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 the place 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.
Today, switching starting electromagnetic devices are used in various electrical equipment. They are an intermediate link between power units and electrical equipment control systems, controlling the switching on and off of electrical circuits. Read below to learn how magnetic starters are designed, what types of devices exist, and what their purpose is.
Magnetic starter: device and principle of operation, equipment
A magnetic electric starter is a low-voltage device for monitoring and distributing current energy. The design of the device is quite simple: the device consists of two parts - upper and lower, combined into a plastic case.
At the top of the starter there is:
- Block of moving contacts;
- Arcing grid;
- The moving part of the electromagnet.
The contact power block, in this case, is closely connected with the moving part of the electromagnet. The arc extinguishing grid in the device acts as a device that serves to prevent and eliminate electric arc fires. A traverse with a magnetic system armature and bridges of power and additional contacts with springs slides along the runners at the top of the device.
The lower part of the electromagnetic device has in its design:
- Retractor coil;
- Return spring;
- Part of an electromagnet.
The retractor coil has a cylindrical shape and a winding made of copper conductor. The number of turns of the coil depends on the calculated supply voltage. The magnet in the device consists of W-shaped, steel, electromagnetic plates. The armature and the core make up the magnetic circuit.
The principle of operation of the device is quite simple: it is based on the effect of a magnetic field on various moving parts of the starter.
So, current is supplied to a coil located on the core. After the current supply is stopped, the magnetic field disappears, the return spring sends the upper part of the device to its original place. At the same time, the contacts that were open are closed, and the closed ones are opened.
Power contact system: magnetic starter device
Modern magnetic starters can be equipped with additional protection and control devices. Most often, starters are equipped with thermal emergency shutdown relays and low-current contact starting control groups. Reproduction of contacts when modifying a magnetic starter is carried out through a contact block. Therefore, the contact system of the starter is called an attachment.
The contact power system of a three-phase magnetic starter consists of three power (main) contacts and one auxiliary.
Power contacts are used to switch powerful loads. Therefore, they are made from copper jumpers coated with technical silver. The additional contact in the block acts as a blocking contact: when using a standard connection diagram, it locks the starter in working condition.
Depending on the type of influence on the electrical circuit, power contacts are divided into:
- Normally closed;
- Normally open.
The contacts are activated when current enters the starter coil. During this, the core pulls the contacts along with it, which makes the normally closed contacts open and the normally open contacts closed.
Auxiliary contacts for time delay starters
To increase the number of power contacts of the electromagnetic device, additional attachments are used. At the same time, the contacts in such attachments are selected taking into account the maximum current of the main ones. So, for starters of the first and second values, the current of the additional contacts must be equal to the current of the main ones or be less than the maximum value. Separately, there are additional contacts (attachments) with a delay. The main task of such set-top boxes is to wait a certain time when turning the device on and off.
Pneumatic attachments are used in control circuits for electric drives:
- With a DC voltage of 440 V and a frequency of 50 Hz;
- With an AC voltage of 660 V and a frequency of 60 Hz.
If a pneumatic PVL attachment is already installed, in order to increase the number of auxiliary contacts of the electrical control circuit, use a contact side attachment of the PKB series. The attachment is mounted using special latches on its body.
What types of magnetic starters are there?
Magnetic electric starters are distinguished by their ability to work with loads of different powers. Domestic starters are divided into 7 groups and can switch power in the range from 7.5 to 45 kW.
In addition, according to their design and operating principle, starters are divided into:
- Reversible (for example, PML 1502, 3100);
- Irreversible (for example, starter PME 211, PAE 311 or “frog”).
Reversing starters have two magnets in their design, thanks to which they are able to spin and move in any direction depending on the operator’s command. Moreover, regardless of how the starter is designed, it may or may not have overload protection.
Depending on the installation location, magnetic starters are of closed and open type.
Separately, dust-proof electromagnetic contactors are distinguished. The first types of starters are installed in standard places that are not characterized by large accumulations of dust or mechanical influences from foreign objects (for example, electrical cabinets). Dust-proof starters are not affected by sunlight and precipitation, and can be installed under canopies on the street. To identify the type of starter, a standardized decoding has been invented that allows you to determine the meaning of each letter and number in the designation on the electrical device.
