# Basic Relays – Electromagnetic Attraction and Induction Relays

- Link: https://studyelectrical.com/2016/10/basic-relays-electromagnetic-attraction-induction-relays.html
- Published: 2016-10-31T07:02:00+00:00
- Author: 

Most of the relays used in the power system operate by virtue of the current and/
or voltage supplied by current and voltage transformers connected in various combinations
to the system element that is to be protected.

Through the individual or relative changes in these two quantities, faults signal
their presence, type and location to the protective relays.

Having detected the fault, the relay operates the trip circuit which results in 
the opening of the circuit breaker and hence in the disconnection of the faulty 
circuit.

Most of the relays in service on electric power system today are of an electro-mechanical
type. They work on the following two main operating principles :

 1. Electromagnetic attraction
 2. Electromagnetic induction

Practically, a relay is an amplifier, since it is able to control a high power output
through a low power input. They were first invented in 1835 by Joseph Henry, the
man who also discovered electromagnetic induction and whose name is currently being
used for the SI unit of inductance.

## Electromagnetic Attraction Relays

[⌊electromagnetic%2Brelay⌉⌊electromagnetic%2Brelay⌉[

Electromagnetic attraction relays operate by virtue of an armature being attracted
to the poles of an electromagnet or a plunger being drawn into a solenoid. Such 
relays may be actuated by DC or AC quantities.

The important types of electromagnetic attraction relays are:

 * Attracted armature type relay
 * Solenoid type relay
 * Balanced beam type relay

![slide 4](https://1.bp.blogspot.com/-QNETJwp2QJI/WBbkFjGnxII/AAAAAAAACDE/4qubc5eZfjIVaACxez8Zwmzil435ZpmygCLcB/
s1600/slide_4.jpg)

## Induction Relays

Electromagnetic induction relays operate on the principle of induction motor and
are widely used for protective relaying purposes involving AC quantities.

They are not used with DC quantities owing to the principle of operation.

[⌊inductionrelay⌉⌊inductionrelay⌉[

An induction relay essentially consists of a pivoted aluminium disc placed in two
alternating magnetic fields of the same frequency but displaced in time and space.

The torque is produced in the disc by the interaction of one of the magnetic fields
with the currents induced in the disc by the other.

The following three types of structures are commonly used for obtaining the phase
difference in the fluxes and hence the operating torque in induction relays :

 * shaded-pole structure
 * watthour-meter or double winding structure
 * induction cup structure

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