Relay & its types:
A relay is an electromagnetic device that breaks or makes a connection.
Introduction:
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.
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 get opened or closed.
DC and 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 use laminated cores to prevent eddy current losses.
Attraction Type Electromagnetic Relays:
These relays can work with both AC and DC supplies 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. These relays don’t have any time 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 contact movement is developed by a moving conductor may be a disc or a cup, through the interaction of electromagnetic fluxes due to fault currents.
These are of several types like a 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 a 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.
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 results from the power gain is 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.
Hybrid Relays:
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 includes an electromagnetic relay.
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.
Reed Relay:
Reed Relays consist of a pair of magnetic strips (also called a 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 which makes these reeds move so that a switching operation is performed.
Relays are also available with different pin configurations like 3 pins, 4 pins, and 5 pin relays. Switching contacts can be SPST, SPDT, DPST, and DPDT types. Some of the relays are normally open (NO) types and the others are normally closed (NC) types.