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It is broadly classified into

Conventional energy

is in practice for long duration of time and well established technology is available to tap and use them. e.g. Coal, oil, natural gas, hydro power, nuclear power etc.

Non-conventional energy

source can be used with advantage for power generation as well as other applications in a large number of locations and situations. These energy sources cannot be easily stored and used conveniently. E.g. Solar, wind, tidal and geothermal etc.

Based upon nature, energy sources are classified as

Renewable energy sources

are inexhaustible and are renewed by nature itself. Solar, wind, tidal, hydro and biomass are few examples.

Non-renewable energy sources

are exhaustible within a definite period of time depending upon its usage. Fossil fuels (coal, oil, gas) and nuclear fuels are few examples.

WORKING OF PLASMA CHAMBERS

The plasma chambers suspend the silicon and the phosphorus or boron molecules in a gaseous state. This mixture is pushed into a chamber which has electrodes at the top and bottom. When the electrodes are charged an electric field is produced between them. The electric field removes some of the electrons from the mixture so creating a gas of positively charged ions, a plasma. A stainless steel or glass sheet with an indium/tin oxide layer is passed into the chamber and between the electrodes. The plasma-like gas deposits charged particles on to the uncharged surface of the steel or glass substrate. Depending upon the chemical nature of the gaseous mixture fed into the chamber, one of the three layers which make up the photovoltaic cell will be formed. The thickness of the layers depends on the speed at which the sub state passes through the chamber, and the size of the chamber.

BREAKDOWN OF SOLID INSULATING MATERIALS

In solid dielectrics, highly purified and free of imperfections, the breakdown strength is high, of the order of 10 MV/cm.

The highest breakdown strength obtained under carefully controlled conditions is known as the "intrinsic strength" of the dielectric. Dielectrics usually fail at stresses well below the intrinsic strength due usually to one of the following causes.

(a) Electro-mechanical breakdown.
(b) Breakdown due to internal discharges.
(c) Surface breakdown (tracking and erosion).
(d) Thermal breakdown.
(e) Electro chemical breakdown.
(f) Chemical deterioration.

BREAKDOWN OF COMPOSITE INSULATION

Almost no complete electrical insulation consists of one insulating phase. Usually more than one insulating material will be involved, either in series, parallel or both.

The simplest form of composite insulation system consists of 2 layers of the same material. In this case advantage is taken of the fact that two thin sheets have a higher electric strength than a single sheet of the same total thickness.

In other cases, composite dielectrics occur either due to design considerations (ex: paper with an impregnating liquid) or due to practical difficulties of fabrication (ex: air in parallel with solid insulation).

In certain cases, the behavior of the composite insulation could be predicted from the behavior of the components.

But in most cases, the system as whole has to be considered. The following considerations determine the performance of the system as a whole.

(i) The stress distribution at different parts of the insulation system is distorted due to the component dielectric constants and conductivities,

(ii) The breakdown characteristics at the surface are affected by the insulation boundaries of various components,

(iii) The internal or partial discharge products of one component invariably affect the other components in the system, and

(iv) The chemical ageing products of one component also affect the performance of other components in the system.

ELECTRO CHEMICAL BREAKDOWN

Since no insulant is completely free of ions, a leakage current will flow when an electric field is applied. The ions may arise from dissociation of impurities or from slight ionisations of the insulating material itself. When these ions reach the electrodes, reactions occur in accordance with Faraday's law of electrolysis, but on a much smaller scale. The insulation and the electrode metal may be attacked, gas may be evolved or substance may be deposited on the electrodes. The products of the electrode reaction may be chemically or electrically harmful and in some cases can lead to rapid failure of the insulation. The reactions are much slower than in normal electrolytic processes due to the much smaller currents. The products of the reactions may be electrically and chemically harmful because the insulation and electrodes may be attacked, and because harmful gases may be evolved.

Typically a 1 F paper capacitor operating at 1 kV at room temperature would require 2 to 3 years to generate 1 cm3 hydrogen. At elevated temperatures, the products of electrolysis would be formed much more rapidly. Also since impurities give rise to an increase in the ion concentration, care must be taken to prevent contamination during manufacture.

