detail information of relays

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Electromechanical Drives:
Relays
Solenoids
Stepper motors
DC Motors & Servomotors.

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A classic example of an electrical relay is the
system used to start a car.
An electrical relay is a switch which is under the
control of another circuit / to allow the automatic
control of a device or circuit.

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When we turn the key to “Start” (or activate the ignition by pushing a
button), a small amount of current leaves the ignition switch then passes
through the neutral start switch to a starter relay. The starter relay sends
power to the starter solenoid that allows high current to flow through the
battery cables to the starter. The starter then cranks the engine so that
the engine will start.

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A relay is an electromechanical device
that uses a small electrical currents
and voltages to control larger electrical
currents and voltages.
Relays have unlimited possibilities,
ranging from industrial applications to
consumer electronics, such as
microwave ovens and television sets.
Definition

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All electromechanical relays have three basic
parts:
1.Electromagnet – consists of an iron core
wrapped with turns of insulated wire.
2.Armature – the moving part of the relay.
3.Switching contacts – at least one must be
stationary and one fastened to the armature.
This is known as a single-pole single-throw
(SPST) arrangement.

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9. Magnetic
Flux

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Example – Relay operated florescent Bulb

11. Schematic Drawing of the Relay Circuit

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Relay Purpose
To isolate the controlling circuit from
controlled circuit.
To Control high voltage system with
low voltage.
To Control high current system with
low current.
To execute Logic Functions

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Relay Types
• Electromagnetic Relays (EMRs)
EMRs consist of an input coil
that's wound to accept a
particular voltage signal, plus a
set of one or more contacts that
rely on an armature (or lever)
activated by the energized coil
to open or close an electrical
circuit.

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Relay Types
• Solid-State Relays (SSRs)
–SSRs use semiconductor output instead
of mechanical contacts to switch the
circuit. The output device is optically-
coupled to an LED light source inside
the relay. The relay is turned on by
energizing this LED, usually with low-
voltage DC power.
• Microprocessor Based Relays
–Use microprocessor for switching
mechanism. Commonly used in power
system monitoring and protection.

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Advantages/Disadvantages
• Electromagnetic Relays (EMRs)
– Simplicity
– Not expensive
– Mechanical Wear
• Solid-state Relays (SSRs)
– No Mechanical movements
– Faster than EMR
– No sparking between contacts
• Microprocessor-based Relay
– Much higher precision and more reliable and durable.
– Improve the reliability and power quality of electrical power
systems before, during and after faults occur.
– Capable of both digital and analog I/O.
– Higher cost

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Relays can be used to control
indoor fan motors, condenser
fan motors, damper motors,
starting capacitors, and control
lockouts.
Relay Applications

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Relay Controlled Pneumatic Circuit
(A+B+C+A-B-C-)

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