Every handyman has a couple of ideas to build some kind of machine, sharpening, lathe or lift. Today we’ll talk about an important element of the electric drive - a thermal relay, which is also called a current relay or a heat relay. This device reacts to the amount of current passing through it and, if the set value is exceeded, switches contacts, turning off the drive or signaling an emergency situation. In one of our articles, we already looked at the types of hot water heaters and the principle of their operation, as well as the parameters by which it occurs. In this article we will look at how to install and connect a thermal relay with your own hands. The instructions will be provided with diagrams, photos and video examples so that you understand all the nuances of installation.
What is important to know?
To avoid repetition and to avoid piling up unnecessary text, I will briefly outline the meaning. The current relay is a mandatory attribute of the electric drive control system. This device responds to the current that passes through it to the engine. It does not protect the electric motor from a short circuit, but only protects it from working with increased current that occurs during or abnormal operation of the mechanism (for example, wedge, jamming, rubbing and other unforeseen moments).
When choosing a thermal relay, they are guided by the passport data of the electric motor, which can be taken from the plate on its body, as in the photo below:
As can be seen on the tag, the rated current of the electric motor is 13.6 / 7.8 Amperes, for voltages of 220 and 380 Volts. According to the operating rules, the thermal relay must be selected 10-20% more than the nominal parameter. The correct choice of this criterion determines the ability of the heater to operate on time and prevent damage to the electric drive. When calculating the installation current for the 7.8 A rating given on the label, we got a result of 9.4 Amperes for the current setting of the device.
When choosing a product in the catalog, you need to take into account that this nominal value was not extreme on the setpoint adjustment scale, so it is advisable to select a value closer to the center of the adjustable parameters. For example, as on the RTI-1314 relay:
Installation features
As a rule, the installation of a thermal relay is carried out together with, which switches and starts the electric drive. However, there are also devices that can be installed as a separate device side by side on a mounting panel or, such as TRN and PTT. It all depends on the availability of the required denomination in the nearest store, warehouse or garage in “strategic reserves”.
The presence of only two incoming connections for the TRN thermal relay should not scare you, since there are three phases. The unconnected phase wire goes from the starter to the motor, bypassing the relay. The current in the electric motor changes proportionally in all three phases, so it is enough to control any two of them. The assembled structure, the starter with the TRN heater will look like this: 
Or like this with RTT:
The relays are equipped with two groups of contacts, a normally closed and a normally open group, which are labeled on the body 96-95, 97-98. The picture below shows a block diagram of the designation according to GOST: 
Let's figure out how to assemble a control circuit that would disconnect the motor from the network in the event of an overload or phase loss emergency. From our article about, you have already learned some nuances. If you haven’t had a chance to check it out yet, just follow the link.
Let's consider the diagram from the article in which a three-phase motor rotates in one direction and the switching control is carried out from one place with two STOP and START buttons.
The machine is turned on and voltage is supplied to the upper terminals of the starter. After pressing the START button, the starter coil A1 and A2 is connected to the network L2 and L3. This circuit uses a starter with a 380-volt coil; look for a connection option with a single-phase 220-volt coil in our separate article (link above).
The coil turns on the starter and additional contacts No(13) and No(14) are closed, now you can release START, the contactor will remain on. This scheme is called “self-retaining start”. Now, in order to disconnect the engine from the network, you need to de-energize the coil. Having traced the current path according to the diagram, we see that this can happen when STOP is pressed or the contacts of the thermal relay are opened (highlighted by a red rectangle).
That is, if an emergency situation arises and the heater operates, it will break the circuit circuit and remove the starter from self-retaining, de-energizing the engine from the mains. When this current control device is triggered, before restarting it is necessary to inspect the mechanism to determine the cause of the shutdown, and not turn it on until it is eliminated. Often the cause of operation is high external ambient temperature; this point must be taken into account when operating the mechanisms and setting them up.
The scope of application of thermal relays in the household is not limited only to homemade machines and other mechanisms. It would be correct to use them in a heating system pump current control system. The specificity of the operation of the circulation pump is that limescale deposits form on the blades and scroll, which can cause the motor to jam and fail. Using the above connection diagrams, you can assemble a pump control and protection unit. It is enough to set the required rating of the heater in the power circuit and connect the contacts.
In addition, it will be interesting to see a diagram for connecting a thermal relay through current transformers for powerful motors, such as a pump for a water irrigation system for holiday villages or farms. When installing transformers in the power circuit, the transformation ratio is taken into account, for example, 60/5 is when the current through the primary winding is 60 amperes, on the secondary winding it will be equal to 5A. The use of such a scheme allows you to save on components without losing performance characteristics.
As you can see, the current transformers are highlighted in red, which are connected to the control relay and ammeter for visual clarity of the processes taking place. The transformers are connected in a star circuit, with one common point. Such a scheme does not pose any great difficulties in implementation, so you can assemble it yourself and connect it to the network.