The rate of electrolysis is much greater with direct stress than with alternating stress. This is due to the fact that the reactions may be wholly or partially reversed when the polarity changes and the extent of reaction depends on the reaction rate and the time for diffusion of the reaction products away from the electrodes as well as on the nature of the reaction products. However at power frequency, electrochemical effects can be serious and are often responsible for long-term failure of insulation. The most frequent source of ions is ionizable impurities in the insulation. Thus contamination of insulation during manufacture and during assembly into equipment must be avoided with great care.

Also, contamination in polar insulating materials should be avoided with still greater care because of the greater degree of dissociation of ionic substance in solution.

The long term lives of capacitors containing chlorinated impregnants under direct stress may be greatly extended by adding small quantities of certain stabilizers, which are hydrogen acceptors and act as depolarizers at the cathode.

Hydrogen ions discharged at the cathode readily react with the stabilizer rather than with the impregnant, a more difficult chemical process. In the absence of the stabilizer, the hydrogen reacts with the chlorine of the impregnant to produce hydrochloric acid, and rapid deterioration occurs due to attack of the acid on the electrodes and cellulose. The extension of the life caused by the stabilizers is proportional to the amount of stabilizer added. For example, with 2% of the stabilizer Azobenzene, mean life may be extended 50 times.

SURFACE BREAKDOWN

Surface flashover

Surface flashover is a breakdown of the medium in which the solid is immersed. The role of the solid dielectric is only to distort the field so that the electric strength of the gas is exceeded.

If a piece of solid insulation is inserted in a gas so that the solid surface is perpendicular to the equipotentials at all points, then the voltage gradient is not affected by the solid insulation. An example of this is a cylindrical insulator placed in the direction of a uniform field. Field intensification results if solid insulation departs even in detail from the cylindrical shape. In particular if the edges are chipped, or if the ends of the cylinder are not quite perpendicular to the axis, then an air gap exists next to the electrode, and the stress can reach up to 0r times the mean stress in the gap. Discharge may therefore occur at a voltage approaching 1/0r times the breakdown voltage in the absence of the cylinder, and these discharges can precipitate a breakdown.

The three essential components of the surface flashover phenomena are

(a) The presence of a conducting film across the surface of the insulation

(b) A mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks,

(c) Degradation of the insulation must be caused by the sparks.

The conducting film is usually moisture from the atmosphere absorbed by some form of contamination. Moisture is not essential as a conducting path can also arise from metal dust due to wear and tear of moving parts. Sparks are drawn between moisture films, separated by drying of the surface due to heating effect of leakage current, which act as extensions to the electrodes. {For a discharge to occur there must be a voltage at least equal to the Paschen minimum for the particular state of the gas. For example, Paschen minimum in air at N.T.P it is 380 V, whereas tracking can occur at well below 100 V. It does not depend on gaseous breakdown.] Degradation of the insulation is almost exclusively the result of heat from the sparks, and this heat either carbonizes if tracking is to occur, or volatilizes if erosion is to occur. Carbonization results in a permanent extension of the electrodes and usually takes the form of a dendrite growth. Increase of creep age path during design will prevent tracking, but in most practical cases, moisture films can eliminate the designed creep age path.

Tracking

Tracking is the formation of a permanent conducting path across a surface of the insulation, and in most cases the conduction (carbon path) results from degradation of the insulation itself leading to a bridge between the electrodes.

The insulating material must be organic in nature for tracking to occur.

Erosion

In a surface discharge, if the products of decomposition are volatile and there is no residual conducting carbon on the surface, the process is simply one of pitting. This is erosion, which again occurs in organic materials.

If surface discharges are likely to occur, it is preferable to use materials with erosion properties rather than tracking properties, as tracking makes insulation immediately completely ineffective, whereas erosion only weakens the material but allows operation until replacement can be made later.

PURIFICATION OF A LIQUID FOR TESTING

(a) Removal of dust

Small dust particles can become charged and cause local stresses which can initiate breakdown. They can also coalesce to form conducting bridges between electrodes. Careful filtration can remove dust particles greater in size than 1 m. The strength of the liquid then increases and greater stability is achieved.

