, and the other, for example, A-C-B.

Mutual interlocking is necessary so that it is impossible to accidentally turn on both contactors at the same time and create a phase-to-phase short circuit.

The reversing magnetic starter circuit looks like this:

A reversible starter can change the phase sequence on the motor by switching the voltage supplying the motor through the contactor KM1 or KM2. Please note that the phase order of these contactors is different.

When you press the “Direct Start” button, the engine starts through the KM1 contactor. In this case, the additional contact of this starter KM1.2 opens. It blocks the start of the second contactor KM2, so pressing the “Reverse start” button will lead to nothing. In order to start the engine in the opposite (reverse) direction, you must first stop it with the “Stop” button.

When the “Reverse start” button is pressed, contactor KM2 is activated, and its additional contact KM2.2 blocks contactor KM1.

Automatic pickup of contactors KM1 and KM2 is carried out using normally open contacts KM1.1 and KM2.1, respectively (see section “Self-retaining circuit of a magnetic starter”).

Connecting a magnetic starter and its small-sized variants is not difficult for experienced electricians, but for beginners it may be a task that requires some thought.

A magnetic starter is a switching device for remote control of high power loads.
In practice, often, the main application of contactors and magnetic starters is the starting and stopping of asynchronous electric motors, their control and reversal of engine speed.

But such devices also find their use in working with other loads, such as compressors, pumps, heating and lighting devices.

For special safety requirements (high humidity in the room), it is possible to use a starter with a 24 (12) volt coil. And the supply voltage of electrical equipment can be high, for example 380 volts and high current.

In addition to the immediate task of switching and controlling loads with high current, another important feature is the ability to automatically “turn off” the equipment when there is a “loss” of electricity.
A good example. While some machine, such as a sawing machine, was operating, the voltage in the network was lost. The engine stopped. The worker climbed to the working part of the machine, and then the tension appeared again. If the machine was controlled simply by a switch, the engine would immediately turn on, resulting in injury. When controlling the machine's electric motor using a magnetic starter, the machine will not turn on until the "Start" button is pressed.

Magnetic starter connection diagrams

Standard scheme. It is used in cases where it is necessary to carry out normal starting of an electric motor. The "Start" button was pressed - the engine turned on, the "Stop" button was pressed - the engine turned off. Instead of a motor, there can be any load connected to the contacts, for example a powerful heater.

In this circuit, the power section is powered by a three-phase alternating voltage of 380V with phases “A” “B” “C”. In cases of single-phase voltage, only two terminals are used.

The power part includes: a three-pole circuit breaker QF1, three pairs of power contacts of a magnetic starter 1L1-2T1, 3L2-4T2, 5L3-6T3 and a three-phase asynchronous electric motor M.

The control circuit receives power from phase “A”.
The control circuit diagram includes the SB1 “Stop” button, the SB2 “Start” button, the magnetic starter coil KM1 and its auxiliary contact 13NO-14NO, connected in parallel to the “Start” button.

When the QF1 machine is turned on, phases “A”, “B”, “C” go to the upper contacts of the magnetic starter 1L1, 3L2, 5L3 and are on duty there. Phase “A”, which supplies the control circuits, comes through the “Stop” button to the “3” contact of the “Start” button, the auxiliary contact of the starter 13NO and also remains on duty on these two contacts.

note. Depending on the voltage rating of the coil itself and the supply voltage used, there will be a different coil connection diagram.
For example, if the coil of a magnetic starter is 220 volts, one of its terminals is connected to the neutral, and the other, through buttons, to one of the phases.

If the coil rating is 380 volts, one output is to one of the phases, and the second, through a chain of buttons, to the other phase.
There are also 12, 24, 36, 42, 110 volt coils, so before you apply voltage to the coil, you must know exactly its rated operating voltage.

When you press the “Start” button, phase “A” hits the coil of the KM1 starter, the starter is triggered and all its contacts are closed. Voltage appears at the lower power contacts 2T1, 4T2, 6T3 and from them goes to the electric motor. The engine starts to rotate.

You can release the “Start” button and the engine will not turn off, since self-retaining is implemented using the auxiliary contact of the starter 13NO-14NO, connected in parallel to the “Start” button.

It turns out that after releasing the “Start” button, the phase continues to flow to the coil of the magnetic starter, but through its 13NO-14NO pair.

If there is no self-retaining, it will be necessary to keep the “Start” button pressed all the time so that the electric motor or other load runs.

