Hydraulic, Pneumatic & Electrical System
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Transcript of Hydraulic, Pneumatic & Electrical System
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Training is the bedrock of success
A HANDOUT ON
HYDRAULIC, PNEUMATIC AND
ELECTRIC SYSTEMS
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Table of contents
Sr. No. Chapter Page No.
1. Introduction 4
1.1 Hydraulic System 4
1.2 Pneumatic System 4
1.3 Electric System 4
1.4 Comparison of Hydraulic, Pneumatic & Electric System 4
2. Hydraulics 5
2.1 Important properties of fluids 5
2.2 Pascal’s Law 5
2.3 Advantages of Hydraulic System 5
2.4 Disadvantages of Hydraulic System 6
2.5 Components of Hydraulic System 6
3. Pneumatics 7
3.1 Characteristics of Compressed Air 7
3.2 Quality of Compressed Air 7
3.3 Production of Compressed Air 8
3.4 Terms used for Compressor Ring 8
3.5 Advantages of Pneumatics 9
3.6 Component classification 9
4. Graphical symbols of various components 12
4.1 Graphical Symbols 15
4.2 Actuator 15
4.3 Direction Control Valve 17
4.4 Poppet Valves 20
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4.5 Spool Valves 22
4.6 Pressure Control Valves 30
4.7 The Pressure Control Valves 405. Electrical System 43
5.1 Contactor 43
5.2 Operating Principle 45
5.3 Ratings 46
5.4 Miniature Circuit Breaker 47
5.5 Actuator Lever 48
5.6 Arc Interruption 49
5.7 Short Circuit Current 50
5.8 Transformer 50
5.9 Operating Principle 51
5.10 Overload Relays 52
5.11 Star- Delta Starter 57
5.12 Control Switches 60
5.13 Gauges 68
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CHAPTER 1. INTRODUCTION
1.1 Hydraulic System:
It deals with generation, transmission and control of power using pressurized
fluid (by virtue of Pascal’s law).
1.2 Pneumatic System:
It deals with the transmission and control of power using pressurized air.
1.3 Electric System:
It deals with the transmission of electrical power and control of hydraulic and
pneumatic systems.
1.4 Comparisons Of Electrical, Hydraulic And Pneumatic
Systems
Electrical HydraulicPneumatic
Energy source Usually from
outside supplier
Electric motor or diesel
driven
Electric motor or diesel
driven
Energy storage Limited (batteries) Limited (accumulator) Good (reservoir)
Energy cost Lowest Medium Highest
Rotary
actuators
AC and DC motors.
Good control on DC
motors.
Low speed, Good control Wide speed range.
Accurate control is
difficult
Linear
actuators
Short motion via
solenoid
Cylinders, Very high
forces
Cylinders, Medium
forces
Controllableforce
Possible withsolenoid & DC
High degree of control andprecision with high forces
Control difficult with highforces, medium forces
can be controllable.
Safety Fire hazard, spark Oil may leak :fire hazard,
chemical/ environmental
problems possible
Explosive failure, Noisy.
The detail study of above three systems is explained below.
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CHAPTER 2. HYDRAULICS
It deals with Generation, Transmission and Control of power using pressurized
fluid. It covers the physical behavior of fluids in motion. It is generally used for
precise control of large forces.
2.1 Important properties of fluids
2.1.1 Shapelessness: liquids have no natural shape. They conform to the shape
of the container. So they can be easily transferred from one location to
other through pipes.
2.1.2 Incompressibility: liquids are essentially incompressible. They regain theiroriginal volume once force is removed. So there is no permanent
distortion.
2.1.3 Transmission of force: the force is transmitted almost equally and
undiminished in all directions.
2.2 Pascal’s Law
Magnitude of force transferred is in direct proportion to its surface area.
i.e. Pressure = Force/Area
2.3 Advantages of hydraulics
Following are some advantages of hydraulic controls.
2.3.1 Liquid does not absorb any of the applied energy.
2.3.2 Capable of moving much higher loads and providing much higher forces
due to the incompressibility.
2.3.3 The hydraulic working fluid is incompressible, leading to a minimum of
spring action. When hydraulic fluid flow is stopped, the slightest motion of
the load releases the pressure on the load; there is no need to "bleed off"
pressurized air to release the pressure on the load.
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2.4 Disadvantages of hydraulics:
2.4.1 A frictional loss in fluid causes the reduction in speed.
2.4.2 Internal leakages causes slow down of motion
2.4.3 The leakage of air into the fluid causes jerky operation
2.4.4 Inflammable mineral oil used in system.
2.5 Components in the hydraulic system
2.5.1 Pump to generate hydraulic power input (flow generator)
2.5.2 Motors or cylinders to obtain useful mechanism output (actuators)
2.5.3 Valves to control the direction, pressure and level of the applied power
2.5.4 Connections to join system components and provide power conductors.
2.5.5 Fluid media, namely liquid providing rigid and stiff control
2.5.6 Fluid storage and conditioning equipments (tank) to ensure quantity and
quality of fluid.
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CHAPTER 3. PNEUMATIC
Pneuma is derived from the ancient Greek word and it means breath, wind and
soul.
Pneumatic systems deal with the transition and control of power using
pressurized Air. Pneumatics is interpreted as systems, machines and devices
operated air pressure. Pneumatic power is used for rapid but light forces. (E.g.
rapid assembly of electrical components in a switch box). It is used in a wide
range of industries such as automated production lines, automated assembly
units, robots, construction, drilling etc.
3.1 Characteristics of Compressed Air
3.1.1 Air is available everywhere for compression. It can be easily
transmitted.
3.1.2 Compressed air is easily storable
3.1.3 Compressed air is clean and air which escapes through
leaking pipes does not cause any contamination.
3.1.4 It is a very fast working medium.
3.1.5 It does not have constant fluidity.
3.2 Quality of Compressed Air
3.2.1 Wet Air:
• Moisture content: 2500 PPM
• Dew point: 0 to 4 °C
3.2.2 Dry Air:
• Moisture content: 350 PPM
• Dew point: -18 to -20 °C
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3.3 Production of Compressed Air
Compressor is used to supply compressed air. A compressor is a
machine which takes in air at a certain pressure and delivers at higher
pressure maintaining constant flow. The capacity of compressor is the
actual quantity of air compressed and delivered.
Fig. 3.1 Types of compressors
3.4 Terms used for Compressor Rating
3.4.1 CFM: Cubic feet per minute: Describes the volume flow rate
of compressed air
3.4.2 ICFM: Inlet CFM- air flow as it enters the compressor intake.
3.4.3 ACFM: Actual CFM- Air flow at some reference point at local
conditions. It is the actual flow rate in the pipe work after the
compressor.
3.4.3 FAD: Free Air Delivery- It is the actual quantity of the
compressed air at the discharge of the compressor. The units for
FAD are cfm in the imperial system, and lpm (litres per minute) in
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the SI system. The units are measured as per the ambient inlet
standard conditions (ISO 1417)- 1 bar abs and 20o C.
3.5 Advantages of Pneumatics:
3.5.1 Pneumatics is used in preference to hydraulics for following
reasons:
3.5.1.1 Easily connected to air supply and needs no
separate power pack.
3.5.1.2 The operation of actuators is fast.
3.5.1.3 No return piping is required; the air is vented to
atmosphere.