That's all you need to know about connecting a thermal relay with your own hands. As you can see, installation is not particularly difficult, the main thing is to correctly draw up a diagram for connecting all the elements in the circuit!
Connection diagram for magnetic starter and thermal relay
A magnetic starter is a special installation that is used to remotely start and control the operation of an asynchronous electric motor. This device is characterized by a simple design, which allows the connection to be made by a technician without relevant experience.
Carrying out preparatory work
Before connecting the thermal relay and the magnetic section, you must remember that you are working with an electrical device. That is why, in order to protect yourself from electric shock, you need to de-energize the area and check it. For this purpose, most often, a special indicator screwdriver is used.
The next stage of preparatory work is to determine the operating voltage of the coil. Depending on the manufacturer of the device, you can see the indicators on the body or on the reel itself.
Important! The operating voltage of the coil can be 220 or 380 Volts. If you have the first indicator, you need to know that phase and zero are supplied to its contacts. In the second case, this means the presence of two opposite phases.
The stage of correctly identifying the coil is quite important when connecting a magnetic starter. Otherwise, it may burn out while the device is operating.
To connect this equipment you must use two buttons:
The first of them can be black or green. This button is characterized by permanently open contacts. The second button is red and has permanently closed contacts.
When connecting a thermal relay, it is necessary to remember that the phases are switched on and off using power contacts. The zeros that approach and depart, as well as the conductors that ground, must be connected to each other in the terminal block area. In this case, the starter must be removed. These devices are not switched.
In order to connect a coil whose operating voltage is 220 Volts, you need to take a zero from the terminal block and connect it to the circuit that is intended for the operation of the starter.
Features of connecting magnetic starters
The magnetic starter circuit is characterized by the presence of:
- three pairs of contacts, through which power is supplied to electrical equipment;
- Control circuit, which includes a coil, additional contacts and buttons. With the help of additional contacts, the operability of the coil is supported, as well as the blocking of erroneous activations.
Attention. The most commonly used circuit is one that requires the use of one starter. This is explained by its simplicity, which allows even an inexperienced master to cope with it.
To assemble a magnetic starter, you need to use a three-core cable that is connected to the buttons, as well as one pair of contacts that are well open.
When using a 220 Volt coil, it is necessary to connect red or black wires. When using a 380 Volt coil, an opposite phase is used. The fourth free pair in this circuit is used as a block contact. Three pairs of power contacts are connected along with this free pair. All conductors are located at the top. If there are two additional conductors, they are placed on the side.
The power contacts of the starter are characterized by the presence of three phases. To turn them on when you press the Start button, you need to apply voltage to the coil. This will allow the circuit to close. To open the circuit it is necessary to disconnect the coil. To assemble the control circuit, the green phase is directly connected to the coil.
Important. In this case, it is necessary to connect the wire that comes from the coil contact to the Start button. A jumper is also made from it, which goes to the closed contact of the Stop button.
The magnetic starter is turned on using the Start button, which closes the circuit, and turned off using the Stop button, which opens the circuit.
Features of connecting a thermal relay
A thermal relay is located between the magnetic starter and the electric motor. Its connection is made to the output of the magnetic starter. Electric current passes through this device. The thermal relay is characterized by the presence of additional contacts. They must be connected in series with the starter coil.
A thermal relay is characterized by the presence of special heaters through which an electric current of a certain magnitude can pass. If dangerous situations arise (current increases above the specified limits), due to the presence of bimetallic contacts, the circuit is broken and subsequently the starter is turned off. In order to start the mechanism, you need to turn on the bimetallic contacts using the button.
Attention. When connecting a thermal relay, it is necessary to take into account the presence of a current regulator on it, which operates within small limits.
Connecting an electromagnetic starter and a thermal relay is quite simple. To do this, you just need to follow the scheme.
A magnetic starter is, in fact, a powerful special-purpose relay. It is designed for switching in electrical circuits with the windings of asynchronous motors. This device does not require special knowledge in order to connect it and use it yourself. A thermal relay is another special design of an electromechanical device. It, paired with a magnetic starter, performs switching in electrical circuits that contain the windings of asynchronous motors.
Installation features
But at the same time, the thermal relay is triggered, unlike a magnetic starter, not by the will of a person, but by an overcurrent of an asynchronous motor. You can also use it with your own hands in the control circuit of an asynchronous engine without any problems. In this regard, it would not be superfluous to remind craftsmen that any work on connecting electrical circuits to the network must begin with a guaranteed disconnection of voltage at the connection point, followed by monitoring this with an indicator screwdriver or tester.
- In order to correctly connect a magnetic starter with a thermal relay, you must first determine the voltage for which they are designed. Its value is indicated both in the technical data sheet and on the nameplate located on the device body.