(b) Removal of dissolved gasses

Liquid insulation will normally contain dissolved gas in small but significant amounts. Some gases such as

Nitrogen and Hydrogen do not appear to upset the electrical properties to a great extent, but oxygen and carbon dioxide can cause the strength to change significantly. Thus it necessary to control the amount of gases present. This is done by distillation and degassing.

(c) Removal of ionic impurities

Ionic impurities in the liquid (particularly residual water which easily dissociates) leads to abnormal conductivity and heating of the liquid. Water can be removed by drying agents, vacuum drying, and by freezing out in low temperature distillation.

For measurements on liquid dielectrics, where test cells are small, electrode preparation is much more critical than it is for measurements on gases or solids. Not only are the surface smoothness important, but surface films, particularly oxides can have a marked influence on the strength.

BREAKDOWN OF COMMERCIAL LIQUIDS

When a difference of potential is applied to a pair of electrodes immersed in an insulating liquid, a small conduction current is first observed. If the voltage is raised continuously, at a critical voltage a spark passes between the electrodes.

The passage of a spark through a liquid involves the following.

(a) Flow of a relatively large quantity of electricity, determined by the characteristics of the circuit,

(b) A bright luminous path from electrode to electrode,

(c) The evolution of bubbles of gas and the formation of solid products of decomposition (if the liquid is of requisite chemical nature)

(d) Formation of small pits on the electrodes,

(e) An impulsive pressure through the liquid with an accompanying explosive sound.

Tests on highly purified transformer oil show that

(a) Breakdown strength has a small but definite dependence on electrode material,

(b) Breakdown strength decreases with increase in electrode spacing,

(c) Breakdown strength is independent of hydrostatic pressure for degassed oil, but increases with pressure if oil contains gases like nitrogen or oxygen in solution.

In the case of commercial insulating liquid, which may not be subjected to very elaborate purifying treatment, the breakdown strength will depend more upon the nature of impurities it contains than upon the nature of the liquid itself.

These impurities which lead to the breakdown of commercial liquids below their intrinsic strength can be divided into the following 3 categories.

(a) Impurities which have breakdown strength lower than that of the liquid itself (ex: bubbles of gas).

Breakdown of the impurities may trigger off the total breakdown of the liquid.

(b) Impurities which are unstable in the electric field (ex: globules of water). Instability of the impurity can result in a low resistance bridge across the electrodes and in total breakdown.

(c) Impurities which result in local enhancement of electric field in a liquid (ex: conducting particles). The enhanced field may cause local breakdown and therefore initiate complete breakdown.

TESTING VOLTAGES

Power systems equipment must withstand not only the rated voltage (Vm), which corresponds to the highest voltage of a particular system, but also over voltages. Accordingly, it is necessary to test high voltages. Equipment during its development stage and prior to commissioning. The magnitude and type of test voltage varies with the rated voltage of a particular apparatus. The standard methods of measurement of high-voltage and the basic techniques for application to all types of apparatus for alternating voltages, direct voltages, switching impulse voltages and lightning impulse voltages are laid down in the relevant national and international standards.

Testing with power frequency voltages

To assess the ability of the apparatus’s insulation withstand under the system’s power frequency voltage the apparatus is subjected to the 1-minute test under 50 Hz or 60 Hz depending upon the country. The test voltage is set at a level higher than the expected working voltage in order to be able to simulate the stresses likely to be encountered over the years of service. For indoor installations the equipment tests are carried out under dry conditions only. For outdoor equipment tests may be required under conditions of standard rain as prescribed in the appropriate standards.

Testing with lightning impulse voltages

Lightning strokes terminating on transmission lines will induce steep rising voltages in the line and set up travelling waves along the line and may damage the system’s insulation. The magnitude of these over voltages may reach several thousand kilo volts, depending upon the insulation. Exhaustive measurements and long experience have shown that lightning over voltages are characterized by short front duration, ranging from a fraction of a microsecond to several tens of microseconds and then slowly decreasing to zero. The standard impulse voltage has been accepted as a periodic impulse that reaches its peak value in 1.2 /sec and then decreases slowly (in about 50 /sec) to half its peak value. In addition to testing equipment, impulse voltages are extensively used in research laboratories in the fundamental studies of electrical discharge mechanisms, notably when the time to breakdown is of interest.

