Electromechanical Relay: Working Principle, Construction, Basics

The last 20 years have seen an increase in competition throughout the world. This new discovery has inspired product designers to include innovation. The automation of the items’ functionality is one strategy these designers utilize to inject innovation. In order to automate the tasks carried out by the items, these designers either select the automating devices that are currently on the market or create brand-new products. In order to manage their operations, many of these automated gadgets rely on joining and disconnecting electrical circuits by turning them on and off the same.

Electromechanical relays are primarily used in this type of automation which involves connecting and disconnecting electrical circuits. Therefore, understanding what a relay is, how it functions and where it might be used is crucial.

What is an Electromechanical Relay?

The electromechanical relay is a remote-controlled switch that responds to input signals given to its coil by opening and closing its contacts. It has the ability to switch several circuits either separately, concurrently or sequentially.

The electromechanical relay serves as a conduit between the load and the control circuit. The relay requires only a little amount of power to activate but it can control devices that consume much more power.

Low power is required to switch the contacts on the coil that operates the relay. High-capacity loads such as contactors and protection devices can be connected to the relay’s output connections.

Other definitions of the electromechanical relay in the automation market are:

• Electromagnetic relay
• Auxiliary relay
• Miniature relay
• Power relay
• Pluggable relay
• Electrical relay
• Control relay

The electromechanical relay has a wide range of applications. The primary goals are:

Switching of large electrical loads with a low voltage control circuit.

Reducing a higher voltage to a control voltage level.

Isolating the load circuit and control circuit galvanically.

Converting a single input to multiple outputs.

Electromechanical Relay Construction

The main components of an electromechanical relay are:

Armature

The armature of the relay is the moving component of the magnetic system that functions through an actuator or the moving relay contacts to close and open the magnetic circuit.

Contacts

Contacts are moved by the magnetic system to switch the load circuit. Contacts carry the main energy.

Connection pins

Pins connect the contact system to the load or the relay sockets.

Coil

The relay coil generates the magnetic field to actuate the armature and the contacts. It can be supplied with AC or DC.

PCB

PCB of a relay consists of protective circuitry and status display.

How Does an Electromechanical Relay Work?

An electromagnet serves as the electromechanical relay’s operating system. A magnetic flux is created through a ferromagnetic core by an electrical current in a coil. The resulting force is applied to the armature which uses mechanical transmission to shift the contacts from the idle state into the working position. Through the ‘de-energizing’ of the coil the armature returns to its idle state.

Relay has a simple working principle. You can check the video below for a better understanding.

Output Contacts of Electromechanical Relays

An electromechanical relay has mechanical contacts which are designed as normally open, normally closed or change-over contacts.

1-Normally open contact

A contact is referred to as a closing, normally open or working contact if it is open when the coil is de-energized. It closes when the coil is charged with current flow.

2-Normally closed contact

A contact that interrupts the circuit when the coil is activated is called an opener, normally closed or break contact.

3-Change-over contact

A combination of normally closed and normally open contact is referred to as a changer, change-over or switch-over contact. The roots of the normally open and normally closed contacts are connected. Therefore the change-over contact has three connections.

Contact Types of Electromechanical Relays

1-Standard contact

The standard contact consists of a pair of contact pills and depending on the contact material is predominantly used for control and power relays.

2-Double contact

The double contact has two pairs of contact pills. The contact reliability is increased by up to a factor of 100 as a result. It is used in signal and control relays.

3-Pre-run contact

The pre-run contact consists of a contact fitted with a highly heat-resistant contact material and of a later closing contact consisting of a further contact material featuring good electrical conductivity at a nominal load. This contact is mainly used for switching high inrush currents.

Contact forms of relays

3 pin, 4 pin and 5 pin relays

Relays are also available with different pin configurations like 3-pin, 4-pin and 5-pin relays. How these relays are operated is shown in the below figure:

Electromechanical Relay Applications

Electromechanical control relays are widely used in most applications and devices that use electricity such as:

• Home electrical appliances: Refrigerators, washing machines
• Industrial machinery: Industrial robots, cutting machines, conveyors
• Plants: Chemical plants, transformer substations, power plants
• Scientific equipment: Laboratories
• Automatic vending machines and entertainment equipment
• Communications and measurement equipment
• OA devices: Copy machines
• Automotive electrics
• Control and automation panels

Advantages of Electromechanical Relays

Electromechanical relays have many benefits in control circuits. Some of them are:

• Contacts can switch AC or DC.
• Small in size and simple construction.
• Low initial cost. (Check price)
• Easy to mount.
• Very low contact voltage drop, thus no heat sink is required.
• High resistance to voltage transients.
• No Off-State leakage current through open contacts.

Disadvantages of Electromechanical Relays

Electromechanical relays have some weaknesses. Some of them are:

• The contacts wear and thus have limited life depending on loads.
• Low speed of operation.
• Low isolation voltage.
• Change in characteristics due to aging.
• Short contact life when used for rapid switching applications or high loads.
• Poor performance when switching high inrush currents.

What Causes an Electromechanical Relay to Fail?

The most common electromechanical relay failures are:

Flashover

The trouble is where discharge between opposing conductors causes a short circuit. This often occurs with contacts used with medium and large power.

Sticking

Welding, locking or gluing make it difficult to open contacts.

Contact wear

The wear of contacts is due to mechanical causes such as wear during repeated operations.

Contact erosion

The expanding of contacts due to electrical, thermal, chemical and other causes all through the repeated operation.

Activation

The failure where contact surfaces become dirty and discharge occurs easily.

Contact film

Metal oxide, sulfide and other films are generated on or attached to contact surfaces and cause boundary resistance.

Fringing effect

The magnetic characteristics are caused by the shape around directly opposing magnetic surfaces.

Humming

Noise due to mechanical vibration caused by AC poles or rectifier wave drives with insufficient smoothing.

Soak

Removing the difference due to the effect of the magnetic history by applying a saturation current to the operating coil during the measurement of the must-operate and must-release voltage (or current) or during testing.