Home Blog

Pneumatic and Hydraulic Actuation Clamps – theteche.com

By
Santhakumar Raja
-
0

Pneumatic and Hydraulic Actuation Clamps

Generally, the above-mentioned pneumatic and hydraulic clamps are actuated by mechanical means. Sometimes, these are actuated by hydraulic or pneumatic methods where large production quantities are required. It facilitates faster clamping, uniform and equalised clamping pressure and less operator fatigue. It also provides an accurate controlling over clamping pressure.

Hydraulic and pneumatic clamps are grouped under fluid power clamping since both use of fluids for generating clamping pressures. Fluid power clamps are generally actuated by cylinders. Image shows a clamping fixture with the clamping links attached to the cylinder piston and clamp.

During clamping of the workpiece the piston inside the cylinder is actuated by oil or air pressure, as shown in image. The piston rod connected to the levers of the clamp operates and exerts necessary pressure to the workpiece. For unclamping, the piston moves back thereby actuating the levers. The clamping pressure on the workpiece is released. Hence, the workpiece can be unloaded.

Fluid Power Clamp – Pneumtic and Hydraulic

Clamping pressure of all clamps will be equal when there are number of clamps actuated by a single control valve. The clamping pressure can be varied by regulating the pressure of the fluid. Higher pressure can be used for rough and heavy cuts. The pressure can be considerably reduced during light finishing cut.

There is a risk of sudden pressure drop when the failure of power will be. This problem can be eliminated by a providing a non-return valve in the pressure supply line. If the power fails, the non-return valve automatically is closed the passage between the pressure line and the cylinder.

Pneumatically operated clamps differ from hydraulically operated clamps in the size of the cylinder. Pneumatically operated clamps require larger cylinder than hydraulic one because of less pressure exerted by the air. A small hydraulic cylinder can exert higher pressure as the hydraulic fluid is incompressible.

Generally, pneumatically operated cylinders are not used where heavy clamping forces are required because extremely large cylinders would be required. But pneumatically operated clamps are convenient because most of the manufacturing Industries have a hi supply of compressed air. In a pneumatic clamping, compressed air is used as the fluid for actuating piston and links. The air is pressurised about 5 to 6 kg/cm². Generally, a large compressor is used to supply air to the entire machine shop. Pneumatic clamps used for light weight applications only and also where there are no load variations.

Air to Hydraulic Booster

In a hydraulic clamping, hydraulic oils are used as the fluid for actuating piston and links. Hydraulic oils are almost incompressible. Therefore, it will not cause much speed variations in hydraulic systems during the variation in load. The operating pressure of hydraulic system ranges from 0.7 to 25N/mm². Small compact cylinders are sufficient for developing nigh clamping force.

However, hydraulic cylinders are slow in operation in comparison with pneumatic actuators. Oil is recirculated in the system through a reservoir. The hydraulic operation of a mechanism requires higher

Pneumatic-and-Hydraulic

Locating and Clamping Principles initial cost since each machine or fixture requires an individual cylinder and power unit.

In modern days, the system of both pneumatic and hydraulic operated clamps is used. For this purpose, air-to-hydraulic booster is used which converts low-pressure air into high hydraulic pressure.

The principle of operation of air-to-hydraulic booster is shown in image. If the air cylinder piston is subjected to 5 N/mm² air pressure and the area of piston is 1000mm², a force of 5000N is placed upon the ram. If the ram diameter is 100mm-. the pressure upon the hydraulic oil must be 50N/mm. A hydraulic pressure of 50N/mm has been produced from 5N/mm² air pressure, i.e., has increased the pressure by a ratio of 10: 1. In general, the ratio of an air-to-hydraulic booster can be determined by dividing the area of air piston by the area of hydraulic ram. The high-pressure output is calculated by multiplying the air pressure by the area ratio.

Alternator Voltage Regulator Excitation System

By
Santhakumar Raja
-
0
Alternator Voltage Regulator

Introduction to Alternator Voltage Regulator Excitation System

When the load on the power system changes, alternator voltage regulator the terminal changes. Therefore, to maintain the terminal voltage of the generator excitation of the generator must be decreased voltage within permissible standards, the or increased depending upon the situation prevailing to protect the devices or apparatus which is operating in the power system. This can be achieved by employing automatic voltage regulator (AVR).

The basic function of an excitation system is to provide direct current to the synchronous machine. In addition, the excitation system performs control and protective functions essentially to the satisfactory performance of the power system by controlling the field voltage, thereby the field current.

The control function include, the control of voltage and reactive power flow and the enhancement of system stability. The protective function ensures that the capability limits of the synchronous machine, excitation system and other equipment are not exceeded.

In addition to voltage regulators at generator buses static shunt capacitors, synchronous compensators, static VAR systems , tap changing transformers are also used in the power system for rapid alternator voltage regulator.

Exciter Ceiling Voltage

It is the maximum voltage that may be attained by an exciter under specific conditions.

Excitation System Ceiling Voltage

It is the maximum D.C. component system output voltage that is able to be attained, by an excitation system under specified conditions.

Excitation System Requirements

The excitation system must satisfy the following requirements.

  1. Meet specified response criteria.
  2. Must be able to prevent damage to itself, generator and its associated equipments.
  3. It should have good operating flexibility.
  4. Meet the desired reliability and availability by incorporating the necessary levels of redundancy, internal fault detection and isclation capability.

Types of Excitation System – Alternator Voltage Regulator

Based on excitation power source used, the excitation system can be classified as :

  1. D.C. Excitation systems.
  2. A.C. Excitation systems.
  3. Static Excitation systems.

D.C. Excitation Systems

This excitation system utilizes D.C generators as sources of excitation power and provide current to the rotor of the synchronous machine through slip rings.

The D.C. excitation systems were used in the earlier days. Now it has been superseded by A.C. exciters.

A.C. Excitation System

This excitation system uses alternators (A.C. machines) as sources of the main generator excitation power. Usually, the exciter is on the same shaft as the turbine generator.

The A.C. output or the exciter is rectified by either controlled or un-controlled rectifiers, to provide the direct current for the generator field.

Static Excitation Systems

The components in these systems are static or stationary. The static rectifier either controlled or un-controlled, supply the excitation current directly to the field of the main synchronous generator through slip rings. The main source of power to the rectifiers is from the main generator through a transformer to step down the voltage to an required level. At the time of starting, the field is supplied through battery power.

Potential Controlled – Rectifier Excitation System

The exciter system shown in image is an example of static excitation system. The field current of the main generator is obtained from the main generator itself through exciter transformer. The variable field voltage is obtained through an controlled rectifier, and the firing angle of thyristors is determined by the main generator terminal voltage.

