Description of various outputs
Some of the output devices are described as follows-
It is an electro-magnetic relay with a wire coil, surrounded by an iron core. A path of very
The diagram shows in figure 3-43 an inner section diagram of a relay. An iron core is
Relay Basics
Energized Relay (ON)
Relay Energized ON
In simple, when a voltage is applied to pins 1 & 3, the electromagnet activates, causing a
Double Pole Single Throw (DPST) – This relay has a total of six terminals. These
The top three contacts are called main contacts. Main contacts switch the respective phases of the incoming 3-phase AC power (terminal A,B & C) to motors (terminal U, V & W). The lowest contacts are “auxiliary” contact which has a current rating much lower than that of the large motor power contacts, but is actuated by the same armature as the power
We can connect auxiliary contacts as shown in figure 
When motor never connects with contactor directly; it always connects thermal overload
There are two common categories of outputs; they are- Digital and ANALOG.
Digital outputs can be- motors, relays, solenoid valves or enunciator windows.
ANALOG outputs can be –Signal to variable speed drives, to input control valves (it
will explained in chapter number 10)
There is some different digital output devices are-
1. Motors (Generally connected with relays and contactors)
2. Valves
3. Actuators
4. Coils
5. Buzzers
6. Bells
7. Alarms
8. Enunciators etc.
Digital output voltage of any PLC depends on the output devices connected to the PLC’s. We
can take any voltage up to the maximum rating of PLC output card.
3.5.1 Motor
AC Motor
AC Motor
An electric motor is an electromechanical device that converts electrical energy
into mechanical energy. Most electric motors operate through the interaction of magnetic
fields and current-carrying conductors to generate force. The output signal from
programmable logic controller is used to operate the motor which in turn is used to run many applications such as industrial fans, blowers and pumps, machine tools, household
appliances, power tools, disk drives etc.
As motor are taking more current then rated current of a PLC output cards. In this
case motor has to be connected through relay or contactor.
Relays
Relays are electromechanical switching devices. Relays consist of an electromagnet and also a set of contacts. The switching mechanism is carried out with the help of the electromagnet. 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.
Relay Design
There are only four main parts in a relay. They are
Electromagnet
Movable Armature
Switch point contacts
Spring
The figures 3-42 show the actual design of a simple relay.
It is an electro-magnetic relay with a wire coil, surrounded by an iron core. A path of very
low reluctance for the magnetic flux is provided for the movable armature and also the switch point contacts. The movable armature is connected to the yoke which is mechanically connected to the switch point contacts. These parts are safely held with the help of a spring. The spring is used so as to produce an air gap in the circuit when the relay becomes deenergized.
Working of relay:
The working of a relay can be better understood by explaining the following diagram given
below in figure 3-43.
The diagram shows in figure 3-43 an inner section diagram of a relay. An iron core is
surrounded by a control coil. The power supply is given to the electromagnet through a
control switch. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
As soon as the coil current is off, the movable armature will be returned by a force back to its
initial position with spring. This spring force will be almost equal to half the strength of the
magnetic force.
Cross sectional diagram of relay without control coil energising condition
Relay Basics
The basics for all the relays are the same. Take a look at a 4 – pin relay shown in figure 3-46.
Terminals 1 & 3 represents, control coil of relay and terminals 2 & 4 are the switching
contacts of a relay where load has to be connected. This switch is controlled by the coil in the control circuit. Now let us take the different steps that occur in a relay.
Energized Relay (ON)
As shown in figure, the current flowing through the coils represented by pins 1 and 3 causes a
magnetic field to be aroused. This magnetic field causes the closing of the pins 2 and 4. Thus the switch plays an important role in the relay working. As it is a part of the load circuit, it is used to control an electrical circuit that is connected to it. Thus, when the relay in energized the current flow will be through the pins 2 and 4.
De – Energized Relay (OFF)
As soon as the current flow stops through pins 1 and 3, the switch opens and thus the open
circuit prevents the current flow through pins 2 and 4. Thus the relay becomes de-energized
and thus in off position.
In simple, when a voltage is applied to pins 1 & 3, the electromagnet activates, causing a
magnetic field to be developed, which goes on to close the pins 2 and 4 causing a closed
circuit. When there is no voltage on pin 1&3, there will be no electromagnetic force and thus no magnetic field. Thus the switches remain open.
Concept of Pole and Throw
Relays have the exact working of a switch. So, the same concept is also applied. A relay is
said to switch one or more poles. Each pole has contacts that can be thrown in mainly three ways. They are
Normally Open Contact (NO) – NO contact is also called a make contact. It closes the
circuit when the relay is activated. It disconnects the circuit when the relay is inactive.
Normally Closed Contact (NC) – NC contact is also known as break contact. This is
opposite to the NO contact. When the relay is activated, the circuit disconnects. When
the relay is deactivated, the circuit connects.
Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used to
control two types of circuits. They are used to control a NO contact and also a NC
contact with a common terminal. According to their type they are called by the names break before make and make before break contacts
Relays are also named with designations like
Single Pole Single Throw (SPST) – This type of relay has a total of four terminals. Out
of these two terminals are acting as switching circuit. The other two terminals are used
as control circuit as shown in figure 3-49
Single Pole Double Throw (SPDT) – This type of a relay has a total of five terminals.
Out of these two terminals are acting as a control circuit in remaining three one is
common or pole other two are switching contacts NO and NC as shown in figure 3-49.
Double Pole Single Throw (DPST) – This relay has a total of six terminals. These
terminals are further divided into two pairs. Thus they can act as two SPST’s which are
actuated by a single coil. Out of the six terminals two of them are coil terminals as
shown in figure 3-49,
Double Pole Double Throw (DPDT) – This relay has eight terminals. these terminals
are further divided into two pairs. Thus they can act as two SPDT’s which are actuated
by a single coil. out of the eight terminals two of them are coil terminals as shown in
figure 3-49.