Main functions and purpose of a magnetic starter
What is a magnetic starter used for? The main purpose of the starting electromagnetic device is to turn the engine on and off. A study of arc suppression in AC contactors indicates that, most often, starters are used to control asynchronous three-phase electric motors. This is explained by the simplicity of the starter design. In addition, when included in the circuit, starters not only turn the electric motor on and off, but also monitor its operation.
Thus, a magnetic starter performs the following functions:
- Provides engine starting and engine acceleration;
- Controls the continuity of work in accordance with a given time period;
- Protects the motor from overloads;
- Changes the direction of rotation of the motor;
- Responsible for countercurrent braking;
- Provides engine shutdown.
At the same time, the starter provides zero protection for the electric drive. So, in the event of an unplanned power outage, the engine ignition switch (controller) may be in a non-zero position. Zero protection prevents the motor from spontaneously turning on when power is restored: the motor starts up only after the operator’s command.
Why do you need a magnetic starter: scope of application
In addition to controlling a three-phase asynchronous motor, a magnetic starter can be used to control the operation of powerful electricity consumers (for example, a pump, an air conditioner). In everyday life, magnetic starters are most often used to turn on a heating system (for example, heating elements).
In addition, starters are used in circuits:
- Remote control of lighting devices;
- Control of thermal furnaces;
- Compressor control.
Thus, the scope of application of starters is extremely wide. This is due to the simplicity of their design and the ease of incorporating devices into the circuit. In addition, finding a starter at an affordable price is not difficult: buying used electrical devices is especially popular today.
Magnetic starter device (video)
Almost no modern electrical equipment can do without a device for turning on and off the electrical circuit - a magnetic starter. A modern magnetic electric starter is a modified two-position electromagnetic electrical contactor. Knowing how a magnetic starter works and what types of devices are distinguished, you can include a contactor in any circuit. And the above recommendations for installing additional contacts will help you improve your device!
The electromagnetic starter is used for switching powerful electricity consumers, mainly in production. This article will discuss why a magnetic starter is needed, what is the principle of operation of a magnetic starter and the design of a magnetic starter. The design and principle of the starter, both for 380V and 220V circuits, are the same for a long time and have been well developed by designers.
As already mentioned, this is a switching device, in other words, a switch, this is its purpose. Starter contacts are designed for high current flowing through heating devices and powerful electric motors. These power contacts are actuated electromagnetically, so the starters can be controlled remotely using relatively low-power circuits. Therefore, a small button or limit switch can be used to connect powerful electric motors and other loads. The reversible starter ensures that asynchronous motors are turned on in any direction - clockwise or counterclockwise, at the choice of the operator or the control system.
Principle of operation
The principle of operation of a magnetic starter actually coincides with a relay. To operate the starter from pushbuttons without latching, self-locking from contacts parallel to the button is used. To turn off, a normally closed button is used, connected in series to the control circuit. When the contacts open, the starter turns off and is ready to be turned on again immediately after the contacts of the stop button are closed.
The “push-button” version of starter control is overwhelming for manual operations. In automation circuits, starters are usually kept in the on state by a continuous signal supplied from the discrete output of the controller to the intermediate relay.
There are different types of starters, among which there are reversible magnetic starters (a “headache” for novice electricians who are trying to understand how an unusual circuit works and are not used to thinking in electrical circuits). In fact, these are two starters operating strictly alternately: if one is turned on, then the other must be turned off, otherwise there will be a short circuit between the phases.
Its principle is as follows: if in one switched-on position the sequence of phases is A, B, C, then in another position there should be, for example, A, C, B, that is, two phases should swap places. This allows you to change the direction of the rotating field in asynchronous motors and run them in different directions, either clockwise or counterclockwise.
All types of magnetic starters are united by such design elements as an alternating current electromagnet, a system of moving and fixed power and auxiliary contacts. The supporting part is a body made of heat-resistant and non-flammable plastics. These plastics must be mechanically strong and not deform at elevated temperatures. Any starter is usually three-phase.
- Contact springs for smooth starting
- Moving contacts (bridges)
- Fixed contacts (plates)
- Plastic traverse
- Anchor
- Starter coil
- W-shaped part of the magnetic circuit
- Additional contacts
The classification of magnetic starters is made according to several criteria, among which the main one is usually the size of the starter. The value does not mean the dimensions or weight of the starter, but what current it can switch and how resistant it is to an arc in circuits with inductances (when the electric motor is turned off). The basis is a non-reversible magnetic starter, since the reversible ones are assembled from the latter. Magnetic starters operate under different conditions, so they are also classified according to the degree of protection: open, protected, dust-splash-proof.
