Download Chapter06_Control_relays - PTEC107 AC/DC Electronics

Chapter 6
Control_relays
Relays and their Types
• Electromagnetic
• Contactors
• Motor Starters
Electromagnetic Control Relays – page 130
The PLC's original purpose was the replacement of
electromagnetic relays with a solid-state switching
system that could be programmed.
The programmable controller was designed
to replace physically small control relays
that make logic decisions but are not
designed to handle heavy current or high
voltage.
Electromagnetic relays, such as the
lighting contactor shown, are still
used as auxiliary devices to switch I/O
field devices.
Electromagnetic Relay Operation
An electromagnetic relay is a magnetic switch. It uses
electromagnetism to switch contacts.
A relay will usually
have only one coil
but may have any
number of different
contacts.
Electromagnetic Relay Operation
With no current flow
through the coil
(coil de-energized),
the armature is held
away from the core
by spring tension.
When the coil is energized,
the electromagnetic field
moves the armature
causing the contact points
of the relay to open or
close.
Relay Symbol
Coil
CR1
CR1-1
CR1-2
Normally open
(NO) contact
Normally closed
(NC) contact
Contacts are open when
no current flows through
the coil but close as soon
as the coil is energized.
Contacts are closed when
no current flows through
the coil but open as soon
as the coil is energized.
Relay Circuit Operation
L1
L2
S
CR1
CR1-1
OFF
R
G
CR1-2
ON
With switch S open:
 coil CR1 is de-energized
 contacts CR1-1 are open
 light R is off
 contacts CR1-2 are closed
 light G is on
Relay Circuit Operation
L1
L2
S
CR1
CR1-1
ON
R
G
CR1-2
OFF
With switch S closed:
 coil CR1 is energized
 contacts CR1-1 are closed
 light R is on
 contacts CR1-2 are open
 light G is off
Contactor - page 131
A contactor is a special type of relay designed to
handle heavy power loads that are beyond the
capability of control relays.
Contactors are designed to operate
such loads as lights, heaters,
transformers, capacitors, and electric
motors for which overload
protection is provided separately
or not required.
Contactor
PLC Used In Conjunction With A Contactor
Contactor
power contacts
Pump
L2
High-current
wiring
L1
Low-current
wiring
L2
Programmable controllers
Coil
have I/O capable of operating
terminals the contactor but they do not
have the capacity to operate
heavy loads directly.
L1
PLC output module
Motor Starter – page 133
A magnetic motor starter is a contactor with an
overload relay attached physically and electrically.
They are electromagnetically operated switches that
provide a safe method for starting large motor loads.
The overload relay will open the
supply voltage to the starter if it
detects an overload on a motor.
Motor overload relay contacts are
normally hardwired in series with
the magnetic starter coil.
Motor Starter
Overload heaters are
connected in series
with the contactor.
If the motor becomes
overloaded they cause
a mechanical latch to
trip. Tripping this
latch opens a set of
contacts that are
wired in series with the
voltage supply and
motor.
Across-The-Line AC Starter Operation
Low-current
control circuit
The
M the
contacts,
in series
When
start
button
is
The
OL
contact
opens
with
the motor,
close
to
pressed,
coil
M
energizes
automatically
when an
complete
the high-current
to close allcondition
M contacts.
overload
path
to the motor. is
sensed, to de-energize
the M coil
and stop
the
Control
contact
M also
motor.to seal-in the coil
closes
circuit when the START
button is released.
High-current
power circuit
PLC Control Of A Large Motor Load
When a PLC needs to
control a large motor, it
must work in conjunction
with a starter.
Motor starters are
available in various
standard National
Electric Manufacturers
(NEMA) sizes and
ratings.
Manually Operated Switches – page 134
Manually operated switches are
controlled by hand. Pushbutton
switches are the most common form of
manual control found in industry.
Normally Open (NO) pushbutton
makes a circuit when it is pressed
and returns to its open position
when the button is released.
