Electromagnetic Relays Basics: Concepts, Types and Advantages

What Are Electromagnetic Relays?

Electromagnetic relays are electrical devices that use electromagnets to control the flow of electric current in a circuit. They are commonly used in various applications to switch or control high-power circuits using low-power control signals.

Figure 1 Electromagnetic Relay

Basic components

Electromagnet: It is the primary component of the relay and consists of a coil of wire wound around a ferromagnetic core. The flow of electric current within the coil results in the creation of a magnetic field. Armature: The armature is a movable component that is attracted to the electromagnet when the coil is energized. It is typically a ferromagnetic material and is mechanically connected to the contacts. Contacts: The contacts are stationary or movable metal pieces that make or break the electrical connection when the relay is operated. There are two types of contacts: normally open (NO) and normally closed (NC). In the resting state, the NO contacts are open, and the NC contacts are closed. Spring: A spring is used to provide a restoring force to the armature when the coil is de-energized. It helps to reset the relay to its initial state when the magnetic field is removed.

Figure 2 Basic Components

Electromagnetic Relay Types

There are various types of electromagnetic relays, each designed for specific applications and operating conditions. Here I present several popular types. Single-Pole Single-Throw (SPST) Relay: This is the simplest type of relay with a single set of contacts. It has one normally open (NO) contact that is either open or closed when the relay is energized. Single-Pole Double-Throw (SPDT) Relay: This relay has one common terminal connected to either of two other terminals. It has one normally open (NO) contact and one normally closed (NC) contact. When the relay is energized, the NO contact closes, and the NC contact opens. Double-Pole Single-Throw (DPST) Relay: This relay consists of two separate SPST relays operated simultaneously by a single coil. It has two sets of contacts, each with a normally open (NO) and a normally closed (NC) contact. Double-Pole Double-Throw (DPDT) Relay: This relay has two separate sets of contacts, each with a common terminal. It provides two normally open (NO) contacts and two normally closed (NC) contacts. When the relay is energized, the contacts change their position simultaneously. Latching Relay: Also known as a bistable relay, this type of relay maintains its state even after the control signal is removed. It has two stable states, set and reset, and requires a pulse of current in one direction to change its state and another pulse in the opposite direction to return to its original state. Reed Relay: Reed relays use a small, sealed glass tube containing two magnetic reeds, which act as the contacts. When a magnetic field is applied, the reeds close, allowing current to flow. Reed relays are compact, have fast switching times, and offer high insulation resistance. Time Delay Relay: These relays incorporate a timing mechanism to introduce a delay between the control signal activation and the switching of contacts. They are used in applications that require time delays, such as motor control, lighting control, or sequencing operations. Solid-State Relay (SSR): Unlike traditional electromagnetic relays, SSRs use semiconductor devices, such as thyristors or triacs, to perform the switching. They have no moving parts, offer faster switching speeds, and high reliability, and are ideal for applications where silent operation and long lifespan are required.

Figure 3 SSR Overload Relay: Overload relays are designed to protect motors and other equipment from excessive current. They typically include current sensing elements that detect the current flowing through the load and trip the relay if the current exceeds a predefined threshold.  

How Do Electromagnetic Relays Works?

The functioning of electromagnetic relays is based on the concept of electromagnetic induction. When an electrical current flows through the coil of a relay, it generates a magnetic field around the coil. This magnetic field then interacts with other components of the relay to control the flow of current in the circuit. So now let’s dive into how they work. Coil energization: A low-power control signal, such as a voltage or current, is applied to the coil of the relay. This control signal energizes the coil and causes an electric current to flow through it. Magnetic field generation: As the current flows through the coil, it creates a magnetic field around the coil. The coil is typically wound around a ferromagnetic core, which enhances the magnetic field's strength. Magnetic attraction: The generated magnetic field attracts the movable component of the relay, known as the armature or plunger. The armature is typically made of a ferromagnetic material that can be magnetized. Contact switching: The movement of the armature is mechanically connected to one or more sets of contacts within the relay. When the armature is attracted by the magnetic field, it moves towards the electromagnet, causing the contacts to change their position.

1. Normally Open (NO) contacts: In their resting state, the NO contacts are open, meaning there is no electrical connection between them. When the armature is attracted, it closes the NO contacts, allowing current to flow through the relay.
2. Normally Closed (NC) contacts: In their resting state, the NC contacts are closed, providing a continuous electrical connection. When the armature is attracted, it opens the NC contacts, breaking the electrical connection.

Current control: By opening or closing the contacts, the relay can control the flow of current in a separate, higher-power circuit connected to the relay's contacts. This enables the relay to switch devices on or off or to control the operation of different parts of a circuit. Release and restoration: When the control signal is removed or de-energized, the magnetic field generated by the coil collapses. This causes the armature to return to its original position due to the force of a spring or other restoring mechanism. Contact state reversal: As the armature returns to its initial position, the contacts also return to their resting state. The NO contacts open, and the NC contacts close, reversing their positions from the previous state. This cycle of energizing the coil, generating a magnetic field, attracting the armature, switching the contacts, and restoring the relay to its initial state repeats as long as the control signal is applied or removed. Electromagnetic relays are widely used in various applications where electrical isolation, high switching capacity, or control of high-power circuits is required.