Alternator Voltage Regulator

The field winding is supplied through slip rings. This system is advantageous as it is small inherent time constant, inexpensive and easily maintainable.

Alternator Voltage Regulator Excitation System

The excitation system consists of main exciter (a D.C. generator) which excites the alternator field to control the output voltage. The exciter field is automatically controlled through error signal

e = V ref – VT
V ref = Reference signal
VT = Potential transformer output voltage.

The error signal is amplified through voltage amplifier. The error signal firing angle of the thyristors which depends upon terminal voltage. Thus, the specified voltage is obtained at the alternator output. The stabilizing transformer is used in addition to the error signal to dump system excitations. The schematic diagram of alternator voltage regulator is shown in image.

Function Declaration in C – User Defined Functions

By
Santhakumar Raja
-
0

Function Declaration in C

Like variables, all functions in a C program must be declared, before they are invoked. A function declaration (also known as function prototype) consists of four parts.

  • Function type (return type)
  • Function name
  • Parameter list
  • Terminating semicolon

They are coded in the following formate : Function-type function-name (parameter list);

This is very similar to the function header line except the terminating semicolon. For example, mul function defined in the previous section will be declared as : int mul (int m, int n); /* Function prototype */

Points to note – Function Declaration

  1. The parameter list must be separated by commas.
  2. The parameter names do not need to be the same in the prototype declaration and the function definition.
  3. The types must watch the types of parameters in the function definition, in number and order.
  4. Use of parameter names in the declaration is optional.
  5. If the function has no formal parameters, the list is written as (void).
  6. The return type is optional, when the function returns int type data.
  7. When the declared types do not match with the types in the function definition, compiler will produce an error.

Equally acceptable forms of declaration of mul function are

int mul (int, int);
mul (int a, int b);
mul (int, int);

When a function does not take any parameters and does not return any value, its prototype is written as : void display (void);

A prototype declaration may be placed in two places in a program.

  1. Above all the functions (including main).
  2. Inside a function definition.

When we place the declaration above all the functions (in the global declaration section), the prototype is referred to as a global prototype. Such declarations are available for all the functions in the program.

When we place it in a function definition (in the local declaration section), the prototype is called a local prototype. Such declarations are primarily used by the functions containing them.

The place of declaration of a function defines a region in a program in which the function may be used by other functions. This region is known as the scope of the function. (Scope is discussed later in this chapter.) It is a good programming style to declare prototypes in the global declaration section before main. It adds flexibility, provides an excellent quick reference to the functions used in the program, and enhances documentation.

Prototypes: Yes or No

Prototype declarations are not essential. If a function has not been declared before it is used, C will assume that its details available at the time of linking. Since the prototype is not available, C will assume that the return type is an integer and that the types of parameters match the formal definitions. If these assumptions are wrong, the linker will fail and we will have to change the program. The moral is that we must always include prototype declarations, preferably in global declaration section.

Parameters Everywhere!

Parameters (also known as arguments) are used in three places:

  1. in declaration (prototypes),
  2. in function call, and
  3. in function definition.

The parameters used in prototypes and function definitions are called formal parameters and those used in function calls are called actual parameters. Actual parameters used in a calling statement may be simple constants, variables or expressions.

The formal and actual parameters must match exactly in type, order and number. Their names, however, do not need to match.

Category of Function Declaration

A function, depending on whether arguments are present or not and whether a value is returned or not, may belong to one of the following categories:

Category 1: Functions with no arguments and no return values.
Category 2: Functions with arguments and no return values.
Category 3: Functions with arguments and one return value.
Category 4: Functions with no arguments but return a value.
Category 5: Functions that return multiple values.

In the sections to follow, we shall discuss these categories with examples. Note that, from now on, we shall use the term arguments (rather than parameters) more frequently.

Electrical Machine Design – Rotating Machines

By
Santhakumar Raja
-
0
Electrical Machine Design

INTRODUCTION – Electrical Machine Design

Electrical machine design involves application of science and technology to produce cost-effective, durable, quality and efficient machines.  Also the machines should be designed as per standard specifications. The requirements like low cost and high quality will be conflicting in nature and so a compromise should be made between them.

The electrical machines can be classified into static and dynamic machines.  The transformer is a static (stationary) machine.  The motors and generators are dynamic (rotating) machines.  The transformer converts electrical energy from one voltage level to another voltage level.  The rotating machines converts electrical energy to mechanical energy or vice-versa.

The conversion in any electrical machine takes place through magnetic  field. The required magnetic field is produced by an electromagnet which requires a core and winding. The basic principle of operation of all electrical machine is governed by Faraday’s law of electromagnetic induction.

CONSTRUCTIONAL ELEMENTS OF TRANSFORMER

The transformer is a static electromagnetic device used to transfer electrical energy from a high potential (voltage) circuit to low potential (voltage) circuit or vice-versa.  It consists of two or more windings which link with a common magnetic field.  An iron core serves as a path for magnetic flux.

The constructional elements of the transformer are windings, core, tank and cooling tubes or radiators.  A simple transformer has two windings and they are called high voltage winding and low voltage winding.  One of the winding is connected to supply and it is called primary.  The other winding is connected to load and it is called secondary.

The two different types of transformer constructions are core type and shell type.  In core type type transformer the windings surround the core and in shell type transformer the core surround the windings.  The core and winding assembly is housed in the tank. Cooling tubes or radiators are provided around the tank surface in order to increase the effective cooling surface.

CONSTRUCTIONAL ELEMENTS OF ROTATING MACHINES

The rotating electrical machines converts electrical energy to mechanical energy or vice-versa.  The energy conversion takes place through magnetic field.  Every rotating machine have the following three quantities. The presence of any two quantities, will produce the third quantity.The rotating electrical machines converts electrical energy to mechanical energy or vice-versa. The energy conversion takes place through magnetic field.  Every rotating machine have the following three quantities. The presence of any two quantities, will produce the third quantity.

  • Magnetic field – I (Field)
  • Magnetic field – II (Armature)
  • Mechanical force

In generator, the armature is rotated by a mechanical force inside a magnetic field or the magnetic field is rotated by keeping armature stationary.  By Faraday’s law of induction, an emf is induced in the armature.  When the generator is loaded,  the armature current flows, which produce another magnetic field (armature magnetic field).  Hence in a generator, by the presence of a magnetic field and mechanical force, an another magnetic field is produced.

