ELECTRIC RELAYS

NAME: ARPIT MAHESHWARI

ROLL NO:03896404914
EEE-6
DATE OF SUBMISSION: 27-9-2017

1
 A relay is an electrically operated switch. Many relays use an electromagnet to
operate a switching mechanism mechanically, but other operating principles are also
used. Relays are used where it is necessary to control a circuit by a low-power signal
(with complete electrical isolation between control and controlled circuits), or where
several circuits must be controlled by one signal. The first relays were used in long
distance telegraph circuits, repeating the signal coming in from one circuit and re-
transmitting it to another. Relays were used extensively in telephone exchanges and early
computers to perform logical operations.

A type of relay that can handle the high power required to directly control an electric
motor or other loads is called a contactor. Solid-state relays control power circuits with
no moving parts, instead using a semiconductor device to perform switching. Relays with
calibrated operating characteristics and sometimes multiple operating coils are used to
protect electrical circuits from overload or faults; in modern electric power systems these
functions are performed by digital
instruments still called "protective
relays".

2
 Contents

Basic design and operation

Types

Latching relay

Reed relay

Mercury-wetted relay

Mercury relay

Polarized relay

Machine tool relay

Ratchet relay

Coaxial relay

Contactor

Solid-state relay

Solid state contactor relay

Buchholz relay

Applications

Relay application considerations

Undesired arcing

References

3
Basic design and operation

A simple electromagnetic relay consists of a coil of wire

wrapped around a soft iron core, an iron yoke which
provides a low reluctance path for magnetic flux, a
movable iron armature, and one or more sets of contacts
(there are two in the relay pictured). The armature is
hinged to the yoke and mechanically linked to one or
Simple electromechanical
more sets of moving contacts. It is held in place by a
relay.
spring so that when the relay is de-energized there is an
air gap in the magnetic circuit. In this condition, one of
the two sets of contacts in the relay pictured is closed, and
the other set is open. Other relays may have more or fewer
sets of contacts depending on their function. The relay in
the picture also has a wire connecting the armature to the
yoke. This ensures continuity of the circuit between the
moving contacts on the armature, and the circuit track
Small "cradle" relay often used in
on the printed circuit board (PCB) via the yoke, which is
electronics. The "cradle" term refers
soldered to the PCB.
to the shape of the relay's armature.
When an electric current is passed through the coil it
generates a magnetic field that activates the armature, and the consequent movement of
the movable contact(s) either makes or breaks (depending upon construction) a
connection with a fixed contact. If the set of contacts was closed when the relay was de-
energized, then the movement opens the contacts and breaks the connection, and vice
versa if the contacts were open. When the current to the coil is switched off, the armature
is returned by a force, approximately half as strong as the magnetic force, to its relaxed
position. Usually this force is provided by a spring, but gravity is also used commonly in
industrial motor starters. Most relays are manufactured to operate quickly. In a low-
voltage application this reduces noise; in a high voltage or current application it reduces
arcing.

4
When the coil is energized with direct current, a diode is often placed across the coil to
dissipate the energy from the collapsing magnetic field at deactivation, which would
otherwise generate a voltage spike dangerous to semiconductor circuit components. Some
automotive relays include a diode inside the relay case. Alternatively, a contact protection
network consisting of a capacitor and resistor in series (snubber circuit) may absorb the
surge. If the coil is designed to be energized with alternating current (AC), a small copper
"shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase
current which increases the minimum pull on the armature during the AC cycle.[1]

A solid-state relay uses a thyristor or other solid-state switching device, activated by the
control signal, to switch the controlled load, instead of a solenoid. An optocoupler (a
light-emitting diode (LED) coupled with a photo transistor) can be used to isolate control
and controlled circuits.

5
Types

Latching relay

A latching relay has two relaxed states (bistable). These are also called "impulse",
"keep", or "stay" relays. When the current is switched
off, the relay remains in its last state. This is achieved
with a solenoid operating a ratchet and cam mechanism,
or by having two opposing coils with an over-center
spring or permanent magnet to hold the armature and
contacts in position while the coil is relaxed, or with a
remanent core. In the ratchet and cam example, the
Latching relay with
first pulse to the coil turns the relay on and the second
permanent magnet
pulse turns it off. In the two coil example, a pulse to
one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of
relay has the advantage that one coil consumes power only for an instant, while it is being
switched, and the relay contacts retain this setting across a power outage. A remanent
core latching relay requires a current pulse of opposite polarity to make it change state.

A stepping relay is a specialized kind of multi-way latching relay designed for early
automatic telephone exchanges.

An earth leakage circuit breaker includes a specialized latching relay.

Very early computers often stored bits in a magnetically latching relay, such as ferreed or
the later memreed in the 1ESS switch.

Some early computers used ordinary relays as a kind of latch -- they store bits in ordinary
wire spring relays or reed relays by feeding an output wire back as an input, resulting in a
feedback loop or sequential circuit. Such an electrically-latching relay requires
continuous power to maintain state, unlike magnetically latching relays or mechanically
racheting relays.

6
In computer memories, latching relays and other relays were replaced by delay line
memory, which in turn was replaced by a series of ever-faster and ever-smaller memory
technologies.

