A Silicon controlled Rectifier (SCR) is a semiconductor device which conducts in only one direction.  It has three terminals.  An anode, a cathode and a gate.  Unlike a diode, however, it needs to be switched on by a pulse applied to the gate.  

The circuit below shows the method of switching on an SCR using a resistor.  The power source is connected across the SCR.  The gate voltage is provided by the voltage divider circuit.  The variable resistor, R4 is used to control the firing angle.  

The Diode D1 prevents negative voltage from reaching the gate during the negative half cycle.  The SCR will be switched off during the negative half cycle by the supply voltage

Resistor switching of an SCR
Circuit Diagram - Resistor switching of an SCR

The Transient Stability Analysis is done to determine the behaviour of a system during transients or sudden changes in a power system. 

Transients occur when there are power flows from one source to another.  They also occur during faults when a load or a generating point is cut off.  This can cause oscillations in the voltage or power.  Many of these oscillations will quickly get resolved and the system will return to its steady state operation.

However, in certain situations, the oscillations may can increase in severity and can cause fluctuations in the voltage or power which can affect the system and can cause trippings.  Hence, it is necessary to ensure that the system will be stable in the event of a transient.

The stability of a system is classified into

Steady State Stability and
Transient Stability

Steady state stability is the ability of the system to respond to small oscillations in the voltage or slow changes in the load.

Transient stability is the ability of the system to respond to sudden, unexpected changes such as the tripping of a power source or a fault in a tranmission line. 

Transient stability analysis is used in relay setting and in determining the clearing time of breakers.  They are also used to determine the voltage level of a power system and the power transfer capability between different systems.

A Fault Analysis is a study which describes the fault currents and the behaviour of a power system during an electric fault.  Faults may be line to line faults or line to ground faults. The fault analysis provides information about the  The Fault Analysis is used to determine the ratings of fuses and circuit breakers. 

Using the fault analysis, we can determine the maximum current which will be developed during a fault.  The bus bars, breakers and other transmission equipment should be equipped to withstand the heavy current which flow during a fault.

The protective relays are set based on the current calculated during the fault analysis.

The Load Flow Analysis is done to determine the voltage, current, real and reactive power in a particular point in a power system as well as the flow from one point to another. 

The Load Flow Analysis helps understand the operation and behaviour of the system when a generator trips or when a big load is suddenly cut off.  Load Flow Analysis also helps identify routes to transfer power when a transmission line has to be isolated due to a fault.  This ensures reliable power supply and ensures quick restoration in the event of blackouts. 

Load Flow Analysis is done during the design of the power system.  It should also be done before any modification of the power system such as the addition of loads or generating units.

A Power system has three main components
They are

  • The Generating System
  • The Transmission System 
  • The Distribution System

Generating System
The Generating System is the source of the power.   The generation can be from generators, solar panels, etc.  Power can be generated from different sources such as hydropower, wind turbines, nuclear plants,etc.

Components: Synchronous Generators, induction generators, solar panels,

Transmission System
The transmission system transmits the generated power over large distances to the distribution centres such as industries and cities.  The distribution areas can be thousands of kilometres away from the generating stations.  The voltage is stepped up to high values to minimize the losses using transformers.  The power is then transmitted through the power lines to the distribution areas.

Transmission systems can be categorized into

Primary Transmission Systems, which transfer power at voltage of 110 kV and above.  These lines are hundreds of miles long.  They are connected to secondary receiving substations

Secondary Tranmission Systems, which receive the power from the primary transmission system send it to the distribution systems.  The voltage levels in the secondary transmission systems are about 33kv to 66kV

Components: Transformers, Circuit Breakers, Overhead Transmission Lines, Underground Cables.

Distribution Systems
The distribution system receives power from the transmission system and distributes the power to the individual customers at the required voltage.  The industrial supply voltage can be 33kV or 11kV.  The domestic supply voltage is 440 or 220V

Components: Transformers, underground and overhead transmission lines.

A Power System Analysis is a very important exercise in the design and operation of a power system.  The Power System Analysis is used to evaluate the performance of a power system. 

Power System Analysis deals, chiefly, with three important parts

They are

  1. Load Flow Analysis
  2. Short Circuit Analysis and 
  3. Stability Analysis

A Power System Analysis helps the following aspects.

  1. Study the ability of the system to respond to small disturbances caused by the application/removal of small or large loads.
  2. Design of the breakers and isolating equipments.
  3. Plan for future expansion of the power system
  4. Study the response of the system to different fault conditions.
  5. Observe and monitor the voltage, real and reactive power betwee different buses.
  6. Calculate the setting of the relays and the design of the protection system.

The power balance equation describes the relation between Power Demand and Power Generation in a power system.

The equation is


PD is the Total Power Demand
PG is the output of individual generating stations

The sum of the power generated should equal the demand for power.

Transient Stability

Transient Stability is the ability of a power system to return to its normal state after a major disturbance, such as a fault or a disconnection or connection of a large load.