Testing with switching impulses

Transient over voltages accompanying sudden changes in the state of power systems, e.g. switching operations or faults, are known as switching impulse voltages. It has become generally recognized that switching impulse voltages are usually the dominant factor affecting the design of insulation in high voltage. power systems for rated voltages of about 300 kV and above. Accordingly, the various international standards recommend that equipment designed for voltages above 300 kV be tested for switching impulses. Although the wave shape of switching over voltages occurring in the system may vary widely, experience has shown that for flashover distances in atmospheric air of practical interest the lowest withstand values are obtained with surges with front times between 100 and 300 /sec. Hence, the recommended switching surge voltage has been designated to have a front time of about 250 /sec and half value time of 2500 /sec. For GIS (gas-insulated switchgear) on-site testing, oscillating switching impulse voltages are recommended for obtaining higher efficiency of the impulse voltage generator.

D.C. voltages

In the past dc voltages have been chiefly used for purely scientific research work. Industrial applications were mainly limited to testing cables with relatively large capacitance, which take a very large current when tested with ac voltages, and in testing insulations in which internal discharges may lead to degradation of the insulation under testing conditions. In recent years, with the rapidly growing interest in HVDC transmission, an increasing number of industrial laboratories are being equipped with sources for producing dc high voltages. Because of the diversity in the application of dc high voltages, ranging from basic physics experiments to industrial applications, the requirements on the output voltage will vary accordingly.

Testing with very low-frequency voltage

In the earlier years when electric power distribution systems used mainly paper-insulated lead covered cables (PILC) on-site testing specifications called for tests under dc voltages. Typically the tests were carried out at 4–4.5V0.

The tests helped to isolate defective cables without further damaging good cable insulation. With the widespread use of extruded insulation cables of higher dielectric strength, the test voltage levels were increased to 5–8V0. In the 1970s premature failures of extruded dielectric cables factory tested under dc voltage at specified levels were noted. Hence on-site testing of cables under very low frequency (VLF) of approximately 0.1Hz has been adopted.

VENTILATION OR COOLING OF AN ALTERNATOR

• The slow speed salient pole alternators are ventilated by the fan action of the salient poles which provide circulating air.

• Cylindrical rotor alternators are usually long, and the problem of air flow requires very special attention.

• The cooling medium, air or hydrogen is cooled by passing over pipes through which cooling water is circulated and ventilation of the alternator.

• Hydrogen is normally used as cooling medium in all the turbine-driven alternators because hydrogen provides better cooling than air and increases the efficiency and decreases the windage losses.

• Liquid cooling is used for the stators of cylindrical rotor generators.

SYNCHRONOUS GENERATORS

Synchronous machines are principally used as alternating current (AC) generators. They supply the electric power used by all sectors of modern societies: industrial, commercial, agricultural, and domestic.

Synchronous generators usually operate together (or in parallel), forming a large power system supplying electrical energy to the loads or consumers.

Synchronous generators are built in large units, their rating ranging from tens to hundreds of megawatts.

Synchronous generator converts mechanical power to ac electric power. The source of mechanical power, the prime mover, may be a diesel engine, a steam turbine, a water turbine, or any similar device.

For high-speed machines, the prime movers are usually steam turbines employing fossil or nuclear energy resources.

Low-speed machines are often driven by hydro-turbines that employ water power for generation.

Smaller synchronous machines are sometimes used for private generation and as standby units, with diesel engines or gas turbines as prime movers.

CRITICAL RESISTANCE FOR A SERIES GENERATOR

Fig. (3.6) shows the voltage build up in a series generator. Here R1, R2 etc. represent the total circuit resistance (load resistance and field winding resistance). If the total circuit resistance is R1, then series generator will build up a voltage OL. The line OC is tangent to O.C.C. and represents the critical resistance RC for a series generator. If the total resistance of the circuit is more than RC (say line OD), the generator will fail to build up voltage. Note that Fig. (3.6) is similar to Fig. (3.5) with the following differences:

(I) In Fig. (3.5), R1, R2 etc. represent the total field circuit resistance. However, R1, R2 etc. in Fig. (3.6) represent the total circuit resistance (load resistance and series field winding resistance etc.).