3.5.1.4 Clean medium with no mess when it leaks.
3.5.1.5 No fire hazard as with oil.
3.5.2 Pneumatics is used in preference to electrics for following
reasons:
3.5.2.1 Will not start a fire through electric fault
(Intrinsically safe)
3.5.2.2 Air motors are safe when overloaded and does not
overheat.
3.5.2.3 Safer for operators (no risk of electrocution)
3.6 Component Classification
Pneumatic circuit elements are classed into four primary groups.
These are:
3.6.1 Air supply and Conditioning elements such as
• Compressor
• Receiver
• Pressure regulator
• Filter
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• Dryer
• Lubricator
3.6.2 Input elements (electrical or pneumatic) such as
• On / off devices (switches)
• Position sensors
• Trip valves
• Air jet sensors
Many pneumatic sensing and switching devices are directional control valves,
plunger operated valves for detecting a cylinder position.
3.6.3 Processing elements such as
• Logic valves (AND/ OR and so on)
• Time delay valves
• Pressure switches
• Direction control valves of many types.
3.6.4 Actuating devices such as
• Cylinders
• Motors
• Semi-rotary actuators
Some of above elements are Mono-stable or Bi-stable.
A mono-stable element only has one stable position and automatically returns to
it when the switching signal is removed. Examples are:
• Direction control valves with spring return
• Pressure switches• Proximity detectors
• Logic valves.
A bi-stable element ahs two switching positions and requires a switching signal to
change it from one to other. Examples are:
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Direction control valves with no spring return such as:
• Pilot/ pilot operation
• Solenoid/ solenoid operation
• Latching relays.
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CHAPTER 4. GRAPHICAL SYMBOLS OF VARIOUS
COMPONENTS
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4.2 Actuator:
It converts hydraulic/ pneumatic energy to mechanical energy. There are two
types of actuators used in hydraulic or pneumatic systems:
• Rotary: Hydro/ Pneumatic rotors. They are used in conveyors. Speed and
torque variations are avaialble.
• Linear: Piston/ cylinder is used when the desired motion is linear.
Hydraulic/ pneumatic pressure moves piston and ram. Load is connected
to ram.
Fig. 4.1 Linear actuator (Cylinder)
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Fig. 4.2 Single and Double acting cylinder
One of the most important functions in any hydraulic and pneumatic power
system is control. If control components are not properly selected, the entire
system will fail to deliver the required output. Elements for the control of energy
and other control in fluid power system are generally called “Valves”.
The selection of these control components not only involves the type, but also
the size, the actuating method and remote control capability. There are 3 basic
types of valves.
1. Directional control valves
2. Pressure control valves
3. Flow control valves
Directional control valves are essentially used for distribution of energy in a
power system. They establish the path through which a fluid/air traverses a given
circuit. For example they control the direction of motion of a hydraulic cylinder or
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motor. These valves are used to control the start, stop and change in direction of
flow of pressurized fluid/air.
Pressure may gradually buildup due to decrease in fluid demand or due to
sudden surge as valves opens or closes. Pressure control valves protect the
system against such overpressure. Pressure relief valve, pressure reducing,
sequence, unloading and counterbalance valve are different types of pressure
control valves.
In addition, fluid/ flow rate must be controlled in various lines of a hydraulic
circuit. For example, the control of actuator speeds depends on flow rates. Thistype of control is accomplished through the use of flow control valves.
4.3 Direction Control Valve
As the name implies directional control valves are used to control the direction of
flow in a hydraulic/pneumatic circuit. They are used to extend, retract, position or
reciprocate cylinder and other components for linear motion. Valves contains
ports that are external openings for fluid to enter and leave via connecting
pipelines, The number of ports on a directional control valve (DCV ) is usually
identified by the term “ way”. For example, a valve with four ports is named as
four-way valve.
Directional control valves can be classified in a number of ways:
4.3.1 According to type of construction :
4.3.1.1 Poppet valves
4.3.1.2 Spool valves
4.3.2 According to number of working ports :
4.3.2.1 Two- way valvesThree – way valves
4.3.2.2 Four- way valves.
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4.3.3 According to number of Switching position:
4.3.3.1 Two - position
4.3.3.2 Three - position
4.3.4 According to Actuating mechanism:
4.3.4.1 Manual actuation
4.3.4.2 Mechanical actuation
4.3.4.3 Solenoid ( Electrical ) actuation
4.3.4.4 Hydraulic ( Pilot ) actuation
4.3.4.5 Pneumatic actuation
4.3.4.6 Indirect actuation
The designation of the directional control valve refers to the number of working
ports and the number of switching positions.
Thus a valve with 2 service ports and 2 switching positions is designated as 2 /
2 way valve.
A
P
Fig. 4.3 2/2 Direction control valve
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A valve with 3 service ports and 2 positions is designated as 2 / 3 way valve.
A
P T
Fig. 4.4 2 / 3 valve symbol
A valve with 4 service ports and 2 positions is designated as 2 / 4 valve.
A B
P T
Fig. 4.5 2 / 4 valve symbol
A valve with 4 Service ports and 3 switching position is designated as 3 / 4 way
valve. Fig 4 shows an example of open centered position.
A B
P T
Fig 4.6 3/ 4 valve symbol
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In directional control valves with 3 spool position, the central position is the
neutral position (or mid position or zero or null position). The neutral position is
the position in which the moving parts are assumed to be inactive, but affected
by a force (e.g. spring).
The ports are designated as follows:
P = Pressure Port (Pump Port)
T = Tank Port
A, B = User Ports
In pneumatic systems the same construction valves are used. The symbols
remain the same. Only while designating the ports the numbers are used such
as:
1 = Pressure Port (compressor Port)
3 = Vent Port
2, 4 = User Ports
4.4 Poppet Valves:
Directional poppet valves consists of a housing bore in which one or more
suitably formed seating elements ( moveable ) in the form of balls, cones are
situated. When the operat
ing pressure increases the valve becomes more tightly seated in this design.
4.4.1 The main advantages of poppet valves are;
4.4.1.1 No Leakage as it provides absolute sealing.
4.4.1.2 Long useful life, as there are no leakages of oil flows.
4.4.1.3 May be used with even the highest pressures, as no hydraulic
sticking (pressure dependent deformation) and leakages occurs in
the valve.
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4.4.2 The disadvantages of these valves are:
4.4.2.1 Large pressure losses due to short strokes
4.4.2.2 Pressure collapse during switching phase due to negative overlap(connection of pump, actuator and tank at the same time).
2 / 2 DCV (Poppet design)
A A
P P
a. Valve Closed b. Valve Opened
Fig 4.7 2 / 2 DCV Poppet Design
Figure 4.7a shows a ball poppet type 2 / 2 DCV. It is essentially a check valve as
it allows free flow of fluid only in one direction (P to A) as the valve is openedhydraulically and hence the pump Port P is connected to port A as shown in fig
4.7b. In the other direction the valve is closed by the ball poppet (note the fluid
pressure from A pushes the ball to its seat) and hence the flow from the port A is
blocked (fig 4.7a.). The symbol for this type of design is same as that of check
valve. (Fig 4.8)
No flow
Free flow
Fig. 4.8 Symbol of 2/2 poppet valve (Check valve)
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4.5 Spool Valves:
The spool valve consists of a spool which is a cylindrical member that has large-
diameter lands machined to slide in a very close- fitting bore of the valve body.