Reed relay

A reed relay is a reed switch enclosed in a

solenoid. The switch has a set of contacts
inside an evacuated or inert gas-filled glass
tube which protects the contacts against
atmospheric corrosion; the contacts are made of magnetic material that makes them move
under the influence of the field of the enclosing solenoid or an external magnet.

Reed relays can switch faster than larger relays and

Top, middle: reed switches, bottom: reed relay
require very little power from the control circuit.
However, they have relatively low switching current and voltage ratings. Though rare, the
reeds can become magnetized over time, which makes them stick 'on' even when no
current is present; changing the orientation of the reeds with respect to the solenoid's
magnetic field can resolve this problem.

Sealed contacts with mercury-wetted contacts have longer operating lives and less
contact chatter than any other kind of relay.

7
Mercury-wetted relay

A mercury-wetted reed relay is a form of reed relay in

which the contacts are wetted with mercury. Such relays are
used to switch low-voltage signals (one volt or less) where
the mercury reduces the contact resistance and associated voltage drop, for low-current
signals where surface contamination may make for a poor
A mercury-wetted reed relay that
contact, or for high-speed applications where the mercury
has AC/DC switching
eliminates contact bounce. Mercury wetted relays are
specifications of 100 W, 500 V, 2
position-sensitive and must be mounted vertically to work
A maximum
properly. Because of the toxicity and expense of liquid
mercury, these relays are now rarely used.

Mercury relay

A mercury relay is a relay that uses mercury as the switching element. They are used
where contact erosion would be a problem for conventional relay contacts. Owing to
environmental considerations about significant amount of mercury used and modern
alternatives, they are now comparatively uncommon.

Polarized relay

A polarized relay places the armature between the poles of a permanent magnet to
increase sensitivity. Polarized relays were used in middle 20th Century telephone
exchanges to detect faint pulses and correct telegraphic distortion. The poles were on
screws, so a technician could first adjust them for maximum sensitivity and then apply a
bias spring to set the critical current that would operate the relay.

8
Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools,
transfer machines, and other sequential control. They are characterized by a large number
of contacts (sometimes extendable in the field) which are easily converted from
normally-open to normally-closed status, easily replaceable coils, and a form factor that
allows compactly installing many relays in a control panel. Although such relays once
were the backbone of automation in such industries as automobile assembly, the
programmable logic controller (PLC) mostly displaced the machine tool relay from
sequential control applications.

A relay allows circuits to be switched by electrical equipment: for example, a timer

circuit with a relay could switch power at a preset time. For many years relays were the
standard method of controlling industrial electronic systems. A number of relays could be
used together to carry out complex functions (relay logic). The principle of relay logic is
based on relays which energize and de-energize associated contacts. Relay logic is the
predecessor of ladder logic, which is commonly used in programmable logic controllers.

Ratchet relay

This is again a clapper type relay which does not need continuous current through its coil
to retain its operation. A ratchet holds the contacts closed after the coil is momentarily
energized. A second impulse, in the same or a separate coil, releases the contacts.

Coaxial relay

Where radio transmitters and receivers share a common antenna, often a coaxial relay is
used as a TR (transmit-receive) relay, which switches the antenna from the receiver to the
transmitter. This protects the receiver from the high power of the transmitter. Such relays
are often used in transceivers which combine transmitter and receiver in one unit. The
relay contacts are designed not to reflect any radio frequency power back toward the
source, and to provide very high isolation between receiver and transmitter terminals.

9
Contactor

A contactor is a heavy-duty relay used for switching electric motors and lighting loads,
but contactors are not generally called relays. Continuous current ratings for common
contactors range from 10 amps to several hundred amps. High-current contacts are made
with alloys containing silver. The unavoidable arcing causes the contacts to oxidize;
however, silver oxide is still a good conductor. Contactors with overload protection
devices are often used to start motors. Contactors can make loud sounds when they
operate, so they may be unfit for use where noise is a chief concern.

A contactor is an electrically controlled switch used for switching a power circuit, similar
to a relay except with higher current ratings. A contactor is controlled by a circuit which
has a much lower power level than the switched circuit.

Contactors come in many forms with varying capacities and features. Unlike a circuit
breaker, a contactor is not intended to interrupt a short circuit current. Contactors range
from those having a breaking current of several amperes to thousands of amperes and 24
V DC to many kilovolts. The physical size of contactors ranges from a device small
enough to pick up with one hand, to large devices approximately a meter (yard) on a side.

Contactors are used to control electric motors, lighting, heating, capacitor banks, thermal
evaporators, and other electrical loads.

10
Solid-state relay

A solid state relay (SSR) is a solid state electronic component that provides a similar
function to an electromechanical relay but does not have any
moving components, increasing long-term reliability. Every
solid-state device has a small voltage drop across it. This voltage
drop limits the amount of current a given SSR can handle. The
minimum voltage drop for such a relay is a function of the
material used to make the device. Solid-state relays rated to
Solid state relay with no moving parts
handle as much as 1,200 amperes have become
commercially available. Compared to electromagnetic relays, they may be falsely
triggered by transients and in general may be susceptible to damage by extreme cosmic
ray and EMP episodes.

