Control Circuits
INC 102 Instrumentation and Process Control
Assoc. Prof. Pakorn Kaewtrakulpong, Ph.D.
INC, KMUTT
Objectives
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State the purpose and general principles of control
components and circuits
Identify various NEC symbols which represent control
components
Explain a basic line diagram
Give examples on how to apply the control circuit for
creating control strategies.
Control Elements
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The elements of a control circuit include all of the
equipment and devices concerned with the circuit
function.
This includes
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enclosures,
conductors,
relays,
contactors,
pilot devices, and
overcurrent-protection devices.
Control Loads and
Control Devices
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The control load is an electrical device that uses
electrical power. E.g.
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Pilot lights,
relays, and Contactors.
Control devices are used to activate the control load.
E.g.
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Pushbuttons and
switches.
Electrical Symbols
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Contact
Switch
Pushbutton
Coil
Pilot Light
Control Transformer
Timing Relays
etc
Contact
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used to indicate an open or closed path of
current flow.
require an action to change their state.
can be
Normally Open (NO) or
 Normally Closed (NC).
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Switch
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also used to indicate an open or closed path of
current flow.
require another device or action to change their
state.
can be
Normally Open (NO) or
 Normally Closed (NC).
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Switch
Selector Switch
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Two-position
Three-position
Etc.
Selector Switch
Pushbutton
Push Button
Pushbutton (contd.)
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2 basic types of pushbuttons:
momentary
 maintained (latches in place when the button is
pressed.)
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can be
Normally Open (NO) or
 Normally Closed (NC).
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Switch (contd.)
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Variations of this symbol are used to represent
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limit switches,
foot switches,
pressure switches,
level switches,
temperature-actuated switches,
flow switches, and
selector switches.
Pilot Lights
Pilot Lights
Pilot Light
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a small electric light used to indicate a specific
condition of a circuit.
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E.g., a red light might be used to indicate a motor is
running.
The letter in the center of the pilot light symbol
indicates the color of the light.
Signaling Column
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can be mounted locally on individual machines
Control Relays
Control Relays (contd.)
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Relays are widely used in control circuits.
They are used
for switching multiple control circuits and
 for controlling light loads e.g. starting coils, pilot
lights, and audible alarms.
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Relay Operation
Coil
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used in electromagnetic starters, contactors, and
relays.
The purpose of contactors and relays is to open
and close associated contacts.
A letter is used to designate the coil; the
associated contacts have the same identifying
letter.
Coil (contd.)
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Associated contacts can be
Normally Open (NO) or
 Normally Closed (NC).
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Coil Example
Magnetic Contactors
Basic Contactor Operation
Magnetic Contactors: Interior
Control Transformers
Timing Relays / Timers
Timing Relays
Counter
Solenoid Valve & Pneumatic
Equipment
Line Diagrams
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also referred to as a ladder diagrams.
show the functional relationship of
components and devices in an electrical circuit,
not the physical relationship.
made up of 2 circuits,
the control circuit and
 the power circuit.
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Line Diagrams (contd.)
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Electrical wires in a line diagram are represented by
lines.
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Control-circuit wiring is represented by a lighter-weight
line and power-circuit wiring is represented by a heavierweight line.
A small dot or node at the intersection of 2 or more
wires indicates an electrical connection.
Line diagrams are read from left to right.
Example: Power Circuit and Control
Circuit
Power circuit
Control circuit
Connecting Loads and
Control Devices
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Control circuits are made up of control loads
and control devices.
Two-Wire Control
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A two-wire control circuit provides low-voltage
release but not low-voltage protection.
low-voltage release
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In the event of a power loss the contactor will deenergize,
stopping the motor; however, when power is restored, the
motor will restart without warning if the control device is still
closed.
This type of control scheme is used for remote or
inaccessible installations such as water-treatment
plants or pumping stations. In these applications it is
desirable to have an immediate return to service when
power is restored.
Two-Wire Control (contd.)
Holding Circuit
Three-Wire Control
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also called self-holding or self-latching circuit
low-voltage protection.
If an overload causes the “OL” contacts in the control
circuit to open,
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the “M” coil is deenergized and
the motor shut down.
When the overload is cleared,
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the motor will not suddenly restart on its own.
An operator must depress the “Start” button to restart the
motor.
Holding Circuit
Three-Wire Control (contd.)
Interposing a Relay
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A 24 VAC coil may not be strong enough to
operate a large starter. E.g.
the electromagnetic coil of the “M” contactor
selected is rated for 460 VAC.
 The electromagnetic coil of the control relay (CR)
selected is 24 VAC.
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Interposing a Relay (contd.)
Schematic Diagram
One-Line Diagram
Wiring Diagram
Some Simple Motor Control
Applications
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Forward-Reverse Motor Control
Star-Delta Motor-Starting Control
Forward-Reverse Motor Control
e.g.
 Jogging circuits
 Reversing-after-stop
circuits
Jogging Circuits
using selector
switch
without shortcircuit protection
with shortcircuit protection
Reversing-After-Stop Circuits
without shortcircuit protection
with shortcircuit protection
Star-Delta Motor-Starting Control
Star-Delta Motor-Starting Control
(contd.)