The spool valves are sealed along the clearance between the moving spool and
the housing. The degree of sealing depends on the size of the gap, the viscosity
of the fluid and especially on the level of pressure. Especially at high pressures
(up to 350 bar) leakage occurs to such a extent that it must be taken into account
when determining the system efficiency. The amount of leakage is primarily
dependent on the gap between spool and housing. Hence as the operating
pressure increases the gap must be reduced or the length of overlap increased.
The radial clearance is usually less than 20µ. The grooves between the lands
provide the flow passage between ports.
4.5.1 Two-Way Valve ( 2/ 2 DCV):
Lever for manual actuation
Bore Port A Valve Body
Spring
Spool
Port P
a) Valve closed
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Port A
Spring
Port P
b) Valve opened by actuation
Fig 4.9 Spool type 2 / 2 DCV
The simplest type of directional control valve is a check valve which is a two way
valve because it contains two ports. These valves are also called as on-off valves
because they allow the fluid flow in only in one direction and the valve is normally
closed. Two – way valves is usually the spool or poppet design with the poppet
design more common and are available as normally opened or normally closed
valves. They are usually actuated by pilot (Hydraulic actuation) but manual,
mechanical, solenoid actuated design are also available. Figure 4.9 above shows
Spool type 2 / 2 DCV manually actuated. In Fig 4.9 a the port P is blocked by the
action of spring as the valve is unactuated (absence of hand force). Hence the
flow from port P to A is blocked. When actuated (Presence of hand force) the
valve is opened, thereby connecting port P to A.
4.5.2 Three – Way Valve :
A directional control valve primary function is alternatively to pressurize and
exhaust one working port is called three-way valve. Generally, these valves are
used to operate single- acting cylinders. Three-way directional valves are
available for manual, mechanical, pilot, solenoid actuation. These valves may be
two-position, or three -position. Most commonly they have only two positions, but
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in some cases a neutral position may be needed. These valves are normally
closed valves (i.e. The pump port is blocked when the valve is not operating ).
The three-way valve ports are inlet from the pump, working ports, and exhaust to
tank. These ports are generally identified as follows: P= pressure (Pump) port; A
or B = working port and T = tank port. Figure 4.10 (a) and (b) shows the two
positions of the three – way valve actuated manually by a push button.
4.5.2.1 Spool Position 1: When the valve is actuated, the spool moves
towards left. In this position flow from pump enters the valve port P
and flows out through the port A as shown by the straight- through line and
arrow (fig a). In this position, port T is blocked by the spool.
4.5.2.2 Spool position 0: when the valve is un-actuated by the absence of
hand force, the valve assumes this position by the action of spring In this
position, port P is blocked by the spool. Flow from the actuator can go to the
tank from A to T as shown by straight – through line and arrow
Spool A Push button for manual
Actuation
Spring P T Valve Body
Fig 4.10a. 1 position: P to A, T blocked Spring
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Spring Port A
Port P T
Fig 4.10b 2 position: Valve closed
4.5.3 Four - Way DCV: -
These valves are generally used to operate cylinders and fluid motors in both
directions hydraulically. The four ways are Port P that is connected to pump, tank
port T, and two working ports A and B connected to the actuator. The primary
function of a four way valve is too alternately to pressurize and exhaust two
working ports A & B. These valves are available with a choice of actuation,
manual, mechanical, solenoid, pilot & pneumatic. Four-way valve comes with
two or three position. One should note that the graphical symbol of the valve
shows only one tank port even though the physical design may have two as it is
only concerned with the function.
4.5.4 Three Positions, Four Way Valve:
These type of DCV consists of three switching position. Most three- position
valves have a variety of possible flow path configurations, but has identical flowpath configuration in the actuated position (position 1 and position 2) and
different spring centered flow paths. When left end of the valve is actuated, the
valve will assume 1 position. In this position the port P to connected to working
port A and working port B is connected to T (in some design P is connected to B,
and A to T when left end is actuated ). Similarly when the right end is actuated,
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the valve will assume 2 positions. In this position port P is connected to B and
working port A to T. When the valve is un-actuated, the valve will assume its
center position due to the balancing opposing spring forces. It should be noted
that a three-position valve is used whenever it is necessary to stop or hold a
actuator at some intermediate position within its stroke range, or when multiple
circuit or functions must be accomplished from one hydraulic power source.
Three- position, four- way DCV have different variety of center configurations.
The common varieties are the open center, closed center, tandem center,
floating center, & regenerative center with open, closed and tandem are the three
basic types A variety of center configurations provides greater flexibility for circuit
design.
Spool Port B A
Bore
T Port P T
Valve body
Fig. 4.11 3/4 DCV
4.5.5 Two- position, Four – way DCV:
These valves are also used to operate double acting cylinder. These valves are
also called as impulse valve as 2 / 4 DCV has only two switching positions, i.e. it
has no mid position. These valves are used to reciprocate or hold and actuating
cylinder in one position. They are used on machines where fast reciprocation
cycles are needed. Since the valve actuator moves such a short distance to
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operate the valve from one position to the other, this design is used for punching,
stamping and for other machines needing fast action.
Bore Spool
Port B A
Actuation
T Port P T
Fig. 4.12 2/4 DCV
4.5.6 Actuation of Directional control valves:
Directional control valves can be actuated by different methods.
4.5.6.1 Manually – actuated Valve:
A manually actuated DCV uses muscle power to actuate the spool. Manual
actuators are hand lever, push button, pedals. The following symbols
shows the DCV actuated manually.
Fig.4.12 Symbol of 2/4 Direction Control Valve
Figure 4.12 shows the symbol of 2 / 4 Direction control valve withmanually operated by roller tappet to 1 and spring return to 2.
Fig.4.13 Symbol of 2/4 DCV
1 2
1 2
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Fig 4.13 shows the symbol of 2 / 4 DCV with manually operated by hand
lever to 1 and spring return to 2.
In the above two symbols the DCV spool is returned by springs which
push the spool back to its initial position once the operating force has
stopped e.g., letting go of the hand lever
4.5.6.2 Mechanical Actuation:
The DCV spool can be actuated mechanically, by roller and cam, roller
and plunger. The spool end contains the roller and the plunger or cam can
be attached to the actuator (cylinder). When the cylinder reaches a
specific position the DCV is actuated. The roller tappet connected to thespool is pushed in by a cam or plunger and presses on the spool to shift it
either to right or left reversing the direction of flow to the cylinder. A spring
is often used to bring the valve to its center configuration when
deactivated.
4.5.6.3 Solenoid-actuated DCV :
A very common way to actuate a spool valve is by using a solenoid is
illustrated in Fig 4.14. When the electric coil (solenoid) is energized, it
creates a magnetic force that pulls the armature into the coil. This caused
the armature to push on the spool rod to move the spool of the valve. The
advantage of a solenoid lies within its less switching time.
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Energized Coil Spool
Armature Bore B A
Spool Rod T P T
Fig. 4.14 Working of solenoid to shift spool of valve.
Figure4.14 show the working of a solenoid actuated valve when left coil is
energized, it creates a magnetic force that pulls the armature into the coil. Since
the armature is connected to spool rod its pushes the spool towards right.
