Exploring the Types of Electromagnetic Relays

July 23, 2025 Jessica

Electromagnetic relays play a crucial role in control systems, providing reliability and safety in various industrial and residential applications, including motor control, overcurrent protection, and circuit breakers.

The conventional types of electromagnetic relays, such as electromechanical, electronic, latching, reed, solid-state, and mercury-wetted relays, employ mechanisms like one or more coils, movable elements, and various contact systems, including form-C, form-M, and form-X. Their operation hinges on whether the operating torque or force, created by electromagnetic attraction or electromagnetic induction, surpasses the restraining torque or power typically provided by springs. The relay activates when the active pressure exceeds the restraining force. These relays can be further classified based on the pole and throw configurations, such as single-pole, double-throw, triple-throw, quadruple-throw, single-pole-double-throw, single-pole-triple-throw, and double-pole-single-throw.

Types of Electromagnetic Relays

Attracted armature type
Balanced Beam type
Induction disc type
Induction cup type
Moving coil relay

Attracted Armature Type Relay

This relay is typically used in overload relays, motor control systems, and auxiliary relays such as indicating relays, alarms, and annunciator relays. This has four types, i.e., plunger, hinged armature, balanced beam, and moving iron polarized. These are the simplest types that respond to AC and DC. The operating principles of these relays make them highly suitable for applications where timing and control signal switching are critical. Industrial settings, such as power plants or factories, often rely on these types of relays to safeguard machinery from faults.

The force of attraction, F = Kl² = K(I_m sin ωt)²

1/2 K (I²_m – I²m cos2ωt)

where k is a constant, and I²_mcos2ωt is pulsating

The coil is energized by a current or voltage, a proportionate operating quantity, which produces a magnetic flux and thereby creates an electromagnetic force. This force is proportional to the square of the air gap flux or the current’s square. This force increases as the armature approaches the pole of the electromagnet.

This type of relay is used to protect small machines and for auxiliary relays such as indicating relays (flags), alarms, and annunciator relays.

Balanced Beam Type Relays

Principle

All these relays operate on the same principle: the electromagnetic force produced by the magnetic flux is generated by the operating quantity. The pressure exerted on the moving element is proportional to the square of the air gap flux or the square of the current. In DC electromagnetic relays, this force is constant. If this force exceeds the restraining force, the relay operates.

F ∝ Φ² (AC), F = K (DC)

This type of balanced beam relay has a fixed beam and an electromagnet (EM), as shown in the figure. The EM has two windings: the voltage energizes one, and the current energizes the other. Under normal conditions, the pull due to the voltage (restraint quantity) will be high; hence, the contact remains open. When a fault occurs, the current increases, and the pull due to the current will be greater than the voltage pull. This closes the circuit of the trip.

The balanced beam-type relay shown in the figure consists of a horizontal beam pivoted centrally, with one armature attached to either side. There are two coils, one on each side. The beam remains horizontal until the operating force exceeds the restraining force. The current in the coil provides the operating power, and on the other hand, it generates the restraining force. When the active force or torque increases, the beam tilts, and the contact closes. The relay then actuates the tripping mechanism of the circuit breaker, controlling the line or equipment. This type of relay finds applications in control signal circuits, especially in motor control and overload protection. It offers advantages in industrial automation, where voltage variations can lead to faults.

Induction Disc Types of Electromagnetic Relays

Watt Meter Type or Non-Directional Relay

The induction disc relay is commonly used for overcurrent protection in industrial power systems. The shaded pole type relay operates efficiently by utilizing a copper band to generate induced eddy currents, while the induction cup relay is highly sensitive and fast, making it an ideal choice for high-speed applications.

It has a metallic disc that can rotate between the poles of two electromagnets.

The spindle of this disc carries a moving contact that bridges two fixed connections when the disc rotates through an adjustable angle, ranging from 0° to 360 °. By adjusting this angle, the travel of the moving contact can be adjusted so that the relay can be set to any desired time indicated by a pointer. The dial is calibrated from 0 to 1. The relay time from the nameplate curve is to be multiplied by the time multiplier setting.

The upper magnet has two windings. The primary coil is connected to the secondary CT through tappings, which are connected to the Settings Bridge. The secondary is connected to the lower electromagnet. The torque exerted on the disc is due to the interaction of the eddy current produced by the flux from the upper EM and the lower EM. The relay setting ranges from 50% to 200% in increments of 25%.

Shaded Pole Type or Directional Relay

The rotating disc is made of aluminum. In the above type, half of each electromagnet shown in Fig. is surrounded by a copper band called the shading ring. The shaded portion of the pole produces a flux displaced in space and time in relation to the change made by the unshaded part. These two alternating fluxes cut the disc and create eddy currents in it. Torques are produced by the interaction of each shift with the eddy current produced by the other change. The resultant torque causes the disc to rotate.

A spring supplies the resetting torque, and a permanent magnet produces eddy current braking on the disc. The braking torque is proportional to the disc’s speed. When the operating current exceeds the pickup value, driving torque is generated, and the disc accelerates to a speed at which the braking torque balances the driving torque. The disc rotates at a rate proportional to the driving torque.

At a current below the pickup value, the disc remains stationary because the tension of the control spring acts against the normal direction of disc rotation. The disc rests against a backstop, which is adjustable in position. This is known as time-setting of the relay, which allows the travel distance of the relay contacts to be varied according to need.

In disc-type relays, several tappings are provided on the coil to select the desired pickup value of the current.

