All or at least most of the starting schemes for asynchronous electric motors, which are used very widely both in industry and in everyday life, are based on a very simple circuit. The bad electrician is the one who doesn't know it.

So, the entire circuit, except for the electric motor, which is installed directly on a specific equipment or device, is mounted either in a panel or in a special box (PML).

The START and STOP buttons can be located either on the front side of this panel or outside it (mounted in a place where it is convenient to control the operation), or maybe both, depending on convenience. Three-phase voltage is supplied to this panel from the nearest power supply point (as a rule, from the distribution board), and from it a cable goes to the electric motor itself.

And now about the principle of operation. Terminals F1, F2, F3 are supplied three phase voltage. To start an asynchronous electric motor, the magnetic starter (PM) requires activation and the closure of its contacts PM1, PM2 and PM3. To trigger the PM, voltage must be applied to its winding. By the way, its value depends on the coil itself, that is, on what voltage it is designed for. It also depends on the conditions and location of the equipment. Coils are available in 380, 220, 110, 36, 24 and 12 V). This circuit is designed for a voltage of 220 V, since it is taken from one of the existing phases and zero.

Power is supplied to the coil of the magnetic starter through such a circuit. From f1, the phase enters the normally closed contact of the thermal protection of the electric motor TP1, then passes through the coil of the starter itself and goes to the START button (KN1) and to the self-retaining contact PM4 (magnetic starter). From them, power goes to the normally closed STOP button and then closes to zero.

To start, you need to press the START button, after which the magnetic starter coil circuit will close and attract (close) contacts PM1-3 (to start the engine) and contact PM4, which will make it possible to continue working when the start button is released and not turn off magnetic starter(called self-recovery). To stop the electric motor, you just need to press the STOP button (KN2) and thereby break the power supply circuit of the PM coil. As a result, contacts PM1-3 and PM4 will turn off and operation will be stopped until the next Start.

For protection, they must be installed (in our diagram this is TP). When the electric motor is overloaded, the current increases and the motor begins to heat up sharply, until it fails. This protection is triggered precisely when the current in the phases increases, thereby opening its contacts TP1, which is similar to pressing the STOP button.

These cases occur mainly when the mechanical part is completely jammed or when there is a large mechanical overload in the equipment on which the electric motor operates. Although often the cause is the engine itself, due to dried bearings, poor winding, mechanical damage, etc.

A simplified version of the above starter circuit is used to start electric motors operating in the same mode, i.e., without changing rotation (pumps, circular drives, fans). But for equipment that must operate in two directions (beam cranes, hoists, winches, opening and closing gates, etc.), a different electrical circuit is required.

For such a scheme, we will need not one, but two identical starters and a three-button START-STOP button, i.e. two START buttons and one STOP. In reverse circuits, remote controls with two buttons can also be used, in areas where operating intervals are very short. For example, for a small winch with operating intervals of 3-10 seconds. For the operation of this equipment, the option with two buttons is more suitable, but both buttons are start buttons, i.e., only with normally open contacts, and the self-retaining block contacts (pm1 and pm2) are not used in the circuit. As long as you keep the button pressed, the equipment works; when you release the button, the equipment stops. Otherwise, the reverse circuit is similar to the simplified version circuit.

Switching the motor from star to delta is used for protection electrical circuits from overloads. Mostly powerful three-phase asynchronous motors from 30-50 kW and high-speed ~3000 rpm, sometimes 1500 rpm, are switched from star to delta.

If the engine is connected in a star, then a voltage of 220 Volts is supplied to each of its windings, and if the engine is connected in a triangle, then a voltage of 380 Volts is supplied to each of its windings. Here Ohm's law I=U/R comes into play: the higher the voltage, the higher the current, but the resistance does not change.

Simply put, when connected to a delta (380), the current will be higher than when connected to a star (220).

When the electric motor accelerates and reaches full speed, the picture completely changes. The fact is that the engine has power that does not depend on whether it is connected to a star or a triangle. Engine power depends largely on the iron and wire cross-section. Another law of electrical engineering applies here: W=I*U.

Power is equal to current times voltage, meaning the higher the voltage, the lower the current. When connected to a delta (380), the current will be lower than to a star (220). In the motor, the ends of the windings are brought out to the “terminal block” in such a way that, depending on how you place the jumpers, you get a star or delta connection. This diagram is usually drawn on the lid. In order to switch from star to delta, we will use contacts instead of jumpers.

