What is electromechanical relay?

Electromechanical relay

Relay is a device that detects the fault and initiates the working of a circuit breaker to isolate the defective or faulty part from rest of the system. Most of the relays used in power system protection are electromechanical type. This type of relay works on the principle of :

  1. Electromagnetic attraction
  2. Electromagnetic repulsion

Electromagnetic attraction relays :

They operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger drawn into a solenoid. Such relays may be actuated by d.c or a.c quantities. The important types of electromagnetic relays are:

  1. Armature attracted type relay
  2. Solenoid type relay
  3. Balance beam type relay

Induction relays :

This type of relay operate on the principle of induction motor and are widely used for protective relaying purpose. It involves a.c quantities.They are not used with d.c quantities. Induction relay consists of an pivoted aluminium disc which is placed in two alternating magnetic fields. These two fields are of same frequency but displaced in time and space. The interaction of one of the magnetic fields with the current induced in the disc by the other results in torque production. There are three types of structures that are used for obtaining the phase difference in fluxes and the operating torque. The structures are :

  1. Shaded-pole structure
  2. Watt hour -meter /double winding structure
  3. Induction cup structure.

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