When there is a disturbance in the system, there are oscillations.  These oscillations are called swings.  Transient stability analysis is concerned with the response of the power system to such oscillations.    A power system with proper response will bring the system back to steady state operations within a short period of time.

Steady State Stability

Steady State Stability is the ability of a power system to respond to slow or gradual changes in its operating parameters.  When a number of power sources and loads are connected to a system, there will be gradual shifting of loads from one generator to another.  These oscillations, if not properly controlled, can develop into large oscillations which can cause bigger disturbances.

Diamond based thermal compounds are used in ICs to transfer heat to the heat sink for dissipation.  Diamond has very high thermal conductivity even greater than that of silver.  This makes this ideal for heat conducting pastes.

The heat generated should be conducted and safely dissipated to the outside environment. This is done using heat sinks.  The heat sink is mounted on the IC.  The  heat conducting compound is applied between the heat sink and the IC.   

Diamond based thermal compounds are used in circuits where a large amoung of heat is generated.  24 carat micronized diamonds are added to thermal grease along with diamond particle loadings.  This makes it an extremely efficient thermal conducting material. 

Integrated Circuits developed large amounts of heat during operation.  This is particularly high in large scale integrated circuits, where thousands of components are crowded within a small area. 

The B-H Analyzer is an instrument which can measure and plot the B-H curve of a given material.  They can also be used to determine the core loss at high frequency.

BH analyzers are used to analyze the behavior of circuit components at different frequencies.  The B-H curves are plotted across a wide frequency spectrum.

BH Analyzers are also used to determine the permeability of materials used in the construction of electric machines.

Coercivity is the strength of the magnetic field required to demagnetise a ferromagnetic material.  It is also described as the ability of a material to resist demagnetization.

Materials with high coercivity are made into permanent magnets, such as Alnico.  The unit of coercivity is ampere/meter.

Materials with low coercivity are made into electromagnets, such as soft iron.

The coercivity can be calculated from the B-H curve of a material.  The horizontal distance between points b and a in the BH curve in the right is the coercivity

The unit of coercivity is Ampere/metre

Arduino is an open source platform used in embedded systems. Arduino has its own hardware and software. Since it is an open source project, it is used for numerous projects by many hundreds of people around the world.  The Layout and the production files are available in the public domain.

The Arduino board is powered by the Atmel 8-bit AVR microcontroller.  The Flash memory and other features may vary among different boards.

Programming the Arduino
The program for the Arduino can be written in any high level programming language with a compiler which can generate machine level code for Arduino.  However, Arduino has its own IDE (Integrated Development Environment).  The Arduino can be used using the IDE.  A program for the Arduino is called the sketch.

Programs can be written using the C and C++ language

Arduino has a well developed ecosystem consisting of numerous manufacturers and developers.  Many professional projects can be built with Arduino.  There are many peripherals such as sensors and actuators which can be linked to the Arduino to create a range of products from robots to security systems.

Many manufacturers and hobbyists create projects based on Arduino.

Useful Links


Software refers to the non physical parts of a computing system.  Examples are the programs which contain the instructions.  The software is written in the programming language such as VB, Java and C

Firmware is the program written on an embedded device such as a microprocessor or a microcontroller.  It controls the functioning of the microprocessor IC

It is written in the assembly level language. It is called firmware as it interfaces between the software and the hardware.

Hardware refers to the physical components of a computing system such as the processor, memory and the peripherals.

The key difference is that in a microcontroller, the memory (ROM and RAM) and the peripherals are fabricated on a single IC. A microprocessor, on the other hand, does not contain the memory and the peripherals in itself.  They are separately mounted and connected.

Microcontrollers are used for specific operations, such as to control and operate a washing machine or a traffic signal.  A microprocessor can be installed for a specific function in a larger system.  It is not designed for a single operation.

The speed of a microprocessor is above 1 GHz while the speed of the microcontroller is around 50 MHz.  

Microprocessors can handle greater complexity as compared to microcontrollers.   They also use more power than microcontrollers.  

Embedded Electronics, as the name suggests, refers to electronic hardware and software that is embedded or attached to the equipment being controlled.  

The component may be a robotic arm in an assembly line or a life support device in an ICU.  Today, Embedded Electronics can be found in all areas of life.  The washing machine and the refrigerator at home are also controlled by embedded electronics.

The advantages of embedded systems are their small size, low cost and power consumption and their rugged construction.  The program and the logic of machine operation can be easily modified.  The cost of embedded systems are lower as they are mass produced which reduces cost.  

Embedded systems can be built using both microprocessors and micro controllers.  Embedded systems can be used as standalone units or as part of a larger network controlling a bigger system.  

Programming Embedded Systems

Embedded systems can be programming using assembly level languages.  The assembly level languages are compiled into machine level using compilers.  The program is stored in the nonvolatile memory of the system.  Microprocessors and microcomputers will have their own programming languages specified by the manufacturers.  A good understanding of the C programming language will be useful in programming embedded systems.

