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INTRODUCTION TO AUTOMATION SYSTEM
1.2. Understand relays, contactors, electromechanical timer and
electromechanical counter
1.2.1 Relay
Introduction
Relays are simple switches which are operated both electrically and
mechanically. Relays consist of an electromagnet and also a set of
contacts. The switching mechanism is carried out with the help of the
electromagnet. There are also other operating principles for its working. But
they differ according to their applications. Most of the devices have the
application of relays.
The main operation of a relay comes in places where only a low-power
signal can be used to control a circuit. It is also used in places where only
one signal can be used to control a lot of circuits. The application of relays
started during the invention of telephones. They played an important role in
switching calls in telephone exchanges. They were also used in long
distance telegraphy. They were used to switch the signal coming from one
source to another destination. After the invention of computers they were
also used to perform Boolean and other logical operations. The high end
applications of relays require high power to be driven by electric motors and
so on. Such relays are called contactors.
a. Symbol
Relay symbols commonly used are:
b. Building diagram
In general, a relay consists of a magnetic core and its associated coil,
contacts, springs, armature, and the mounting. Figure 1.0 illustrates the
construction of a relay.
Figure 1.0 —Relay construction.
A relay can have many different types of contacts. The relay shown in figure
1.0 has contacts known as "break-make" contacts because they break one
circuit and make another when the relay is energized. Figure 1.1 shows five
different combinations of relay contacts and the names given to each.
Figure 1.1 : Contact
combination
c. Operation
Refer to figure 1.0, when the coil is energized, the flow of current through
the coil creates a strong magnetic field which pulls the armature
downward to contact C1,completing the circuit from the common terminal
to C1. At the same time, the circuit to contact C2, is opened.
Figure 1.2 : SPST relay used to control a ac load
In the above schematic, the relay's coil is energized by the low-voltage (12
VDC) source, while the single-pole, single-throw (SPST) contact interrupts
the high-voltage (480 VAC) circuit. It is quite likely that the current required
to energize the relay coil will be hundreds of times less than the current
rating of the contact. Typical relay coil currents are well below 1 amp,
while typical contact ratings for industrial relays are at least 10 amps.
d. Various types of relay (SPDT, DPDT, 4PDT)
Since relays are switches, the terminology applied to switches is also
applied to relays. A relay will switch one or more poles, each of whose
contacts can be thrown by energizing the coil in one of three ways:



Normally-open (NO) contacts connect the circuit when the relay is
activated; the circuit is disconnected when the relay is inactive. It is also
called a Form A contact or "make" contact.
Normally-closed (NC) contacts disconnect the circuit when the relay is
activated; the circuit is connected when the relay is inactive. It is also
called a Form B contact or "break" contact.
Change-over (CO), or double-throw (DT), contacts control two circuits:
one normally-open contact and one normally-closed contact with a
common terminal. It is also called a Form C contact or "transfer" contact
("break before make"). If this type of contact utilizes a "make before
break" functionality, then it is called a Form D contact.
The following designations are commonly encountered:




SPST - Single Pole Single Throw. These have two terminals which can
be connected or disconnected. Including two for the coil, such a relay
has four terminals in total. It is ambiguous whether the pole is normally
open or normally closed. The terminology "SPNO" and "SPNC" is
sometimes used to resolve the ambiguity.
SPDT - Single Pole Double Throw. A common terminal connects to
either of two others. Including two for the coil, such a relay has five
terminals in total.
DPST - Double Pole Single Throw. These have two pairs of terminals.
Equivalent to two SPST switches or relays actuated by a single coil.
Including two for the coil, such a relay has six terminals in total. The
poles may be Form A or Form B (or one of each).
DPDT - Double Pole Double Throw. These have two rows of changeover terminals. Equivalent to two SPDT switches or relays actuated by a
single coil. Such a relay has eight terminals, including the coil.
The "S" or "D" may be replaced with a number, indicating multiple switches
connected to a single actuator. For example 4PDT indicates a four pole
double throw relay (with 14 terminals).
Fig. 1.3: Circuit symbols of relays.
coil relay
Fig. 1.4: The package of a DPDT AC
Figure 1.3 shown Circuit symbols of relays. "C" denotes the common
terminal in SPDT and DPDT types while, the Figure 1.4 shown The diagram
on the package of a DPDT AC coil relay
e. Application examples
Relays are used to and for:






Control a high-voltage circuit with a low-voltage signal, as in some types
of modems or audio amplifiers.
Control a high-current circuit with a low-current signal, as in
the starter solenoid of an automobile,
Detect and isolate faults on transmission and distribution lines by opening
and closing circuit breakers (protection relays),
Isolate the controlling circuit from the controlled circuit when the two are at
different potentials, for example when controlling a mains-powered device
from a low-voltage switch. The latter is often applied to control office
lighting as the low voltage wires are easily installed in partitions, which
may be often moved as needs change. They may also be controlled by
room occupancy detectors in an effort to conserve energy,
Logic functions. For example, the boolean AND function is realised by
connecting normally open relay contacts in series, the OR function by
connecting normally open contacts in parallel. The change-over or Form C
contacts perform the XOR (exclusive or) function. Similar functions for
NAND and NOR are accomplished using normally closed contacts.