Connection diagram for a three-phase asynchronous motor, in the starting position of which the stator windings are connected by a star, and in the operating position by a triangle.

There are six ends suitable for the engine. The KM magnetic starter is used to turn the motor on and off. The contacts of the magnetic starter KM1 work as jumpers to turn on the asynchronous motor in a triangle. Please note that the wires from the motor terminal block must be connected in the same order as in the motor itself. The main thing is not to confuse.

The KM2 magnetic starter connects jumpers for star connection to one half of the terminal block, and voltage is supplied to the other half.

When you press the “START” button, power is supplied to the KM magnetic starter. It is triggered and voltage is supplied to it through the block contact. The button can now be released. Next, voltage is applied to the radio, it counts down the set time. Also, voltage is supplied through the closed contact of the time relay to the magnetic starter KM2, and the engine starts in the “star”.

After the set time, the RT time relay is activated. Magnetic starter P3 is turned off. The voltage is supplied through the time relay contact to the normally closed (closed in the off position) block contact of the magnetic starter KM2, and from there to the coil of the magnetic starter KM1. The electric motor is connected to a triangle.

The KM2 starter should also be connected through a normally closed contact block of the KM1 starter to protect against simultaneous activation of the starters.

It is better to take double magnetic starters KM1 and KM2 with a mechanical lock for simultaneous activation.

The “STOP” button turns off the circuit.

The scheme consists of:

Automatic switch.
Three magnetic starters KM, KM1, KM2.
Start - stop button; - Current transformers TT1, TT2; - Current relay RT; - Time relay RV.
BKM, BKM1, BKM2 are block contacts of their starter.

It is better to supply power to electric motors through magnetic starters (also called contactors). Firstly, they provide protection against inrush currents. Secondly, the normal connection diagram of a magnetic starter contains controls (buttons) and protection (thermal relays, self-retaining circuits, electrical interlocks, etc.). Using these devices, you can start the engine in the opposite direction (reverse) by pressing the corresponding button. All this is organized using diagrams, and they are not very complicated and can be assembled independently.

Magnetic starters are built into power networks to supply and disconnect power. They can work with alternating or direct voltage. The work is based on the phenomenon of electromagnetic induction; there are working (power is supplied through them) and auxiliary (signal) contacts. For ease of use, Stop, Start, Forward, Back buttons are added to the magnetic starter switching circuits.

Magnetic starters can be of two types:

With normally closed contacts. Power is supplied to the load constantly and is turned off only when the starter is triggered.
With normally open contacts. Power is supplied only while the starter is running.

The second type is more widely used - with normally open contacts. After all, basically, devices should work for a short period of time, the rest of the time they should be at rest. Therefore, next we will consider the principle of operation of a magnetic starter with normally open contacts.

Composition and purpose of parts

The basis of a magnetic starter is an inductance coil and a magnetic circuit. The magnetic core is divided into two parts. Both of them have the shape of the letter “W”, installed in a mirror image. The lower part is motionless, it middle part is the core of the inductor. The parameters of the magnetic starter (the maximum voltage with which it can operate) depend on the inductor. There may be starters of small ratings - 12 V, 24 V, 110 V, and the most common - 220 V and 380 V.

The upper part of the magnetic circuit is movable, with movable contacts attached to it. The load is connected to them. The fixed contacts are fixed to the starter body and are supplied with supply voltage. In the initial state, the contacts are open (due to the elastic force of the spring that holds the upper part of the magnetic circuit), power is not supplied to the load.

Operating principle

In the normal state, the spring lifts the upper part of the magnetic circuit, the contacts are open. When power is applied to a magnetic starter, the current flowing through the inductor generates an electromagnetic field. Compressing the spring, it attracts the moving part of the magnetic circuit, the contacts close (the picture on the right). Through closed contacts, power is supplied to the load, it is in operation.

When the power to the magnetic starter is turned off, the electromagnetic field disappears, the spring pushes the upper part of the magnetic circuit up, the contacts open, and power is not supplied to the load.