Manually Operated Switches
Normally Closed (NC) pushbutton opens
the circuit when it is pressed and returns
to the closed position when the button is
released. The abbreviations NO and NC
represent the state of the switch when it is
not actuated.
Manually Operated Switches
The break-make pushbutton is used for
interlocking controls. In this switch the top
section is NC, while the bottom section is
NO. When the button is pressed, the
bottom contacts are closed as the top
contacts open.
NOT PRESSED
PRESSED
ON
OFF
R
R
L2
L1
L2
L1
G
OFF
G
ON
Selector Switch (Manually Operated)
Selector switch positions are made by
turning the operator knob – not
pushing it.
Contacts
Off
Hand
Position
A
Hand
X
Auto
A
Off
B
Auto
B
X
Selector switch
positions may have
two or more selector
positions with either
maintained contact
position or spring
return to give
momentary contact
operation.
An Example of a Selector Switch
Used for Motor Reversing
Selector switch used in conjunction with a reversing
motor starter to select forward or reverse operation
of the motor.
Dual In-Line Package (DIP)
Switches (Manually Operated)
ON
OFF
Are small switch assemblies
designed for mounting on printed
circuit board modules.
Switch settings are seldom
changed, and the changes occur
mainly during installation or
configuration of the system.
Mechanically Operated Switches – page 136
A mechanically operated switch is controlled automatically
by factors such as pressure, position, and temperature.
The limit switch is a type of
mechanically operated switch
designed to operate only when a
predetermined limit is reached,
and is usually actuated by
contact with an object such as a
cam.
Mechanically Operated Switches
Limit Switch Operation
Symbols
Limit switches take
the place of a human
operator.
NO Contact
NC Contact
They are often used in the control of machine processes
to govern the starting, stopping, or reversal of motors.
Mechanically Operated Switches
Typical Limit Switch Circuit
L1
Stop
Forward
L2
FWD Limit
F
F
R
OLs
Limit switches
Reverse
R
F
REV Limit
R
Control circuit for starting and stopping a motor in
forward and reverse with limit switches providing
over travel protection.
Mechanically Operated Switches
Temperature Switch
The temperature switch or thermostat is used to
sense temperature changes and is actuated by some
specific environmental temperature change.
Responds to changes in
temperature by opening or
closing an electric circuit.
Symbols
NO Contact
NC Contact
Mechanically Operated Switch
An Example of a Temperature Switch Controlling a Motor
Closing ON
Rising
temperature
Motor
Mechanically Operated Switches
Pressure Switch
Pressure switches are used to control the pressure of
liquids and gases and are activated when a specific
pressure is reached.
Opens or closes an
electric circuit in
response to a
change in pressure.
Symbols
NO Contact
NC Contact
Bellows
Switch
Mechanically Operated Switch
An Example of a Starter Operated By A Pressure Switch
Closing ON
Rising
pressure
Mechanically Operated Switch
Level Switch
Level or float switches are used to sense the height of
a liquid.
Switch
Opens or closes an electric
circuit in response to a
change in liquid level.
Symbols
Float
NO Contact
NC Contact
Two-wire level switch
control of starter.
1. The programmable controller was designed
to replace relays designed to handle heavy
current or high voltage. (True/False)
2. The NO (normally open) contacts of a relay are
open when the relay coil is energized.
(True/False)
3. A contactor is a special type of relay designed to
switch heavy power loads. (True/False)
4. Motor overload relay contacts are normally
hardwired in series with the magnetic starter coil.
(True/False)
5. A _______________ pushbutton opens the
circuit when it is pressed and returns to the
closed position when the button is released.
(a) normally open
(b) normally closed
(c) break-make pushbutton
(d) make-break pushbutton
6. The abbreviations NO and NC represent the
state of the switch when it is actuated.