The mechanical force developed by the motor is due to the reaction of two magnetic fields.  A current carrying conductor has a magnetic field around it. When it is placed in another magnetic field it experiences a mechanical force due to the reaction of two magnetic field.  Hence in a motor by the presence of two magnetic fields a mechanical force is developed.

From the above discussion it is clerar that any rotating machine requires two magnetic field and one of the field is rotating.  Hence a rotating machine will have a stationary and rotating electromagnet, each consisting of a core and winding.  The stationary electromagnet is called stator and the rotating electromagnet is called rotor.

The basic constructional elements of a rotating electrical machine are stator and rotor.  In dc machines the stator consists of field core and winding.  The rotor comprises of armature core and winding.  In ac machines the stator has armature core and winding.  The rotor consists of field core and winding.  The constructional elements of various electrical machines are listed here.

Constructional elements of dc machine

Stator   –   Yoke or Frame           Rotor     –   Armature core

  • Field pole –   Armature winding
  • Pole shoe     –  Commutator
  • Field winding    Others     –  Brush
  • Interpole –  Brush holder

Constructional elements of salient pole synchronous machine

Stator  –  Frame     Rotor    –  Field pole

  • Armature core –  Pole shoe
  • Armature winding –   Field windin   –    Damper winding

Constructional elements of cylindrical rotor synchronous machine

Stator  –  Frame                 Rotor –  Rotor core

  • Stator core                   –  Rotor bars
  • Stator winding               –   End ring

Constructional elements of slip-ring induction motor

Stator  –  Frame      Rotor   –  Rotor core

  • Stator core –  Rotor winding
  • Stator winding  –  Slip rings

CLASSIFICATION OF DESIGN PROBLEMS

The design of an electrical machine involves solution of many complex and diverse engineering problems.  The design problems may be classified under the following four headings.

  • Electromagnetic design
  • Mechanical design
  • Thermal design
  • Dielectric design

Each problem may be solved separately and the results are combined to give overall solution.  Each of these three major problems may be further divided into simple problem are combined to give the solution of a major design problem.

The electromagnetic design problem in rotating machines involves the design of stator & rotor core dimensions, stator & rotor teeth dimensions, air-gap length, stator and rotor windings.  In transformer it is the problem of designing the core and the windings.

The mechanical design in rotating machine involves the design of frame (enclosure), shaft and bearings.  In transformer it is the design of tank (i.e., housing for core and winding assembly).

The thermal design in rotating machine involves the design of cooling ducts in core and cooling fans.  In case of large machines coolants like air or hydrogen may be forced to circulate in the ducts and air-gap.  In transformer it involves the design of cooling tubes or radiators.

Another important design problem, that may require great attention is the design of insulations (Dielectric design).  Dielectric material are used to insulate one conductor from other and also the windings from the core.  The dielectric materials are designed to withstand high voltage stresses.  The breakdown of dielectric materials may lead to failure of machine.

STANDARD SPECIFICATIONS – Electrical Machine Design

Every country has a standards organization to fix standard specification for the manufacturers.  The specifications are guidelines for the manufacturers to produce economic products without compromising quality.  The manufacturers who are compiling with the standards will be issued a certification for their products.  The quality of the certified products will be periodically monitored by the standards organisations.

The standard specifications issued for electrical machines includes the following.

  • Standard ratings of machines.
  • Types of enclosure.
  • Standard dimensions of conductors to be used.
  • Method of marking ratings and name plate details.
  • Performance specifications to be met.
  • Types of insulation and permissible temperature rise.
  • Permissible loss and range of efficiency.
  • Procedure for testing of machine parts and machines.
  • Auxiliary equipments to be provided.
  • Cooling methods to be adopted.

In India, the Indian Standards Organisation (ISO) has laid down their specifications (ISI) for various produicts. The standards will be amended time to time, in order to include the latest developments in technology. Recently they have released revised standards ISO 9002,  to comply with international standards electrical machine design.

Electrical Machine Design

The name plate of a rotating machine has to bear the following details as per ISI specifications.

  • kW or kVS rating of machine
  • Rated working voltage
  • Operating speed
  • Full load current
  • Class of insulation
  • Frame size
  • Manufacturers name
  • Serial number of the product

Some of the Indian standard specifications numbers along with year of issue for elect4rical machines are listed here.

IS 325 – 1966  :   Specifications for three phase induction motor.

IS 1231-1974  :   Specification for foot mounted induction motor.

IS 4029-1967 ;  Guide for testing three phase induction motor.

IS 996-1979   :  Specification for single phase ac and universal motor.

IS 1885-1993 :  Specifications for electric and magnetic circuits.

IS 9499-1980  :  Conventions concerning electric and magnetic circuits.

IS 7538-1996  :   Specifications for three phase induction motor for centrifugal pumps and Agricultural applications.

IS 12615-1986:  Specifications for energy efficient induction motor.

Is 9320-1979 :    Guide for testing dc machines.

IS 4722-1992 :    Specification for rotating electrical machines.

IS 12802-198 :     Temperature rise measurement of rotating electrical machines.

IS 4889-1968  :    Method of determination of efficiency of rotating electrical machines.

IS 13555-1993:   Guide for selection and application of three phase induction motor for different types of driven equipment.

IS 7132-1973 :   Guide for testing synchronous machines.

IS 5422-1996  :   Turbine type generators.

IS 7572-1974  :   Guide for testing single-phase ac and universal motors.

IS 8789-1996  :  Values of performance characteristics for three phase induction motors.

IS 12066-1986 :  Three phase induction motors for machine tools.

IS 1180-1989  :  Specifications for outdoor 3-phase distribution transformer upto 100 kVA (Sealed And Non-sealed type)

IS 2026-1994  :  Specifications of power transformers.

IS 11171-1985:  Dry type power transformers

IS 5142-1969  :  Continuously variable voltage auto transformers.

IS 10028-1985:  Code of practice for selection, installation and maintenance of transformers

IS 10561-1983 :   Applications guide for power transformers.

IS 13956-1994 :  Testing transformers.

IS 9678-1980 :  Methods of measuring temperature rise of electrical equipment.

IS 12063-1987  :  Classification of degree of protections provided by enclosures of electrical Equipment.

IS 3855-1966  :  Standard dimensions of rectangular enameled copper conductor.

IS 449-1962  :   Standard dimensions of enameled round copper conductor (oleo resinous enamel).

IS 1595-1960  :  Standard dimensions of enameled round copper conductor (synthetic enamel).

IS 1897-1962  :  Standard dimensions of bare copper strip.

IS 1666-1961  :  Standard dimension of paper covered rectangular copper conductor for transformer Windings.