The Ladder programming language is often used for designing relay logic
networks.
Early computing. Before vacuum tubes and transistors, relays were used
as logical elements in digital computers. See ARRA (computer), Harvard
Mark II, Zuse Z2, and Zuse Z3.


Safety-critical logic. Because relays are much more resistant than
semiconductors to nuclear radiation, they are widely used in safety-critical
logic, such as the control panels of radioactive waste-handling machinery.
Time delay functions. Relays can be modified to delay opening or delay
closing a set of contacts. A very short (a fraction of a second) delay would
use a copper disk between the armature and moving blade assembly.
Current flowing in the disk maintains magnetic field for a short time,
lengthening release time. For a slightly longer (up to a minute) delay,
a dashpot is used. A dashpot is a piston filled with fluid that is allowed to
escape slowly. The time period can be varied by increasing or decreasing
the flow rate. For longer time periods, a mechanical clockwork timer is
installed.
Figure 1.5 : A DPDT AC coil relay with "ice
packaging
Examples of the use of relay:
i.
Electrical Circuit (automobile)
Figure 1.6 Relay used in automobile
cube"
ii.
Electric/electronic circuit. (DC to AC)
Figure 1.7 : Relay used to control as load in electronic circuit
iii.
Motor control circuit
Figure 1.8 : Relay memory circuit (Latching circuit)
1.2.2 Contactors
Introduction
Figure 1.9 : a contactor
A contactor is an electro-magnetic switching device (a relay) used for
remotely switching a power or control circuit.[1] A contactor is activated by a control
input which can be a lower or higher voltage / currentthan that which the contactor
is switching. Contactors come in many forms with varying capacities and features.
Unlike a circuit breaker a contactor is not intended to interrupt a short
circuit current.
Contactors range from having a breaking current of several amps and 110
volts to thousands of amps and many kilovolts. The physical size of contactors
ranges from a device small enough to pick up with one hand, to large devices
approximately a metre (yard) on a side.
Contactors
are
used
to
control electric
motors, lighting, heating, capacitor banks, and other electrical loads.
a. Basic Construction & components
A contactor is basically composed of three different items. The contact
item/s are the current carrying part of the contactor. This includes Power
Contacts, Auxiliary Contacts, and Contact Springs. The electromagnet item
provides the driving force to close the contacts. The enclosure item is a frame
housing the contact and the electromagnet. Enclosures are made of insulating
materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate
the contacts and to provide some measure of protection against personnel
touching the contacts. Open-frame contactors may have a further enclosure to
protect against dust, oil, explosion hazards and weather.
A basic contactor will have a coil input (which may be driven by either an AC
or DC supply depending on the contactor design). The coil may be energized at
the same voltage as the motor, or may be separately controlled with a lower coil
voltage better suited to control by programmable controllers and lower-voltage pilot
devices. Certain contactors have series coils connected in the motor circuit; these
are used, for example, for automatic acceleration control, where the next stage of
resistance is not cut out until the motor current has dropped.
Figure 1.10: Three-phase contactor
principle: 1. Coil, 2. Spring, 3. Armature, 4. Moving contact
b. Symbol
A1, A2
- Coil
1,2,3,4,5,6 – Main Contact
11,12,23,24 – Additional Contact (auxiliary contact)
c. Operation
Unlike general-purpose relays, contactors are designed to be directly
connected to high-current load devices. Relays tend to be of lower capacity and
are usually designed for both Normally Closed and Normally Open applications.
Devices switching more than 15 amperes or in circuits rated more than a few
kilowatts are usually called contactors. Apart from optional auxiliary low current
contacts, contactors are almost exclusively fitted with Normally Open contacts.
Unlike relays, contactors are designed with features to control and suppress the
arc produced when interrupting heavy motor currents.
When current passes through the electromagnet, a magnetic field is produced
which attracts ferrous objects, in this case the moving core of the contactor is
attracted to the stationary core. Since there is an air gap initially, the electromagnet
coil draws more current initially until the cores meet and reduce the gap, increasing
the inductive impedance of the circuit. The moving contact is propelled by the
moving core; the force developed by the electromagnet holds the moving and fixed
contacts together. When the contactor coil is de-energized, gravity or a spring
returns the electromagnet core to its initial position and opens the contacts.
For contactors energized with alternating current, a small part of the core is
surrounded with a shading coil, which slightly delays the magnetic flux in the core.
The effect is to average out the alternating pull of the magnetic field and so prevent
the core from buzzing at twice line frequency.
Most motor control contactors at low voltages (600 volts and less) are "air
break" contactors, since ordinary air surrounds the contacts and extinguishes the
arc when interrupting the circuit. Modern medium-voltage motor controllers use
vacuum contactors.