Similarly when right coil is energized spool is moved towards left. When both coil
is de-energized the spool will come to the mid position by spring force Figure
4.16 a shows a symbol for single solenoid used to actuate 2- position ,4 way
valve and b shows symbol for 2 solenoids actuating a 3- position valve, 4 way
valve.
Fig 4.14 a) Symbol for Single solenoid-actuated, 2- Position, 4-way spring centered
DCV
Fig 4.14 b) Symbol for Solenoid actuated, 3- position.
1 0
1 2
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4.6 Pressure Control Valve
4.6.1 These are the units ensuring the control of pressure. A throttling orifice is
present in the valve and by variation of orifice, the pressure level can be
controlled or at a particular pressure, a switching action can be influenced.
Pressure regulation valves are for maintaining a constant pressure in a
system. Pressure switching valves, apart from a definite control function
they also perform a switching action. Such valves not only provide a
switching signal, as in the case of pressure switches, but also operate
themselves as a DCV type of switching within the hydraulic system. In the
case of pressure switching valves the piston or spool of the valve remains
at a definite position either open or closed depending on the control signal
(Yes or No). The control signal is generally external to the valve. In the
case of pressure regulating valves the piston or spool takes up in between
position depending on the variable pressure and flow characteristics.
As in DCV these valves can also have the valve element either poppet or
spool. With poppet the sealing is good. But small movement of poppet
allows large flows thereby excessive drop of pressure than required. This
result is impact effect.
The spool type of valves allows very fine control or throttling of flows. But
of course, the sealing is not very good.
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4.6.2 Opening and closing pressure difference
The minimum pressure at which the valve action starts is called as the opening
or cracking pressure. The difference between the cracking pressure(commencement of flow) and the pressure obtained at maximum flow (normal
flow without change of spring force) is referred as the “opening pressure
difference”.
Similarly the difference between the pressure corresponding to nominal flow and
no flow during closing of the valve is referred as “closing pressure difference”.
This is larger than the opening due to the flow forces acting in the opening
direction as also the hysteresis in the spring.
4.6.3 Different types of pressure control valves
Pressure control valves are usually named for their primary function such as
relief valve, sequence valve, unloading valve, pressure reducing valve and
counterbalance valve.
4.6.3.1 Pressure Relief valve
One of the most important pressure controls is the relief valve. Its primary
function is to limit the system pressure. Relief valve is found in practically
all the Hydraulic system. It is normally a closed valve whose function is to
limit the pressure to a specified maximum value by diverting pump flow
back to the tank. There are two basic design, a) direct operated or inertia
type, b) the pilot operated design (compound relief valve).
4.6.3.2 Direct type of relief valve
The direct type of relief valve has two basic working port connections. One
port is connected to pump and the other to the tank. The valve consists of
a spring chamber (control chamber) with an adjustable bias spring which
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pushes the poppet to its seat, closing the valve. A small opening
connecting the tank is provided in the control chamber to drain the oil that
may collect due to leakage, thereby preventing the failure of valve. System
pressure opposes the poppet, which is held on its seat by an adjustable
spring. The adjustable spring is set to limit the maximum pressure that can
be attained within the system. The poppet is held in position by spring
force plus the dead weight of spool. When pressure exceeds this force,
the poppet is forced off its seat and excess fluid in the system is bypassed
back to the reservoir. When system pressure drops to or below
established set value, the valve automatically reseats. Fig 4.15a shows a
direct pressure relief valve. Fig 4.15b shows the symbol.
Screw
(for pressure setting)
Spring Control
Poppet Chamber
Drain
(to remove oil from
Tank
Pump (When Pressure here is less than
The valve setting, the valve is closed)
Fig 4.15a Pressure Relief Valve
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Fig 4.15b. Symbol of Pressure Relief Valve
4.6.3.3 Unloading Valve:
A unloading valve is used to permit a pump to operate at minimum load.
The unloading valve is normally closed valve with the spool closing the
tank port. It operates on the principle that pump delivery is diverted to the
tank, when sufficient pilot pressure is applied to move the spool against
the spring force. The valve is held open by pilot pressure until the pump
delivery is again needed by the circuit. The pilot fluid applied to move the
spool upwards becomes a static system. In other words, it merely pushes
the spool upward and maintains a static pressure to hold it open. When
the pilot pressure is relaxed, the spool is moved down by the spring, and
flow is diverted through the valve into the circuit. The spool type unloading
valve is shown in fig 16a. The valve consists of a spring chamber (control
chamber) with an adjustable bias spring which pushes the spool closing
the tank port. The valve has 2 ports one connecting the pump and other
connecting the tank. The movement of the spool inside the bore opens or
closes the ports. Drain is provided to remove the oil that may collect in
control chamber due to leakage, thereby preventing valve failure.
Unloading valves also helps to prevent heat buildup in a system, which is
caused by fluid being discharged over the relief valve at its pressure
setting. The unloading valve is used in system having one or more fixed
delivery pump to control the amount of flow at any given time. A well
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designed hydraulic circuit uses the correct amount of fluid for each phase
of a given cycle of machine operations. When pressure builds up during
the feed phase of the cycle, the pilot pressure opens the unloading valve,
causing the large discharge pump to bypass its flow back to the tank.
Screw
(for pressure setting)
Spring Control
Chambe
Drain
Tank
Pump
Remote
Pilot Pressure Signal
Fig 4.16a Unloading Valve
Fig 4.16b. Symbol of unloading valve
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4.6.3.4 Sequence valve:
A sequence valve’s primary function is to divert flow in a predetermined
sequence. It is a pressure- actuated valve similar in construction to a relief
valve and normally a closed valve. The sequence valve operates on the
principle that when main system pressure overcomes the spring setting,
the valve spool moves up allowing flow from the secondary port. A
sequence valve may be direct or remote pilot- operated. These valves are
used to control the operational cycle of a machine automatically.
Sequence valve may be directly operated as shown in the fig 17b. The
valve consists of a spring chamber (control chamber) with an adjustablebias spring for setting the pressure. It consists of 2 ports, one main port
connecting the main line and other (secondary port) connected to the
secondary circuit. Usually the secondary port is closed by the spool. A
small opening connecting the tank is provided in the control chamber to
drain the oil that may collect due to leakage, thereby preventing the failure
of valve. The pressure is effective on the end of the spool. This pressure
will urge the spool against the spring force and at the preset value of the
spring it allows a passage from the primary to the secondary port. For
remote operation it is necessary to close the passage used for direct
operation by plugging and provide a separate pressure source as required
for the operation of the spool in the remote operation mode.
Fig 4.17a Symbol of Sequence Valve
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Screw
(for pressure setting)
Spring Control chamber
Drain
Secondary Port
Pressure ( Main )
Line Spool
Direct Operation(Control Signal)
Remote
Pilot operation
(Plugged)
Fig 4.17b Sequence valve
4.6.3.5 Counterbalance Valve:
A Counterbalance valve is used to maintain back pressure to prevent a
load from failing. One can find application in vertical presses, lift trucks,
loaders and other machine tool that must position or hold suspended
loads. The counterbalance valve shown in the figure 4.18a. The valve
consists of a spring chamber (control chamber) with an adjustable bias
spring which controls the movement of spool. It has two ports, one
connected to load and the other to the tank. A small opening connecting
the tank is provided in the control chamber to drain the oil that may
collected due to leakage, thereby preventing the failure of valve.