You can supply alternating or constant voltage. Only its size is important - it should not exceed the nominal value specified by the manufacturer. For alternating voltage the maximum is 600 V, for direct voltage - 440 V.

Connection diagram for a starter with a 220 V coil

In any magnetic starter connection diagram there are two circuits. One power line through which power is supplied. The second is a signal one. This circuit controls the operation of the device. They need to be considered separately - it’s easier to understand the logic.

At the top of the magnetic starter housing there are contacts to which the power for this device is connected. The usual designation is A1 and A2. If the coil is 220 V, 220 V is supplied here. It makes no difference where to connect “zero” and “phase”. But more often the “phase” is supplied to A2, since here this output is usually duplicated in the lower part of the case and quite often it is more convenient to connect here.

Below on the case there are several contacts labeled L1, L2, L3. The power supply for the load is connected here. Its type is not important (constant or alternating), it is important that the rating is not higher than 220 V. Thus, voltage from a battery, wind generator, etc. can be supplied through a starter with a 220 V coil. It is removed from contacts T1, T2, T3.

The simplest scheme

If you connect a power cord (control circuit) to pins A1 - A2, apply 12 V voltage from the battery to L1 and L3, and lighting devices (power circuit) to pins T1 and T3, we get a lighting circuit operating on 12 V. This is only one of the options for using a magnetic starter.

But more often, these devices are used to supply power to electric motors. In this case, 220 V is also connected to L1 and L3 (and the same 220 V is removed from T1 and T3).

The simplest diagram for connecting a magnetic starter - without buttons

The disadvantage of this scheme is obvious: to turn the power off and on, you will have to manipulate the plug - remove/insert it into the socket. The situation can be improved if you install an automatic machine in front of the starter and turn on/off the power supply to the control circuit with its help. The second option is to add buttons to the control circuit - Start and Stop.

Diagram with “Start” and “Stop” buttons

When connected via buttons, only the control circuit changes. The strength remains unchanged. The entire connection diagram of the magnetic starter changes slightly.

The buttons can be in a separate case, or in one. In the second version, the device is called a “push-button post”. Each button has two inputs and two outputs. The “start” button has normally open contacts (power is supplied when it is pressed), the “stop” button has normally closed contacts (the circuit breaks when pressed).

Connection diagram of a magnetic starter with “start” and “stop” buttons

Buttons are built in series in front of the magnetic starter. First - “start”, then - “stop”. Obviously, with such a connection scheme for a magnetic starter, the load will only work while the “start” button is held down. As soon as she is released, the food will disappear. Actually, in this version the “stop” button is superfluous. This is not the mode that is required in most cases. It is necessary that after releasing the start button, power continues to flow until the circuit is broken by pressing the stop button.

Connection diagram of a magnetic starter with a self-recharging circuit - after closing the contact shunting the “Start” button, the coil becomes self-feeding

This operating algorithm is implemented using auxiliary contacts of the starter NO13 and NO14. They are connected in parallel with the start button. In this case, everything works as it should: after releasing the “start” button, power flows through the auxiliary contacts. Stop the load operation by pressing “stop”, the circuit returns to the operating state.

Connection to a three-phase network via a contactor with a 220 V coil

Through a standard magnetic starter operating from 220 V, three-phase power can be connected. This magnetic starter connection diagram is used with asynchronous motors. There are no differences in the control circuit. One of the phases and “zero” are connected to contacts A1 and A2. The phase wire goes through the “start” and “stop” buttons, and a jumper is also placed on NO13 and NO14.

The differences in the power circuit are minor. All three phases are supplied to L1, L2, L3, and a three-phase load is connected to outputs T1, T2, T3. In the case of a motor, a thermal relay (P) is often added to the circuit, which will prevent the motor from overheating. The thermal relay is placed in front of the electric motor. It controls the temperature of two phases (placed on the most loaded phases, the third), opening the power circuit when critical temperatures are reached. This magnetic starter connection diagram is used often and has been tested many times. See the following video for assembly procedure.

Reverse motor connection diagram

Some devices require the motor to rotate in both directions to operate. The direction of rotation changes when the phases are transferred (two arbitrary phases must be swapped). The control circuit also requires a push-button station (or separate buttons) “stop”, “forward”, “backward”.

