During the past two decades, the world has been witnessing the intensification of competition. This development has been triggering the designers to infuse innovation in products. One of the ways that these designers adopt to infuse innovation is the automation of the functioning of the products. While making this effort, these designers either choose the automating devices that are already available in the market or design new such devices to automate the operations performed by the products. Many of these automating devices depend upon connecting and disconnecting the electrical circuits by switching on and switching off the same for managing the operations. Most of this kind of automation involving the connection and disconnection of electrical circuits is carried out by using electromechanical relays. Hence, it is really important to know what a relay is, how it works, and the areas of application.
Let’s start with its definition.
What is an electromechanical relay?
The electromechanical relay is a remote-controlled switch that opens and closes its contacts as the result of an input signal applied to its coil. It is capable of switching multiple circuits, either individually, simultaneously, or in sequence.
The electromechanical relay is used as an interface between the control circuit and the load. It takes a relatively small amount of power to turn on a relay but the relay can control something that draws much more power.
The coil that controls the relay needs low power to switch the contacts. Output contacts of the relay can be connected with high capacity loads such as contactors, protection devices, etc.
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 can be used for many purposes. The main purposes 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.
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Electromechanical relay construction
The main components of an electromechanical relay are:
The armature of the relay is the moving part of the magnetic system that closes and opens the magnetic circuit and acts via an actuator or the moving relay contacts.
Contacts are moved by the magnetic system to switch the load circuit. Contacts carry the main energy.
Pins connect the contact system to the load or the relay sockets.
The relay coil generates the magnetic field to actuate the armature and the contacts. It can be supplied with AC or DC.
PCB of a relay consists of protective circuitry and status display.
How does an electromechanical relay work?
The electromechanical relay works on the principle of an electromagnet. An electrical current in a coil generates a magnetic flux through the ferromagnetic core. The resulting force is exerted on the armature, which moves the contacts into the working position from the idle position using the mechanical transmission. The armature falls back into the idle position by the so-called de-energizing of the coil.
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.
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
The standard contact consists of a pair of contact pills and, depending on the contact material, is predominantly used for control and power relays.
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.
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 the control circuits. Some of them are:
- Contacts can switch AC or DC.
- Small 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:
The trouble where discharge between opposing conductors causes a short-circuit. This often occurs with contacts used with medium and large power.
Welding, locking, or gluing make it difficult to open contacts.
The wear of contacts is due to mechanical causes, such as wear during repeated operation.
The expending of contacts due to electrical, thermal, chemical, and other causes all through the repeated operation.
The failure where contact surfaces become dirty and discharge occurs easily.
Metal oxide, sulfide, and other films are generated on or attached to contact surfaces and cause boundary resistance.
The magnetic characteristics are caused by the shape around directly opposing magnetic surfaces.
Noise due to mechanical vibration caused by AC poles or rectifier wave drives with insufficient smoothing.
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.