The document discusses relays, contactors, and their uses in pneumatic control systems. Relays are electrically actuated switches that use coils to open and close contacts. Contactors are similar but are used for high voltage switching. Relays and contactors are used with solenoids to control pneumatic cylinders and valves. Pneumatic relays can indicate process conditions, perform logic functions, and regulate pressure and air flow in control applications.

1. Use of relays and
contractors in pneumatic
control
Jayanth Kumar L
Nikhil AS
Raveesh Rai

2.  Relays switches whose contact open or close when their corresponding coils are energized.
 These relays are commonly used for the energizing of solenoid because they operate at a
high current level. In this way a manually actuated switch can be operated at low voltage
levels to protect the operator.
 This lower circuit can be used to energize relay coils that control high voltage contacts used
to open and close circuit containing solenoids.
 The use of relay also provides interlock capability, which prevents the accidental energizing
of two solenoids at the opposite ends of a valve spool.
 This safety features can therefore prevent the burnout of one or both of these solenoids.
Relays

3.  A relay is an electrically actuated switch.
 As shown in the schematic figure (a), when SW1 is closed, the coil (electromagnet) is
energized.
 This pulse on the springs-loaded relay arm to open the upper set of the normally closed
contacts and close the lower set of normally open contacts.
Working principle:
Fig (a) Relay Fig (b) Component symbols

4. Simple circuit to control the actuation of double acting cylinder using solenoid, limit
switch, start and stop switch and relay.
Ladder logic diagram
Hydraulic circuit

5. Box Sorting System:
• An electropneumatic system for sorting two different-sized boxes moving on a conveyor is
presented in Figure(a).
• Low boxes are allowed to continue on the same conveyor, but high boxes are pushed on to a
second conveyor by a pneumatic cylinder.
Schematic diagram

7.  When the START button is momentarily depressed, coil 2-CR is energized to close its two
normally open sets of contacts. This turns on the compressor and conveyor motors.
 When a high box actuates 1-LS, coil 1-CR is energized. This closes the 1-CR (NO) contacts
and opens the 1-CR (NC) contacts. Thus, the conveyor motor stops, and SOL A is energized.
 Air then flows through the solenoid-actuated valve to extend the sorting cylinder to the right
to begin pushing the high box onto the second conveyor.
 As 1-LS becomes deactuated, it does not de- energize coil 1-CR because contact set 1-CR
(NO) is in its closed position.
 After the high box has been completely positioned onto the second conveyor, 2-LS is
actuated. This de-energizes coil 1-CR and SOL A.
 The valve returns to its spring-offset mode, which retracts the cylinder to the left. It also
returns contact set 1-CR (NC) to its normally closed position to turn the conveyor motor
back on to continue the flow of boxes.
 When the next high box actuates 1-LS, the cycle is repeated.
 Depressing the STOP button momentarily turns off the compressor and conveyor motors
because this causes coil 2-CR to become de-energized.
 The production line can be put back into operation by the use of the START button.
The operation as follows:

8. CONTACTORS AND SWITCHES
 These are control members used to make or break the electrical circuit for the solenoid
operated valves.
 There are varieties of contactors available in the industry. The essential qualities of a
contactor are as follows:
1. It should be a good conductor and should be made of brass or copper
2. It should have an insulated base or casing
3. The operating mechanism (i.e., handle, key, button, etc.) should be suitably
insulated, so that they are safe to handle
The switches are mostly made of the following types:
(i)Single pole switch
(ii) Double pole switch
(iii) Triple pole switch
(iv) Four pole switch

9.  pneumatic systems, push button switches are used where an electrical signal is to be fed
to start or stop an operation manually.
 They make or break contacts only as long as they are held pressed and hence are termed
'momentary' switches.
 Under pushed condition, they may generate 'arching' if used for heavy loads and hence
should not be used directly to energise loads.

10. Construction of a Contactor:
 The contactor consists of two iron cores, where one is fixed and the other one is the movable
coil and it is an insulated copper coil. Where the copper coil is located on the fixed core.
 There are six main contacts for power connection, where three are fixed cores and the other
three are movable cores.
 These contacts are made from pure copper, and the contact points are made from special alloy
to withstand high starting current and temperature.
 A spring which is located between coil and the movable core, auxiliary contacts it could be
normally open or closed.
 The main contacts cut on and off the light current loads such as contactors coil, relays, timers
and many other control circuit parts are linked to contact mechanism.
 A three-phase AC power supply provided to the circuit which is shown below

11. Working Principle of Contactors:

12.  An electromagnetic field is generated whenever current flows where the moving coils
attract each other. A large amount of current is drawn initially by an electromagnetic coil.
 The moving contact is pushed forward by moving core, as a result, the force created by the
electromagnet holds the moving and fixed contacts together.
 On de-energizing, the contactor coil gravity or spring moves back the electro-magnetic coil
to its initial position and there is no flow of current in the circuit.
 If contactors are energized with AC current, a small portion of the coil is the shaded coil,
where the magnetic flux in the core is slightly delayed. This effect is too average as it
prevents the core from buzzing at twice line frequency.
 There are internal tipping point processes to ensure rapid action so that contactors can open
and closed very rapidly.
 The supply is given using a switch, that is when the switch is closed current flows through
the contactor coil and attaches the moving core.
 The contactor attached to the moving core closes and the motor starts running. When the
switch is released the electromagnetic energizes spring arrangement pause the moving coil
back to its initial position and power supply to the motor is cut off

13. Relay Contactor
A relay is used for low voltage
switching purpose
It is used for high voltage
switching purpose
The relay contactor is similar to
the axillary contactor
There are two types of contactors
auxiliary and power
The size of the relay is small The size of the contactor is large
Cannot be repaired Can be repaired
Difference between Relay and contactor:

14. Applications of Pneumatic relays :
 An indicator relay is typically composed of a two-position valve that is actuated by a pneumatic
signal from a remote sensor or transmitter. They are used to indicate if there is an unsafe
condition present in a particular variable or process.
 A logic relay is used for a variety of functions such as diverting the flow from one pipeline to
another.
 A ratio relay is a pneumatic switch that can perform tasks based on a predetermined ratio, such
as addition, subtraction, biasing, and reversing.
 A pneumatic reducing relay is a ratio relay capable of reducing the output pressure based on a
particular input to output ratio.
 An air flow switch is used as a volume booster, using a low flow pneumatic signal to produce an
output pressure with a higher flow capacity.
 An air pressure switch uses adjustable set points that are tripped when the pneumatic signal
pressure increases or decreases.