Electromechanical Relays

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What Is Electromechanical Relays?

Electromechanical relays are electrically operated switches used to isolate circuits or batteries, detect faults on transmission and distribution lines, and control a high powered circuit using a low power signal.

Benefits of Electromechanical Relays

Simple and robust design

Electromechanical relays are built with straightforward components that make them strong and reliable for various uses.

Low cost for basic models

Even the most basic versions of these relays are quite affordable, making them a cost-effective choice for many electrical applications.

High power switching capability

They can manage large amounts of electricity, allowing them to control big machines and equipment without trouble.

Visible switching state

You can actually see the contacts move when the relay switches on or off, which helps in understanding and diagnosing the system.

Wide availability

These relays are commonly found in stores and supply chains, making it easy to get one when needed for repairs or new projects.

Types of Electromechanical Relays

Electromechanical relays can be classified into various types based on their structure, operation, and application. Some common types include:

General purpose relays

These are the most common type of electromechanical relays and are used in a wide range of applications. They typically have multiple contacts and can handle a variety of voltages and currents.

Reed relays

Reed relays consist of two or more reed switches sealed in a glass tube. They offer fast switching times and are used in applications requiring low power consumption and high speed.

Power relays

Power relays are designed to handle higher voltages and currents. They are used in applications such as motor control and high-power switching.

Time delay relays

These relays incorporate a timer circuit that delays the opening or closing of the contacts for a specific period. They are used in applications requiring time-based control, such as industrial automation and process control.

Relay Applications

Relays are used to protect the electrical system and to minimize the damage to the equipment connected in the system due to over currents/voltages. The relay is used for the purpose of protection of the equipment connected with it. These are used to control the high voltage circuit with low voltage signal in applications audio amplifiers and some types of modems. These are used to control a high current circuit by a low current signal in the applications like starter solenoid in automobile. These can detect and isolate the faults that occurred in power transmission and distribution system. Typical application areas of the relays include:

Lighting control systems

Telecommunication

Industrial process controllers

Traffic control

Motor drives control

Protection systems of electrical power system

Computer interfaces

Automotive

Home appliances

Components of Electromechanical Relays

Typically, an electromechanical relay consists of four main components:

Coil

The coil generates the magnetic field when an electric current flows through it.

Armature

This is a movable metal piece which responds to the magnetic field generated by the coil.

Spring

The spring returns the armature to its original position when the coil is de-energized.

Contacts

These are the switch parts that open or close the circuit depending on the armature position.

Protective Relay Testing Procedure

The proper cleaning and testing of protective relays is usually found in the manufacturer’s instruction leaflet. However, if this type of documentation is not available, follow the procedure below to test medium voltage electromechanical devices.

1. Visual Inspection

A: Remove the relay cover

Inspect the gasket of the cover
Inspect for cracks or frame tightness
Clean the covers and glass thoroughly

B: Remove the relay assembly from the case

Short circuit the CT terminals for safety
Open all of the trip circuits

C: Foreign objects like metal bits and dust should be remove from the case and the relay. These may cause problems on the mechanical parts and erratic operation of the relay.
D: Blow dust by blowing air gently using a hand syringe.
E: Metal bits or corrosion should be remove from the magnet poles or disc using a brush or magnet cleaner.
F: Hold the relay up to the light to ensure that the gap has good clearance and that the disc does not rub.
G: Check for moisture problems. If you see rust spots on the relay, it is important to check if the relay is in proper operational environment. Moisture can cause severe corrosion and problems in the mechanical components.
H: Check for loose connections. Taps, screws, bolts, nuts and pivotal joints should be tight.
I: The bearings should be smooth. To check, the disc is rotated manually to close the contacts and letting the action of the spiral spring to the relay disc to its de-energized position. You should observe for smoothness and should not be sluggish. Clean and put oil on the mechanism. However, if cleaning and oiling fails, the relay must be reconditioned or replaced.
J: The operation of targets should be manually checked. This is done by fitting the armatures and checking if there is a showing target.
K: The relay coil must be inspected to ensure that it is not subjected to high currents for a long time.
L: The components that touch together during a relay’s normal de-energized position must be cleaned. This is to prevent the relay from getting stuck or operate erroneously especially on low current faults.