- If the voltage is 220 V, the device must be connected to phase voltage, that is, to the phase and neutral wires. If a voltage of 380 V is indicated, linear voltage is used for connection, that is, to the phase wires of any two phases.
- If the voltage does not correspond to the device's rating data, it may either be damaged by overheating or malfunction due to an insufficiently strong magnetic field in the control coil.
A special feature of the operation of a magnetic starter is its contact, which, when closed, bypasses the power button of its control coil. This allows you to switch electrical circuits by briefly pressing the “start” button, which is convenient and easy for the user. When connecting the starter, you will need to connect a normally open contact and a normally closed contact. Their appearance in the device itself and on the electrical circuit is shown in the image. They are used to control the starter coil and are located in the starter control unit. It's called a "button post". It has two buttons. Each of them actuates: one normally closed contact and one normally open contact. The buttons are usually painted black (used to start or reverse), and red (used to stop the engine by disconnecting the starter coil).
Circuit with phase voltage (220 V)
The voltage to power the control circuit of the KM1 coil of the magnetic starter comes from phase L3 and neutral N. The button contacts for controlling the operation of the coil are connected in series. This allows contact SB2, actuated by the “start” button, to close the electrical circuit. The coil will activate the KM1 contacts and they will close the circuit with the motor windings. Voltage will appear on the motor windings, and its shaft will begin to rotate. Stopping the engine is possible either when the thermal relay is activated, or by pressing the “stop” button, which opens the circuit of the KM1 coil. 
Contact P of the thermal relay opens due to heating of a special element located in it. As the current increases, the heating of this element also increases. The thermal relay passes the current of one of the motor phases through each pair of its terminals. In this case, a corresponding heating element is associated with each pair of terminals. When a given temperature is reached, which corresponds to a given electrical power, the KM1 coil is de-energized from the mechanical action of the heated element by contact P. Temperature deformation of elements is achieved by using bimetallic materials.
The KM1 contacts open the electrical circuits with the windings of the asynchronous motor, which then stops. Structurally, different models of thermal relays may differ from each other in the design of the main six terminals, the design of heating elements, contacts and additional regulators. Therefore, when installing thermal relays, it is necessary to connect and configure them in accordance with the technical data sheet and accompanying documentation.
As can be seen from the diagram, the voltage for the electrical circuit of the KM1 coil is obtained from two phase wires L2 and L3. The voltage between them for a three-phase electrical network is 380 V. There are no other differences, both in the connections of the circuit elements and in its operation in comparison with a circuit with phase voltage.
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.
Normally open and normally closed contacts 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).
Connection diagram of the TP to the contactor in the magnetic starter 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.
Thermal relay in the circuit of reversible connection of contactors 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.
Re-platoon control 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.
Adjusting the response setting relative to the mark 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.
Time-current characteristics graphs 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,
Settings protection and marking 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.
Thermal relay TRN with current control in only two phases 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.
Connecting a thermal relay to a 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.
Adjusting the terminals for the contactor terminals For additional fixation of the TP, corresponding protrusions are provided on the device itself and on the contactor.
Fastening element on the thermal relay housing 
Special mounting groove 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.
TR disassembled 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.
Bending bimetallic strip 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.
Leverage If you remove the levers, the contact groups of the thermal relay will be visible.
Switching node TR 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.
The connection diagram for a magnetic starter seems complicated at first glance, but dealing with such a device will not be difficult if you follow the installation rules and recommendations.
At its core, a magnetic starter (push-button or contactless) is a device that can be classified as a type of electromagnetic contact that allows it to cope with current loads.
It works during constant switching on and off of circuits.
With the connection of a magnetic starter, it becomes possible to remotely control the start, stop and general operation of a three-phase electric motor.
However, such a relay is so unpretentious that it allows you to control other mechanisms: lighting, compressors, pumps, taps, a thermal heater or furnace, and air conditioners.
When buying such a mechanism, pay attention: after all, a push-button magnetic starter is not much different from a modern contactor.
Their functions are almost identical, so there should not be any special difficulties when connecting.
The principle of operation of the circuit is quite simple. Voltage is applied to the starter coil, after which a magnetic field appears in it.
It is due to this that the metal core is drawn into the coil.
We attach power contacts to the core, which close when activated, allowing current to flow freely through the wires.
The magnetic starter circuit contains a post where buttons are installed that activate the starting and stopping mechanisms.
How does the starter mechanism work?
Before connecting a magnetic starter, you need to understand its configuration diagram: it includes the device itself and a post (block) with the most important contacts.
Although it is not included in the main part of the relay circuit, when working in a circuit with additional wire elements, for example, with a motor reverser, it is necessary to provide branching of the wires.
This is where a block is needed, which is also called a contact-type attachment to the circuit.
