A relay device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element/section from the rest of the system.
We have covered the fundamentals and working principle of relay before.
There are two basic types of relay
- ·
Electromechanical
Relays (EMR)
- · Solid-State Relays (SSR)
- · Reed Relays
What is Electromechanical Relays (EMR)
Electromechanical Relays (EMRs) are switches
that use an electromagnet to mechanically operate contacts, allowing a
low-power signal to control a high-power circuit. They consist of a coil, an
armature, a spring, and contacts, which move when the coil is energized by an
electric current. EMRs are widely used for their ability to provide electrical
isolation between control and high-power circuits and are found in applications
ranging from automotive systems to industrial machinery. While they are durable
and versatile, they have moving parts that can wear out over time and are
slower than solid-state relays. Despite these drawbacks, their robustness and
ability to handle high inrush currents make them indispensable in many
electrical applications.
Most of the relays in
service on electric power system are of electro-mechanical type.
They
work on the following two main operating principles:
i.
Electromagnetic attraction
ii. Electromagnetic induction
Electromagnetic Attraction Relays
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 d.c. or
a.c. quantities.
The important types of electromagnetic attraction relays are:
i. Attracted armature type relay.
ii. Solenoid type relay.
iii. Balanced beam type relay.
Attracted armature type relay: An attracted
armature type relay operates by using an electromagnet to control its contacts.
In its resting state, with no current flowing through the coil, the armature
remains stationary, keeping the normally open (NO) contacts open and the
normally closed (NC) contacts closed. When a control signal energizes the coil,
it generates a magnetic field that attracts the armature toward the coil. This
movement changes the state of the contacts, causing the NO contacts to close
and the NC contacts to open. Once the control signal stops and the coil is
de-energized, the magnetic field dissipates, and a spring pulls the armature back
to its original position, returning the contacts to their default states. This
process allows the relay to control high-power circuits with a low-power
signal.
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| attracted armature type relay |
Solenoid type relay: A solenoid type relay operates using an
electromagnet to move a plunger or armature, which controls the electrical
contacts. When the relay is at rest and the coil is not energized, the contacts
are in their default state (normally open or normally closed). When a control
signal energizes the coil, it generates a magnetic field that pulls the plunger
or armature into the coil. This movement either closes or opens the contacts,
depending on the relay's design, allowing current to flow or stopping it. When
the control signal is removed, the coil is de-energized, the magnetic field
disappears, and a spring returns the plunger or armature to its original
position, resetting the contacts to their default state. This simple mechanism
enables the relay to control high-power circuits using a low-power input.
 |
| solenoid type relay |
Balanced beam type relay: A balanced beam type relay operates by using
a balance beam mechanism to control electrical contacts. In this type of relay,
a beam is pivoted in the center, with electrical contacts on either end. When
no current flows through the relay’s coil, the beam remains balanced and the
contacts stay in their default positions. When a control signal energizes the
coil, it creates a magnetic field that exerts force on one end of the beam,
causing it to tilt. This tilting motion either closes the normally open contacts
or opens the normally closed contacts, depending on the relay's design. When
the control signal is removed and the coil is de-energized, the magnetic field
disappears, and the beam returns to its balanced position due to gravity or a
spring mechanism, resetting the contacts to their original states. This design
allows for precise control of the contacts in response to a low-power input.
 |
| balanced beam type relay |
Electromagnetic induction relay
Electromagnetic
induction relays operate based on the principle of electromagnetic induction,
where a varying current induces a magnetic field that actuates the relay
mechanism. These relays typically consist of a coil, a core, and a moving
element like a disc or a drum. When alternating current (AC) flows through the
coil, it generates a time-varying magnetic field, which induces eddy currents
in the moving element. These currents produce their own magnetic field,
creating a force that moves the element. This movement either opens or closes
the relay contacts, allowing the relay to control an external circuit.
Electromagnetic induction relays are often used in applications requiring
reliable and precise switching, such as in protective relays for electrical
power systems.
Working Principle
An electromagnetic induction relay operates
on the principle of electromagnetic induction, where an alternating current
(AC) in the relay's coil generates a varying magnetic field. This magnetic
field induces eddy currents in a nearby metal disc or drum, producing a
secondary magnetic field that interacts with the original field. This
interaction creates a force that moves the metal element, which in turn opens
or closes the relay's electrical contacts. When the AC current stops or
decreases, the magnetic field weakens, and a spring or gravity returns the
moving element to its original position, resetting the contacts. This allows
the relay to control circuits by switching them on or off in response to AC
signals.
The two AC fluxes f2 and f1 differing in phase by an angle a induce e.m.f.s’ in the disc and cause the circulation of eddy currents i2 and i1 respectively. These currents lag behind their respective fluxes by 90o.
The following points may be noted from exp. (i):
- a.
The greater the phase angle α between the fluxes, the greater is
the net force applied to the disc. Obviously, the maximum force will be
produced when the two fluxes are 90o out of phase.
- b.
The net force is the same at every instant.
- c.
The direction of net force and hence the direction of motion of
the disc depends upon which flux is leading.
The following three types of structures are used to displaced
the two flaxes:
i. Shaded-pole structure
ii. Watthour-meter or double winding structure
iii. Induction cup structure
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