The connection diagram for a magnetic starter for engine reversal is assembled on two identical devices. It is advisable to find ones that have a pair of normally closed contacts. The devices are connected in parallel - to reverse the rotation of the motor, the phases on one of the starters are swapped. The outputs of both are fed to the load.

Signal circuits are somewhat more complex. The “stop” button is general. Next to it is a “forward” button, which connects to one of the starters, and a “back” button to the second. Each of the buttons must have bypass circuits (“self-catch”) so that there is no need to keep one of the buttons pressed all the time (jumpers are installed on NO13 and NO14 on each of the starters).

To avoid the possibility of power being supplied through both buttons, an electrical interlock is implemented. To do this, after the “forward” button, power is supplied to the normally closed contacts of the second contactor. The second contactor is connected in the same way - through the normally closed contacts of the first.

If the magnetic starter does not have normally closed contacts, they can be added by installing an attachment. When installed, the attachments are connected to the main unit and their contacts work simultaneously with others. That is, while power is supplied through the “forward” button, an open normally closed contact will not allow reverse motion to be activated. To change direction, press the “stop” button, after which you can turn on the reverse by pressing “back”. The reverse switching occurs in the same way - through “stop”.

MAGNETIC STARTER CONNECTION DIAGRAM

Before we begin the practical connection of the starter, let us recall a useful theory: the magnetic starter contactor is turned on by a control pulse emanating from pressing the start button, which supplies voltage to the control coil. Keeping the contactor in the on state occurs according to the self-retaining principle - when an additional contact is connected in parallel with the start button, thereby supplying voltage to the coil, as a result of which there is no need to hold the start button pressed.

Disabling the magnetic starter in this case is possible only if the control coil circuit is broken, which makes it obvious that it is necessary to use a button with a break contact. Therefore, the starter control buttons, which are called push-button posts, have two pairs of contacts - normally open (open, normally closed, NO, NO) and normally closed (closed, normally closed, NC, NC)

This universalization of all the buttons of the push-button station was made in order to anticipate possible schemes for providing instant engine reverse. It is generally accepted to call the shutdown button the word: “ Stop"and mark it in red. The switching button is often called the start button, start button, or is designated by the word “ Start», « Forward», « Back».

If the coil is designed to operate from 220 V, then the control circuit switches the neutral. If operating voltage electromagnetic coil 380 V, then the current flows in the control circuit, “removed” from the other supply terminal of the starter.

Connection diagram for a 220 V magnetic starter

Here, the current is supplied to the magnetic coil KM 1 through a thermal relay and terminals connected in a chain of buttons SB2 for turning on - “start” and SB1 for stopping - “stop”. When we press start electric current goes to the reel. At the same time, the starter core attracts the armature, resulting in the closure of the moving power contacts, after which voltage is supplied to the load. When the “start” is released, the circuit does not open, since the KM1 block contact with closed magnetic contacts is connected parallel to this button. Thanks to this, phase voltage L3 is supplied to the coil. When you press “stop,” the power is turned off, the moving contacts return to their original position, which leads to de-energization of the load. The same processes occur when the thermal relay P operates - a break in the zero N supplying the coil is ensured.

Connection diagram for a 380 V magnetic starter

Connecting to 380 V is practically no different from the first option, the only difference is in the supply voltage of the magnetic coil. In this case, power is provided using two phases L2 and L3, whereas in the first case - L3 and zero.

The diagram shows that the starter coil (5) is powered from phases L1 and L2 at a voltage of 380 V. Phase L1 is connected directly to it, and phase L2 is connected through button 2 “stop”, button 6 “start” and button 4 of the thermal relay, connected in series to each other. The principle of operation of such a circuit is as follows: After pressing the “start” button 6, through the switched on button 4 of the thermal relay, the voltage of phase L2 reaches the coil of the magnetic starter 5. The core is retracted, closing the contact group 7 to a certain load (electric motor M), and current is supplied, voltage 380 V. If the “start” is turned off, the circuit is not interrupted, the current passes through contact 3 - a movable block that closes when the core is retracted.

In the event of an accident, thermal relay 1 must be activated, its contact 4 is broken, the coil is turned off and the return springs bring the core to its original position. The contact group opens, relieving the voltage from the emergency area.

Connecting a magnetic starter via a push-button post

This circuit includes additional start and stop buttons. Both “Stop” buttons are connected in the control circuit in series, and the “Start” buttons are connected in parallel. This connection allows switching with buttons from any position.

