Electromagnetic Relay
These relays are constructed from electrical, mechanical and magnetic components, and possess operating coil and mechanical contacts. Therefore, when a coil gets activated by a power supply source, these mechanical contacts get opened or closed. The type of supply can be AC or DC.
Types of an Electromagnetic Relay
By their working principle, the electromagnetic relay is mainly classified into two types. These are
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Attraction Type Electromagnetic Relays
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Induction Type Electromagnetic Relays
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.
These can be typed like
Attracted Armature Relay
Solenoid Type Relay
Balanced Beam Relay

Induction Type Electromagnetic Relays
These are used as protective relays in AC systems alone and are unusable with DC systems.The actuating force of 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 Type Relays
Watt-hour Meter Type Relays
Induction Cup Structure Relays
Shaded Pole Type Relays
Shaded pole relay consists of aluminum disc, which is pivoted such that it freely rotates in the air gap of an electromagnet. Shaded ring or copper ring is surrounded on one half of each pole. The induced currents in this shaded ring produce another flux called as shaded flux, which lags behind the flux produced by the unshaded pole by some angle. The phase difference between these two fluxes produces necessary torque to rotate the disc.