What does the installation (practical) diagram for connecting a magnetic starter look like?

In order not to pull an extra wire to the “Start” button, you can place a jumper between the coil output and one of the nearest auxiliary contacts, in this case these are “A2” and “14NO”. And from the opposite auxiliary contact the wire runs directly to the “3” contact of the “Start” button.

How to connect a magnetic starter in a single-phase network

Electric motor connection diagram with thermal relay and circuit breaker

How to choose a circuit breaker (circuit breaker) to protect the circuit?

First of all, we choose how many “poles”; in a three-phase power supply circuit, a three-pole circuit breaker will naturally be needed, and in a 220 volt network, as a rule, a two-pole circuit breaker will be sufficient, although a single-pole circuit breaker will be sufficient.

The next important parameter will be the operating current.

For example, if the electric motor is 1.5 kW. then its maximum operating current is 3A (real operating current may be less, it must be measured). This means that the three-pole circuit breaker must be set to 3 or 4A.

But we know that the starting current of the engine is much higher than the operating current, which means that a regular (household) automatic machine with a current of 3A will operate immediately when starting such an engine.

The characteristic of the thermal release must be selected D so that the machine does not trip when starting.

Or, if such a machine is not easy to find, you can select the current of the machine so that it is 10-20% greater than the operating current of the electric motor.

You can also go into a practical experiment and use a clamp meter to measure the starting and operating current of a particular motor.

For example, for a 4kW motor, you can install a 10A automatic.

To protect against motor overload, when the current increases above the set value (for example, phase loss), the contacts of the thermal relay RT1 open and the power circuit of the electromagnetic starter coil is broken.

In this case, the thermal relay acts as a “Stop” button, and is in the same circuit, in series. Where to put it is not particularly important, it can be in the section of the L1 - 1 circuit, if it is convenient for installation.

With the use of a thermal release, there is no need to so carefully select the current of the input circuit breaker, since the thermal protection of the motor should be quite adequate.

Connecting an electric motor via a reversing starter

This need arises when it is necessary for the engine to rotate alternately in both directions.

Changing the direction of rotation is implemented in a simple way; any two phases are swapped.

Magnetic starters are most often used to control electric motors. Although it has other areas of application: control of lighting, heating, switching of powerful loads. They can be turned on and off either manually, using control buttons, or using automatic systems. We'll talk about connecting control buttons to a magnetic starter.

Starter control buttons

In general, you will need two buttons: one to turn it on and one to turn it off. Please note that they use contacts with different purposes to control the starter. For the “Stop” button they are normally closed, that is, if the button is not pressed, the group of contacts is closed, and opens when the button is activated. The Start button is the opposite.

These devices can either contain only a specific element needed for operation, or be universal, including one closed and one open contact. In this case, you need to choose the right one.

Manufacturers usually provide their products with symbols that make it possible to determine the purpose of a particular contact group. The stop button is usually painted red. The launcher color is traditionally black, but green is welcome, which corresponds to the “On” or “Turn on” signal. Such buttons are mainly used on cabinet doors and machine control panels.

For remote control, push-button stations are used, containing two buttons in one housing. The station is connected to the starter installation location using a control cable. It must have at least three cores, the cross-section of which may be small. The simplest working circuit of a starter with a thermal relay

Magnetic switch

Now about what you should pay attention to when examining the starter itself before connecting it. The most important thing is the voltage of the control coil, which is indicated either on it itself or nearby. If the inscription reads 220 V AC (or there is an AC icon next to 220), then a phase and a zero are required for the control circuit to operate.

Watch an interesting video about the operation of a magnetic starter below:

If it is 380 V AC (the same alternating current), then the starter will be controlled by two phases. In the process of describing the operation of the control circuit, it will become clear what the difference is.

With any other voltage values, the presence of a direct current sign or the letters DC, it will not be possible to connect the product to the network. It is intended for other circuits.

We will also need to use an additional contact of the starter, called a block contact. For most devices, it is marked with the numbers 13NO (13NO, simply 13) and 14NO (14NO, 14).

The letters NO mean “normally open”, that is, it closes only when the starter is pulled in, which can be checked with a multimeter if desired. There are starters that have normally closed additional contacts; they are not suitable for the control circuit under consideration.

Power contacts are designed to connect the load, which they control.

Their markings vary from manufacturer to manufacturer, but there are no difficulties in identifying them. So, we attach the starter to the surface or DIN rail in the place of its permanent location, lay the power and control cables, and begin the connection.

