types of relays

Understand the various types of relays

The protection and switching of numerous control systems and other electrical elements depend on relays. To open or close the contacts or electronics, all relays respond to voltage or current.

Because of the effectiveness of relays, several types are available out there to offer different effects to applications. These relays are either operated electromechanical or electronically.

In this article, you’ll get to know popular types of relays used in many applications, diagram, functions, and their working principle.

types of relays

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Types of Relays

There are different types of relays including electromagnetic relays, latching relays, electronic relays, non-latching relays, multi-dimensional relays, and thermal relays which are classified based on the function, application type, configuration or structural features, etc. Below are the various types of relays used and suitable for different applications:

Solid-state relays (SSRs)

Solid-state types of relays use components such as BJTs, thyristors, IGBTs, MOSFETs, and TRIACs. These components carry out switching operations. Compared to electromechanical relays, the power obtained in solid-state relays is much higher because the power needed to control the circuit is much lower. These relays can work for both AC and DC supply.

Solid-state types of relays have high switching speeds since there are no mechanical contacts. There is a sensor in a solid-state relay which is also an electronic device. This sensor helps to switch on or off the power to load after responding to a control signal.

These relays which are abbreviated as SSR are classified into different types, but the major types include photo-coupled SSRs and transformer-coupled SSRs. The transformer-coupled SSR allows a small DC to be supplied to the primary of the transformer through a DC-to-AC converter.

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This current is then converted to AC to operate the solid-state device and trigger the circuit. The level of isolation between the input and output depends on the design of the transformer.

solid state relay
Solid State Relays (SSRs)

In the photo-coupled SSR, a photosensitive semiconductor device is used for the switching operation. The control signal is applied to the LED to turn the photosensitive device into conduction mode. This is done by detecting the light emitted from the LED.

Isolation in these types of solid-state relays is relatively high compared with transformer-coupled SSR. This is because of a photodetection principle.

solid state relay
Solid State Relays (SSRs)

Solid-state relays have faster switching speeds when compared with electromechanical relays. It has a higher life expectancy because there are no moving parts and they tend to create very little noise.

Latching relays:

Latching types of relays maintain their state after being actuated. This is why they are also called impulse relays, keep relays, or stay relays. It’s used in most applications to limit power consumption and dissipation.

Latching relay types consist of internal magnets so that when current is supplied to the coil, the internal magnet holds the contact position. With this, the system requires no power to maintain its position. This is why after being actuated, it manages to maintain the last contact position even if the current is removed from the coil.

Thus, considerable energy is saved with these types of relays.
latching relays

Just like other relays, a latching relay can be made with one or two coils, which are responsible for the armature position in the relay. Just as shown in the above figure, the latching relay doesn’t have any default position. In the one coil type, the armature position is determined by the direction of the current flow in the coil.

In the case of two coil types, the position of the armature depends on the coil current flows through. These relays maintain their position once they are actuated but their position is controlled by the system.

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Reed relays:

Just like electromechanical types of relays, reed relays also work with the mechanical actuation of physical contacts to open or close a circuit path. However, the reed relays have low mass and much smaller contacts compared to the electromechanical types.

Reed switch is wounded as it acts as an armature. It’s a glass tube or capsule filled with an inert gas contained in two overlapping reeds or ferromagnetic blades, which is hermetically sealed.

Its overlapping ends are contacts that allow input and output terminals to be connected to them. A magnetic field is produced when power is supplied to the coils causing the reeds to be drawn together. Their contacts make a closed path through the relay.

Reeds are separated apart by the pulling force of the spring attached to them. this occurs during the de-energizing process of the coil.

reed relays

Comparing switching speed between the reed and electromechanical types of a relay. Reed relays are 10 times faster than their comparative, due to their less massive, different actuating medium, and smaller contacts. But electrical arcing will suffer this due to smaller contacts.

In the switching event, the arc jumps across the contacts which causes the contact surface to melt over a small section. Also, this results in joining the contacts if both contacts are still closed.

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If this happens, the de-magnetization of the coil spring force may not be sufficient to separate them. well, this issue can be escaped by placing a series of impedances like a resistor or ferrite between the relay and system capacitance. This helps inrush currents to be reduced thereby avoiding arcing in the relay. The reed types of relays are used in many switching applications because of their small size and high-speed effect.

Polarized relay:

Just as it’s named, the polarized types of relays are very sensitive to the direction of the current by which it’s energized. It’s a DC electromagnetic relay provided with an additional source of a permanent magnetic field to move the armature in the relay.

In polarized relays, magnetic circuits are built with permanent magnets, electromagnets, and an armature. Instead of spring force, these types of relays use magnetic force to attract or repel the armature. This armature is a permanent magnet, positioned between the pole faces formed by an electromagnet.

