An electromagnetic relay is an electrically operated switch that uses an electromagnet to mechanically operate a switching mechanism. When an electric current flows through the coil of the relay, it generates a magnetic field that attracts a movable armature, thereby opening or closing a set of contacts. This allows one electrical circuit to control another circuit without direct electrical connection between them.
In simple terms, an electromagnetic relay allows a low-power control circuit to switch high-power electrical loads safely and efficiently. It acts as an automatic switch controlled by an electrical signal.
Construction and Parts of Electromagnetic Relay
Referring to the diagram, the major parts of an electromagnetic relay are described below:
Iron Core
- The iron core is a soft iron piece placed inside the coil.
- It enhances the magnetic effect produced by the coil.
- It helps in attracting the armature efficiently.
Coil
- The coil is made of several turns of insulated copper wire wound around the iron core.
- When an electric current passes through the coil, it produces a magnetic field that attracts the armature.
Armature
- The armature is a movable soft iron piece pivoted at one end.
- It moves when the coil is energized, causing the contact points to open or close.
Contact Points
- These are electrical terminals that open or close the circuit.
- There are generally two types:
- Normally Open (NO): Contacts that close when the relay is energized.
- Normally Closed (NC): Contacts that open when the relay is energized.
Spring
- The spring returns the armature to its normal position when the coil is de-energized.
- It ensures the contacts return to their default state.
Air Gap
- The air gap is the small distance between the armature and the core.
- It allows free movement and prevents sticking due to residual magnetism.
Terminals
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The terminals are external connections for the coil supply and the contact circuits.
Adjustable Back Rest
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It adjusts the distance between the armature and the contact points for fine-tuning the relay operation.
Insulators
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They provide electrical insulation between the conducting parts to prevent short circuits.
Iron Frame
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The iron frame supports all components and provides a magnetic path for flux.
Diagram Explanation
The diagram of an electromagnetic relay shows The coil wound on the iron core, connected to the supply terminals.
- The armature positioned over the core with an air gap.
- The contact points connected via terminals and controlled by the armature.
- A spring attached to bring the armature back when the coil is de-energized.
Working Principle of Electromagnetic Relay
The working principle of an electromagnetic relay is based on electromagnetism. When current flows through a coil (electromagnet), it creates a magnetic field. This field attracts a movable iron armature. The armature either makes or breaks contact with fixed points, thereby switching the circuit ON or OFF.
Step-by-Step Working:
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De-energized condition (OFF state): When no current flows through the coil, the armature is held by a spring in its normal position. The contact points remain open (in normally open relays) or closed (in normally closed relays).
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Energized condition (ON state): When a voltage is applied to the coil terminals, current passes through the coil and produces a magnetic field. This magnetic field magnetizes the iron core and attracts the armature. The movement of the armature either closes or opens the contact points, depending on the relay configuration. When the contacts close, the circuit is completed, and current flows through the load.
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Return to normal state: When the supply to the coil is cut off, the magnetic field disappears. The spring pulls the armature back to its original position. The contacts return to their normal state, thus disconnecting the load circuit.
This process enables control of a high-power circuit using a small control signal.
Types of Electromagnetic Relays
| Category | Type | Description |
|---|---|---|
| Based on Operation Principle | Attracted Armature Relay | Uses an armature attracted toward an electromagnet; commonly used in DC circuits. |
| Based on Operation Principle | Induction Type Relay | Operates using electromagnetic induction; mainly used in AC circuits. |
| Based on Contacts | Normally Open (NO) | Contacts remain open in de-energized state and close when the relay is energized. |
| Based on Contacts | Normally Closed (NC) | Contacts remain closed in de-energized state and open when the relay is energized. |
| Based on Poles & Throws | SPST / SPDT / DPST / DPDT | Single or double pole and throw configurations for different switching requirements. |
| Based on Application | Overcurrent Relay | Protects circuits from excessive current by disconnecting the supply. |
| Based on Application | Undervoltage Relay | Activates when supply voltage drops below a preset level. |
| Based on Application | Differential Relay | Detects faults by comparing current between two or more points in a system. |
Advantages of Electromagnetic Relay
- Electrical Isolation: Provides isolation between the control and load circuits.
- High Load Capacity: Can switch high voltages and currents using small control signals.
- Reliability: Mechanical design ensures long service life under proper use.
- Simplicity: Simple construction and easy to troubleshoot.
- Flexibility: Can be used in both AC and DC circuits.
- Cost-effective: Inexpensive compared to solid-state alternatives for basic control needs.
Disadvantages of Electromagnetic Relay
- Mechanical Wear: Contacts and springs wear out over time due to mechanical movement.
- Slow Operation: Slower response compared to solid-state relays.
- Noise: Produces clicking sounds during switching.
- Arcing: Contact points can produce electrical arcs, especially with high current loads.
- Limited Switching Frequency: Not suitable for high-speed switching applications.
- Size: Larger than electronic switching devices.
Applications of Electromagnetic Relay
- Automotive Industry: Used in car starters, horns, and lighting systems.
- Home Appliances: Employed in washing machines, microwave ovens, and HVAC systems.
- Industrial Control Systems: Used for motor control, automation, and process control circuits.
- Power Systems: For overcurrent, undervoltage, and fault detection protection.
- Communication Equipment: Switching and routing signals in telecom networks.
- Computers and Electronics: Isolation and interfacing between microcontrollers and heavy loads.
- Alarm and Safety Circuits: To activate alarms or safety mechanisms during fault conditions.
Electromagnetic Relay Summary
| Parameter | Details |
|---|---|
| Device Name | Electromagnetic Relay |
| Principle | Electromagnetism |
| Main Parts | Coil, Armature, Contacts, Iron Core, Spring |
| Function | Opens or closes circuits using electromagnetic force |
| Control Signal | Low voltage/current |
| Output Load | High voltage/current |
| Applications | Protection, Control, Automation, Power Systems |
Conclusion
An electromagnetic relay is a fundamental and widely used switching device in electrical and electronic systems. It bridges the gap between low-power control circuits and high-power load circuits, ensuring both safety and efficiency. Despite the rise of solid-state relays, electromagnetic relays remain popular due to their robustness, simplicity, and reliability. Understanding its working, construction, and applications is essential for anyone studying or working in electrical engineering or automation fields.