Inside such a set-top box there is a contact circuit connected, which is tightly connected to the conventional contact system of a magnetic starter.
Such a mechanism for a three-phase motor, for example, consists of two pairs of closed and two pairs of open contacts.
To remove the blocking component (during repair or connection), it is enough to move away the special slides holding the cover.
The diagram consists of two parts: upper and lower. The push-button mechanism for a three-phase motor is easy to distinguish by color. For example, the Stop button is red.
It has a break contact connected through which voltage will pass into the circuit. The button that will be responsible for launching is colored green.
It uses a normally open contact that, when connected, conducts electrical current through the circuit.
The connection diagram for a reversing magnetic starter is usually protected against accidental pressing.
To do this, additional side contacts are installed, where when one is triggered, the second will be blocked.
The wiring diagram is performed in a couple of steps, but in practice it turns out to be a convenient push-button mechanism.
Device connection diagram
Before the magnetic starter circuit is connected, you must:
- Ensure de-energization on the entire front of our work (de-energization of the engine, parts of the wiring). You can check the absence of voltage using special indicator tools, the simplest of which is a screwdriver, sold in any hardware store;
- Find out the operating voltage, this is especially true for the coil element. It is written not on the starter packaging itself, but directly on the device. There are only two options: 380v or 220 volts. When we choose 220 volts and not 380v, then when connecting a photo relay, phase and zero are supplied to the coil. If we are talking about 380V, and not 229, then we use two opposite phases. If you do not understand between 220 and 380 volt relays, then the circuit may simply burn out from the voltage difference;
- We select the appropriate buttons of the corresponding colors;
- For a relay, all zeros, which are incoming and outgoing, as well as elements that make it possible to achieve grounding, are connected in the circuit on the terminal block through the device without touching it. For a 220 volt coil, zero is taken during connection, which should not be done for 380 volts.
The connection sequence consists of the following parts:
- three pairs of power elements that will be responsible for supplying power, be it an electric motor circuit or any device;
- control circuit, including a coil, additional wires and buttons.
The simplest process is considered to be connecting a reversing magnetic starter in the amount of one unit. This is the simplest circuit (220 or 380 volts), most often it is used in engine operation.
For a photo relay we need a three-wire wire, which we will connect to the buttons, as well as a pair of open contacts.
Consider a typical 220 volt connection diagram. If you have chosen a 380 volt connection scheme, then instead of the blue zero it is important to connect another opposite phase.
The photo relay contact post is the fourth free phase. Three phases go to the power contacts through the circuit.
So that they can be connected normally, we supply 220 volts to the coil (or 380, depending on the choice of relay). The circuit will close and we will be able to control the operation of the electric motor.
Connecting the thermal relay
A thermal relay can be placed between the magnetic starter and the motor device, which may be needed to safely supply current to the motor device.
Why do you need to connect a thermal relay? It doesn’t matter what voltage is in our circuit, 220 or 380 volts: during surges, any motor can burn out. That is why it is worth setting up a post for protection.
The photo relay allows the circuit to operate even if one of the phases has burned out.
Connect a photo relay at the output of the magnetic starter to the motor device. Then a current of 220 or 380 volts passes through the post from the photorelay heater and gets inside the engine.
On the photo relay itself you can find contacts that should be connected to the coil.
Thermal relay heaters (photo relays) do not last forever and have their own operating limits.
Thus, the post of such a magnetic starter will be able to pass through itself only a certain current indicator, which may have a maximum limit.
Otherwise, the consequences of the photo relay operation for the engine will be disastrous - despite the protective post, it will burn out.
If an unpleasant situation arises when a current above the specified limits is passed through the post, then the heaters begin to act on the contacts, disrupting the general circuit in the device.
As a result, the starter turns off.
When choosing a photo relay for a motor, pay attention to its characteristics. The current of the mechanism must be suitable for the engine power (designed for 220 or 380 volts).
It is not recommended to install such a protective post on ordinary devices - only on motors.
How to choose the right magnetic starter?
To prevent the device from burning out after connecting it after a couple of weeks, you need to be careful about your choice. The most popular starter series are PML and PM12.
They are supplied by both domestic and foreign companies.
Each digit of the value indicates the current that the post can conduct through the circuit without breakdowns or fires. If the load current is higher than 63 A, then it is better to buy contactors to connect to the circuit.
An important characteristic when connecting is the wear resistance class. It shows how many times the device can be pressed without difficulty.
An important indicator if the mechanism has to be turned on and off frequently. If there are a lot of clicks per hour, then contactless starters are chosen.
In addition, devices can be sold with or without reverses. Used for reversible engines, where rotation occurs in two directions at once.
A starter of this type has two coils and two pairs of power contacts. Additional elements include a security mechanism, a light bulb, and buttons.