IS IS 2068-1962  : Standard dimensions of cotton covered rectangular copper conductor for Transformer windings.

IS 3454-1996  :  Standard dimensions of paper covered round conductors used for transformer Windings.

IS 450-1964:  Standard dimensions of cotton covered round conductors used for transformer Windings.

GENERAL DESIGN PROCEDURE – Electrical Machine Design

     In general any electrical machine has two windings.  The transformer has primary and secondary winding.  The dc machine and synchronous machine has armature and field winding.  The induction machine has stator and rotor winding.  The basic principle of operation of all electrical machine is governed by Faraday’s law of induction.  Also in every electrical machine the energy is transferred through the magnetic field.   Hence a general design procedure can be developed for the design of electrical machines.

The general design procedure is to relate the main dimensions of the machine to its rated power output.  An electrical machine is designed to deliver a certain amount of power called rated power.  The rated power output of a machine is defined as the maximum power that can be delivered by the machine safely.  In dc machine the power rating is expressed in kW and in ac machine in kVA.  In case of motor the output power is expressed in HP.

In electrical machine the core and winding of the machine are together called active part.  (Because the energy conversion takes place only in the active part of the machine).  A general output equation can be developed for dc machine which relates the power output to volume of active part (D2L),speed magnetic and electric loading.  Similarly a general  output equation can be developed for ac machine which relates kVA rating to volume of active part(D2L),speed, magnetic and electric loading.

MAIN DIMENSIONS OF ROTATING MACHINES

In rotating machine the active part is cylindrical in shape.  The volume of the cylinder is given by the product of area of cross section and length.  If D is the diameter and L is the length of cylinder, then the  volume is given by πD2L/4.  Therefore D and L are specified as main dimensions.

In case of dc machine, D represent the diameter of armature and L represent the length of armature.  In case of ac machine,  D represent the inner diameter of stator and L represent the length of stator core.  The fig1.1 shows the main dimensions of rotating machines.

Here,  Dr  =  Diameter of rotor.

Lg =  Length of air-gap.

Automatic Voltage Regulator – System Level Control

By
Santhakumar Raja
-
0
Automatic Voltage Regulator

Automatic Voltage Regulator

The function of an electric power system is to convert energy from one of the naturally available forms to electrical form and to transport it to the points of consumption automatic voltage regulator.

A properly designed and operated power system should, therefore, meet the following fundamental requirements.

  1. Adequate ‘spinning reserve’ must be present to meet the active and receive power demand.
  2. Minimum cost with minimum ecological impact.
  3. The power quality must have certain minimum standards within the tolerance or limit such as.
  • Constancy of frequency.
  • Constancy of voltage (Voltage magnitude and load angle).
  • Level of reliability.

Factor affecting power quality

The factors affecting power quality are:

  • Switching surges.
  • Flickering of voltages.
  • Load shedding.
  • Electromagnetic interference.
  • Line capacitance and line inductance.
  • Operation of heavy equipment.
  • Welding machine operation, etc.

The three main controls involved in power systems are:

  • Plant Level Control (or) Generating Unit Control.
  • System Generation Control.
  • Transmission Control.

Plant Level Control or Generating Unit Control

The plant level control consists of:

  1. Governor control or Prime mover control.
  2. Automatic voltage regulator (AVR) or excitation control.

Governor Control or Prime Mover Control

Governor control or Prime mover controls are concerned with speed regulation of the governor and the control of energy supply system variables such as boiler pressure, temperature and flows.  Speed regulation is concerned with steam input to turbine.  With variation in load, speed of governor varies as the load is inversely proportional to speed.  The speed of the generator varies and the governor senses the speed and gives a command signal, so that, the steam input of the turbine is changed relative to the load requirement.

Automatic Voltage Regulator (AVR) or Excitation Control

     The function of Automatic voltage regulator (AVR) excitation control is to regulate generator voltage and relative power output.  As the terminal voltage varies the excitation control, it maintain the terminal voltage to the required standard and the demand of the reactive power is also met by the excitation control unit.

System Level Control

The purpose of system generation control is to balance the total system generation against system load and losses, so that, the desired frequency and power interchange with neighbouring systems are maintained.  The comprises of:

  • Load frequency control (LFC)
  • Economic dispatch control (EDC)
  • System voltage control.

Load Frequency Control (LFC)

The involves the sensing of the bus bar frequency and compares with the tie-line power frequency.  The difference of the signal is fed to the integrator and it is given to speed changer which generates the reference speed for the governor.  Thus, the frequency of the tie-line is maintained as constant.

Economic Dispatch Control (EDC)

When the economical load distribution between a number of generator units is considered, it is found that the optimum generating schedule is affected when an incremental increase at one of the units replaces a compensating decrease at every other unit, in term of some incremental cost.  Optimum operation of generators at each generating station at various station load levels is known as unit commitment.

System Voltage Control

The involves the process of controlling the system voltage within tolerable limits.  This includes the devices such as static VAR compensators, synchronous condenser, tap-changing transformer, switches, capacitor and reactor.

The controls described above contribute to the satisfactory operation of the power system by maintaining system voltages, frequency, and other system variables within their acceptable limits.  They also have a profound effect on the dynamic performance of power system and on its ability to cope with disturbances.

Security Control

     The main objective of real time power system operation requires a process guided by control and decisions based on constant monitoring of the system condition.  The power system operation is split into two levels.

Level 1: Monitoring and Decision

The condition of the system is continuously observed in the control centres by protective relays for faults or contingencies caused by equipment trouble and failure.  If any of these monitoring devices identifies a sufficiently severe problem at the sample time, then the system is in an abnormal condition.  If no such abnormality is observed, then the system is in a normal condition.

Level 2: Control

At each sample, the proper commands are generated for correcting the abnormality on protecting the system from its consequences.  If no abnormality is observed, then the normal operation proceeds for the next sample interval.

Central controls also play an important role in modern power systems.  Today systems are composed of interconnected areas, where each area has its own control centre.  There are many advantages to interconnections.  The interconnected areas can share their reserve power to handle anticipated load peaks and unanticipated generator outages.  Interconnected areas can also tolerate larger load changes with smaller frequency deviations at spinning reserve and standby provides a reserves margin.

The central control centre monitors information including area frequency, generating unit outputs, and tie-line power flows to interconnected areas.  This information is used by automatic load frequency control in order to maintain area frequency at its scheduled value.

OVERVIEW OF SYSTEM OPERATION – Automatic Voltage Regulator

Load forecasting

The load on their systems should be estimated in advance.  This estimation in advance is known at load forecasting.  Load forecasting based on the previous experience without any historical data.