Here's another option. The circuit consists of a two-button post “Start” and “Stop” with two pairs of contacts, normally closed and open. Magnetic starter with a control coil for 220 V. The power supply for the buttons is taken from the terminal of the power contacts of the starter, number 1. The voltage approaches the “Stop” button, number 2. It passes through a normally closed contact, along the jumper to the “Start” button, number 3.

We press the “Start” button, the normally open contact number 4 closes. The voltage reaches the target, number 5, the coil is triggered, the core is retracted under the influence of the electromagnet and sets in motion the power and auxiliary contacts highlighted in dotted lines.

The auxiliary block contact 6 bypasses the contact of the “start” button 4, so that when the “Start” button is released, the starter does not turn off. The starter is turned off by pressing the “Stop” button, number 7, the voltage is removed from the control coil and the starter is turned off under the influence of the return springs.

Connecting the motor via starters

Irreversible magnetic starter

If it is not necessary to change the direction of rotation of the engine, then the control circuit uses two non-fixed spring-loaded buttons: one in the normal position is open - “Start”, the other is closed - “Stop”. As a rule, they are manufactured in a single dielectric housing, and one of them is red. Such buttons usually have two pairs of contact groups - one normally open, the other closed. Their type is determined during installation work visually or using a measuring device.

The control circuit wire is connected to the first terminal of the closed contacts of the Stop button. Two wires are connected to the second terminal of this button: one goes to any of the closest open contacts of the “Start” button, the second is connected to the control contact on the magnetic starter, which is open when the coil is turned off. This open contact is connected by a short wire to the controlled terminal of the coil.

The second wire from the “Start” button is connected directly to the terminal of the retractor coil. Thus, two wires must be connected to the controlled “pull-in” terminal - “direct” and “blocking”.

At the same time, the control contact closes and, thanks to the closed “Stop” button, the control action on the retractor coil is fixed. When the Start button is released, the magnetic starter remains closed. Opening the contacts of the “Stop” button causes the electromagnetic coil to be disconnected from the phase or neutral and the electric motor is turned off.

Reversing magnetic starter

To reverse the motor, two magnetic starters and three control buttons are required. Magnetic starters are installed next to each other. For greater clarity, let’s conditionally mark their supply terminals as 1-3-5, and those to which the motor is connected as 2-4-6.

For reverse circuit control starters are connected as follows: terminals 1, 3 and 5 with the corresponding numbers of the adjacent starter. And the “output” contacts are crosswise: 2 with 6, 4 with 4, 6 with 2. The wire feeding the electric motor is connected to three terminals 2, 4, 6 of any starter.

With a cross connection scheme, simultaneous operation of both starters will result in a short circuit. Therefore, the conductor of the “blocking” circuit of each starter must first pass through the closed control contact of the adjacent one, and then through the open one of its own. Then turning on the second starter will cause the first one to turn off and vice versa.

Not two, but three wires are connected to the second terminal of the closed “Stop” button: two “blocking” and one supplying the “Start” button, connected in parallel to each other. With this connection scheme, the “Stop” button turns off any of the connected starters and stops the electric motor.

Before assembling the circuit, you need to free the working area from the current and check that there is no voltage with a tester.
Set the core voltage symbol that is mentioned on it and not on the starter. It can be 220 or 380 volts. If it is 220 V, phase and zero go to the coil. Voltage marked 380 means different phases. This is an important aspect, because if connected incorrectly, the core may burn out or will not fully start the necessary contactors.
Starter button (red) You need to take one red “Stop” button with closed contacts and one black or green button with the inscription “Start” with invariably open contacts.
Please note that power contactors force or stop only the phases, and the zeros that come and go, conductors with grounding are always combined at the terminal block, bypassing the starter. To connect a 220 Volt core to the addition, 0 is taken from the terminal block into the design of the starter organization.

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 picture, 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, but 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 ones, 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 it looks like simplest scheme connecting the motor through the starter. The power contacts of the KM1 magnetic starter are connected to the electric motor terminals. Installed in front of the contactor circuit breaker QF1 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 direct 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 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”).