Counterbalance valve acts on the principle that fluid is trapped under
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pressure until pilot pressure, either direct or remote (only one opened at a
time and other blocked), has to overcome the spring force setting in the
valve. Fluid is then allowed to escape, letting the load to descend under
control. This valve can be used as a “braking valve” for decelerating
heavy inertia load.
A counterbalance valve is normally closed valve and will remain closed
until acted upon by a remote pilot pressure source. Therefore, a much
lower spring force is sufficient to allow the valve to operate at a lesser pilot
pressure.
Screw
for pressure setting)
Spring Control chamber
Drain
Load
Tank Spool
Direct Operation
Control signal Line
Remote
Pilot operation
(Plugged)
Fig 4.18a Counter Balance Valve
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Direct pilot
Remote pilot
Fig 4.18b. Symbol of Counterbalance Valve
4.6.3.6 Pressure Reducing Valve:
Pressure reducing valve is used to limit its outlet pressure. Reducing
valves are used for the operation of branch circuits, where pressure may
vary from the main system pressures.
The pressure reducing valve is normally an open type valve. Figure 19a
shows the pressure reducing valve. The valve consists of a spring
chamber (control chamber) with an adjustable spring to set the pressure
as required by the system. A small opening is provided in the control
chamber to drain the oil that may be collected due to leakage, thereby
preventing the failure of valve. A free flow passage is provided through the
valve from inlet to secondary outlet until a signal from the outlet side tends
to throttle the passage through the valve. The valve operates on the
principle that pilot pressure from the controlled pressure side opposes an
adjustable bias spring normally holding the valve open. When the two
forces are equal, the pressure downstream is controlled at the pressure
setting. Thus, it can be visualized that if the spring has greater force, the
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valves open wider and if the controlled pressure has greater force, the
valves moves towards the spring and throttles the fluid.
Screw
(For pressure setting)
Spring Control Chamber
Drain
Spool
Pump orMain Pressure
Out
(Controlled Pressure)
Control Signal Line
Fig 4.19a Pressure Reducing Valve
Fig 4.19b Symbol of Pressure Reducing Valve
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4.7 THE PNEUMATIC VALVES
4.7.1 Logic Valves:
The two main logic valves are OR valves and AND valves.
4.7.2 OR valve:
The OR valve is also called as SHUTTLE valve. The air always comes out
of port C when air is applied to port A OR port B.
Fig 4.20 OR Valve
4.7.3 AND valve
The AND valve only gets air at port C when air is supplied to port A AND
port B.
Fig. 4.21 AND valve
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4.7.4 Quick Exhaust Valve
These valves are used to increase the piston speeds in cylinders. This
enables lengthy return times to be avoided particular with single actingcylinders. The air comes in through the inlet and pushes the flapper back
blocking the exhaust and letting air through the holes around the edge and
out through the cylinder port.
When air enters the cylinder port, the rush throws the flapper against the
flat surface and blocks the holes in it so preventing air going back to inlet.
This action opens the exhaust port and air leaves that way.
Fig. 4.22 Quick Exhaust Valve
4.7.5 Time Delay Valve:
These are pilot operated valves in which the pilot air is supplied through a
variable restrictor so that it takes time for operating pressure to build up.
The time delay is adjusted by adjusting the variable restrictor. The symbol
is shown below. When a pressure is applied to port 1, a time delay occurs
and then pressure is obtained from port 3. A permanent pressure source
is connected to port 2.
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Fig. 4.23 Symbol of Time Delay Valve
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CHAPTER 5. ELECRTICAL SYSTEM
The component employed in case of electrical system for control & switching are
as follows:
5.1 Contactor
A contactor is an electro-magnetic switching device used for remotely switching a
power or control circuit. A contactor is activated by a control input which is a
lower voltage / current than 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 voltsto 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 contactor is composed of three different systems. The contact system is the
current carrying part of the contactor. This includes Power Contacts, Auxiliary
Contacts, and Contact Springs. The electromagnet system provides the driving
force to close the contacts. The enclosure system 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.
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Contactors used for starting electric motors are commonly fitted with overload
protection to prevent damage to their loads. When an overload is detected the
contactor is tripped, removing power downstream from the contactor.
High voltage contactors (greater than 1000 volts) often have arc suppression
systems fitted (such as a vacuum or an inert gas surrounding the contacts).
Magnetic blowouts are sometimes used to increase the amount of current a
contactor can successfully break. The magnetic field produced by the blowout
coils force the electric arc to lengthen and move away from the contacts. This is
especially useful in contactors used in DC power circuits; AC arcs have periods
of low current, during which the arc can be extinguished with relative ease, butDC arcs have continuous high current, so blowing them out requires the arc to be
stretched further than an AC arc of the same current. The magnetic blowouts in
the pictured Albright contactor (which is designed for DC currents) more than
double the current it can break, increasing it from 600 amps to 1500 amps.
Sometimes an economizer circuit is also installed to reduce the power required to
keep a contactor closed. A somewhat greater amount of power is required to
initially close a contactor than is required to keep it closed thereafter. Such a
circuit can save a
substantial amount of power and allow the energized coil to stay cooler.
Economizer circuits are nearly always applied on direct-current contactor coils
and on large alternating current contactor coils.
Contactors are often used to provide central control of large lighting installations,
such as an office building or retail building. To reduce power consumption in the
contactor coils, latching contactors are used, which have two operating coils.
One coil, momentarily energized, closes the power circuit contacts, which are
then mechanically held closed; the second coil opens the contacts.
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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.
5.2 Operating Principle
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 reduct 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.
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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 acombination starter. In large industrial plants many contactors may be assembled
in motor control centers.
5.3 Ratings
Contactors are rated by designed load current per contact (pole), [3] maximum
fault withstand current, duty cycle, voltage, and coil voltage. A general purposemotor control contactor may be suitable for heavy starting duty on large motors;
so-called "definite purpose" contactors are carefully adapted to such applications
as air-conditioning compressor motor starting. North American and European
ratings for contactors follow different philosophies, with North American general
purpose machine tool contactors generally emphasizing simplicity of application
while definite purpose and European rating philosophy emphasizes design for
the intended life cycle of the application.
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5.4 Miniature circuit breaker
A circuit breaker is an automatically-operated electrical switch designed to
protect an electrical circuit from damage caused by overload or short circuit.
Unlike a fuse, which operates once and then has to be replaced, a circuit breaker
can be reset (either manually or automatically) to resume normal operation.
Circuit breakers are made in varying sizes, from small devices that protect an
individual household appliance up to large switchgear designed to protect high
voltage circuits feeding an entire city.
Low voltage (less than 1000 VAC) types are common in domestic, commercial
and industrial application, include:
5.4.1 MCB (Miniature Circuit Breaker)-rated current not more than 100 A.
Trip characteristics normally not adjustable. Thermal or thermal-
magnetic operation. Breakers illustrated above are in this category.
5.4.2 MCCB (Molded Case Circuit Breaker)—rated current up to 1000 A.
Thermal or thermal-magnetic operation. Trip current may beadjustable in larger ratings.
Low voltage power circuit breakers can be mounted in multi-tiers in LV
switchboards or switchgear cabinets.
The characteristics of LV circuit breakers are given by international standards
such as IEC 947. These circuit breakers are often installed in draw-out
enclosures that allow removal and interchange without dismantling theswitchgear.