Induction of Cup Relay or Directional Relay

It is a double-acting quantity relay (current and voltage)
Highly sensitive
High speed
Steady, non-vibrating torque
The ratio of reset to pickup is high (above 95%)
It has an operating time of less than 0.01 sec

The operation is similar to that of the induction motor. It comprises a stationary iron core and a moving rotor conductor (cup). The moving element is a hollow cylinder or cup that turns on its axis. The driving factor is a four—or eight-pole structure placed radially around the outside of the cylinder and joined by a yoke. A stationary iron core is placed inside the rotating cylinder to shorten the air gap.

It is a high-speed induction relay since there is less inertia. The two pairs of coils are displaced by 90 °. When these coils are excited, a rotating magnetic field is produced. The rotor cuts the magnetic field, inducing an electromotive force (emf) in the rotor, which results in a current due to the short-circuit nature of the rotor. A torque is produced due to the interaction between the rotating flux and the induced current, which causes the rotation of the cylinder. The movement of the rotor closes the trip circuit. The torque produced is proportional to |I₁| |I₂| sin α or Φ₁ Φ₂ sin α, where Φ₁ Φ₂ are the fluxes due to excitation currents I₁ and I₂ of coil pairs, and α is the phase difference between the two fluxes.

Due to the rotor’s low weight and efficient magnetic system, its torque is three times that of an induction disc type. This type of relay features a high torque-to-weight ratio, enabling high-speed operation.

Moving Coil Relays

It has two types:

Rotary Moving Coil
Axial moving coil

Rotary Moving Coil Relay

This relay is widely used in high-torque applications for motor control and industrial automation systems. The force generated by the interaction of the magnetic fields is used to actuate trip circuits, especially in overload relays.

negative time current characteristics rotary moving coil

It consists of a permanent magnet coil wound on a non-magnetic former, spring, spindle, etc., as shown in Fig. The fault current energizes the coil. Due to the interaction between the permanent magnetic field and the field generated by the ring, a moving torque is developed. Due to this, the spindle rotates and closes the trip circuits. It has negative time-current characteristics, as shown in the figure. It has a high torque/weight ratio.

F α NHIL

where F = Force, N= No. of turns, H=Magnetic Field, I=Current in the coil, L=Length of coil

Axial Moving Coil Relay

It has an axially suspended coil wound on a former. The coil has only axial movement. When the coil is energized by the current, a magnetic field develops, and the existing permanent magnet repels this magnetic field. Due to this, contacts close.

This relay is more sensitive and faster than the rotary moving coil relay and has inverse operating time-current characteristics.

Future of Electromagnetic Relays

The future of electromagnetic relays is shaped by technological advancements, such as solid-state relays (SSRs) and smart relays, which are transforming control systems and automation. Let’s explore the details of SSRs, Smart Relays, and Automation:

Solid-State Relays (SSRs)

SSRs replace mechanical parts with semiconductors, offering faster switching, higher reliability, and longer lifespans compared to traditional electromagnetic relays. They are ideal for high-speed applications in industrial automation, telecommunications, and home appliances, providing overload protection and enhanced sensitivity.

Smart Relays and IoT Integration

Smart relays are integral to IoT-enabled systems, offering remote monitoring, real-time diagnostics, and predictive maintenance. They enhance energy management and smart grid applications by providing advanced control and diagnostics for industrial systems and automated manufacturing.

Automation and the Role of Relays

As automation systems evolve, relays play a crucial role in motor control, voltage regulation, and energy efficiency in industrial environments. Smart relays and solid-state technologies enable faster and more precise operations in robotic systems and electric vehicles, thereby enhancing system efficiency.

Conclusion

This article covers various types of electromagnetic relays, including attracted armature, balanced beam, induction disc (including watt meter, shaded pole, and induction cup), and moving coil (rotary and axial) relays. These relays operate on electromagnetic forces, switching contacts when the operating pressure exceeds the restraining force, and are essential in control circuits and protection systems for various applications, including power systems, automation, motor control, and signaling.

The design ensures reliable operation, featuring components such as terminals and heat-resistant materials for insulation. They control the flow of signals and energy, with crucial roles in overload protection and high-voltage systems. Electromagnetic relays are vital in electrical engineering, offering control, overload protection, and temperature protection. Their delay parameters, sensitivity, and input sources make them indispensable in power relay systems, telecommunications, and motor control applications.

FAQs

1. How does the operating force in an electromagnetic relay work?

The electromagnetic attraction between coils and a movable armature generates the operating force in an electromagnetic relay. This force must exceed the restraining force (usually provided by springs or other mechanical means) for the relay to switch its contacts. The force increases as the armature moves closer to the electromagnet, allowing the relay to activate and control the circuit.

2. What is the role of delay parameters in electromagnetic relays?

Delay parameters in electromagnetic relays are used to control the timing of relay operation, such as the time it takes for the relay to respond to changes in the control signal. These delays can be crucial for overload protection and for preventing frequent switching due to transient voltage fluctuations, ensuring more stable and reliable control systems.

3. How do Reed relays differ from other electromagnetic relays?

Reed relays operate using a glass tube containing two ferromagnetic reeds that close when exposed to a magnetic field. Unlike other types of electromagnetic relays, reed relays have no moving parts except for the reeds, making them faster and more reliable for low-power applications, such as telecommunications and signal control.

4. What are the advantages of moving coil relays in industrial applications?

Moving coil relays, including rotary and axial types, offer a high torque-to-weight ratio, making them ideal for motor control and industrial automation applications. These relays are renowned for their high sensitivity and rapid switching, ensuring reliable operation in overload protection systems and high-speed control circuits.

5. How do induction disc relays protect electrical systems?

Induction disc relays, including watt meter, shaded pole, and induction cup types, protect electrical systems by responding to overcurrent conditions. They use eddy currents generated by alternating magnetic fields to create a rotating disc that activates the relay. These relays are especially useful for overload protection in high-voltage systems and power relay systems.

Supporting Article

Differences Between Electromagnetic and Solid-State Relays