The operation of a magnetic starter very often requires a thermal relay. All types of magnetic starters have structurally compatible thermal relays. They are often produced by the same manufacturer. A particularly important application of thermal relays is to protect electric motors from overheating. The thermal relay consists of two-phase bimetallic conductors (conductors with different coefficients of thermal expansion) - one for each phase.
From an electrical point of view, they are resistors with very low resistance, and thus serve as current sensors. When too much current flows through the phases (or one of them), the bimetallic strip bends and opens the magnetic contacts, that is, the contacts in the starter coil circuit. Thermal relays are connected between the starter and the load.
Modular starters are becoming more and more common. These are DIN rail mounted starters. This is a metal profile strip fixed in cabinets on a panel. The simplicity and ease of installation are exceptional. Next to the starter (contactor) you can attach thermal relays, circuit breakers, RCDs (residual current devices), microprocessor controllers and much more. Modular devices are very easy to assemble into circuits, thanks to the wire channels laid between the DIN rails. Installation is carried out with stripped wires of the required cross-section and crimped lugs. The tips are inserted into the holes of the device terminals according to the circuit diagram and clamped with screws.
Markings required for installation and repair are applied to the top side of the starters. There is a type designation, a contact diagram and in some cases manufacturers leave space for a sticker or signature of consumer data.
Great advances in power electronics over the past decades have meant that most major manufacturers now offer consumers contactless starters containing high-power semiconductor switches. They have certain advantages. They operate silently, do not spark, and have a high switching frequency.
Some models, thanks to PWM controllers, allow smooth starting of electric motors, and even network interfaces are provided for automation. Disadvantages include high price, highly qualified repair personnel and unsafe galvanic connection to the network, which can threaten repair electricians.
Conclusion
Despite the introduction of electronic switches: already outdated thyristors and triacs, powerful field-effect transistors, and promising IGBT transistors, magnetic starters retain their importance. They are the ones who reliably break circuits, without any residual currents or leaks dangerous to personnel or equipment. In fact, this is the same immortal “switch” that is guaranteed to de-energize the electrical installation. high-quality starters never jam and you need to purchase just such ones.
The connection diagram for a magnetic starter (small-sized contactor “KM”) is not difficult for experienced electricians, but for beginners it can cause many difficulties. Therefore, this article is for them.
The purpose of the article is to show as simply and clearly as possible the very principle of operation (operation) of a magnetic starter (hereinafter referred to as MP) and a small-sized contactor (hereinafter referred to as KM). Go.
MP and KM are switching devices that control and distribute operating currents along the circuits connected to them.
MP and KM are mainly used for connecting and disconnecting asynchronous electric motors, as well as their reverse switching using remote control. They are used for remote control of lighting groups, heating circuits and other loads.
Compressors, pumps and air conditioners, heating furnaces, conveyor belts, lighting circuits are where and not only you can find MP and KM in their control systems.
What is the difference between a magnetic starter and a small-sized contactor, according to the principle of operation - nothing. Essentially, these are electromagnetic relays.
The found difference for a contactor - power - is determined by the dimensions, and for a starter it is determined by the values, and the maximum power of the MP is greater than that of the contactor.
Visual diagrams of MP and CM
Rice. 1
Conventionally, MP (or CM) can be divided into two parts.
In one part there are power contacts that do their job, and in the other part there is an electromagnetic coil that turns these contacts on and off.
- In the first part there are power contacts (movable on the dielectric traverse and stationary on the dielectric body), they then connect the power lines.
A traverse with power contacts is attached to a movable core (anchor).
In the normal state, these contacts are open and no current flows through them; the load (in this case, the lamp) is at rest.
A return spring keeps them in this state. Which is depicted as a snake in the second part (2)
- In the second part we see an electromagnetic coil, which is not supplied with its operating voltage, as a result of which it is at rest.
When voltage is applied to the coil winding, an electromagnetic field is created in its circuit, forming an EMF (electromotive force), which attracts a moving core (the moving part of the magnetic circuit - the armature) with power contacts attached to it. They, accordingly, close the circuits connected through them, including the load (Fig. 2).