Large low-voltage molded case and power circuit breakers may have electrical
motor operators, allowing them to be tripped (opened) and closed under remote
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control. These may form part of an automatic transfer switch system for standby
power.
Low-voltage circuit breakers are also made for direct-current (DC) applications,
for example DC supplied for subway lines. Special breakers are required for
direct current because the arc does not have a natural tendency to go out on
each half cycle as for alternating current. A direct current circuit breaker will have
blow-out coils which generate a magnetic field that rapidly stretches the arc when
interrupting direct current. Small circuit breakers are either installed directly in
equipment, or are arranged in a breaker panel.
The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker isthe most common style in modern domestic consumer units and commercial
electrical distribution boards throughout Europe. The design includes the
following components:
5.5 Actuator lever - used to manually trip and reset the circuit breaker.
Also indicates the status of the circuit breaker (On or Off/tripped). Most
breakers are designed so they can still trip even if the lever is held or
locked in the "on" position. This is sometimes referred to as "free trip" or
"positive trip" operation.
5.5.1 Actuator mechanism - forces the contacts together or apart.
5.5.2 Contacts - Allow current when touching and break the current when
moved apart.
5.5.3 Terminals
5.5.4 Bimetallic strip
5.5.5 Calibration screw - allows the manufacturer to precisely adjust the
trip current of the device after assembly.
5.5.6 Solenoid
5.5.7 Arc divider / extinguisher
5.5.8 Operation
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All circuit breakers have common features in their operation, although details
vary substantially depending on the voltage class, current rating and type of the
circuit breaker.
The circuit breaker must detect a fault condition; in low-voltage circuit breakers
this is usually done within the breaker enclosure. Once a fault is detected,
contacts within the circuit breaker must open to interrupt the circuit; some
mechanically-stored energy (using something such as springs or compressed air)
contained within the breaker is used to separate the contacts, although some of
the energy required may be obtained from the fault current itself. Small circuit
breakers may be manually operated; larger units have solenoids to trip the
mechanism, and electric motors to restore energy to the springs.The circuit breaker contacts must carry the load current without excessive
heating, and must also withstand the heat of the arc produced when interrupting
the circuit. Contacts are made of copper or copper alloys, silver alloys, and other
materials. Service life of the contacts is limited by the erosion due to interrupting
the arc. Miniature circuit breakers are usually discarded when the contacts are
worn, but power circuit breakers and high-voltage circuit breakers have
replaceable contacts.
When a current is interrupted, an arc is generated - this arc must be contained,
cooled, and extinguished in a controlled way, so that the gap between the
contacts can again withstand the voltage in the circuit. Different circuit breakers
use vacuum, air, insulating gas, or oil as the medium in which the arc forms.
Finally, once the fault condition has been cleared, the contacts must again be
closed to restore power to the interrupted circuit.
5.6 Arc interruptionMiniature low-voltage circuit breakers use air alone to extinguish the arc. Larger
ratings will have metal plates or non-metallic arc chutes to divide and cool the
arc. Magnetic blowout coils deflect the arc into the arc chute
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5.7 Short circuit current
Circuit breakers are rated both by the normal current that are expected to carry,
and the maximum short-circuit current that they can safely interrupt.
Under short-circuit conditions, a current many times greater than normal can
exist. When electrical contacts open to interrupt a large current, there is a
tendency for an arc to form between the opened contacts, which would allow the
current to continue. Therefore, circuit breakers must incorporate various features
to divide and extinguish the arc.
The maximum short-circuit current that a breaker can interrupt is determined by
testing. Application of a breaker in a circuit with a prospective short-circuit current
higher than the breaker's interrupting capacity rating may result in failure of the
breaker to safely interrupt a fault. In a worst-case scenario the breaker may
successfully interrupt the fault, only to explode when reset.
Miniature circuit breakers used to protect control circuits or small appliances may
not have sufficient interrupting capacity to use at a panelboard; these circuit
breakers are called "supplemental circuit protectors" to distinguish them from
distribution-type circuit breakers.
5.8 Transformer
Transformer is basically a device employed in system to step-down or step-up
the value of operating voltage in system. As a electrical system consist of various
equipment which operate at different voltages the need of transformer generally
arises in the system. Transformers are available with different KVA (Kilo volt
Ampere) ratings & different operating voltages as per the requirement.
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5.9 Operating principle
The transformer is based on two principles: firstly, that an [electric current] can
produce a [magnetic field] ( [electromagnetism] ) and secondly that a changing
magnetic field within a coil of wire induces a voltage across the ends of the coil (
[electromagnetic induction] ). Changing the current in the primary coil changes
the magnitude of the applied magnetic field. The changing magnetic flux extends
to the secondary coil where a voltage is induced across its ends.
A simplified transformer design is shown to the left. A current passing through the
primary coil creates a [magnetic field] . The primary and secondary coils are
wrapped around a [core] of very high [magnetic permeability] , such as [iron] ;
this ensures that most of the magnetic field lines produced by the primary current
are within the iron and pass through the secondary coil as well as the primary
coil.
VSP D2
Introduction:
It is a Negative Sequence Voltage Sensing Phase Failure Relay. It is best
suitable for 3 phase loads or 3 phase motor / pump loads only. It offers
protection against Phase Failure, Phase Unbalance, Phase Sequence Reversal,
Under Voltage & Over Voltage faults. It is useful for incoming side, Mains Supply
monitoring, General voltage faults, AMF/Transfer Switch Panels & Pump Control
Panels. It is available in DIN Rail mounting enclosure.
Features:
· Monitors all 3 Phases of Voltage Supply.
· Available for any System Voltage.
· External Aux.supply.
· Auto Reset.
· Sleek DIN Rail mounted Enclosure.
· 2 CO output contact.
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Application Areas:
Protection for LT, HT Motors Auto Transformer Starter Panels General Purpose
machines Motor Starter Panels / MCC s Crane Motor Control Panels Air-
conditioning Machines Compressor Control Panels Centrifuge Machines
Technical Specifications:
Sr. Parameter Specifications
1 Supply VoltageSystem 240 / 380 / 415 V AC ±20%; Auxiliary 110
/ 240 / 380 / 415 V AC ±20%
2 Output Contacts 2 CO
3 Trip Setting (Volts)
Phase unbalance 30 V to 70 V ±6 V (adjustable);
Under Voltage N.A.; Over Voltage N.A.; Water
Level N.A.
4 Trip Time Delay On Phase Failure/Seq. 3.5 sec. ±1.5 sec.
5 Resetting Mode Auto / Manual / Remote
6 Weight 400 gms.
7 Dimensions (mm) 76 56.5 x 117.5; Mounting (L x W) 67 x 46
5.10 Overload Relays
Overload relays are electrical switches typically employed in industrial settings to
protect electrical equipment
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from damage due to overheating in turn caused by excessive current flow.