Relay Applications
Relays are used to realize logic functions. They play a very important role in providing
safety critical logic.
Relays are used to control high voltage circuits with the help of low voltage signals.
Similarly they are used to control high current circuits with the help of low current
signals.
They are also used as protective relays. By this function all the faults during
transmission and reception can be detected and isolated.
Contactors:
When a relay is used to switch a large amount of electrical power through its contacts, it is
designated by a special name: contactor. Contactors typically have multiple contacts, and those contacts are usually normally-open, so that power to the load is shut off when the coil is de-energized. The most common industrial use for contactors is the control of electric motor
The top three contacts are called main contacts. Main contacts switch the respective phases of the incoming 3-phase AC power (terminal A,B & C) to motors (terminal U, V & W). The lowest contacts are “auxiliary” contact which has a current rating much lower than that of the large motor power contacts, but is actuated by the same armature as the power
contacts. The auxiliary contact is often used in a relay logic circuit, or for some other part of
the motor control scheme, typically switching is 230 V AC or 24 V DC Voltage instead of
the motor voltage. One contactor may have several auxiliary contacts, either normally-open
or normally-closed, if required
When motor never connects with contactor directly; it always connects thermal overload
protection series with main contacts of contactor.
The three “opposed-question-mark” shaped devices in series with each phase going to the
motor are called overload heaters. Each “heater” element is a low-resistance strip of metal intended to heat up as the motor draws current. If the temperature of any of these heater elements reaches a critical point (equivalent to a moderate overloading of the motor), a normally-closed switch contact (not shown in the diagram) will spring open. This normallyclosed contact is usually connected in series with the relay coil, so that when it opens the relay will automatically de-energize, thereby shutting off power to the motor. Overload heaters are intended to provide over current protection for large electric motors, unlike circuit breakers and fuses which serve the primary purpose of providing over current
protection for power conductors.
Overload heaters are designed to thermally mimic the heating characteristic of the particular electric motor to be protected. All motors have thermal characteristics, including the amount of heat energy generated by resistive dissipation (I2R), the thermal transfer characteristics of heat “conducted” to the cooling medium through the metal frame of the motor, the physical mass and specific heat of the materials constituting the motor, etc. These characteristics are mimicked by the overload heater on a miniature scale: when the motor heats up toward its critical temperature, so will the heater toward its critical temperature, ideally at the same rate and approach curve. Thus, the overload contact, in sensing heater temperature with a thermo-mechanical mechanism, will sense an analogue of the real motor. If the overload contact trips due to excessive heater temperature, it will be an indication that the real motor has reached its critical temperature (or, would have done so in a short while). After tripping, the heaters are supposed to cool down at the same rate and approach curve as the real motor, so that they indicate an accurate proportion of the motor’s thermal condition, and will not allow power to be re-applied until the motor is truly ready for start-up again. Shown in figure 3-54, it is a contractor for a three-phase electric motor, installed on a panel as part of an electrical control system;
Three-phase, 480 volt AC power comes in to the three normally-open contacts at the top of
the contactor via screw terminals labeled “L1,” “L2,” and “L3” (The “L2” terminal is
hidden behind a square-shaped “snubber” circuit connected across the contactor’s coil
terminals). Power to the motor exits the overload heater assembly at the bottom of this
device via screw terminals labeled “T1,” “T2,” and “T3.”
The overload heater units labeled “W34,” indicating a particular thermal response for a
certain horsepower and temperature rating of electric motor. If an electric motor of differing
power and/or temperature ratings were to be substituted for the one presently in service, the
overload heater units would have to be replaced with units having a thermal response
suitable for the new motor. The motor manufacturer can provide information on the
appropriate heater units to use.
A white pushbutton located between the “T1” and “T2” line heaters serves as a way to
manually re-set the normally-closed switch contact back to its normal state after having
been tripped by excessive heater temperature. Wire connections to the “overload” switch
contact may be seen at the lower-right of the photograph, near a label reading “NC
(normally-closed). On this particular overload unit, a small “window” with the label
“Tripped” indicates a tripped condition.
Heater elements may be used as a crude current shunt resistor for determining whether or not a motor is drawing current when the contactor is closed. There may be times when
you’re working on a motor control circuit, where the contactor is located far away from the motor itself. How do you know if the motor is consuming power when the contactor coil is
energized and the armature has been pulled in? If the motor’s windings are burnt open, you
could be sending voltage to the motor through the contactor contacts, but still have zero
current, and thus no motion from the motor shaft. If a clamp-on ammeter isn’t available to
measure line current, you can take your multimeter and measure millivoltage across each heater element: if the current is zero, the voltage across the heater will be zero (unless the heater element itself is open, in which case the voltage across it will be large); if there is
current going to the motor through that phase of the contactor, you will read a definite
millivoltage across that heater
This is an especially useful trick to use for troubleshooting 3-phase AC motors, to see if one phase winding is burnt open or disconnected, which will result in a rapidly destructive condition known as “single-phasing.” If one of the lines carrying power to the motor is open, it will not have any current through it (as indicated by a 0.00 mV reading across its heater), although the other two lines will (as indicated by small amounts of voltage dropped across the respective heaters).
This is an especially useful trick to use for troubleshooting 3-phase AC motors, to see if one phase winding is burnt open or disconnected, which will result in a rapidly destructive condition known as “single-phasing.” If one of the lines carrying power to the motor is open, it will not have any current through it (as indicated by a 0.00 mV reading across its heater), although the other two lines will (as indicated by small amounts of voltage dropped across the respective heaters).