Classification of load forecasting:

         Forecast                 Load Time               Application
Very short time Few minutes to half an hour Real time control, real time security evalution
Short term Half an hour to a few hours Allocation of spinning reserve, unit commitment, maintenance scheduling
Medium term Few days to a few weeks Planning or seasonal peak-winter, summer
Long term Few months to a few years To plan the growth of the generation capacity

Need for load forecasting:

Need for load forecasting are:

  • To meet out the future demand
  • Long-term forecasting is required for preparing maintenance schedule of the generating units, planning future expansion of the system.
  • For day-to-day operation, short term load forecasting is needed in order to commit enough generating capacity for the forecasting demand and for maintaining the required spinning reserve.
  • Very short term load forecasting are used for generation and distribution. (i.e.,) economic generation scheduling and load dispatching.
  • Medium term load forecasting is needed for predicted monsoon acting and hydro availability and allocating spinning reserves.

Unit Commitment

The unit commitment problem is to minimize system total operating costs while simultaneously providing sufficient spinning reserve capacity to satisfy a given security level.  In unit commitment problems, we consider the following terms.

  • A short term load forecast
  • System reserve requirements.
  • System security
  • Startup costs for all units.
  • Minimum level fuel costs for all units.
  • Incremental fuel costs of units.
  • Maintenance costs.

Load Scheduling (Load Dispatching)

Loading of units are allocated to serve the objective of minimum fuel cost is known as load scheduling.

Load scheduling problem can be divided into:

  • Thermal scheduling.
  • Hydrothermal scheduling.

Thermal scheduling:

The loading of steam units are allocated to serve the objective of minimum fuel cost.  Thermal scheduling will be assumed that the supply undertaking has got only from thermal or from steam stations.

Hydrothermal Scheduling:

Loading of hydro and thermal units are allocated to serve the objective of minimum fuel cost is known as hydrothermal scheduling.

Scheduling of hydro units are complex because of natural differences in the watersheds, manmade storage and release elements used to control the flow of water are difficult.

During rainy season, we can utilize hydro generation to a maximum and the remaining period, hydro generation depends on stored water availability.  If availability of water is not enough to generates power, we must utilize only thermal power generation.  Mostly hydroelectric generation is used to meet out peak loads.

There are two types of hydrothermal scheduling.

Long-Range Hydro-Scheduling:

     Long-range hydro scheduling problem involves the long-range forecasting of water availability and the scheduling of reservoir water releases for an interval of time that depends on the reservoir capacities.  Long-range hydro-scheduling involves from 1 week to 1 year or several years.  Long-range hydro-scheduling involves optimization of statistical variables such as load, hydraulic inflows, and unit availabilities.

Short-range hydro-scheduling:

     Short-range hydro-scheduling involves from one day to one week or hour-by-hour scheduling of all generation on a system to achieve minimum production cost for a given period.
Assuming load, hydraulic inflows and unit availabilities are known.
For a given reservoir level, we can allocate generation of power using hydro plants to meet out the demand, to minimize the production cost.
The largest category of hydrothermal system includes a balance between hydroelectric and thermal generation resources.  Hydrothermal scheduling is developed to minimize thermal generation production cost.

Sensors and Transducers – Type of Sensors

By
Santhakumar Raja
-
0

Sensors and Transducers

Sensors transducers are devices which produce proportional output signal (mechanical, electrical. magnetic, ctc.) when exposed to a physical phenomenon (pressure, temperature, displacement, force, etc.). Many devices require sensors for accurate measurement of pressure, position, speed, acceleration or volume. Transducers are devices which converts an input of one form of energy into an output of another form of energy. The term transducer is often used synonymously with sensors. However, ideally. the word ‘transducer’ is used for the sensing element itself whereas the term ‘sensor’ is used for the sensing element plus any associated signal conditioning circuitry. Typically, a transducer may include a diaphragm which moves or vibrates in response to some form of energy, such as sound.

Some common examples of transducers with diaphragms are microphones, loudspeakers, thermometers, position and pressure sensors. Sensors are transducers when they sense one form of energy input and output in a different form of energy.

For cxample, a thermocouple responds to a temperature change (thermal energy) and outputs a proportional change in electromotive force (electrical energy). Therefore, thermocouple can be called a sensor and or transducer.

Illustrates a sensor with sensing process in terms of energy conversion. The form of the output signal will often be a voltage analogous to the input signal, though sometimes it may be a wave form whose frequency is proportional to the input or a pulse train containing the information in some other form.

Classification of Sensors and Transducers

Sensors are generally classified into two types based on its power requirement: passive and active. In active sensors, the power required to produce the output is provided by the sensed physical phenomenon itself (Examples: thermocouples, photovoltaic cells, piezoelectric transducers, thermometer etc.) whereas the passive sensors require external power source (Examples: resistance thermometers, potentiometric devices, differential transformers, strain gage etc.). The active sensors are also called as self-generating transducers. Passive sensors work based on one of the following principles: resistance, inductance and capacitance.

Sensors can also be classified as analog or digital based on the type of output signal. Analog sensors produce continuous signals that are proportional to the sensed parameter. These sensors generally require analog-to-digital conversion before sending output signal to the digital controller (Examples: potentiometers, LVDTs (linear variable differential transformers), load cells, and thermistors, bourdon tube pressure sensor, spring type force sensors, bellows pressure gauge etc.). Digital sensors on the other hand produce digital outputs that can be directly interfaced with the digital controller (Examples: incremental encoder, photovoltaic cells, piezoelectric transducers, phototransistors, photodiodes etc.). Often, the digital outputs are produced by adding an analog-to-digital converter to the sensing unit. If many sensors are required, it is more economical to choose simple analog sensors and interface them to the digital controller equipped with a multichannel analog-todigital converter.

Another way of classifying sensor refers to as primary or secondary sensors. Primary sensors produce the output which is the direct measure of the input phenomenon. Secondary sensors on the other hand produce output which is not the direct representation of the physical phenomenon. Mostly active sensors are referred as primary sensors where as the passive sensors are referred as secondary sensors.