(II) In Fig (3.5), field current alone is represented along X-axis. However, in Fig. (3.6) load current IL is represented along Y-axis. Note that in a series generator, field current = load current IL.

CRITICAL FIELD RESISTANCE FOR A SHUNT GENERATOR

Voltage build up in a shunt generator depends upon field circuit resistance. If the field circuit resistance is R1 (line OA), then generator will build up a voltage OM as shown in Fig. (3.5). If the field circuit resistance is increased to R2 (tine OB), the generator will build up a voltage OL, slightly less than OM. As the field circuit resistance is increased, the slope of resistance line also increases. When the field resistance line becomes tangent (line OC) to O.C.C., the generator would just excite. If the field circuit resistance is increased beyond this point (say line OD), the generator will fail to excite. The field circuit resistance represented by line OC (tangent to O.C.C.) is called critical field resistance RC for the shunt generator. It may be defined as under:

The maximum field circuit resistance (for a given speed) with which the shunt generator would just excite is known as its critical field resistance.

It should be noted that shunt generator will build up voltage only if field circuit resistance is less than critical field resistance.

DC GENERATOR CHARACTERISTICS

The following are the three most important characteristics of a dc generator:

1. Open Circuit Characteristic (O.C.C.)

This curve shows the relation between the generated emf at no-load (E0) and the field current (If) at constant speed. It is also known as magnetic characteristic or no-load saturation curve. Its shape is practically the same for all generators whether separately or self-excited. The data for O.C.C. curve are obtained experimentally by operating the generator at no load and constant speed and recording the change in terminal voltage as the field current is varied.

2. Internal or Total characteristic (E/Ia)

This curve shows the relation between the generated emf on load (E) and the armature current (Ia). The emf E is less than E0 due to the demagnetizing effect of armature reaction. Therefore, this curve will lie below the open circuit characteristic (O.C.C.). The internal characteristic is of interest chiefly to the designer. It cannot be obtained directly by experiment. It is because a voltmeter cannot read the emf generated on load due to the voltage drop in armature resistance. The internal characteristic can be obtained from external characteristic if winding resistances are known because armature reaction effect is included in both characteristics.

3. External characteristic (V/IL)

This curve shows the relation between the terminal voltage (V) and load current (IL). The terminal voltage V will be less than E due to voltage drop in the armature circuit. Therefore, this curve will lie below the internal characteristic.

This characteristic is very important in determining the suitability of a generator for a given purpose. It can be obtained by making simultaneous measurements of terminal voltage and load current (with voltmeter and ammeter) of a loaded generator.

PARALLEL OPERATION OF DC MOTORS

As in the case of generators motors may also be required to operate in parallel driving a common load. The benefits as well as the problems in both the cases are similar. As the two machines are coupled to a common load the speed of the load is the common parameter in the torque speed plane. The torque shared by each machine depends on the intersection of the torque speed curves. If the torque speed lines are drooping the point of intersection remains reasonably unaltered for small changes in the characteristics due to temperature and excitation effects. However if these curves are flat then great changes occur in torque shared by each machine. The machine with flatter curve shares a larger portion of the torque demand. Thus parallel operation of two shunt motors is considerably more difficult compared to the operation of the same machines as generators. The operation of level compounded generators is much more difficult compared to the same machines working as cumulative compounded motor. On a similar count parallel operation of cumulative compounded motors is easier than shunt motors. Series motors are, with their highly falling speed with the load torque, are ideal as far as the parallel operation is considered. Considerable differences in their characteristics still do not affect adversely their parallel operation.

One application where several series motors operate in parallel is in electric locomotives. Due to the uneven wear and tear of the wheels of the locomotive the speeds of the rotation of these motors can be different to have the same common linear velocity of the locomotive. The torque developed by each machine remains close to the other and there is no tendency for derailment. The torque speed curves for parallel operation of series motors are given in Fig.