Rice. 2
Naturally, if you stop supplying voltage to the coil, the electromagnetic field (EMF) will disappear, the armature will no longer be held and, under the action of the spring (together with the movable contacts attached to it), returns to its original state, opening the circuits of power contacts (Fig. 1).
From this it can be seen that the starter (and contactor) are controlled by applying and cutting off voltage to their electromagnetic coil.
MP scheme
- MP power contacts
Schematic diagram of MP connection
Scheme of linking the main elements of the circuit diagram with MP
As can be seen from Figure 5 with the diagram, the MP also includes additional block contacts, which are normally open and normally closed; they can be used to control the supply of voltage to the coil, as well as for other actions. For example, turn on (or turn off) a signal indication circuit that will show the operating mode of the MP as a whole.
Connection diagram in fact with connection of contact groups to the circuit diagram of the MP
- MP power contacts
- Coil, return spring, additional MP contacts
- Push-button station (start and stop buttons)
Schematic diagram of KM connection
Scheme for linking the main elements of the circuit diagram with the CM
Connection diagram in fact with connection of contact groups to the circuit diagram of the CM
- “STOP” button – “Stop” button
- “START” button – “Start” button
- Kn MP – power contacts MP
- BC – block contact MP
- KTR – thermal relay contact
- M – electric motor
Connection diagrams for MP (or KM) with a 220 V coil
- “STOP” button – “Stop” button
- “START” button – “Start” button
- KMP – coil MP (magnetic starter)
- Kn MP – power contacts MP
- BC – block contact MP
- Tr – heating element of thermal relay
- KTR – thermal relay contact
- M – electric motor
The designation of elements is similar to cx. Higher
Please note that the circuit involves a thermal relay, which, through its additional contact (normally closed), duplicates the function of the “Stop” button in the push-button station.
The principle of operation of a magnetic starter and a small-sized contactor + Video explanation
Important: for clarity, in the diagrams the magnetic starter is shown without an arc-extinguishing cover, without which its operation is prohibited!
Sometimes the question arises: why use MP or KM at all, why not just use a three-pole machine?
- The machine is designed for up to 10 thousand shutdowns and starts, and for MP and KM this figure is measured in millions
- During power surges, the MP (KM) will turn off the line by playing
- The machine cannot be controlled by remotely applying a small voltage
- The machine will not be able to perform additional functions of turning on and off additional circuits (for example, signal circuits) due to the lack of additional contacts
In a word, the machine perfectly copes with its main function of protection against short circuits and overvoltages, and MP and PM do theirs.
That's all, I think that the principle of operation of MP and CM is clear, for a more clear explanation, see the video.
Happy and safe installation!
In addition to the article, I attach technical documentation for KMI series contactors
KMI series contactors
Regulatory and technical documentation
In terms of their design and technical characteristics, contactors of the KMI series meet the requirements of Russian and international standards GOST R 50030.4.1,2002, IEC60947,4,1,2000 and have a certificate of conformity ROSS CN.ME86.B00144. According to the All-Russian Product Classification, contactors of the KMI series are assigned code 342600.
terms of Use
Application categories: AC,1, AC,3, AC,4. Ambient temperature
– during operation: from –25 to +50 °С (lower limit temperature –40 °С);
– during storage: from –45 to +50 °С.
Height above sea level, no more than: 3000 m.
Working position: vertical, with a deviation of ±30°.
Type of climatic modification according to GOST 15150.96: UHL4.
Degree of protection according to GOST 14254.96: IP20.
Designation structure
When selecting KMI contactors, pay attention to the structure of the symbol
Main technical characteristics
Power Circuit Specifications
Control Circuit Specifications
Connecting the power circuit
Control circuit connection
Technical characteristics of built-in auxiliary contacts
| Options | Values | |
| Rated voltage Ue, V | AC current | up to 660 |
| fast. current | ||
| Rated insulation voltage Ui, V | 660 | |
| Thermal resistance current (t°≤40°) Ith , A | 10 | |
| Minimum making capacity | Umin, V | 24 |
| Imin, mA | 10 | |
| Overcurrent protection - gG fuse, A | 10 | |
| 100 | ||
| Insulation resistance, not less, MOhm | 10 | |
Electrical circuits
Typical electrical circuits
Contactors of the KMI series can be used to create standard electrical circuits.