Quality to be measured

Type of Sensors
Linear/Rotational displace met

Linear/Rotational variable diffrential
transformer (LVDT/RVDT)
Optical encoder
Electrical tachometer
Hall effect sensor
Capacitive transducer
Strain gauge elements
Interferometer
Magnetic pickup
Gyroscope
Proximity

Inductance sensor
Eddy current sensor
Hall effect sensor
Photoelectric sensor
Capacitance sensor
Force, torque, and pressure

Strain gauge
Dynamometers/load cells
Piezoelectric load cells
Tactile sensor
Ultrasonic stress sensor
Velocity, and acceleration

Electromagnetic sensor
Ultrasonic sensor
Tachogenerators
Resistive sensor
Capacitance sensor
Piezoelectric sensor
Photoelectric sensor
Electron tube
Flow

Pitot tube
Orifice plate
Flow nozzle
Venturi tubes
Rotameter
Ultrasonic flow meter
Turbine flow meter
Electromagnetic flow meter
Level

Float Level Sensor
Pressure Level Sensor Resistive sensor
Variable Capacitance sensor
Piezoelectric sensor
Photoelectric sensor
Temperature

Thermocouples
Thermistors
Thermodiodes
Thermo transistors
Resistance temperature detector (RTD)
Infrared thermography
Light

Photoresistors
Photodiodes
Photo transistors
Photo conductors
Charge-coupled diode

Microprocessor Based Controllers in Mechatronics

By
Santhakumar Raja
-
0
Microprocessor Based Controllers
Microprocessor Based Controllers

Microprocessor Based Controllers

Microprocessors are essential to many of the products we use every day such as TVs, cars, radios, home appliances and of course, computers. Microprocessor-based controllers are also called as microcontrollers. Microcontroller is a digital integrated circuit which serves as a heart of many modern control applications.

Microprocessors and microcontrollers are similar but the architecture of both differs in the applications domains. Microprocessors are employed for high speed applications such as desktop and laptop computers where as the microcontrollers are employed in automation and control applications such as microwave ovens, automatic washing machines, dish washers, engine management systems, DVD players etc.

Microcontrollers are embedded inside some other device (often a consumer product) so that they can control the features or actions of the product. Therefore, it is also called as embedded controller.

Because of its relatively low cost, it is a natural choice for design. It performs many of the functions traditionally done by simple logic circuitry, sequential control circuits, timers or a small microcomputer. Microcontrollers are generally compact in construction, small in size, flexibility and consume less power.

A microcontroller generally has the main CPU core, ROM/ EPROM / RAM and some accessory functions (like timers, pulse width modulator, A/D convertor and I/O controllers) all integrated into one chip. Microcontroller is a computer on a chip that is programmed to perform almost any control, sequencing, monitoring and display function.

Another more adaptable form of microcontroller is the programmable logic controller (PLC). image shows the basic structure of a PLC. The PLC is a microprocessor based controller consists of the CPU, memory and I/O devices. These components are integral to the PLC controller. Additionally the PLC has a connection for the programming unit, and printer. The CPU used in PLC system iS a standard CPU present in many other microprocessor controlled systems. The choice of the CPU depends on the process to be controlled. Memory in a PLC system is divided into the program memory which is usually stored in EPROM/ROM, and the operating memory. The RAM memory is necessary for the operation of the program and the temporary storage of input and output data. Input/output units are the interfaces between the internal PLC systems and the external processes to be monitored and controlled. Programming unit in the PLC systems is a essential component and are used only in the development/testing stage of PLC program, they are not permanently attached to the PLC. Programming unit can be a dedicated a device or a personal computer.

Microprocessor Based Controllers
Microprocessor Based Controllers

The following example illustrates how microprocessor-based controllers are used to control the processes in the system.

Example 1: Automatic camera

The modern automatic camera using film has the features of automatic focusing and exposure. The basic elements of the microprocessor based control system used in ar. automatic camera for focusing and exposure are shown in image.

The working of auto focusing and aperture control for auto-exposure 1S explained as follows:

Auto focusing 

The auto focusing is achieved by using range sensor. When the system is switch on to activation mode, the camera is pointed at the object to take the snap. The microprocessor takes the input signal from the range sensor. This signal is processed to send output signal to the lens position drive to move the lens for achieve auto focusing. The microprocessor gets the feedback signal about the lens position from the range sensor which is then used to modify the lens poaition to get the desired position of focus.

Aperture control for auto-exposure

The light sensor is used to achieve aperture control for auto-exposure. When the shutter switch is pressed to the initial position the microprocessor calculates the shutter speed and aperture settings based on the input from light sensor. It then gives output signal to the view finder. When the shutter switch is pressed to the final position the microprocessor gives signal to the aperture control drive to open the shutter to the required position. The shutter is kept open for the preset amount of time and then closed. After photograph has been taken, the microprocessor sends an output signal to the motor drive to advance the film for the next snap.

Example 2: Engine management system

Engine management system is used in many of the modern cars such as Benz, Mitsubishi, Ford, and Toyota etc. This system uses many electronic control systems involving microcontrollers. The objective of the system is to ensure that the engine is operated at its optimum settings. Most of the modern medium range cars use 4-stroke 4-cylinder SI engine. As the name implies it consists of 4 cylinders, each of which has a piston and a connecting rod which are connected to a common crank shaft. Illustrates the sequence of operations of the 4-stroke spark ignition engine.

Working of a four stroke SI engine

At the beginning of the suction stroke, the piston is at the top most position and is ready to move downwards. As the piston moves downwards, a vacuum is created inside the cylinder. Due to this vacuum, air fuel mixture from the carburetor is sucked into the cylinder through inlet valves till the piston reaches the bottom most position. During the suction stroke, exhaust valve remains in closed condition and the inlet valve remains open. During the compression stroke, both the inlet and exhaust valves are in closed condition and the piston moves upwards from bottom to top to compress the air fuel mixture. It leads to an increase in pressure and temperature of the mixture instantaneously. At the end of the stroke, the spark plug ignites the mixture which increases the pressure of the mixture suddenly. The sudden rise in pressure of the mixture exerts an impulse on the piston and pushes it downwards. Thus, the piston moves from top to bottom and produces power.

This stroke is known as a power stroke. During the exhaust stroke, Microprocessor based controllers the piston moves from bottom to top, the exhaust valve is opened and the inlet valve is closed. The burnt gases are pushed out through the exhaust valve when the piston moves upwards.

Microprocessor Based Controllers

Basic elements of an electronic engine management system are shown in image. The system consists of many sensors for observing vehicle speed, engine temperature, oil and fuel pressure, airflow etc. These sensors supply input signals to the microprocessor after suitable signal conditioning and provide output signals via drivers to actuate corresponding actuators.