2. Electrical Testing

A: Disconnect the relay from the trip and power circuits for testing.
B: Secondary injection testing.

This allows you to check the operation of the circuit breaker, relay connections and the relay assembly.
This testing method is conducted by injecting current directly into the relay.

3. Other Tests

Insulation resistance measurement: This test ensures that the insulation of the relay is within the acceptable operating limits. Ideally, the relay insulation should have a resistance of a few MΩ to TΩ.
Seal-in operation and target testing : Most protective relays installed on electrical facilities are the seal-in and target combination. The test verifies that the contacts will seal-in to its close position with the minimum DC current.
Zero Check: This test is conducted on relays that has a time dial. This test is done to determine the time dial reading when the moving contacts are closed by turning the time dial to zero position and the relay is fixed.
Testing Relay Pick-up: This will test the min and max frequency, voltage, current to close the relay.
Instantaneous Testing: There are some protective relays that operate instantaneously These types usually have a separate instantaneous component. This type of relay doesn’t have any intentional delay. An instantaneous components at twice the pick-up must operate between 0.016 and 0.30 seconds.

History

Relays were first developed back in the 1800s when it became necessary to use them for electrical protection. In the 1980s, static relays became the norm, and mechanical relays were being replaced by them. Static relays operated with analog circuitry and were considered more straightforward, as they had no moving parts. Digital relays are now replacing older types of relays. They are more accurate and reliable, since they use microprocessors that use numeric systems of counting, which are more exact. Electromechanical relays are considered the basis of electrical protection systems. Even though electromechanical relays are being replaced in some instances with microprocessors which are number-based devices, there still are a lot of relays in use today. The very first electromechanical relays were invented and used in circuits for long-distance telegraph as means of amplifying. These relays repeated the signal that came in from one circuit and then transmitted it on to a second circuit. Electromechanical relays were most commonly used in the first computers and also in telephone exchanges in order to perform logical computations and operations.

Relay Switch Basics

A relay is a form of electrical switch that is operated by electromagnet which changes over the switching when current is applied to the coil.

These relays may be operated by switch circuits where the switch cannot take the high current of the electrical relay, or they may be operated by electronic circuits, etc. In either circumstance they provide a very simple and attractive proposition for electrical switching.
Relays have a number of basic parts that form the relay.

Frame: A mechanical frame is required to hold the components in place. This frame is normally quite robust so that it can firmly support the additional elements of the electromechanical relay without relative movement.

Coil: A coil wound round an iron core to increase the magnetic attraction is needed. The coil of wire causes an electromagnetic field to be created when the current is switched on an causes the armature to be attracted.

Armature: This is the moving part of the relay. This element of the relay opens and closes the contacts and it has a ferromagnetic metal to be attracted by the electromagnet. The assembly has an attached spring which returns the armature to its original position.

Contacts: The contacts are operated by the action of the armature movement. Some of the electrical switching contacts may close the circuit when the relay is activated where as others may open a circuit. These are known as normally open and normally closed.
Relay design involves a number of aspect. It is a key element of the design to obtain the required magnetic flux to attract the armature sufficiently quickly, without consuming excessive current. Also it is necessary to ensure that the relay can open quickly once the energising current is removed. Magnetic retention in the materials needs to be low.

When a current flows through the coil an electro-magnetic field is set up. The field attracts an iron armature, whose other end pushes the contacts together, completing the circuit. When the current is switched off, the contacts open again, switching the circuit off.
When specifying electromechanical relays, it will be seen that the electrical switch contacts come in a variety of formats. Like ordinary electrical switches, electromechanical relays are defined in terms of breaks, poles and throws that the device has.

Basic knowledge of electromechanical relay switches

Break: Whilst may of the terms applied to electromechanical relays also apply to low power electrical switches, this one is more applicable to higher power switching. It is the number of separate places or contacts where a switch is used to open or close a single electrical circuit.

All relays are either single break or double break. A single break, SB contact breaks an electrical circuit in only one place. Then as the name indicates, a double break, DB contact breaks the circuit in two places.

Single break contacts are normally used when switching lower power devices, possibly electronic circuits or low power electrical switching applications. Double break contacts are used for the electrical switching of high power devices. If one of the contacts sticks, then the other one is likely to still switch and break the circuit.

