Transcript Document

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Chapter 7
PLC and System Interfacing
Systems • Primary Systems • System Interfacing •
Electrical Circuits • Basic Electrical Circuits •
Improving Basic Electrical Circuits • Complex
Electrical Circuits • Interfacing Circuits • Interface
Devices • Electromechanical Relays • Solid-State
Relays • Contactor Interfaces • Motor Starter
Interfaces • Electric Motor Drive Interfacing
Chapter 7 — PLC and System Interfacing
Primary systems transmit and control
the movement of energy.
Chapter 7 — PLC and System Interfacing
Electrical systems use
alternating current (AC)
or direct current (DC) to
produce power.
Chapter 7 — PLC and System Interfacing
Electronic systems monitor and control electricity to
send and/or receive information, produce sound or
vision, store data, control circuits, or perform other
work.
Chapter 7 — PLC and System Interfacing
Fluid power systems produce work
by transmitting fluid under pressure
through pipework.
Chapter 7 — PLC and System Interfacing
Mechanical systems transmit
power using gears, belts, chains,
shafts, couplings, and linkages.
Chapter 7 — PLC and System Interfacing
Informational systems display electrical, electronic,
fluid power, and mechanical quantities and conditions
to indicate the status of a circuit, process, or
application.
Chapter 7 — PLC and System Interfacing
Interfacing systems
interconnect primary
systems of various types.
Chapter 7 — PLC and System Interfacing
Combination systems such as
industrial robots interconnect
two or more primary systems
(hydraulic, pneumatic,
mechanical, electrical,
electronic, digital, and welding),
combining the individual
advantages of each system to
meet the requirements of a
given application.
Chapter 7 — PLC and System Interfacing
System interfacing permits
devices and components of
various levels of voltage,
current, and power to work
together as a system.
Chapter 7 — PLC and System Interfacing
Circuit conditioning is required any time the existing
electrical power is not at the proper phase, voltage, or
current level for the application.
Chapter 7 — PLC and System Interfacing
Interface devices are
required any time an
input device or output
component is not directly
compatible with a PLC.
Chapter 7 — PLC and System Interfacing
Interface devices
change or condition a
system in a variety of
ways.
Chapter 7 — PLC and System Interfacing
Basic electrical circuits must
include a source of electricity,
a method of controlling the
flow of electricity, and a
component that converts
electrical energy into some
other usable form of energy. A
circuit must also include
protection device(s) to ensure
that the circuit operates safely
and within designed electrical
limits.
Chapter 7 — PLC and System Interfacing
Basic PLC-controlled
circuits with multiple loads
wired in parallel have the
loads wired separately to
the PLC output terminals
and the output terminals
programmed to be parallel.
Chapter 7 — PLC and System Interfacing
Basic circuits are improved
by adding additional
protection, instrumentation
for monitoring circuit
parameters, and finer
controls, and by
interconnecting all basic
circuits into a system.
Chapter 7 — PLC and System Interfacing
PLCs and PCs are
interconnected through
communication lines to
form various types and
levels of networks.
Chapter 7 — PLC and System Interfacing
Sensors and switches are typical input devices to a
PLC, with lighting, motors and information displays
being typical output components for a PLC.
Chapter 7 — PLC and System Interfacing
Relays, contactors,
and motor starters are
the most common
interface devices used
to control high-power
loads.
Chapter 7 — PLC and System Interfacing
Electromechanical relays have sets of contacts that
are closed by magnetic force.
Chapter 7 — PLC and System Interfacing
Solid-state relays have an input
voltage range, such as 3 VDC to
32 VDC, that allows a single solidstate relay to be used with most
electronic circuits and PLC output
modules.
Chapter 7 — PLC and System Interfacing
The type of relay used depends
on the life expectancy, electrical
requirements, and cost
requirements of the application.
Chapter 7 — PLC and System Interfacing
Any load that can cause
a problem to the output
section or module of a
PLC must include an
interface device between
the PLC and the load.
Chapter 7 — PLC and System Interfacing
The output side of a solid-state relay can be used to
control high-voltage medium-power loads such as 3f
heating elements.
Chapter 7 — PLC and System Interfacing
Contactors are designed
to control high-power,
non-motor loads.
Chapter 7 — PLC and System Interfacing
Contactors allow PLC output circuitry to be rated
much lower than the loads that the PLC is controlling.
Chapter 7 — PLC and System Interfacing
Magnetic motor starters
use a small control current
to energize a coil to send
high power to a motor.
Chapter 7 — PLC and System Interfacing
When the coil of a motor starter and the control circuit
require lower voltage, a step-down transformer is
used to reduce the high voltage to a low voltage.
Chapter 7 — PLC and System Interfacing
Electric motor drives control motor speed by
controlling the frequency of the electricity to a motor.
Chapter 7 — PLC and System Interfacing
Any type of switch contacts
(pushbutton, pressure switch,
PLC output contacts) can be
used to control an electric
motor drive.