Electromagnetic relay: device, types, marking, connection and adjustment

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The conversion of electrical signals into the corresponding physical quantity - movement, force, sound, etc., is carried out with the help of drives. The drive should be classified as a transducer, since this device changes one type of physical quantity to another.

The drive is usually activated or controlled by a low voltage command signal. It is additionally classified as a binary or continuous device based on the number of stable states. So, the electromagnetic relay is a binary drive, considering two existing stable states: on - off.

In the present article, the principles of operation of the electromagnetic relay and the scope of use of the devices are discussed in detail.

The content of the article:

  • Basics of the drive
  • Fundamental Principles of EWM
    • General structure of the device
    • Action relay electromagnetic system
    • Electrical contact relay groups
    • Features of the passage of load currents
    • Type of material contacts relay
  • Typical execution of EMR contacts
    • Features of execution of connecting elements
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    • Subtleties of application of devices
    • Reverse Voltage Protection Techniques
    • Marking of electromagnetic relay devices
  • Conclusions and useful video on the topic

Basics of the drive

The term "relay" is characteristic of devices that provide electrical connection between two or more points by means of a control signal.

The most common and widely used type of electromagnetic relay (EMR) is an electromechanical design.

Electromagnetic relay

It looks like one design of the numerous series of products, referred to as electromagnetic relays. Shown here is a closed version of the mechanism with a transparent Plexiglas lid.

The scheme of fundamental control over any equipment always provides for the possibility of switching on and off. The easiest way to do this is to use the power interlock switches.

Manual action switches can be used to control, but have disadvantages. Their obvious disadvantage is the setting of states “on” or “disabled” by physical means, that is, manually.

Manual switching devices, as a rule, are large-sized, slow-motion, capable of switching small currents.

Cam switch

The manual switching mechanism is a “distant relative” of electromagnetic relays. It provides the same functionality - switching of working lines, but is controlled exclusively by hand.

Meanwhile, electromagnetic relays are represented mainly by electrically controlled switches. Devices have different shapes, dimensions and are divided by the level of nominal power. The possibilities of their application are extensive.

Such devices, equipped with one or several pairs of contacts, can be included in a single structure of larger power actuators - contactors that are used for switching mains voltage or high-voltage devices.

Fundamental Principles of EWM

Traditionally electromagnetic type relays are used as part of electrical (electronic) switching control circuits. In this case, they are installed either directly on printed circuit boards, or in a free position.

General structure of the device

The load currents of the products used are usually measured from fractions of an amp to 20 A or more. Relay circuits are widespread in electronic practice.

Variety of electromagnetic relays

Devices of very different configuration, designed for installation on electronic circuit boards or directly as a separately installed device

The design of the electromagnetic relay converts the magnetic flux created by the applied AC / DC voltage into mechanical force. Due to the obtained mechanical force, the control of the contact group is performed.

The most common design is the shape of the product, including the following components:

  • excitation coil;
  • steel core;
  • support chassis;
  • contact group.

The steel core has a fixed part, called a rocker, and a movable spring-loaded part, called an anchor.

In fact, the anchor complements the magnetic field circuit, closing the air gap between the fixed electric coil and the moving armature.

Electromagnetic relay design

Detailed layout design: 1 - spring pressing; 2 - metal core; 3 - anchor; 4 - contact normally closed; 5 - contact is normally open; 6 - common contact; 7 - copper wire coil; eight - rocker

The armature moves on hinges or rotates freely under the action of the generated magnetic field. This closes the electrical contacts attached to the valve.

As a rule, the spring (spring) of the reverse stroke located between the rocker and the anchor returns the contacts to the initial position when the relay coil is in the de-energized state.

Action relay electromagnetic system

A simple classic EMR design has two sets of electrically conductive contacts.

Based on this, two states of the contact group are realized:

  1. Normally open contact.
  2. Normally closed contact.

Accordingly, a pair of contacts is classified as normally open (NO) or, being in a different state, normally closed (NC).

For relays with normally open contacts, the state "closed" is achieved only when the excitation current passes through an inductive coil.