Pole: The number of poles that an electrical switch posses is the number of different sets of switching contacts that it has. A single pole switch can only switch one circuit, whereas a double pole switch can switch two different and isolated circuits at the same time. A single pole switch is often denoted by the letters SP, and a double pole by DP. Relays can have one, two or more poles.

Throw: The number of throws on an electrical switch is the number of positions that are available. For an electromechanical relay, there are normally only one or two throws. A single throw relay will make and break a circuit, whereas a double throw relay will act as a changeover routing a connection from one end point to a different one. Single throw and double throw are often denoted by the letters ST and DT.
For example an electrical relay specification may quote a single pole, single throw: SPST or one may be described as double pole single throw: DPST, etc. These terms enable the number of sets of switch contacts and whether they are an open / close or whether they provide a change-over function.

The Trends in Electromechanical Relay Tech

As technology advances, electromechanical relays are evolving to meet the ever-changing demands of various industries. Some noteworthy trends and developments include:

01/

Miniaturization: The ongoing trend of miniaturization has led to the development of compact, high-performance relays that can fit in smaller spaces without compromising on functionality.

02/

Energy efficiency: As the world moves toward greener and more sustainable energy solutions, electromechanical relay manufacturers are focusing on designing energy-efficient products that consume less power while still delivering excellent performance.

03/

Enhanced reliability: New materials and manufacturing techniques are being employed to improve the reliability and durability of electromechanical relays, extending their service life and reducing maintenance requirements.

04/

Integration with IOT and industry 4.0: With the advent of the Internet of Things (IOT) and Industry 4.0, electromechanical relays are being integrated into smart systems, allowing for real-time monitoring, remote control, and advanced diagnostics.

Differences Between Solid State Relay and Electromechanical Relay

The following table shows the comparison between electromechanical relay (EMR) and solid state relay (SSR).

Electromechanical Relay	Solid State Relay
An electromechanical relay is a type of relay that uses physical contacts to switch circuits.	Solid state relay is a type of relay that uses a semiconductor PN junction to switch circuits.
It uses an electromagnetic coil to generate magnetic force to pull the armature down and switch the contacts.	It has an optocoupler that generates a light signal. It triggers the photosensitive SCR or TRIAC and allows the flow of current.
It has physical contact or moving parts.	It has semiconductor switches, therefore it does not have moving parts.
It consumes relatively large power to switch the load circuit.	It requires very low power and provides an energy-efficient solution.
It has usually multipole and multi-throw contacts	It has only a single set of contacts.
Its contacts can switch both AC and DC as well.	It can either switch AC or DC.
It has a low switching speed with a very low initial cost.	It has a high initial cost with a very high switching speed even compatible with digital logic circuits.
Vibrations and mechanical shocks affect its performance.	Since there are no moving parts, vibration or mechanical shock does not affect it.
Arcs are generated during the switching of physical contacts.	There are no arcs generated as there are no mechanical contacts.
It generates switching noise as well as electromagnetic interference in the system.	There is no arcing to generate EMI in the power system.
Its contacts get eroded and wear over time. Thus it has a lower lifespan.	There are no contacts to wear thus it has a longer lifespan.
The arcs create hazards in the environment having volatile and combustible materials.	They are safe to operate in a volatile combustible environment.
It has poor performance for frequent switching of large currents.	It has better performance for rapid switching of large currents.
It has no leakage current in the open state.	It has leakage current through its PN junction in the open state.
It has a negligible contact voltage drop.	It has a large ON state voltage drop at its PN junction.
It does not require heat sinks.	Voltage drops generate a large amount of heat thus they require large heat sinks.

FAQ

Q: What are electromagnetic relays used for?

A: Electromagnetic relays are components that control circuits using an electrical signal. They are used in applications where it is necessary to control one or more circuits with one electrical supply circuit. The supply may or may not be isolated from the relay circuit.

Q: Where are electro mechanical relays used?

A: Our electromechanical relays are typically used for – among many other applications – electrical isolation, controlling power in manufacturing and transportation applications, and for switching smaller current values in a control circuit, such as in building automation technology and control panels.

Q: What is the benefit of using an electro mechanical relay?

A: They are the oldest and simplest type of relays, but they are still widely used in some power systems. The advantages of electromechanical relays are their reliability, durability, and low cost. They can also withstand harsh environments and operate without external power sources.