Conductivity is an important parameter of industrial liquids.  Conductivity is measured for liquids almost all liquids.  The conductivity of the liquid gives an idea of the ions in the liquid.

The conductivity of a liquid is measured using special conductivity sensors.  The unit of conductivity is siemen/cm.  A siemen is 1/ohm.  The unit of conductance is sometimes referred to mho (ohm written in reverse).

The conductance is usually a very low value for conducting liquids such as water.  It will be of the order of a millionth of a siemen, in microsiemens.  Highly pure water, for instance, will have a conductivity of 1microsiemen/cm.

Measurement of conductivity
Conductivity is measured by measuring the conductivity of a liquid between two electrodes whose area and distance between each other is fixed.  This is known as a cell constant.

A cell constant of 1 implies that the electrodes will have a surface area of 1 cm2 and will be spaced 1 cm apart.

Magnetic flow meters are used to measure flow of liquids that are conductive.  Magnetic flow meters do not have to physically be in contact with the medium.

Magnetic Flow meters, or Magmeters as they are otherwise called work on the basis of Faraday's law which states that the voltage produced by a moving conductor in a magnetic field is proportional to the velocity of the conductor.

In a Magnetic flowmeter, the conductive liquid such as water is passed through a constant magnetic field.

As the conductive liquid flows between a magnetic field, a voltage is induced in direction perpendicular to the magnetic field.  This voltage is measured by a pair of probes.

The flowrate can be calculated from the voltage induced in these probes.

The magnetic field is produced by a pair of electromagnets whose polarity is constantly reversed. The reversal of polarity is essential to prevent interference due to electrochemical potentials induced where the probes come in contact with the liquid.

The voltage is proportional to the velocity of the liquid, the width of the pipe (diameter), and the magnetic field strength.

Latching current is the minimum current which is required to flow from the anode to the cathode to switch "ON" the SCR.

Holding current is the minimum current which needs to keep flowing to keep the SCR in the 'ON' state.

The Latching current will be greater than the Holding current for an SCR.

The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes
Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.
Anti-friction bearings are divided into two categories,
a) ball bearings
b) Roller bearings.

Ball bearings have five general types:
Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types
Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.
Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.
Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.
Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.
Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

A compound motor is a combination of shunt and series motor i.e., a series field winding, wound with heavy copper conductor on top of the shunt field winding. The series field winding is connected in series with the armature. So that its mmf will be proportional to the armature current and in the same direction as the shunt field mmf

Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.

There are two types of compound motors connection,

If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.

If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.

Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.

Torque in DC
The direction of the developed torque may be determined from an end view of the conductors and magnet poles. The direction of flux due to the known direction of current was determined by the right – hand rule and the direction of the mechanical force on each conductor, due to the interaction of the magnetic fields, was determined by the flux bunching effect.

BP = Flux density in air gap produced by shunt field poles (tesla)
IA = Armature current (ampere)
KM = Constant
Constant KM depends on the design of the motor and include the number of turns, effective length of armature conductors, number of poles, type of internal circuitry and units used.
The torque developed by a DC motor is proportional to the flux density in the air gap and the current in the armature.

Torque in AC
The torque developed by AC motors has two components: A Reluctance – torque component and a magnet – torque component. The reluctance – torque component is due to the normal characteristic of magnetic materials in a magnetic field to align themselves so that the reluctance of the magnetic circuit is minimum

The magnet – torque component is due to the magnetic attraction between the field poles (magnets) on the rotor and the corresponding opposite poles of the rotating stator flux.
It is also justified for salient – pole motors operating from 50 percent rated load to above 100 percent rated load, with power factors ranging from unity to leading, the reluctance torque for such loads is significantly smaller than the magnet torque.

Direct current (DC) generators are machines that convert mechanical energy into electrical energy. This conversion of energy is based on the principle of the production of dynamically induced electromotive force.

Types of DC Machines
Homo polar machines: These types of machines are used where low voltage and high currents are required e.g., Faraday’s disc dynamo
Hetero polar machines: The DC machines that are commonly used fall under this category.

The type of generator used in welding is homo polar machine. Differential compound generator also belongs to homo polar machines. The generator is so designed that it delivers a voltage high enough to start the arc and reduce the voltage as required to maintain the arc during the welding.
Unlike AC, DC flows continuously in one direction from a negative charge to positive charge. Although DC flows only one way, you can manipulate it to flow in the appropriate direction, which is its polarity.

DC welding is preferred when using high welding speeds and when welding is out – of – position.

Fusing Current refers at which the fuse is designed to melt and disconnect the circuit.  Wiring rules specify the fusing current for different circuits.  It is also known as minimum fusing current.  

Fusing Factor
The Fusing Factor is the ratio of the rated current and the fusing current

Fusing Current = Fusing Factor x Rated Current

The fusing current will always be more than the rated current.  Thus, the fusing factor will always be greater than one.