The power and speed of the engine are controlled by varying the air-fuel mixture and spark ignition timing. The engine speed sensor is an inductive type sensor attached with the fly wheel. It consists of a coil and sensor wheel. The inductance of the coil changes as the teeth of the sensor wheel pass it and so results in an oscillating voltage. The ignition coil supplies the electrical signal to the spark plug to produce a spark which ignites the mixture. The feedback signal from a spark plug is sent to a microprocessor to adjust the timing if it is wrong. The solenoid driver attached to the carburetor is used to control the air-fuel mixture supplied to the cylinder based on input received from an engine temperature sensor and throttle position sensor. Hot wire anemometer is used as a sensor for measuring mass airflow rate. The basic principle is that the heated wire will be cooled as air passes over it.

The amount of cooling is dependent on the mass rate of flow. The engine temperature sensor is generally a thermocouple which is made of bimetallic strip or a thermister. The oil and fuel pressure sensors are diaphragm type sensors. According to the pressure variation, the diaphragm may contract or expand and activate strain gauges which produce voltage variation in the circuit. The various drivers such as fuel injector drivers, ignition coil drivers, solenoid drivers are used to actuate actuation according to the signals by various sensors.

Definition of Function in C – User Defined Functions

By
Santhakumar Raja
-
0

Definition of Function in C

A function definition, also known as function implementation shall include the following elements
1. function name;
2. function type;
3. list of parameters;
4. local variable declarations;
5. function statements; and
6. a return statement.

All the six elements are grouped two parts, namely,
♦ function header (First three elements);and
♦ function body (Second three elements).

A general format of a function definition to implement these two parts is given below

function_type function_name(parameter list)
 {
    local variable declaration;
    executable statement1;
    executable statement2;
    . . . . .
    . . . . .
    return statement;
 }

The first line function_type function_name(paramreter list) is known as the function header and the statements within the opening and closing braces constitute the function body, which is a compound statement.

Function Header

The function header consists of three parts: the function type (also known as return type), the function name and the parameter list. Note that a semicolon is not used at the end of the function header.

Name and Type

The function type specifies the type of value (like float or double) that the function is expected to return to the program calling the function. If the return type is not explicitly specified, C will assume that it is an integer type. If the function is not returning anything, then we need to specify the return type as void. Remember, void is one of the fundamental data types in C. It is a good programming practic to code explicitly the return type, even when it is an integer. The value returned is the output produced by the function.

The function name is any valid C identifier and therefore must follow the same rules of formation as other variable names in C. The name should be appropriate to the task performed by the function. However, care must be exercised to avoid duplicating library routine names or operating system commands.

Formal Parameter List – Definition of Function

The parameter list declares the variables that will receive the data sent by the calling program. They serve as input data to the function to carry out the specified task. Since they represent actual input values, they are often referred to as formal parameters. These parameters can also be used to send values to the calling programs. This aspects will be covered later when we discuss more about functions. The parameters are also known as arguments.

The parameter list contains declaration of variables separated by commas and surrounded by parentheses.

Examples :
float quadratic (int a, int b, int c) {. . . .}
double power (double x, int n) {. . . .}
float mul (float x, float y) {. . . .}
int sum (int a, int b) {. . . .}

Remember, there is no semicolon after the closing parenthesis. Note that the declaration of parameter variables cannot be combined. That is, int sum (int a,b) is illegal.

A function need not always receive values from the calling program. In such cases, functions have no formal parameters. To indicate that the parameter list is empty, we use the keyword void between the parentheses as in
void printline (void)
{
. . . .
}
This function neither receives any input values nor returns back any value. Many compilers accepts an empty set of parentheses, without specifying anything as in
void printline ()
But, it is a good programming style to use void to indicate a nill parameter list.

Function Body

The function body contains the declarations and statements necessary for performing the required task. The body enclosed in braces, contains three parts, in the order given below

  1. Local declarations that specify the variables needed by the function.
  2. Function statements that perform the task of the function.
  3. A return statement that returns the value evaluated by the function.

If a function does not return any value (like the printline function), we can omit the return statement. However, note that its return type should be specified as void. Again it is nice to have a return statement even for void functions.

Some examples of typical function definitions are :

(a)  float mul (float x, float y)
     {
       float result;                /* local variable*/
       result = x * y;              /* computes the product */
       return (result);             /* returns the result */
     }
(b)  void sum (int a, int b)
     {
       printf ("sum = %s", a+b);     /* no local variables */
       return;                       /* optional */
     }
(C)  void display (void)
        {                            /* no local variables */
           printf ("No type, no parameters");
                                     /* no return statement */
        }

Note  1. When a function reaches its return statement, the control is tranferred back to the calling program. In the absence of a return statement, the closing brace acts as a void return.

2. A local variable is a variable that is defined inside a function and used without having any role in the communication between functions.

Principles of Clamping – Mechanical Actuation Clamp

By
Santhakumar Raja
-
0
Principles of Clamping

Principles of Clamping

Principles of clamping devices are used to hold the workpiece in the correct relative position in the jig or fixture. It should ensure that the workpiece is not displaced under the action of cutting forces. For efficient operation, firm clamping of the workpiece is a must. An inadequate clamping may always prove dangerous. Clamping devices are designed for minimum operating and handling time. The following design and operational factors should be considered to achieve best results.
(i) The applied clamping pressures against the workpiece must counteract the tool forces.
(ii) The clamping force should be kept minimum. It must only hold the workpiece and should never be great enough so as to damage the workpiece.
(iii) The clamping pressure should be exerted on the solid supporting part of the workpiece to prevent distortion.
(iv) Clamping should be simple, quick operating and foolproof.
(v) The clamping pressure should be directed parallel to cutting operation. i.e., it should not be directed towards the cutting operation.
(vi) The clamping pressure should be directed towards the support surface in order to prevent lifting of workpiece from its support.
(vii) The movement of the clamp for loading and unloading purposes should be kept limited and if possible, it should be positively guided.
(viii) As far as possible, a suitable device, such as a spring should always be incorporated to avoid lifting of the clamp by hand.
(ix) The clamp should be arranged on the work to perform as many operations as possible in one setting.
(x) The clamp should be of robust construction so that it will not bend under pressure.
(xi) The clamp should be case hardened to prevent the wear of clamping faces.
(xii) The clamping faces should always be arranged directly above the work supports to avoid distortion of work.
(xiii) The clamping parts should be designed to make it. non. detachable from the jig.
(xiv) In case of handling soft or fragile workpiece, fibre pads are provided in clamping face to prevent damage to the work.
(xv) The design should facilitate a complete separation of the clamp from the work to enable quick and unrestricted unloading and loading.

Clamping Devices

There are large numbers of different types of clamps. Clamps can be broadly classified into two types:
(i) Mechanical actuating clamps,
(ii) Power clamps.

