hydraulic, pneumatic & electrical system

7/24/2019 Hydraulic, Pneumatic & Electrical System

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KIRLOSKAR PNEUMATIC CO. LTD.

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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


Spring Control

Chambe

Drain

Tank

Pump

Remote

Pilot Pressure Signal

Fig 4.16a Unloading Valve

Fig 4.16b. Symbol of unloading valve




35


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




36


Screw


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




37


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




38


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




39


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




40


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




41


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.




42


Fig. 4.23 Symbol of Time Delay Valve




43


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.




44


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.




45


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.




46


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.




47


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




48


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




49


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




50


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.




51


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.




52


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




53


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




54


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




55


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




56


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.




57


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.




58


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.




59


Fig 5.1 Circuit diagram for star-delta starter is as below




60


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





The legends employed for P& I diagram are as follows: -

hydraulic, pneumatic & electrical system

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