Overload relays are provided for protecting components connected to an
electrical circuit in the event the current flowing through the circuit exceeds a
predetermined level. An overload relay monitors the current flowing in the
protected circuit and sends a signal to cause a contactor in the protected circuit
to open when the current flowing in the protected circuit is higher than a
preselected level. Overload relays are more than simple circuit interrupters; they
are sensors which, upon determining the existence of an overload or other
undesirable circuit condition, break a circuit and in turn provide a control or an
indicating function. Overload relays are specialized circuit breakers used with
industrial motors to protect the motors from damages caused by overload orelectrical faults. In a typical case, the electrical equipment is a three-phase motor
which is connected to a power source through another relay commonly referred
to as a contactor. The contactor is controlled by another switch which is typically
remotely located. Overload relays of various sorts have long been utilized in
connection with the operation of electrical equipment, particularly electrical
equipment drawing relatively high levels of power. Single-phase and multi-phase
(e.g., three-phase) power systems typically include an overload relay for
interrupting power in the power conductors when a fault condition occurs, such
as a ground fault, phase loss, overcurrent, or undercurrent condition. For
instance, a three phase induction motor is often linked to a power source through
a relay commonly referred to as a contactor. A typical contactor includes a
separate power path for each of the three motor phases. Contactor motion is
typically provided magnetically as the result of power flow through a coil where
the current though the coil is controlled by a control switch. In this case, the
contactor is a heavy duty relay having three contact sets for breaking each of the
three-phases of power upon movement of a yoke member within a contactor coil,
the yoke member and coil together forming an electrical solenoid. With an
electronic relay, it is possible to protect multiphase motors by cutting off their
power supply for example when a current overload arises on at least one phase
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of the motor or an imbalance between the phase currents occurs.
Overload relays are normally used in conjunction with an electromechanical
contactor, that may be used to disconnect power from equipment, for example,
from a three-phase motor, when an overload condition exists. Electric motors are
one type of electrical load which can be started and stopped using a contactor.
The contactor includes a contact associated with each phase conductor
connected to the motor. A contact of an overload relay is typically connected in
series with the coil of the contactor to cause the contactor to open when an
overload condition is sensed. The overload relay senses an overload condition
by monitoring the current in each of the three-phases received by the motorwindings. For a three-phase motor, the contactor would include three contacts
which are opened and closed in unison. The overload relay includes current
sensing elements that are wired in series with the three phases passing through
the contactor. In this way, the overload relay can monitor current flowing in the
three phases through the contactor, and based on current magnitude and
duration, may interrupt the current flow through the contactor armature circuit to
open the contactor contacts when an overload occurs. The mechanical motion
required to open and close the contacts is provided by a solenoid including a coil.
The coil is controlled by a basic circuit which includes a normally closed stop
button, a normally open start button, and an overload switch. When an overload
condition is experienced,
power is supplied to a solenoid in the electromechanical trip mechanism causing
plunger to retract, which subsequently, through a series of levers or other
mechanical components, causes the normally closed contacts to open. Many
overload relays have been designed such that, once tripped, the relay remains
open to prevent current flow to the contactor until the relay is manually reset by a
system operator. A common resetting device is a reset push button selectable by
an operator to reset the relay thereby allowing current to flow to and to close the
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contactor coil which in turn provides current to the linked equipment. An overload
relay is usually designed to operate over a wide range of values and the user
must set the trip current based upon the specifications of the motor in use. The
trip current defines the value at which the relay is triggered into breaking the
circuit between the load and the power. The trip point of the overload relay is
selected by moving a pointer from one position on a scale to another position on
the scale. The pointer is connected to a variable resistor in the overload sensing
circuit such that as the pointer is moved from one position to another along the
scale the resistance of the variable resistor changes. The two most critical
elements in the overload sensing circuit are the current transformers through
which a current proportional to that flowing in the protected circuit is induced andthe variable resistor which changes circuit characteristics such that the relay will
initiate a trip at the selected overload current. In addition to the mechanical
components, a fully featured relay assembly also typically includes a printed
circuit board (PCB) including control circuitry for tripping and automatically
resetting the relay, current sensors and various types of terminals for linking to
power lines, the contactor and LEDs.
A variety of types of overload relays are available, ranging from simple thermal
overload relays to more complex, solid-state relays which may include some
intelligence and/or reporting capabilities. A thermal overload relay is a bimetallic
device which provides motor protection for running and stalled rotor overloads. A
strip bimetal in the overload relay is electrically heated by heater elements which
carry the motor currents. Bimetal overload relays include a snap action electrical
switch which has a contact that is movable between an unactuated and an
actuated position to make or break electrical connection with a stationary contact.
This movable contact is mechanically coupled to a main bimetal element that is
responsive to changes in temperature to operate the electrical switch. Excess
heat is generated in the heater elements by an overloaded motor. The bimetals
deflect to thermally open the normally closed contact, thereby opening a coil
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circuit of a magnetic contactor which disconnects the overloaded motor from the
line. Thereafter the relay may be reset by pressing and releasing a reset rod.
With advances in electronic circuitry, the bi-metallic element has been replaced
with more complex circuitry. Overload relays have been designed to utilize
electronic circuitry responsive to signals derived from the secondary windings of
current transformers whose primary windings carry the motor phase currents.
Such circuitry may sample current flow to the motor on a periodic basis and
provide sophisticated overload prediction based not only on a simple
thresholding but on more complex trend analyses. The output of this circuitry is
typically a low-powered overload signal. The electronic circuitry processes these
signals on a current-time integral basis to determine when a current overloadcondition is sufficiently persistent to require interruption of the motor circuit. In
order for this overload signal to control the contactor coil current, a solid state
switch may
be required, adding to the complexity and cost of the overload relay. The
electronic circuitry can be readily designed to recognize not only overload
conditions, but also high fault current conditions calling for circuit interruption
without delay and hazardous ground fault conditions. Bigmetal and eutectic
overload relays include heater elements in each phase which open when an
excessive current flowing through the heater elements causes the element to
exceed a specific temperature. Solid-state relays, on the other hand, include
electronic devices for monitoring phase current and for determining, based on the
monitored current, whether a fault condition has occurred. Solid-state relays
typically can be configured to provide protection for ground fault, undercurrent
and phase loss conditions, in addition to overcurrent conditions. Solid state
overload relays are commonly available in relatively compact, affordable
packages that can be easily installed and serviced. In addition to circuitry for
detecting fault conditions, such relays also commonly include power supply
circuitry for storing energy from the load circuit being controlled.
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5.11 Star-Delta Starter
The star delta starter is employed in system due to the reason that the motor
takes high value of starting current during the starting, the starting current is
almost 6-8 times the full load current & hence to control this starting current the
motor is connected initially in star & then in delta for normal operation.
The Star/Delta starter is probably the most commonly used reduced voltage
starter,
The Star/Delta starter requires a six terminal motor that is delta connected at the
supply voltage. The Star Delta starter employs three contactors to initially start
the motor in a star connection, then after a period of time, to reconnect the motor
to the supply in a delta connection. While in the star connection, the voltage
across each winding is reduced by a factor of (1 /. ' / '3) [1 divided by root three].
This results in a start-current reduction to (1 /. ' / '3) [1 divided by root three] of the
DOL start current and a start torque reduction to one third of the DOL start
torque.
If there is insufficient torque available while connected in star, the motor can only
accelerate to a partial speed compared to the full speed it would reach if
connected in delta. When the timer operates (set normally from 5-10 seconds),
the motor is disconnected from the supply and then reconnected in delta,
resulting in full line voltage running currents and the torque.
The transition from star connection to delta connection requires that the current
flow through the motor is interrupted. This is termed "Open Transition Switching"
and with an induction motor operating at a partial speed compared to full loadspeed, there is a large current and torque transient produced at the point, unless
proper protection methods are used, can cause severe damage to the supply
service's infrastructure and to other connected equipment.