Relay with normally closed contact

One of two possible options for setting the default contact group. Here, in the de-energized state of the “default” coil, the normally closed (closed) position is set

In another embodiment, the normally closed position of the contacts remains constant when the excitation current is absent in the coil circuit. That is, the switch contacts return to their normal closed position.

Therefore, the terms “normally open” and “normally closed” should be referred to the state of the electrical contacts when the relay coil is de-energized, that is, the relay supply voltage is turned off.

Electrical contact relay groups

Relay contacts are usually represented by electrically conductive metal elements that are in contact with each other, closing the circuit, acting like a simple switch.

When the contacts are open, the resistance between normally open contacts is measured by a high value in megohms. This creates an open circuit condition when the passage of current in the coil circuit is excluded.

Contact resistance relay

The contact group of any electromechanical switch in open mode has a resistance of several hundred mega. The magnitude of this resistance may be slightly different for different models.

If the contacts are closed, the contact resistance should theoretically be zero - the result of a short circuit.

However, this condition is not always noted. The contact group of each individual relay has a certain contact resistance in the "closed" state. Such resistance is called steady.

Features of the passage of load currents

For the practice of installing a new electromagnetic relay, the contact resistance of inclusion is marked by a small value, usually less than 0.2 Ohm.

The reason is simple: new tips remain clean for now, but over time, tip resistance will inevitably increase.

For example, for contacts under a current of 10 A, the voltage drop will be 0.2 x 10 = 2 volts (Ohm's law). From here it turns out - if the supply voltage applied to the contact group is 12 volts, then the voltage for the load will be 10 volts (12-2).

When contact metal tips wear, being not properly protected from high inductive or capacitive loads, it becomes inevitable that damage from the effect of electric arc.

Electric arc on the relay contacts

An electric arc on one of the contacts of an electromechanical switching device. This is one of the causes of damage to the contact group in the absence of appropriate measures.

Electric arc - sparking at the contacts - leads to an increase in contact resistance of the tips and, consequently, to physical damage.

If you continue to use the relay in this condition, the contact tips can completely lose the physical property of the contact.

But there is a more serious factor when, as a result of an arc damage, the contacts eventually weld, creating short circuit conditions.

In such situations, the risk of damage to the circuit that is monitored by the magnetic field is not excluded.

So, if the contact resistance increased from the influence of an electric arc by 1 ohm, the voltage drop across the contacts for the same load current increases to 1 × 10 = 10 volts DC.

Here, the magnitude of the voltage drop at the contacts may be unacceptable for the load circuit, especially when working with supply voltages of 12-24 V.

Type of material contacts relay

In order to reduce the influence of an electric arc and high resistances, the contact tips of modern electromechanical relays are made or coated with various silver-based alloys.

In this way, it is possible to significantly extend the life of the contact group.

Silver contact tips

Tips of contact plates of electromechanical devices of switching. Here are the silver-coated tips. A coating of this kind reduces the damage factor.

In practice, the use of the following materials is noted, with which the tips of the contact groups of an electromagnetic (electromechanical) relay are processed:

  • Ag - silver;
  • AgCu - silver-copper;
  • AgCdO - silver-cadmium oxide;
  • AgW - silver-tungsten;
  • AgNi - silver-nickel;
  • AgPd - silver-palladium.

Increase the service life of tips of contact groups of relays by reducing the number of formations electric arc, is achieved by connecting resistor-capacitor filters, also called RC dampers.

These electronic circuits are connected in parallel with the contact groups of electromechanical relays. The voltage peak, which is observed at the moment of opening the contacts, with this solution seems to be safely short.

The use of RC-dampers can suppress the electric arc, which is formed on the contact tips.

Typical execution of EMR contacts

In addition to the classic normally open (NO) and normally closed (NC) contacts, the mechanics of relay switching also suggests classification based on the action.

Features of execution of connecting elements

The design of the electromagnetic type relay in this embodiment allows the presence of one or more individual switch contacts.

Relay with SPST configuration

This is the device technologically configured for SPST execution - unipolar and unidirectional. There are also other variants of execution.

The execution of contacts is characterized by the following set of abbreviations:

  • SPST (Single Pole Single Throw) - unipolar unidirectional;
  • SPDT (Single Pole Double Throw) - unipolar bidirectional;
  • DPST (Double Pole Single Throw) - bipolar unidirectional;
  • DPDT (Double Pole Double Throw) - Bipolar bidirectional.

