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Different Types of Relays and their Working Principles
Listed on Oct 01,2018
By admin
Relays are the primary protection as well as switching devices in most of the control processes or equipments. All the relays respond to one or more electrical quantities like voltage or current such that they open or close the contacts or circuits. A relay is a switching device as it works to isolate or change the state of an electric circuit from one state to another.

Classification or the types of relays depend on the function for which they are used. Some of the categories include protective, reclosing, regulating, auxiliary and monitoring relays.

Protective relays continuously monitor these parameters: voltage, current, and power; and if these parameters violate from set limits they generate alarm or isolate that particular circuit. These types of relays are used to protect equipments like motors, generators, and transformers, and so on.

Reclosing relays are used to connect various components and devices within the system network, such as synchronizing process, and to restore the various devices soon after any electrical fault vanishes, and then to connect transformers and feeders to line network. Regulating relays are the switches that contacts such that voltage boosts up as in the case of tap changing transformers.

Auxiliary contacts are used in circuit breakers and other protective equipments for contact multiplication. Monitoring relays monitors the system conditions such as direction of power and accordingly generates the alarm. These are also called directional relays.

This article’s main aim is to give a brief idea about various relays that are employed for a wide variety of control applications. Some of these relays are described below.

Different Types of Relays
Depending on the operating principle and structural features relays are of different types such as electromagnetic relays, thermal relays, power varied relays, multi-dimensional relays, and so on, with varied ratings, sizes and applications.

1. Electromagnetic Relays
These relays are constructed with electrical, mechanical and magnetic components, and have operating coil and mechanical contacts. Therefore, when the coil gets activated by a  supply system, these mechanical contacts gets opened or closed. The type of supply can be AC or DC.

DC vs AC Relays
Both AC and DC relays work on the same principle as electromagnetic induction, but the construction is somewhat differentiated and also depends on the application for which these relays are selected. DC relays are employed with a freewheeling diode to de-energize the coil, and the AC relays uses laminated cores to prevent eddy current losses.

The very interesting aspect of an AC is that for every half cycle, the direction of the current supply changes; therefore, for every cycle the coil loses its magnetism since the zero current in every half cycle makes the relay continuously make and break the circuit. So, to prevent this – additionally one shaded coil or another electronic circuit is placed in the AC relay to provide magnetism in the zero current position.

Attraction Type Electromagnetic Relays
These relays can work with both AC and DC supply and attract a metal bar or a piece of metal when power is supplied to the coil. This can be a plunger being drawn towards the solenoid or an armature being attracted towards the poles of an electromagnet as shown in the figure. These relays don’t have any time delays so these are used for instantaneous operation.

delays so these are used for instantaneous operation.

Induction Type Relays
These are used as protective relays in AC systems alone and are usable with DC systems. The actuating force for contacts movement is developed by a moving conductor that may be a disc or a cup, through the interaction of electromagnetic fluxes due to fault currents.

These are of several types like shaded pole, watt-hour and induction cup structures and are mostly used as directional relays in power-system protection and also for high-speed switching operation applications.

Magnetic Latching Relays
These relays use permanent magnet or parts with a high remittance to remain the armature at the same point as the coil is electrified when the coil power source is taken away.

2. Solid State Relays
Solid State uses solid state components to perform the switching operation without moving any parts. Since the control energy required is much lower compared with the output power to be controlled by this relay that results the power gain higher when compared to the electromagnetic relays. These are of different types: reed relay coupled SSR, transformer coupled SSR, photo-coupled SSR, and so on.

The above figure shows a photo coupled SSR where the control signal is applied by LED and it is detected by a photo-sensitive semiconductor device. The output form this photo detector is used to trigger the gate of TRIAC or SCR that switches the load.

3. Hybrid Relay
These relays are composed of electromagnetic relays and electronic components. Usually, the input part contains the electronic circuitry that performs rectification and the other control functions, and the output part include electromagnetic relay.

4. Thermal Relay
These relays are based on the effects of heat, which means – the rise in the ambient temperature from the limit, directs the contacts to switch from one position to other. These are mainly used in motor protection and consist of bimetallic elements like temperature sensors as well as control elements. Thermal overload relays are the best examples of these relays.

5. Reed Relay
Reed Relays consist of a pair of magnetic strips (also called as reed) that is sealed within a glass tube. This reed acts as both an armature and a contact blade.  The magnetic field applied to the coil is wrapped around this tube that makes these reeds move so that switching operation is performed.

Based on dimensions, relays are differentiated as micro miniature, subminiature and miniature relays. Also, based on the construction, these relays are classified as hermetic, sealed and open type relays. Furthermore, depending on the load operating range, relays are of micro, low, intermediate and high power types.

Relays are also available with different pin configurations like 3 pin, 4 pin and 5 pin relays. The ways in which these relays are operated is shown in the below figure. Switching contacts can be SPST, SPDT, DPST and DPDT types. Some of the relays are normally open (NO) type and the other are normally closed (NC) types.

These are some of the different types of relays that are employed in most of the electronic as well as electrical circuits. The information about the different types of relays serves readers’ purpose and we hope that they will find this basic information very useful. Considering the huge significance of relays with zvs in circuits, this particular article on them deserves its readers’ feedback, queries, suggestions and comments. Therefore, readers can post their comments here.






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