Mechanical actuating clamps can be further classified into screw clamps, strap clamps, latch clamps, wedge or key clamps, cam clamps, hinged clamps, edge clamp, toggle clamps.

Mechanical Actuation Clamps – Principles of Clamping

Screw Clamps

Screw clamps are the simple type of clamps and quite commonly used in jigs and fixtures. This clamp comprises of a screw, a hand knob, and a pressure pad. It works on the principle of bolt and nut.

The basic screw clamp uses the torque developed by a screw thread to hold a component in place.

The clamping area of a screw clamp can be increased by a provision for a pad. The clamping pad is free to rotate on the pivot. This eliminates friction between workpiece and pad. The clamping pad remains stationary on the workpiece while the screw rotates and rubs on the conical seat of the pad.

These types of clamp are also known as clamping screws or clamp screws.

Principles of Clamping

The force developed by the screw can be calculated by the following formula : F = PL/R tan(α+θ)

F = Force developed by screw
P = Pull or push applied to spanner
R = Pitch radius of screw thread
L = Leangth of spanner or lever
α = Helix angle of thread
θ = Friction angle of thread

Even though it has many advantages, they have following limitations also,

  1. Clamping force is not constant.
  2. Relatively larger effort is needed for clamping, resulting in fatigue to the operator.
  3. Time taken by clamping is more.
  4. There may be a tendency of producing indentations of the screw tip on workpiece in the absence of floating pads.

Strap clamp or Lever clamp

Strap clamps are simple in construction and operation, and can be economically manufactured. They operate on the principle of a lever.

The various designs in the strap type clamp used in jigs and fixtures are discussed below:
Bridge clamp:
It is one of the very simple and reliable clamping devices. The basic elements of a bridge clamp are a solid heel, a strap, a stud, a spring and spherical washers. The clamping force is applied by the springloaded nut, The stud is designed to have a safe diameter to withstand the clamping and cutting pressures and also the cross section of the strap. In this type of a clamp, a pillar pin is integrated as a part of the jig or fixture body. Sometimes, it may also a separate part. When the nut is unscrewed, the spring pushes the clamp uowards. It facilitates the loading and unloading operation, Principles of Clamping.

In these clamps, the ratio of compressive force of the nut shared between the workpiece and clamp support depends on the following factors:

  • Relative positions of the nut,
  • The point of contact of the clamps with the work and with outer support.

Bend Test for Steel – Bend Weld Testing

By
Santhakumar Raja
-
0
Bend Test

Bend Test – Bend weld

Bend test is one of the commonly used destructive tests to determine the ductility and soundness (for the presence porosity, inclusion, penetration and other macro-size internal weld discontinuities) of the weld joint produced using under one set of welding conditions.

There are following two types of bend test:

Free bend test:

The free bend weld testing approach has been devised to measure the ductility of the weld metal deposited in a weld joint. A physical weld testing specimen is machined from the welded plate with the weld located as shown.

Each corner lengthwise of the specimen shall be rounded in a radius not exceeding one-tenth of the thickness of the specimen.  Two scribed lines are placed on the face 1.6 mm from the edge of the weld.  The distance between these lines is measured and recorded as the initial distance x as.  The ends of the test specimen are then bent through angles of about 300 and these bends are being approximately one-third of the length in from each end.  The weld is thus located centrally to ensure that all of the bending occurs in the weld.

The specimen bent initially is then placed in a machine capable of exerting a large compressive force as shown and bent until a crack greater than 1.6 mm in any dimension appears on the face of the weld.  If no cracks appears, bending is continued until the specimens 6.4 mm thick or under can be tested in vise.

Bend test

After bending the specimen to the point where the test bend is concluded, the distance between scribed lines on the specimen is again measured and recorded as the distance y.  Then percentage elongation is calculated.  The usual requirements for passing this test are that the minimum elongation be 15% and that no cracks greater than 1.6 mm in any dimension exist on the face of the weld.

The free bend test is being largely replaced by the guided bend test where the required testing equipment is available.

Guided bend test:

The quality of the weld metal at the face and root of the welded joint as well as the degree of penetration and fusion to the base metal are determined by means of guided bend tests.  It also shows the efficiency of the weld.

This type of physical weld testing is made in a jig.  These test specimens are machined from welded plates and the thickness of which must be within the capacity of the bending jig.  The test specimen is placed across the supports of the die which is the lower portion of the jig.  The plunger operated from above by a hydraulic jack or other device causes the specimen to be forced to assure the shape of the die.

For bend test, the load is increased until cracks start to appear on face or root of the weld for face and root bend test respectively and angle of bend at this stage is used as a measured of ductility of weld joints.  Higher is bend angle greater is ductility of the weld.  To fulfill the requirements of this test, the specimens must bend 1800and, to be accepted as passable, no cracks greater than 3.2 mm in any dimension should appear on the surface.  Fraction surface of the joint from the face/root side due to bending reveals the presence of internal weld discontinuities if any.

Free bending of the weld joint can be done from face or root side depending upon the purpose.  The face bend tests are made in the jig with the face of the weld in tension (i.e., on the outside of the bend).  The root bend tests are made with the root of the weld in tension (i.e., on outside of the bend).  The root side bending shows the lack of penetration and fusion if any at the root.

Nick Bend Test

The nick break test has been devised to determine if the weld metal of a welded but joint has any internal defects such as slag inclusions, gas pockets, poor fusion and oxidized or burnt metal.

The specimen is obtained from a welded butt joint either by machining or cutting with an oxyacetylene torch.  Each edge of the weld at the joint is slotted by means of a saw cut through the center.  The piece thus prepared is bridged across two steel blocks and stuck with a heavy hammer until the section of the weld between slots fractures.

Bend Test

The metal thus exposed should be completely fused and free from slag inclusions.  The size of any gas pocket must not be greater than 1.6 mm across the greater dimension and the number of gas pockets or pores per square inch (64.5 mm3) should not exceed 6.

Bend Weld Testing

Another break test method is used to determine the soundness of fillet welds.  It is the fillet weld break test. This type of testing involves breaking a sample fillet weld that is welded on one side only.  A force, by means of a press or testing machine blows of a hammer, is applied to the apex of the V shaped specimen until the filled weld ruptures. The surfaces of the fracture will then be examined for soundness. This type of weld inspection can detect such items as lack of fusion, internal porosity and slag inclusions.  This testing method is often used in conjunction with the acid etch test.

123...7Page 1 of 7

EDITOR PICKS

POPULAR POSTS

POPULAR CATEGORY

©