SERIES OPERATION OF DC MOTORS

In the case of series operation the motors shafts of the two machines are connected to the same load and also the two armatures are series connected. This forces a common armature current through both the machines and the torques developed by the machines are proportional to the flux in each machine. Series operation of series motors is adopted during starting to improve the energy efficiency. This method is ideally suited for shunt and compound machines with nearly flat torque speed characteristics. Such machines can go through high amount of dynamics without the fear of becoming unstable. This configuration is used in steel mills. Having two smaller machines connected to the shaft is preferred over there in place of one large machine as the moment of inertia of the motors is much reduced, thus improving the dynamics.

THE FIRST LAW OF ELECTROSTATICS

By Engr. Aneel Kumar January 16, 2014 No comments:

The negative charge of the electron is equal, but opposite to, the positive charge of the proton. These charges are referred to as electrostatic charges. In nature, unlike charges (like electrons and protons) attract each other, and like charges repel each other. These facts are known as the First Law of Electrostatics and are sometimes referred to as the law of electrical charges. This law should be remembered because it is one of the vital concepts in electricity.

Some atoms can lose electrons and others can gain electrons; thus, it is possible to transfer electrons from one object to another. When this occurs, the equal distribution of negative and positive charges no longer exists. One object will contain an excess of electrons and become negatively charged, and the other will become deficient in electrons and become positively charged. These objects, which can contain billions of atoms, will then follow the same law of electrostatics as the electron and proton example shown above. The electrons that can move around within an object are said to be free electrons and will be discussed in more detail in a later section. The greater the number of these free electrons an object contains, the greater its negative electric charge. Thus, the electric charge can be used as a measure of electrons.

THERMIONIC EMISSION

A thermionic energy converter is a device consisting of two electrodes placed near one another in a vacuum. One electrode is normally called the cathode, or emitter, and the other is called the anode, or plate. Ordinarily, electrons in the cathode are prevented from escaping from the surface by a potential-energy barrier. When an electron starts to move away from the surface, it induces a corresponding positive charge in the material, which tends to pull it back into the surface. To escape, the electron must somehow acquire enough energy to overcome this energy barrier. At ordinary temperatures, almost none of the electrons can acquire enough energy to escape. However, when the cathode is very hot, the electron energies are greatly increased by thermal motion. At sufficiently high temperatures, a considerable number of electrons are able to escape. The liberation of electrons from a hot surface is called thermionic emission.

The electrons that have escaped from the hot cathode form a cloud of negative charges near it called a space charge. If the plate is maintained positive with respect to the cathode by a battery, the electrons in the cloud are attracted to it. As long as the potential difference between the electrodes is maintained, there will be a steady current flow from the cathode to the plate.

The simplest example of a thermionic device is a vacuum tube diode in which the only electrodes are the cathode and plate, or anode, as shown in Figure 17. The diode can be used to convert alternating current (AC) flow to a pulsating direct current (DC) flow.

PHOTOELECTRIC EFFECT

Light is a form of energy and is considered by many scientists to consist of small particles of energy called photons. When the photons in a light beam strike the surface of a material, they release their energy and transfer it to the atomic electrons of the material. This energy transfer may dislodge electrons from their orbits around the surface of the substance. Upon losing electrons, the photosensitive (light sensitive) material becomes positively charged and an electric force is created, as shown in Figure 16.

This phenomenon is called the photoelectric effect and has wide applications in electronics, such as photoelectric cells, photovoltaic cells, optical couplers, and television camera tubes. Three uses of the photoelectric effect are described below.

Photovoltaic: The light energy in one of two plates that are joined together causes one plate to release electrons to the other. The plates build up opposite charges, like a battery (Figure 16).

Photoemission: The photon energy from a beam of light could cause a surface to release electrons in a vacuum tube. A plate would then collect the electrons.

Photoconduction: The light energy applied to some materials that are normally poor conductors causes free electrons to be produced in the materials so that they
become better conductors.

