An electromagnetic attraction type relay operates on ______.

SOLUTION

​Electromechanical relay:

• These relays were the earliest forms of relay used for the protection of the power system.
• They work on the principle of mechanical force causing an operation of the relay.

The operating principles of electromechanical relays are

1. Electromagnetic attraction
2. Electromagnetic induction

Electromagnetic attraction relay:

Figure: electromagnetic attracted armature relay

• An electromagnetic attraction relay is an electromechanical relay that operates on the principle of electromagnetic attraction.
• Whenever there is a fault in the system the current measured by the current transformer(C.T) is large, that large current will energize the relay operating coil.
• Now, the magnetic field produced by an electromagnetic is so high. Hence, it attracts the movable armature, making contact with the trip circuit contacts.
• Then the circuit breaker will isolate the faulty section from the healthy section.
• A restraining force has to be produced exactly in the opposite direction of the operating force to restrict relay operation under normal conditions. Spring unit provides the restraining force.
• This type of relay will be used for both AC and DC relaying purposes.

Operating torque of electromagnetic attraction relay is given by 

\({{\bf{T}}_{{\bf{op}}}} = \frac{1}{4}\frac{{dL}}{{dx}}.I_m^2\)

Resultant torque of electromagnetic relay is given by

T_R = K₁ I² - K

As the operating torque produced is proportional to the square of the current. Hence, the electromagnetic attraction relay can be used for a.c as well as d.c.

Note:

• ​Electromagnetic induction relay is applicable for AC relaying purposes only.
• Induction relay can be used as the inverse time overcurrent relay.