Motor control contactors can be fitted with short-circuit protection (fuses or
circuit breakers), disconnecting means, overload relays and an enclosure to make
a combination starter. In large industrial plants many contactors may be assembled
in motor control centers.
d. Application examples
Contactors
are
used
to
control electric
motors, lighting, heating, capacitor banks, and other electrical loads.
Examples:
1. Direct On-Line Motor Starer (DOL):
Figure 1.11: DOL Box (consist of contactor and overload) used as
motor starter
Figure 1.12 : DOL schematic circuit
2. Star-Delta Stater:
Figure 1.13 : Star-Delta Stater schematic circuit
3. AC Motor Control (Forward – Reverse):
Figure 1.14 : Forward-Reverse starter schematic circuit
1.2.3 Electromechanical Timer
Introduction
A timer is a control device that outputs a signal at a preset time after an
input signal is received.
A timer also known as time delay relay is a combination
of an
electromechanical output relay and a control circuit. The control circuit is
comprised of solid state components and timing circuits that control operation of
the relay and timing range. Typical time delay functions include On-Delay, Repeat
cycle (starting off), Interval, Off-Delay, Retriggerable One Shot, Repeat cycle
(starting on), Pulse Generator, One Shot, On/Off Delay, and Memory Latch.
Time delay relays have a broad choice of timing ranges from less than one
second to many days. There are many choices of timing adjustments from
calibrated external knobs, DIP switches, thumbwheel switches, or recessed
potentiometer. The output contacts on the electromechanical output relay are
direct wired to the output terminals. The contact load ratings are specified for each
specific type of time delay relay.
Figure 1.15 : Various types of timers
a. Symbol
The symbol for relay and timer used in electrical circuit is shown in figure 1.16.
Figure 1.16 : coil and contact symbols for timer and relay
From figure 1.16, symbol for timer can be simplified as follows:
Figure 1.17 : shows the timer used in the control circuit.
b. Building diagram
Figure 1.18 : Timer mechanisms
c. Operation
DELAY ON OPERATE
The time delay period begins when power is applied to the input
terminals of the relay. When the time delay has completed, the contacts of
the relay will switch to the energized position. The relay contacts will remain
in the energized position until power is removed. The contacts of the relay
transfer to the unenergized position upon removal of power. The time delay
of the relay will reset if power is removed before the delay has completed.
INTERVAL TIME DELAY
When power is applied to the input terminals of the relay, the contacts of
the relay switch to the energized position and the timing period begins.
When the time delay interval has completed, the relay contacts will transfer
to the unenergized position. Once the interval time delay has completed,
power must be removed in order to reset the time delay of the relay. If
power is removed before the time delay has completed, the contacts will deenergize and the time delay will terminate.
d. Examples in conventional control system
Example 1:
An electric motor is used to power a large conveyor belt. Before the motor actually
starts, a warning siren activates to alert workers of the conveyor's forthcoming
action. The following relay circuit accomplishes both tasks (motor control plus siren
alert):
Example 2:
Large electric motors are often equipped with some form of soft-start control, which
applies power gradually instead of all at once (as in across the line" starting). Here
is an example of a simple soft start" control system:
1.2.4 Counter
Introduction
Counter is a control device that receives a pulse signal and counts the pulses
to display a number. This includes Preset Counters that output a signal at a preset
value.
The term counter is derived from the word count. It is fairly simple for people
to count ten or twenty objects, but larger numbers make counting increasingly
difficult. Counters outperform people when it comes to counting accurately.
For example, the devices that are used by people on street corners to count
pedestrians in traffic surveys are one type of counter. They are a good
replacement for people because they accurately count pedestrians and remember
the count even with very large numbers.
Figure 1.19 : Various types of counter
Electromechanical counters are used to detect, totalize and indicates a
sequence of event. They typically accept electrical and electronic input to operate
mechanical output such as rotating wheels or knobs. This device may count up
and/or down support multiple function and provide reset capabilities.
a. Symbol
b. Counter operation in conventional control system
The following example shows the use of Counters in automated machinery.
Pudding Production Line
On a pudding production line, a Photoelectric Sensor detects the finished
products. If the number three is preset in the Counter and it receives a
Photoelectric Sensor signal three times, the Pusher (see note) pushes the three
pudding containers into a box.
The following section describes the input, control, and output signal sequence in a
boxing process.
1. Input
A Photoelectric Sensor is an input device that detects an object when that object
blocks light. Each time the light is blocked, the Photoelectric Sensor sends a signal
to the Counter.
2. Control
Preset the number three in the Counter. The Counter will then count the number of
signals from the Photoelectric Sensor and sends a signal to the Pusher (i.e., the
output device) after the Counter receives a signal for the third time.
3. Output
When the Pusher receives the output signal from the Counter, it pushes the three
pudding containers into a box.
As you can see, the Counter is involved in every step of the input,control, and
output signal sequence.