1. Chapter 6
Control_relays

2. Relays and their Types
Electromagnetic
Contactors
Motor Starters

3. 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.

4. 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.

5. 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.

6. Relay Symbol Normally open Normally closed (NO) contact (NC) contact
CR1-1
CR1-2
Normally open
(NO) contact
CR1
Coil
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.

7. Relay Circuit OperationS
CR1
CR1-1
CR1-2
OFF R ON G
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

8. Relay Circuit OperationS
CR1
CR1-1
CR1-2
OFF R ON G
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

9. 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.

10. Contactor

11. PLC Used In Conjunction With A Contactor
Pump
Contactor
power contacts L2 L1
High-current
wiring
Programmable controllers
have I/O capable of operating
the contactor but they do not
have the capacity to operate
heavy loads directly.
Coil
terminals
Low-current
wiring L2 L1
PLC output module

12. 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.

13. 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.

14. Across-The-Line AC Starter Operation
The OL contact opens
automatically when an
overload condition is sensed, to de-energize
the M coil and stop the motor.
When the start button is
pressed, coil M energizes
to close all M contacts.
The M contacts, in series
with the motor, close to
complete the high-current
path to the motor.
Control contact M also
closes to seal-in the coil
circuit when the START
button is released.
Low-current
control circuit
High-current
power circuit

15. 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.

16. 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.

17. 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.

18. 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. L1 L2 R G ON
OFF
NOT PRESSED
PRESSED

19. Selector Switch (Manually Operated)
Selector switch positions are made by turning the operator knob – not pushing it.
Contacts
Position A B
Off
Hand
Auto
Selector switch
positions may have
two or more selector
positions with either
maintained contact
position or spring
return to give
momentary contact
operation.
Hand X
Off X
Auto

20. 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.

21. 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.

22. 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.

23. Mechanically Operated Switches Limit Switch Operation
Symbols
NO Contact
NC Contact
Limit switches take
the place of a human
operator.
They are often used in the control of machine processes
to govern the starting, stopping, or reversal of motors.

24. Mechanically Operated Switches Typical Limit Switch Circuit
Stop
Forward F R
Reverse
OLs
Limit switches
FWD Limit
REV Limit
Control circuit for starting and stopping a motor in
forward and reverse with limit switches providing
over travel protection.

25. 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

26. Mechanically Operated Switch
An Example of a Temperature Switch Controlling a Motor
Closing ON
Rising
temperature
Motor

27. 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.
Switch
Bellows
Symbols
NO Contact
NC Contact

28. Mechanically Operated Switch
An Example of a Starter Operated By A Pressure Switch
Closing ON
Rising
pressure

29. 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
NO Contact
NC Contact
Float
Two-wire level switch
control of starter.

30. 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)

31. 5. A _______________ pushbutton opens the
circuit when it is pressed and returns to the
closed position when the button is released.
normally open
normally closed
break-make pushbutton
make-break pushbutton
6. The abbreviations NO and NC represent the
state of the switch when it is actuated.
(True/False)

32. 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

33. 9. The device shown is that of a:
Float Switch
Limit Switch
Temperature Switch
Pressure Switch

34. 10. The symbol shown is that
of a:
NO float switch
NC float switch
NO temperature switch
NC temperature switch

35. Transducer – page 138
A transducer is any device that converts energy from
one form to another.
Input transducer
(microphone) converts
sound energy to electric
energy
Output transducer
(speaker) converts
electric energy to sound
energy
Amplifier

36. 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

37. Proximity sensors or switches detect the presence of
Type of Sensor:
Proximity Sensor
Proximity sensors or switches detect the presence of
an object without making physical contact with it.

38. 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.

39. Proximity Sensor Connections
The method of connecting and exciting a proximity
sensor varies with the type of sensor and its
application.
Target L1 L2
Load
Two-wire sensor connection

40. 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".

41. 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.
20 Ohms Light resistance 5,000 Ohms Dark resistance
Ohms

42. Reflective Photoelectric Sensor
Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected.
Retro-reflective type
Operating
range
Reflector
Operating
range
Diffused-reflective type
Emitter/receiver
Target

43. 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.
Operating
range
Target
Emitter
Receiver

44. 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.

45. 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
Solenoid
valve
Control
relay
Alarm
Heater
Motor starter
Small motor

46. 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.
The solenoid converts electric current into linear motion.
AIR
Coil
Plunger
Solenoid
Symbol

47. 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
When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward
SOL A
Forward motion of piston
Directional
solenoid
valve
FWD CR

48. 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
Motor
starter coil
Start
Stop M M
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.
Seal-in contact