Each such connecting element is designated as a “pole”. Any of them can be connected or reset, simultaneously activating the relay coil.

Subtleties of application of devices

With all the simplicity of the design of electromagnetic switches, there are some subtleties in the practice of using these devices.

Thus, experts categorically do not recommend connecting all relay contacts in parallel in order to switch the load circuit with a high current in this way.

For example, connect the load to 10 A by parallel connection of two contacts, each of which is designed for a current of 5 A.

These subtleties of installation are due to the fact that the contacts of mechanical relays never close or open in a single moment of time.

As a result, one of the contacts will be overloaded in any case. And even with the short-term overload, a premature failure of the device in such a connection is inevitable.

Burnt relay

Incorrect operation, as well as connecting the relay outside the established installation rules, usually ends with this outcome. Inside almost all the contents burned out.

Electromagnetic products can be used in the composition of electrical or electronic circuits with low energy consumption as switches for relatively high currents and voltages.

However, it is strictly not recommended to pass different load voltages through adjacent contacts of the same device.

For example, switch AC voltage of 220 V and DC 24 V. You should always use separate products for each of the options in order to ensure safety.

Reverse Voltage Protection Techniques

A significant detail of any electromechanical relay is the coil. This part belongs to the discharge of a load with a high inductance, since it has a wound winding.

Any wire wound coil has some impedance, consisting of inductance L and resistance R, thus forming a series circuit LR.

As the current flows through the coil, an external magnetic field is created. When the current in the coil stops in the “off” mode, the magnetic flux increases (transformation theory) and a high reverse voltage EMF (electromotive force) occurs.

This induced value of the reverse voltage can be several times higher than the switching voltage.

Accordingly, there is a risk of damage to any semiconductor components located near the relay. For example, a bipolar or field effect transistor used to apply voltage to a relay coil.

Control protection schemes

Circuit options that provide protection for semiconductor controls - bipolar and field-effect transistors, microcircuits, microcontrollers

One way to prevent damage to a transistor or any switching semiconductor devices, including microcontrollers, is the option of connecting a reverse biased diode to a coil circuit relay.

When the current flowing through the coil immediately after tripping, generates an induced reverse EMF, this reverse voltage opens the reverse biased diode.

Through the semiconductor, the accumulated energy is dissipated, which prevents damage to the control semiconductor - the transistor, thyristor, microcontroller.

Often included in the coil circuit semiconductor is also called:

  • diode flywheel;
  • shunt diode;
  • inverted diode.

However, there is not much difference between the elements. They all perform the same function. In addition to the use of diodes with reverse bias, other devices are used to protect the semiconductor components.

The same chain RC-dampers, metal oxide varistors (MOV), zener diodes.

Marking of electromagnetic relay devices

Technical designations that carry partial information about devices are usually indicated directly on the chassis of an electromagnetic switching device.

Such designation in the form of an abbreviated abbreviation and numeric set looks like.

Marking of electromagnetic relays

Each electromechanical switching device is traditionally labeled. On the case or on the chassis is applied about such a set of characters and numbers, indicating certain parameters

Example of case marking of electromechanical relays:

RES32 RF4.500.335-01

This record stands for: low-current electromagnetic relay, 32 series, corresponding to the execution according to the RF Passport4.500.335-01.

However, such designations are rare. More often abbreviated versions are found without explicit indication of GOST:

RES32 335-01

Also, the chassis (on the case) of the device is marked with the date of manufacture and batch number. Details are contained in the technical data sheet for the product. Each device or batch is completed with a passport.

Conclusions and useful video on the topic

The video tells a lot about how electromechanical switching electronics work. The subtleties of structures, features of connections and other details are clearly marked:

Electromechanical relays for quite a long time are used as electronic components. However, this type of switching devices can be considered obsolete. Mechanical devices are increasingly being replaced by more modern devices - purely electronic ones. One such example is solid state relay.

Have any questions, found shortcomings, or are there interesting facts on the topic of becoming which you can share with the visitors of our site? Please leave your comments, ask questions, share your experience in the block for communication under the article.

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