The current that flows through the electromagnet allows the magnetic flux to be created. The armature changes its position when the force exerted by the electromagnet exceeds the force exerted by the permanent magnet.

polarized relays

When the current is interrupted, the electromagnetic force is reduced to less than the permanent magnet. With this, the armature returns to its normal position.

this permanent magnet produces magnetic flux Φm, which passes through the armature of two parts; Φ1 and Φ2. The flux Φ1 is designed to pass through the left working gap of the magnet, while Φ2 passes through the right working gap of the magnet.

watch the video to learn how polarized relays work:

Even if there is no current in the coil, these two fluxes armature will stay either left or right of the neutral position. This is because the magnetic system is not stable. However, when a current is supplied to the coils of the relay, an additional working magnetic flux Φ passes through the working gap of the magnet. These magnetic field interactions cause a force effect on the armature which depends on the magnitude of the current, the initial position of the armature, the polarity of the current, the value of the working gap, and the power of the magnet. Due to these parameters’ combination, the armature of the relay turns to a new stable state. Thus, closes the right contact and the relay picks up.

Polarized relays are of different types depending on the magnetic circuit configuration. Differential and bridge types of polarized relays are the most used ones. In a differential magnetic system, the difference of two fluxes of permanent magnet acts on the armature. whereas in bridge magnetic types, a field is created by the coils divided into two fluxes with opposite signs in the working gap area. However, the magnetic flux of the permanent magnet is not divided into two fluxes.

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Differential relays:

Differential types of relays start working when the phasor difference of two or more similar electrical quantities exceeds a predetermined value. Current differential relays operate when the system experiences a comparison between the magnitude and phase difference of the currents entering and leaving the system to be protected.

if the system is working at normal operating conditions, currents entering and leaving are equal in magnitude and phase. This causes the relay to be inactive. But if a fault occurs in the system, the currents are no longer equal in magnitude and phase.

These types of relays are designed in a way that the difference between the current entering and the current leaving flows through the operating coil of the relay. these allow the relay coil to be energized only under fault conditions due to the difference in the quantity of the current. Thus, operating the relay and opening the circuit breaker so that the circuit can be a trip.

The figure below shows the principle of differential relays where there are two CTs connected to either side of the power transformer. For instance, one CT is on the primary side and the other at the secondary side of the power transformer. The relay then compares the currents on both sides and if there is any unbalance then it tends to operate. Differential relays can be current differential relays, voltage balance differential relays, and biased differential relays.

differential relays

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Overload protection relays:

Overload protection types of relays are purposely designed to offer over-current protection to electrical motors and circuits. These overload relays are of different types such as fixed bimetallic strip type, and electronic or interchangeable heater bimetallic, etc.

Whenever an electric motor is overloaded, such a motor will require these relay types to protect the system from overcurrent. For this reason, overload sensing equipment such as a heat-operated relay must be employed. This heat-operated relay contains a coil that heats a bimetallic strip or solder pot, which then melts.

The melted bimetallic strip releases the spring for operating auxiliary contacts which are in series with the coil. This coil gets de-energized by sensing excess current in the load because of the overload.

The temperature of the motor winding can be calculated using the motor armature thermal model, and electronic overload protection relay by measuring the motor current. Thus, allowing accurate protection of the motor.


overload protection relays
Overload Protection Relays

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Inverse definite minimum time relays (IDMT Relays):

Inverse definite minimum time relays are types of relays that offer definite-time current characteristics of a fault current at a higher value. And also, an inverse time-current characteristic of a fault current at a lower value.

These IDMT relays are widely used for protecting distribution lines and they help to set limits for current and time settings. In these types of a relay, their operating time is approximately inversely proportional to the fault current near the pickup value. They become constant slightly above the pickup value of the relay. This can be achieved by using the core of the magnet which gets saturated for the current slightly greater than the pickup current.

The pickup value in this system is the point where the actuating quantity or fault current initiates the relay to operate. This is known as the pickup value. These types of relays are called so because of their characters that when the actuating quantity reaches their infinity value the time does not approach zero.

At a lower value of the fault current, inverse time characteristics will be offered, whereas at higher values it gives definite time characteristics. Its operating time will become constant from a particular value till the actuating quantity becomes infinity.

time relay

Buchholz relays:

The Buchholz types of relays are gas-operated or actuated relays. They are widely used to detect incipient faults or internal faults that are minor initially but could cause major faults with time. These relays are mostly used for transformer protection and they are mounted in a chamber between the transformer tank and conservator.

These relay types are only used for an oil-immersed relay that is specifically utilized for power transmission and distribution systems. The figure below shows the working of a Buchholz relay.

In the working of this relay, when there is a slow-developing fault inside a transformer,

Buchholz relays

the oil level falls because of gas accumulation. Because of this, the hollow float tilts, and the mercury contacts are closed. These mercury contacts complete the path of the alarm circuit that allows the operator to know something is wrong with the transformer.

If the fault in a transformer is severe, the pressure inside the tank suddenly increases due to a fast reduction in the level of the oil. That is, the oil rushes towards the conductor which then causes the lower side flap valve to get deflected.

When this happens, mercury switch contacts close thereby enabling its trip circuit. Thus, the transformer is disconnected from the supply source.

In conclusion, we’ve explained the various and most used types of relays in this article. The function and working of these relays were also explained.

I hope you enjoyed the reading, if so, kindly comment on your favorite types of relays in our comment section. And please don’t forget to share with other students, it might be helpful. Thanks!


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