Q: What is the purpose of a mechanical relay?

A: Mechanical relays are devices that can turn on or turn off the power supplied to another device, like a switch. However, instead of having a person flip the switch, mechanical relays switch when provided with a small amount of power.

Q: What are the examples of electromagnetic relays?

A: Depending upon working principle these can be divided into following types of electromagnetic relays. Attracted Armature type relay, Induction Disc type relay, Induction Cup type relay, Balanced Beam type relay, Moving coil type relay, Polarized Moving Iron type relay.

Q: Why do people use relays?

A: The relay permits a small amount of electrical current to control high current loads. When voltage is supplied to the coil, small current passes through the coil, resulting in a larger amount of current passing through the contacts to control the electrical load.

Q: Are electromechanical relays still used?

A: Electromechanical Relays are the oldest type of relays. Electromechanical, have been gradually been replaced by Static and then by Numerical Relays. However, Electromechanical Relays are still being used. They are quick acting and can be reset fast.

Q: Which kind of device is an electromechanical relay?

A: A relay is an electromagnetic switch that can open and close circuits electromechanically or electronically. There is a relatively small current required to operate a relay. Usually, they are used to regulate low currents in a control circuit. However, we can also use relays to control high electric currents.

Q: What is the difference between electromechanical and electronic relays?

A: Solid-state relays operate silently and produce little electrical interference. They are selected for quiet operation, because they do not make noise when the output contacts change states. Electromechanical relays create electromagnetic noise, because of contact arcing and can create interference in the power lines.

Q: What device can be used to replace electromechanical relays because it is faster?

A: SSRs are a faster alternative to electromechanical relays because their switching time is dependent on the time required to power the LED on and off - approximately 1 ms and 0.5 ms respectively. Because there are no mechanical parts, their life expectancy is higher than an electromechanical or reed relay.

Q: What is the main difference between a typical electromechanical relay and an electrical contactor?

A: Load Capacity

The first difference among a relay and contactor is that both have different load capacities. Relays are used with electrical loads at a range of about 10 amperes or less, while a contactor load capacity is greater than 10 ampere.

Q: What are the problems with mechanical relays?

A: Mechanical wear and tear: Relays that are used frequently can experience mechanical wear and tear, which can cause the contacts to wear out or the actuator to fail. Environmental factors: Environmental factors such as temperature, humidity, vibration, and dust can cause a relay to fail.

Q: Why use a relay instead of a switch?

A: The primary difference between relays and switches lies in the way they operate. A switch will open or close the circuit, while a relay uses electromagnets to control the flow of electricity. This allows for more sophisticated control over power distribution and protection from potential overloads.

Q: How do mechanical relays fail?

A: Physical wear can also occur to other elements within the relay. Some relays contain springs to provide a mechanical resistance against electrical contact when a switching current is not applied. Springs will loose resiliency with time. Relays can also fail due to poor contact alignment and open coils.

Q: What device allows relays to be controlled?

A: A relay controller is a device that is used to control a bank of switches. A relay controller works by turning on and off magnetic coils under logic control. A computer controlled relay driver allows your computer to send simple commands to activate a switch or a group of switches.

Q: How many types of electromechanical relays are there?

A: Electromechanical relays which are common to commercial and industrial applications can be generally subdivided into three classifications: general purpose relays, reed relays, and machine control relays.

Q: Where are relays used in everyday life?

A: For example, when you push the button on a TV remote to watch TV, it sends an electrical signal to the “relay” inside the TV, turning the main power ON. There are various types of relays used in many applications to control different amounts of currents and number of circuits.

Q: Does a relay increase voltage?

A: Relays may be able to sustain higher voltages across their contacts than the maximum switching voltage, provided no attempt is made to operate the relay while the signal is applied.

Q: What is a 12v relay used for?

A: Automotive relays are for use in 12 volt electrical systems. Relays are used when you need to switch higher currents than a switch can handle, or when you want to isolate an electrical circuit.

Q: Are the relays are AC or DC?

A: DC relays use the wire resistance to limit current. AC relays must rely on the AC impedance to limit current, There is a shader ring on the magnetics. This shifts the phase of the current to lag by 90 degrees and keep the field energized.

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