THERMOELECTRICITY

Some materials readily give up their electrons and others readily accept electrons. For example, when two dissimilar metals like copper and zinc are joined together, a transfer of electrons can take place. Electrons will leave the copper atoms and enter the zinc atoms. The zinc gets a surplus of electrons and becomes negatively charged. The copper loses electrons and takes on a positive charge. This creates a voltage potential across the junction of the two metals. The heat energy of normal room temperature is enough to make them release and gain electrons, causing a measurable voltage potential. As more heat energy is applied to the junction, more electrons are released, and the voltage potential becomes greater, as shown in Figure 15. When heat is removed and the junction cools, the charges will dissipate and the voltage potential will decrease. This process is called thermoelectricity. A device like this is generally referred to as a "thermocouple."

The thermoelectric voltage in a thermocouple is dependent upon the heat energy applied to the junction of the two dissimilar metals. Thermocouples are widely used to measure temperature and as heat-sensing devices in automatic temperature controlled equipment.

Thermocouple power capacities are very small compared to some other sources, but are somewhat greater than those of crystals.

Generally speaking, a thermocouple can be subjected to higher temperatures than ordinary mercury or alcohol thermometers.

PIEZOELECTRIC EFFECT

By applying pressure to certain crystals (such as quartz or Rochelle salts) or certain ceramics (like barium titanate), electrons can be driven out of orbit in the direction of the force. Electrons leave one side of the material and accumulate on the other side, building up positive and negative charges on opposite sides, as shown in Figure 14. When the pressure is released, the electrons return to their orbits. Some materials will react to bending pressure, while others will respond to twisting pressure. This generation of voltage is known as the piezoelectric effect. If external wires are connected while pressure and voltage are present, electrons will flow and current will be produced. If the pressure is held constant, the current will flow until the potential difference is equalized.

When the force is removed, the material is decompressed and immediately causes an electric force in the opposite direction. The power capacity of these materials is extremely small.

However, these materials are very useful because of their extreme sensitivity to changes of mechanical force.

Example: One example is the crystal phonograph cartridge that contains a Rochelle salt crystal. A phonograph needle is attached to the crystal. As the needle moves in the grooves of a record, it swings from side to side, applying compression and decompression to the crystal. This mechanical motion applied to the crystal generates a voltage signal that is used to reproduce sound.

MAGNETIC INDUCTION

A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. Magnetic induction is used to produce a voltage by rotating coils of wire through a stationary magnetic field, as shown in Figure 13, or by rotating a magnetic field through stationary coils of wire. This is one of the most useful and widely employed applications of producing vast quantities of electric power. Magnetic induction will be studied in more detail in the next two chapters "Magnetism," and "Magnetic Circuits."

STATIC ELECTRICITY

Atoms with the proper number of electrons in orbit around them are in a neutral state, or have a "zero charge." A body of matter consisting of these atoms will neither attract nor repel other matter that is in its vicinity. If electrons are removed from the atoms in this body of matter, as happens due to friction when one rubs a glass rod with a silk cloth, it will become electrically positive as shown in Figure 12. If this body of matter (e.g., glass rod) comes near, but not in contact with, another body having a normal charge, an electric force is exerted between them because of their unequal charges. The existence of this force is referred to as static electricity or electrostatic force.

Example: Have you ever walked across a carpet and received a shock when you touched a metal door knob? Your shoe soles built up a charge by rubbing on the carpet, and this charge was transferred to your body. Your body became positively charged and, when you touched the zero-charged door knob, electrons were transferred to your body until both you and the door knob had equal charges.

ELECTROCHEMISTRY

Chemicals can be combined with certain metals to cause a chemical reaction that will transfer electrons to produce electrical energy. This process works on the electrochemistry principle.

One example of this principle is the voltaic chemical cell, shown in Figure 11. A chemical reaction produces and maintains opposite charges on two dissimilar metals that serve as the positive and negative terminals. The metals are in contact with an electrolyte solution.

Connecting together more than one of these cells will produce a battery.

Example: A battery can maintain a potential difference between its positive and negative terminals by chemical action. Various types of cells and batteries will be studied in more detail in Module 4, Batteries.

METHODS OF PRODUCING ELECTRICITY

Following are few methods of producing electricity.