Reversing electrical circuit
This circuit is assembled from two contactors and a blocking mechanism MB 09.32 or MB 40.95 (depending on the type), designed to prevent the simultaneous activation of contactors.
This starting method is intended for motors whose rated voltage corresponds to the delta connection of the windings. Star-delta starting can be used for motors starting without load, or with reduced load torque (no more than 50% of the rated torque). In this case, the starting current when connected to a “star” will be 1.8–2.6 A of the rated current. Switching from star to delta must be done after the engine reaches its rated speed.
Design and installation features
Connecting clamps ensure reliable fixation of conductors:
– for dimensions 1 and 2 – with hardened disc washers;
– for sizes 3 and 4 – with a clamping bracket that allows you to connect a contact with a larger cross-section.
There are two ways to install contactors:
- Quick installation on DIN rail:
KMI from 9 to 32 A (dimensions 1 and 2) – 35 mm;
KMI from 40 to 95 A (dimensions 3 and 4) – 35 and 75 mm.
- Installation with screws.
For the needs of industrial enterprises and companies, a fairly large amount of equipment and devices are produced to ensure uninterrupted operation that meets the standards. One such device is a magnetic starter.
Special purpose
An electromagnetic starter is an electromechanical device used to distribute supply voltage and control the operation of connected loads, the operation of which is regulated by a low voltage circuit. The list of tasks for which a magnetic starter is needed looks like this:
- Starting the electric motor and then accelerating to rated speed;
- Maintaining continuous engine operation;
- Stopping the supply voltage to the engine;
- Protective disconnection of the load from the network in case of overloads or unusual situations.
Since magnetic starters are structurally simple devices and, according to their parameters, are capable of switching fairly powerful loads with huge currents, they are also used in controlling the operation of melting furnaces, ventilation and air conditioning units, liquid electric pumps, pneumatic blowers and other similar consumers.
Design and technical parameters
Magnetic starter device:
- Core;
- Electromagnet coil;
- Anchor;
- Polymer frame;
- Mechanical work sensors;
- Central and additional group of contactors.
Main parameters displayed in the technical documentation:
- The measure of current passing through the central terminals is the magnitude of currents at which the device is operational over a long period of time with specified parameters;
- The maximum current value that the device can operate;
- The voltage of the connected circuit is the voltage of the operated circuit at which the insulation between the central terminals retains its technical parameters;
- The control voltage of the electric magnet coil is the alternating or constant supply voltage of the electromagnet;
- Relay and electromechanical resistance to wear - the indicator is expressed in the number of cycles for closing and opening the terminals. Relay wear resistance is determined according to the corresponding graph displayed in the accompanying documentation for the device. By substituting the values of the supply voltage and current of the operated network, it is possible to determine the parameter yourself;
- Limit number of operations per unit of time;
- Number of additional terminals and method of their implementation;
- Time period for connection and disconnection.
In addition, the electromagnetic starter can be supplemented with:
- Protective relay to prevent overheating and electrical overloads of the end user;
- Additional set of terminals;
- Starting device for the engine;
- Electrical fuses.
Types of magnetic starters
The following types of magnetic starters stand out from the general assortment:
- Reversible - ensuring rotation of the engine rotor in the direction opposite to the initial one;
- Non-reversible – supporting rotation of the engine rotor in one direction;
- Enclosing type – designed for installation in areas with a small amount of dust;
- Dustproof - used for outdoor placement and can be exposed to sunlight, rain and snow;
- Open type – used in rooms free of dust and foreign objects.
Operating principle of a magnetic starter
The principle of operation of a magnetic starter is as follows. When a control signal is applied to the winding of the electromagnet coil (6), it is magnetized and, together with the stationary W-shaped part of the core (7), attracts the armature (5) on the plastic cross-arm (4), of which the contact bridges (2) smoothly close the contact plates ( 3), thanks to contact springs (1), which, in turn, create the necessary pressing force. Additional contacts (8) can be used at the discretion of the consumer.
The group of terminals is designed in the form of a three-pole AC electric magnet with block contacts made of silver-containing metal; it switches main circuits, the current amplitude of which varies from 3 Amperes to 200 Amperes. Based on the fact that the main terminals carry the operating load current for a long time and produce a large number of connection and disconnection cycles, cermets are used as the material for the main contacts. To simplify use, stationary and moving terminals are usually mounted easily removable.
