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WHAT IS RELAY ?

A Relay is a simple electromechanical switch. While we use


normal switches to close or open a circuit
manually, a Relay is also a switch that connects or
disconnects two circuits. But instead of a manual
operation, a relay uses an electrical signal to control an
electromagnet, which in turn connects or disconnects another
circuit.
Construction of relay

The relay operates both electrically and mechanically. It consists


electromagnetic and sets of contacts which perform the operation
of the switching. The construction of relay is mainly classified into
four
groups. They are the contacts, bearings, electromechanical design,
terminations and housing.

Contacts – The contacts are the most important part of the relay
that affects the reliability. The good contact gives limited contact
resistance and reduced contact wear. The selection of the contact
material depends upon the several factors like nature of the current
to be interrupted, the magnitude of the current to be interrupted,
frequency and voltage of operation.

Bearing – The bearing may be a single ball, multi-ball, pivot-ball and


jewel bearing. The single ball bearing is used for high sensitivity and
low friction. The multi-ball bearing provides low friction and greater
resistance to shock.

Electromechanical design – The electromechanical design includes


the design of the magnetic circuit and the mechanical attachment
of core, yoke and armature. The reluctance of the magnetic path is
kept
minimum for making the circuit more efficient. The electromagnet is
made up of soft iron, and the coil current is usually restricted to 5A

and the coil voltage to 220V.

Terminations and Housing – The assembly of an armature with the


magnet and the base is made with the help of spring. The spring is
insulated from the armature by moulded blocks which provide
dimensional stability. The fixed contacts are usually spot welded on
the terminal link.
Operation of relay

It works on the principle of an electromagnetic


attraction. When the circuit of the relay senses the fault current ,
it energises the electromagnetic field which
produces the temporary magnetic field.This magnetic
field moves the relay armature for opening or closing the
connections. The small power relay has only one
contacts, and the high power relay has two contacts for
opening the switch The inner section of the relay is
shown in the figure below. It has an iron core which is
wound by a control coil. The power supply is given to the coil
through the contacts of the load and the control
switch. The current flows through the coil produces the
magnetic field around it Due to this magnetic field, the
upper arm of the magnet attracts the lower arm. Hence close
the circuit, which makes the current flow through the load. If the
contact is already closed, then it moves
oppositely and hence open the contacts.
Specification of Relay

1. Coil Specifications

Coil Voltage: The voltage required to energize the coil, commonly available
in AC (e.g., 6V, 12V, 24V, 120V, 240V) and DC (e.g., 5V, 12V, 24V).
Coil Resistance: The resistance of the coil, affecting the current drawn
when energized.
Coil Power Consumption: The power consumed by the coil when energized,
typically measured in watts.
2. Contact Specifications

Contact Configuration: The arrangement of contacts, such as Normally


Open (NO), Normally Closed (NC), or Double Throw (DT).
Contact Rating: The maximum voltage and current the contacts can
handle, usually specified in terms of AC and DC ratings (e.g., 10A at 250V
AC).
Contact Material: The material of the contacts (e.g., silver, gold) affects
conductivity and durability.
3. Physical Specifications

Size and Mounting: Dimensions and mounting style (e.g., PCB mount, DIN
rail mount).
Enclosure Type: Open, sealed, or weatherproof enclosures for
environmental protection.
4. Performance Specifications

Switching Time: The time it takes for the relay to switch from one state
to another.
Life Expectancy: The expected number of operations (mechanical and
electrical life).
Dielectric Strength: The voltage that the relay can withstand between the
contacts and the coil without breakdown.
5. Environmental Specifications

Operating Temperature Range: The range of temperatures in which the


relay can operate effectively.
Humidity Rating: The level of humidity the relay can withstand without
performance degradation.
Types of relay

1.electromechanical relays
2.solid state relays
3.magnetic latching relays
4.thermal relays
5.reed relays
6.differential relays
7.distance relays
8.electromagnetic relay

1. Electromechanical relays

When an EMR receives a low-power input signal, an


electromagnetic coil creates a magnetic field that
pulls an armature towards it. This movement closes or opens the
contacts, which can then switch
electrical circuits on or off.
2. Solid state relays

A solid-state relay (SSR) is an electronic device that switches on or off


when an external voltage is applied to its control terminals. SSRs work
by converting an electrical control signal into an optical signal, which is
then used to trigger a semiconductor switch that allows current to
flow.

3. Magnetic latching relays

Magnetic Latching relays can have either single or dual coils. On a single
coil device, the relay will operate in one direction when power is applied
with one polarity, and will reset when the polarity is reversed. On a dual
coil device, when polarized voltage is applied to the reset coil the
contacts will transition.
4.Thermal relays

A thermal relay works depending upon the above mentioned property


of metals. The basic working principle of thermal relay is that, when
a bimetallic strip is heated up by a heating coil carrying over current
of the system, it bends and makes normally open contacts.
5.differential relays

A differential relay is a protective device that monitors the


difference in current between two or more points in an electrical
system and triggers a circuit breaker to isolate a faulty section

6. Distance relays

A distance relay is a type of protection relay most often used for


transmission line protection. Distance relays measure the
impedance from the installation side to the fault location and
operates in response to changes in the ratio of measured current
and voltage.
7. Electromagnetic relays

An electromagnetic relay is an electrical switch that uses


an electromagnetic mechanism to control circuits. It
works by passing an electric current through a coil of wire
, which generates a magnetic field that attracts a movable
armature. The armature's movement then opens or closes
the relay's contacts, which allows or stops the flow of
current in the circuit
Relays by Ampere rating
1. 1a
2. 2a
3. 5a
4. 10a
5. 20a
6. 30a
7. 40a
8. 50a
9. 60a
10. 100a
11. 200a

Manufacturing companies

1. Siemens
2. ABB.Ltd
3. Schneider electric
4. Adlite Electricals
5. LEELAVATI AUTOMATION PVT.LTD
6. Larsen and Toubro limited
7. H.K.Electric co
Application

1. The primary purpose of a relay is to protect the electrical


system from too high of a voltage or current, allowing the safe
operation of any equipment it

2.Relays are used in a variety of applications, including


commercial, industrial, home, and consumer products. They are
used to protect electrical systems from high voltages or
currents, allowing equipment to operate safely.

Advantages

Electromechanical relays: These relays are reliable, durable, and


low cost. They can withstand harsh environments and operate
without external power sources.

Other relays: Relays can handle high voltages and currents,


provide electrical isolation, and offer simple, reliable switching.

Disadvantages

Electromechanical relays: These relays have a slow response


time, require high maintenance, and are prone to wear and tear.

Other relays: Relays can have mechanical limitations, switching


delays, contact wear, electromagnetic interference, and
increased power consumption.

Different types of relays: Different types of relays have different


disadvantages, such as solid state relays being sensitive to
temperature variations, and thermal relays having limited
applications.

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