Solid state contactor relay

A solid state contactor is a heavy-duty solid state relay, including the necessary heat
sink, used where frequent on/off cycles are required, such as
with electric heaters, small electric motors, and lighting
loads. There are no moving parts to wear out and there is no
contact bounce due to vibration. They are activated by AC
control signals or DC control signals from Programmable
logic controller (PLCs), PCs, Transistor-transistor logic
(TTL) sources, or other microprocessor and microcontroller
25 A or 40 A solid state
controls.
contactors

11
Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled
transformers, which will alarm on slow accumulation of gas or shut down the transformer
if gas is produced rapidly in the transformer oil.

Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linked together,
so that when the relay coil is energized or de-energized, all of the linked contacts move
together. If one set of contacts in the relay becomes immobilized, no other contact of the
same relay will be able to move. The function of forced-guided contacts is to enable the
safety circuit to check the status of the relay. Forced-guided contacts are also known as
"positive-guided contacts", "captive contacts", "locked contacts", "mechanically-linked
contacts", or "safety relays".

Overload protection relay

Electric motors need overcurrent protection to prevent damage from over-loading the
motor, or to protect against short circuits in connecting cables or internal faults in the
motor windings.[6] The overload sensing devices are a form of heat operated relay where a
coil heats a bimetallic strip, or where a solder pot melts, releasing a spring to operate
auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload
senses excess current in the load, the coil is de-energized.

This thermal protection operates relatively slowly allowing the motor to draw higher
starting currents before the protection relay will trip. Where the overload relay is exposed
to the same environment as the motor, a useful though crude compensation for motor
ambient temperature is provided.

The other common overload protection system uses an electromagnet coil in series with
the motor circuit that directly operates contacts. This is similar to a control relay but
requires a rather high fault current to operate the contacts. To prevent short over current
spikes from causing nuisance triggering the armature movement is damped with a

12
dashpot. The thermal and magnetic overload detections are typically used together in a
motor protection relay.

Electronic overload protection relays measure motor current and can estimate motor
winding temperature using a "thermal model" of the motor armature system that can be
set to provide more accurate motor protection. Some motor protection relays include
temperature detector inputs for direct measurement from a thermocouple or resistance
thermometer sensor embedded in the winding.

Vacuum relays

A sensitive relay having its contacts mounted in a highly evacuated glass housing, to
permit handling radio-frequency voltages as high as 20,000 volts without flashover
between contacts even though contact spacing is but a few hundredths of an inch when
open.

Applications

Relays are used for:

Amplifying a digital signal, switching a large amount of power with a small

operating power. Some special cases are:

o A telegraph relay, repeating a weak signal received at the end of a long

wire

o Controlling a high-voltage circuit with a low-voltage signal, as in some

types of modems or audio amplifiers,

o Controlling a high-current circuit with a low-current signal, as in the

starter solenoid of an automobile,

Detecting and isolating faults on transmission and distribution lines by opening

and closing circuit breakers (protection relays),

13
 Isolating the controlling circuit from the controlled circuit when the two are at
different potentials, for example when controlling a mains-powered device from a
low-voltage switch. The latter is often applied to control office lighting as the low
voltage wires are easily installed in partitions, which may be often moved as
needs change. They may also be controlled by room occupancy detectors to
conserve energy,

Undesired arcing

Switching while "wet" (under load) causes undesired arcing between the contacts,
eventually leading to contacts that weld shut or contacts that fail due to a build up of
contact surface damage caused by the destructive arc energy.

Inside the 1ESS switch matrix switch and certain other high-reliability designs, the reed
switches are always switched "dry" to avoid that problem, leading to much longer contact
life.

Without adequate contact protection, the occurrence of electric current arcing causes
significant degradation of the contacts in relays, which suffer significant and visible
damage. Every time a relay transitions either from a closed to an open state (break arc) or
from an open to a closed state (make arc & bounce arc), under load, an electrical arc can
occur between the two contact points (electrodes) of the relay. The break arc is typically
more energetic and thus more destructive.

14
References

Jump up ^ Mason, C. R. "Art & Science of Protective Relaying, Chapter 2, GE

Consumer & Electrical". Retrieved October 9, 2011.

a b
^ Jump up to: A. C. Keller. "Recent Developments in Bell System Relays --
Particularly Sealed Contact and Miniature Relays". The Bell System Technical
Journal. 1964.

Jump up ^ Ian Sinclair, Passive Components for Circuit Design, Newnes, 2000
ISBN 008051359X, page 170

Jump up ^ Kenneth B. Rexford and Peter R. Giuliani (2002). Electrical control

for machines (6th ed.). Cengage Learning. p. 58. ISBN 978-0-7668-6198-5.

Jump up ^ Terrell Croft and Wilford Summers (ed), American Electricans'

Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-
013932-6 page 7-124

Jump up ^ Zocholl, Stan (2003). AC Motor Protection. Schweitzer Engineering

Laboratories, Inc. ISBN 0-9725026-1-0, 978-0972502610 Check |isbn= value
(help).

15