220 V starter control circuit

One wise man said: there are 44 schemes for connecting buttons to a magnetic starter, of which 3 work, and the rest do not. But there is only one correct one. Let's talk about it (see diagram below).
It is better to leave connecting the power circuits for later. This will make it easier to access the coil screws, which are always covered by the main circuit wires. To power the control circuits, we use one of the phase contacts, from which we send a conductor to one of the terminals of the “Stop” button.

This can be either a conductor or a cable core.

Two wires will go from the stop button: one to the “Start” button, the second to the block contact of the starter.

To do this, a jumper is placed between the buttons, and a cable core to the starter is added to one of them at the point where it is connected. There are also two wires from the second terminal of the “Start” button: one to the second terminal of the block contact, the second to terminal “A1” of the control coil.

When connecting buttons with a cable, the jumper is already placed on the starter, and the third core is connected to it. The second output from the coil (A2) is connected to the zero terminal. In principle, there is no difference in what order you connect the outputs of the buttons and the block contact. It is advisable to connect only the “A2” terminal of the control coil to the neutral conductor. Any electrician expects that zero potential will only be there.

Now you can connect the wires or cables of the power circuit, not forgetting that next to one of them at the input there is a wire to the control circuit. And only from this side is power supplied to the starter (traditionally - from above). Trying to connect buttons to the starter output will lead to nothing.

380V Starter Control Circuit

Everything is the same, but in order for the coil to work, the conductor from terminal “A2” must be connected not to the zero bus, but to any other phase that has not been used before. The entire circuit will operate from two phases.

Connecting a thermal relay to the starter circuit

Thermal relay is used for overload protection. Of course, it is still protected by an automatic switch, but its thermal element is not enough for this purpose. And it cannot be adjusted exactly to the rated current of the motor. The operating principle of a thermal relay is the same as in a circuit breaker.

The current passes through the heating elements; if its value exceeds the specified value, the bimetallic plate bends and switches the contacts.

This is another difference from a circuit breaker: the thermal relay itself does not turn off anything. It simply gives a signal to turn off. Which needs to be used correctly.
The power contacts of the thermal relay allow you to connect it to the starter directly, without wires. To achieve this, each product range complements each other. For example, IEK produces thermal relays for its starters, ABB produces its own. And so it is with every manufacturer. But products from different companies do not fit together.

Thermal relays can also have two independent contacts: normally closed and normally open. We will need a closed one - as in the case of the “Stop” button. Moreover, functionally it will work the same way as this button: breaking the power supply circuit of the starter coil so that it falls off.

Now you need to embed the found contacts into the control circuit. In theory this can be done almost anywhere, but traditionally it is connected after the coil.

In the case described above, this will require sending a wire from pin “A2” to the contact of the thermal relay, and from its second contact to where the conductor was previously connected. In the case of control from 220 V, this is the zero bus; with 380 V, this is the phase on the starter. The thermal relay is not noticeable in most models.

To return it to its original state, there is a small button on the instrument panel that resets when pressed. But this should not be done immediately, but let the relay cool down, otherwise the contacts will not engage. Before putting it into operation after installation, it is better to press the button, eliminating possible switching of the contact system during transportation due to shaking and vibration.

Another interesting video about the operation of a magnetic starter:

Checking the functionality of the circuit

In order to understand whether the circuit is assembled correctly or not, it is better not to connect the load to the starter, leaving its lower power terminals free. This way you will protect your switched equipment from unnecessary problems. We turn on the circuit breaker that supplies voltage to the object under test.

It goes without saying that it must be turned off while editing is in progress. And also, in any available way, accidental activation by unauthorized persons is prevented. If after applying voltage the starter does not turn on on its own, that’s good.

Press the “Start” button, the starter should turn on. If not, check the closed position of the “Stop” button contacts and the state of the thermal relay.

When diagnosing a malfunction, a single-pole voltage indicator helps, which can easily check the passage of a phase through the “Stop” button to the “Start” button. If, when you release the “Start” button, the starter does not lock and falls away, the block contacts are incorrectly connected.

Check - they should be connected parallel to this button. A correctly connected starter should be locked in the on position when mechanically pressing on the moving part of the magnetic circuit.

Now we check the operation of the thermal relay. Turn on the starter and carefully disconnect any wiring from the relay contacts. The starter should fall off.