A contactor is an electromagnetic device designed for switching, that is, turning on and off electrical equipment. It is a two-position mechanism that is used for frequent switching. The main elements of its design are:

Power contact group, which can be two or three-pole depending on the voltage required for the operation of the actuator.
Arc suppression chambers, which are aimed at reducing the arc that occurs when an electric current breaks;
Electromagnetic drive. It is designed to move the moving part of the power contact. Depending on the design, it can be designed for different voltages, both direct and alternating current. Made from a U-shaped or W-shaped core;
A system of block contacts required for signaling and controlling the operational circuits of the contactor. Using them, you can connect a sound or light alarm indicating the position of the contactor, as well as for the self-retaining circuit.

A distinctive feature of the design of an electromagnet operating with alternating current is the presence of a short-circuited coil, which prevents its iron from humming during operation. If the electromagnet operates from DC, then between its disconnected parts there must be a non-metallic gasket that prevents the core from sticking. A contactor differs from a magnetic starter or relay, only in working with a more powerful load; the dimensions of the device itself depend on its size. It is very important to choose the right contactor corresponding to the current that it will switch.

Modern devices of the KMI series have good reliability indicators and are intended for general industrial use. Due to their design, they have an easy method of fastening and small dimensions.

Operating principle

When voltage is applied to the electromagnet coil, the moving part of the device is set in motion under the influence of electromagnetic forces and is attracted to the stationary part. In this case, the power contacts are closed and voltage is supplied to the actuator. And also at the same time there is movement of block contacts, which can be making or breaking.

How to connect a contactor

When connecting a contactor, you immediately need to decide on the mechanism that it will turn on. This could be a motor, pump, fan, heating elements, compressors, etc. Main feature What distinguishes a contactor from a machine is the absence of any protection. Therefore, when thinking through the circuits for switching on electrical equipment through a contactor, it is necessary to take into account the current-limiting and heating elements. To limit and shut down equipment in case of short circuits and loads many times higher than the rated load, fuses and circuit breakers are used. Thermal relays are used to prevent long-term slightly exceeding the rated currents of operating equipment.

In order to correctly connect a contactor to a circuit, you need to clearly understand which of the contacts are power and which of them are auxiliary, that is, block contacts. You also need to look at the ratings of the switching coil. The voltage, its type and magnitude, as well as the currents that flow through it for normal operation must be indicated there. During operation, power contacts may burn, so they must be inspected and cleaned regularly.

How to connect a modular contactor

A modular contactor is a type of conventional switching devices of the same type, only they are mainly used for switching switchboards on and off remotely. That is, turning it on, power is supplied to a group of machines, each of which is responsible for its own specific circuit. It is installed on a DIN rail. Can switch both direct and alternating current circuits.

Connecting a contactor via a button

To connect the contactor via a button, you need to study the attached diagram below. It is designed to start a load, in this case a motor, from a contactor whose coil is designed for 220 Volts alternating voltage. Depending on the voltage, it is worth considering its power supply. Therefore, when purchasing and selecting a contactor, it is worth taking this nuance into account. Since if the electromagnet is designed for constant voltage, then such a source will be needed.

When you press the start button, the contactor electromagnet coil will receive power and it will turn on. The power contacts will close, thereby applying voltage to asynchronous motor. The block contact of contactor K1, which is connected in parallel to the stop button, will also close. Electricians call it a self-retaining contact, since it is this that supplies power to the switching coil after the start button is released. When you press the stop button, the power from the electromagnet is turned off, the power elements of the contactor break the circuit and the engine turns off.

Connecting a contactor with a thermal relay

The thermal relay is designed to prevent prolonged minor current overloads during operation of electrical equipment, because overheating negatively affects the condition of the insulation. Frequent excesses of temperature and current will lead to its destruction, and therefore to a short circuit and failure of an expensive actuator.

When the current in the stator circuit of the electric motor increases, the elements of the thermal relay KK will heat up. When the set temperature, which can be adjusted, is reached, the thermal relay will operate and its contacts will break the circuit of the electromagnet coil of the KM contactor.

For safety reasons, you must remember that work in the contactor circuit must be carried out when it is completely de-energized. In this case, the power supply must be locked with a key or prohibiting sign from unauthorized or erroneous activation. And also you cannot turn on this device with the arc chutes removed, this will lead to a short circuit.