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Update: Electronic motor-control systems, which offer soft-starts in DELTA
configuration, are now replacing the use of manual or semi-automatic star-delta
starters.
Technical explanation
When the windings of a 3-phase motor are connected in STAR: the
voltage applied to each winding is reduced to only (1 /. ' / '3) [1 divided by
root three] of the voltage applied to the winding when it is connected
directly across two incoming power service lines in DELTA, the current
per winding is reduced to only (1 /.' / '3) [1 divided by root three] of the
normal running current taken when it is connected in DELTA.
So, because of the Power Law V [in volts] x I [in amps] = P [in watts],
the total output power when the motor is connected in STAR is:
PS = [VL x (1/.' / '3)] x [ID x (1/.' / '3)] = PD x (1/3) [one third of the power in DELTA]
where:
VL is the line-to-line voltage of the incoming 3-phase power service
ID is the line current drawn in DELTA
PS is the total power the motor can produce when running in STAR
PD is the total power it can produce when running in DELTA.
a further disadvantage when the motor is connected in STAR is that its total
output torque is only 1/3 of the total torque it can produce when running in
DELTA.
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Fig 5.1 Circuit diagram for star-delta starter is as below
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5.12 Control Switches
The control switches are used in system to control the system parameters in the
system the switches which are mainly used in system are as follows:
2.12.1 Pressure Switch
Pressure Switch are employed in the system to keep the value of pressure of the
system within the safe limit of operation.the general diagram ofpressure switch is
shown below.The operating pricnicple of the pressure switch is bascially
dependent on snap action against the spring operation.when the value of
pressure in the system reaches the cut-in or cut-out value, the bellow is activated
& thus due to the spring contraction or expansion the contacts changeover & the
signal is given as a feedback to the control panel if the system is healthy or
faulty.
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Fig. 5.2 Pressure switch
5.12.2 Differential Pressure switch
Differential pressure switch is employed in system for maintaining the
value of the pressure in the two headers at the required level, these headers may
be of water, air or oil circuit.
For example in case of water circuit if the input line pressure is 3psi &
output pressure is 2 psi then the differential pressure between two is 1 psi &
hence the setting of the DPS can be kept at 1.5 psi. if an leakage occurs in the
water circuit the output pressure reduces causing the differential pressure of the
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system to rise beyond 1.5 psi & hence the switch will operate as it is a unhealthy
condition. This is the basic principle of operation of DPS.
The cross-sectional diagram of the differential switch is as shown below.
Fig 5.3 Differential pressure switch
5.12.3 Resistance temperature detector
Fig 5.4 Resistance Temperature Detector
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5.12.4 Flow switch
Flow switch are employed in system to ensure the flow of the charge in the
system for eg.in water & oil circuit the flow has to be maintained upto a certain
limit so the flow switch are employed for the same.the construction of the flow
switch is as shown below.
The flapper of the flow switch is located in the charge line for
operation,depending on the flow of the charge in the system the flapper position
changes & the contacts changeover giving either the healthy orfault command in
the system. The clearance between the flapper & pipe should be minimum 4 to
5mm also the charge should be free from impurities.
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Fig 5.5 Flow switch
5.12.5 Level Switch
Level switch are basically employed in system to conrol the level of the charge in
the system,the electrical connection of the level switch are given to control panel
& depending on the healty & fault condition the NO(Normally Open) &
NC(Normally closed) connection are done.
The internal construction & the specifcation for the level switch are as below.
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Fig 5.6 Internal construction & the specifcation for the level switch
5.12.6 Solenoid Valves
Solenoid valves are basically employed in system to control the flow of the
charge in the system. The requirement in case our system mainly arise in loading
& unloading system of compressor. Solenoid valves are available in the format
required i.e. Normally open (NO), Normally closed (NC) & Universal operation.
Also depending on requirement as per the site condition the solenoid valve are
also available in simple & flameproof version as shown in diagram.
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Fig 5.7 solenoid valve
5.12.7 Auto Drain Valve
For various reasons, it is always advisable to drain the moisture from the air, and
you can rely on the Auto- Drain Valve for the same.It can drain the moisture at
regular interbals on its own, eliminating the human errors that can cause system
failure. ‘On Time’ of the Auto – Drain Valve can be set as per rate of moisture
accumulated and so also the ‘Off Time’.
Connection details
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Fig. 5.8 Auto drain valve
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5.13 Gauges
The gauges are employed in the system for the indication of parameters for the
charge in the system i.e temeperatute, pressure, flow,level are the different
indication provided by the gauges someof the frequently used gauges are
described below.
5.13.1 Pressure Gauges
Pressure gauges are employed in system to indicate the value of the pressure of
the charge in the system. pressure gauges are available in local & gauge board
mounting type.
5.13.2 Operating Principle
Most standard dial type pressure gauges use a bourdon tube-sensing element
generally made of a copper alloy (brass) or stainless steel for measuring
pressures 15 PSI and above. Bourdon tube gauges are widely used in all
branches of industry to measure pressure and vacuum. The construction is
simple yet rugged and operation does not require any additional power source.
The C-shaped or spirally wound bourdon tube flexes when pressure is applied
producing a rotational movement, which in turn causes the pointer to indicate the
measured pressure. These gauges are generally suitable for all clean and non-
clogging liquids and gaseous media. Low pressure gauges typically use an
extremely sensitive and highly accurate capsule design for measuring gaseous
media from as low as 15 INWC to 240 INWC (10 PSI). Digital gauges use an
electronic pressure sensor to measure the pressure and then transmit it to adigital display readout.
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Fig. 5.9 various types of pressure gauges
5.13.2 Temperature Gauge
Structure & Operating Principle
The working principle of bimetal thermometer is to utilize two different metals with
different thermal linear expansion coefficient. One end is welded on a fixed point;
the other end will bend when the temperature changes. This torsion will rotate
the pointer to indicate the temperature.
Core-extractable style bimetal thermometer means that the sensing element canbe replaced by taking it out of the protective thermowell. It is indicating
thermometer used in a wide-range area on site. (See DWG-1)
The working principle of the RTD is based on the resistance of metal wire varied
by temperature. The working principle of the thermocouple thermometer is to
have one end of two different kinds of metal welded in one point, the output
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voltage of the other end will be varied by the change of temperature. The
thermocouple and RTD with remote signal are used in wide-range area.
Bimetal together with RTD integral thermometer is to have the sheathed platinum
thermal resistance installed in the protective thermowell of bimetal thermometer;
it can output a remote platinum resistance signal.
For the bimetal, thermal resistance integral temperature transmitter, the sheathed
platinum thermal resistance and platinum thermal resistance temperature
transfer modules are installed in the protective thermowell of bimetal
thermometer, thus it can not only indicate on site, but also output a standard
signal of 4-20mA.
Fig. 5.10 Temperature gauge
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5.13.4 Differential Pressure Gauge
Differential pressure gauge is used to indicate the difference of pressure between
two lines of charge. For e.g. in case of a water circuit the gauge is located suchthat the input to the gauge is from the inlet header & other from outlet header
thus the gauge indicates the difference of two pressures in the line.
The working principal of the gauge is dependent on bellow operation depending
on the bellow movement depending on the two input pressures the pointer
moves on scale indicating the pressure
Fig. 5.11 Differential Pressure Gauge
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The legends employed for P& I diagram are as follows: -