Due to the use of arc extinguishing elements in contactors, it became possible to reduce the distance between the working terminals and, accordingly, weaken the power of the electromagnet, reduce the dimensions and weight of the electromagnetic starter as a whole. An arc extinguishing device is used to prevent the occurrence of sparking at the terminals at the moment of closing and opening the contacts. For operating currents of more than 10 Amps, the arc extinguishing device is implemented in the form of an arc extinguishing grate for each opening. Arc-extinguishing grates are implemented on the principle of compensation of the electric arc by a transverse magnetic field in chambers with longitudinal holes. The negative consequences of sparking are burning, charring, and excessive heating of contacts.
To move the armature with contacts, directly directed systems of electromagnets with U- and Sh-shaped stacked magnetic cores are used. Since when a magnetic starter is triggered, an alternating current passes through the retractor coil, its value significantly exceeding the retracted state current, for such starters the manufacturer sets a limit on the number of connections and disconnections per hour.
Depending on the throughput currents of the magnetic starter, contacts of various shapes and with different contact planes are used, as shown in the picture below.
For the control circuits of the magnetic starter, point contacts (a) are used, namely:
- Point-plane (1);
- Point-sphere (2);
- Sphere-plane (3);
- Sphere-sphere (4);
For the power circuits of the electromagnetic starter, longitudinal contacts (b) are used, namely:
- Prism-plane (5);
- Cylinder-plane (6);
- Cylinder-cylinder (7);
- Plane-plane (8).
An additional bridge contactor is used to switch low-current control circuits and is driven by the same pull-in coil as the main contacts. The basis of the auxiliary contacts is copper coated with a thin layer of silver or bimetal. Manufactured magnetic starters include from two to four additional contacts, which can also operate for both closing and opening.
An integral part of the operation of asynchronous motors is the presence of a magnetic starter, the main task of which is to protect the device from overloads. When the engine is running, there are cases where one of the phases breaks due to blown fuses or for other reasons. It is clear that this phenomenon leads to a sharp increase in the current on the stator windings, which leads to overheating and failure of the electric motor. To prevent such breakdowns, magnetic starters with thermal relays are used. The bulk of thermal relays are built on the basis of bimetallic elements. The principle of operation of a bimetallic element is inherent in its design, the essence of which is the rigid fastening, by hot rolling or welding, of two metal plates with different expansion coefficients. Since when such an element is heated, the metal plate on one side will linearly expand faster than the plate on the opposite side, the plate will physically bend. Accordingly, thermal energy is converted into mechanical work by turning off the load when overheated.
Note! Since the thermal process is inertial, thermal relays cannot be a means of protecting equipment from short circuit currents. Even a short time to disconnect the load during a short circuit can be enough for the load to burn out or fail.
The metals with different linear expansion coefficients used in bimetallic elements are chromium-nickel steel and invar.
Types of magnetic starters
Typical magnetic starters include:
- The PML class is operated with electric motors with a power of up to 75 kW. The main mechanism can be supplemented with a temperature relay and surge suppressors;
- The PMA series is used in conjunction with electric asynchronous motors, the rotor of which is squirrel-cage, and has a power of up to 100 kW with an operating voltage from 380V to 660V. The mechanism is complemented by a temperature relay, voltage limiter and positronic protection;
- The operation of asynchronous motors with a power of up to 11 kW, with a supply voltage of up to 660 V, is complemented by magnetic starters of the PME series. This series is equipped with AC-3, AC-4 class terminals and thermal relays;
- The ships' equipment is equipped with electromagnetic starters of the PMM class. For areas of activity with more stringent safety conditions, magnetic starters in a waterproof or drip-proof housing have been created;
- The purpose of the magnetic starter of the PM-12 group is to connect to the network, reverse and turn off asynchronous motors with a squirrel-cage rotor, with a power of up to 125 kW and with a supply voltage of 380V to 660V.
Understanding the structure and operating principle of a magnetic starter, it will not be difficult to select a specific device to perform a specific task. When operating the device, do not forget about maintenance and regular inspection of the magnetic starter, while the device will serve for a long time with the specified characteristics.
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