(True/False)
7. Which of the following would be classified
as a manually operated switch?
a. pressure switch
c. level switch
b. temperature switch
d. none of these
8. A limit switch is usually actuated by:
(a) hand
(b) contact with an object
(c) pressure
(d) an electromagnet
9. The device shown is that of a:
(a) Float Switch
(b) Limit Switch
(c) Temperature Switch
(d) Pressure Switch
10. The symbol shown is that
of a:
(a) NO float switch
(b) NC float switch
(c) NO temperature switch
(d) NC temperature switch
Transducer – page 138
A transducer is any device that converts energy from
one form to another.
Amplifier
Input transducer
(microphone) converts
sound energy to electric
energy
Output transducer
(speaker) converts
electric energy to sound
energy
Sensors – page 138
Sensors are input transducers used for detecting and
often measuring the magnitude of something. They
convert mechanical, magnetic, thermal, optical, and
chemical variations into electric voltages and currents.
Photoelectric
sensor
Type of Sensor:
Proximity Sensor
Proximity sensors or switches detect the presence of
an object without making physical contact with it.
Proximity Sensor Applications
The object being detected is too small, lightweight, or
soft to operate a mechanical switch.
Rapid response and high switching rates are required.
An object has to be sensed through nonmetallic barriers
such as glass, plastic, and paper cartons.
Hostile environments conditions exist.
Long life and reliable service are required.
A fast electronic control system requires a bounce-free
input signal.
Proximity Sensor Connections
The method of connecting and exciting a proximity
sensor varies with the type of sensor and its
application.
L1
Target
L2
Load
Two-wire sensor connection
Capacitive Proximity Sensor
A capacitive proximity sensor can be actuated by both
conductive and nonconductive material such as wood,
plastics, liquids, sugar flour and wheat.
Operation is similar to that of inductive
proximity sensor. Instead of a coil, the
active face of the sensor is formed by
two metallic electrodes – rather like an
"opened capacitor".
Photoconductive Or Photoresistive Cell
The photoconductive cell, or
photoresistive cell, is is another
popular type of light transducer.
Light energy falling on this device
will cause a change in the
resistance of the cell.
Ohms
20 Ohms Light resistance
5,000 Ohms Dark resistance
Reflective Photoelectric Sensor
Emits a light beam (visible,
infrared, or laser) from its light
emitting element and detects
the light being reflected.
Diffused-reflective
Retro-reflective
typetype
Emitter/receiver
Operating
Operating
range
range
Target
Reflector
Through-Beam Type Photoelectric Sensor
A through-beam photoelectric
sensor is used to measure the
change in light quantity caused
by the target's crossing the
optical axis.
Emitter
Operating
range
Target
Receiver
Ultrasonic Sensor – page 147
An ultrasonic sensor operates by
sending sound waves towards the
target and measuring the time it
takes for the pulses to bounce
back.
The returning echo signal
is electronically converted
to a 4 mA to 20 mA output,
which supplies flow rate to
external control devices.
Output Control Devices – page 150
A variety of output control devices can be operated by the
controller output module to control traditional processes.
These include:
Pilot light
Heater
Control
relay
Solenoid
Motor starter
Alarm
Solenoid
valve
Small motor
Actuators – page 150
An actuator is any device that converts an electrical
signal into mechanical movement. The principle
types of actuators are relays, solenoids, and motors.
Solenoid
The solenoid converts
electric current into
linear motion.
Symbol
Plunger
AIR
Coil
Solenoid Valve
A solenoid valve is a combination of:
 a solenoid with its core or plunger
 a valve body containing an orifice
in which a disc or plug is positioned
to restrict or allow flow
Forward motion of piston
Directional
solenoid
valve
When SOL A is energized, the
valve spool is shifted to redirect
the fluid and move the cylinder
forward
CR
SOL A
FWD
SOL A
CR
Seal-In Circuits – page 153
A seal-in circuit is a method of maintaining
current flow after a momentary switch has
been pressed and released.
Hardwired Seal-In Circuit
Start
Stop
Motor
starter coil
M
M
Seal-in contact
The seal-in auxiliary contact of the
starter is connected in parallel with
the Start button to keep the starter
coil energized when the Start button is
released.