Electromagnetic Relay VS Solid State Relay

Electromagnetic relays and solid-state relays are two different types of relays that serve similar functions but operate in different ways. Here is a chart about their differences.  

Criteria	Electromagnetic Relay	Solid-State Relay
Working Principle	Uses an electromagnet to control the switching	Relies on semiconductor devices for switching
Moving Parts	Yes	No
Switching Speed	Slower	Faster
Noise	Mechanical switching noise	Silent operation
Contact Wear	Yes	No
Lifespan	Limited	Longer
Shock and Vibration Resistance	Moderate	High
Temperature Resistance	Moderate	High
Voltage and Current Handling	High	Moderate to High
Suitable for AC and DC Loads	Yes	Yes
Applications	Motor control, industrial automation, power distribution	Control of heaters, motor drives, HVAC systems, photovoltaic systems

 

Electromagnetic Relay VS Electromechanical Relay

Electromagnetic relays and electromechanical relays are the same thing and the terms are used interchangeably. These relays use an electromagnet to control contact switching. Consider the following key points when comparing electromagnetic relays with other types of relays: Working Principle: Electromagnetic relays operate based on the principle of electromagnetic induction. When an electric current flows through the coil, it generates a magnetic field, which attracts or repels a movable armature. This movement of the armature causes the contacts to open or close, allowing or interrupting the flow of current in the load circuit. Construction: Electromagnetic relays consist of several components, including a coil, armature, spring, and contacts. The coil is wound around a core and generates the magnetic field when energized. The armature is typically a movable iron or steel component that is attracted or repelled by the magnetic field. The contacts are typically made of conductive materials and are connected to the load circuit. Advantages

• Good electrical isolation between the control circuit and the load circuit.
• Capable of handling high currents and voltages.
• Can switch both AC and DC loads.
• Suitable for applications with varying voltage levels or inrush currents.

Applications

• Motor control and protection.
• Industrial automation and process control.
• Power distribution and switching in electrical systems.

Electromechanical relays are often distinguished from solid-state relays, which don't have moving parts and rely on semiconductors for switching. However, both terms are generally used interchangeably. Electromechanical relays are widely used for their electrical isolation, high current/voltage handling, and compatibility with different load types. They remain a reliable choice despite the emergence of solid-state relays in some applications.

The Advantages of Electromagnetic Relay

Electromagnetic relays offer several advantages that make them a popular choice in various applications. Here are some key advantages of electromagnetic relays: Electrical Isolation: One significant advantage of electromagnetic relays is the electrical isolation they provide. The control circuit, which energizes the coil, is completely isolated from the load circuit connected to the contacts. This isolation helps protect sensitive control circuitry from voltage spikes, surges, or other electrical disturbances in the load circuit. Versatility: Electromagnetic relays are versatile and can handle a wide range of voltages, currents, and power levels. They can be used in both AC and DC circuits, making them suitable for various applications. Additionally, electromagnetic relays can switch both low-power and high-power loads, making them adaptable to different power requirements. Reliability: Electromagnetic relays are known for their reliability. They have a long operating life and can withstand high temperatures, vibrations, and other harsh environmental conditions. This reliability makes them suitable for demanding industrial applications where durability is crucial. Easy Control: Electromagnetic relays are easy to control. When the coil is energized, the relay switches its contacts, either opening or closing the circuit. This simplicity of control makes them compatible with a wide range of control systems, including microcontrollers, PLCs (Programmable Logic Controllers), and other electronic devices. Cost-Effective: Electromagnetic relays are generally more cost-effective compared to solid-state relays or other switching devices. They are widely available, and their simplicity in design and operation contributes to their affordability. This cost-effectiveness makes them a preferred choice, especially in applications where budget constraints are a consideration. Compatibility with Different Loads: Electromagnetic relays can handle various types of loads, including resistive, inductive, and capacitive loads. Whether it's controlling lights, motors, solenoids, or other devices, electromagnetic relays can effectively handle the switching requirements. This versatility makes them suitable for a wide range of applications. Fail-Safe Operation: In the event of a power loss or failure, electromagnetic relays typically return to their default state. For example, a normally closed (NC) relay will automatically close its contacts when power is lost. This fail-safe operation ensures that critical systems or safety functions can be maintained even in unforeseen circumstances. These advantages make electromagnetic relays a reliable and versatile choice for various applications, from industrial control systems to automotive electronics and beyond. However, it's important to consider the specific requirements of your application and consult with an expert to determine the most suitable relay type for your needs.