Electro chemistry
Static (friction)
Induction (magnetism)
Piezoelectric (pressure)
Thermal (heat)
Light
Thermionic emission

ELECTRIC CURRENT

The density of the atoms in copper wire is such that the valence orbits of the individual atoms overlap, causing the electrons to move easily from one atom to the next. Free electrons can drift from one orbit to another in a random direction. When a potential difference is applied, the direction of their movement is controlled. The strength of the potential difference applied at each end of the wire determines how many electrons change from a random motion to a more directional path through the wire. The movement or flow of these electrons is called electron current flow or just current.

To produce current, the electrons must be moved by a potential difference. The symbol for current is (I). The basic measurement for current is the ampere (A). One ampere of current is defined as the movement of one coulomb of charge past any given point of a conductor during one second of time.

If a copper wire is placed between two charged objects that have a potential difference, all of the negatively-charged free electrons will feel a force pushing them from the negative charge to the positive charge. This force opposite to the conventional direction of the electrostatic lines of force is shown in Figure 9.

The direction of electron flow, shown in Figure 10, is from the negative (-) side of the battery, through the wire, and back to the positive (+) side of the battery. The direction of electron flow is from a point of negative potential to a point of positive potential. The solid arrow shown in

Figure 10 indicates the direction of electron flow. As electrons vacate their atoms during electron current flow, positively charged atoms (holes) result. The flow of electrons in one direction causes a flow of positive charges. The direction of the positive charges is in the opposite direction of the electron flow. This flow of positive charges is known as conventional current and is shown in Figure 10 as a dashed arrow. All of the electrical effects of electron flow from negative to positive, or from a higher potential to a lower potential, are the same as those that would be created by a flow of positive charges in the opposite direction. Therefore, it is important to realize that both conventions are in use and that they are essentially equivalent; that is, all effects predicted are the same. In this text, we will be using electron flow in our discussions.

Generally, electric current flow can be classified as one of two general types: Direct Current (DC) or Alternating Current (AC). A direct current flows continuously in the same direction.

An alternating current periodically reverses direction. We will be studying DC and AC current= in more detail later in this text. An example of DC current is that current obtained from a battery. An example of AC current is common household current.

VOLTAGE

The basic unit of measure for potential difference is the volt (symbol V), and, because the volt unit is used, potential difference is called voltage. An object’s electrical charge is determined by the number of electrons that the object has gained or lost. Because such a large number of electrons move, a unit called the "coulomb" is used to indicate the charge. One coulomb is equal to 6.28 x 1018 (billion, billion) electrons. For example, if an object gains one coulomb of negative charge, it has gained 6,280,000,000,000,000,000 extra electrons.

A volt is defined as a difference of potential causing one coulomb of current to do one joule of work.

A volt is also defined as that amount of force required to force one ampere of current through one ohm of resistance.

The latter is the definition with which we will be most concerned in this module.

BOOLEAN ALGEBRA

For a basic introduction to sets, Boolean operations, Venn diagrams, truth tables, and Boolean applications, see Boolean logic.

For an alternative perspective see Boolean algebras canonically defined.

In abstract algebra, a Boolean algebra is an algebraic structure (a collection of elements and operations on them obeying defining axioms) that captures essential properties of both set operations and logic operations. Specifically, it deals with the set operations of intersection, union, complement; and the logic operations of AND, OR, NOT.

For example, the logical assertion that a statement a and its negation ¬a cannot both be true.

POWER ENGINEERING

Power engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors and power electronics.

In many regions of the world, governments maintain an electrical network that connects a variety electric generators together with users of their power. This network is called a power grid. Users purchase electricity from the grid avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both.

Power engineers may also work on systems that do not connect to the grid. These systems are called off-grid power systems and may be used in preference to on-grid systems for a variety of reasons. For example, in remote locations it may be cheaper for a mine to generate its own power rather than pay for connection to the grid and in most mobile applications connection to the grid is simply not practical.

Today, most grids adopt three-phase electric power with an alternating current. This choice can be partly attributed to the ease with which this type of power can be generated, transformed and used. Often (especially in the USA), the power is split before it reaches residential customers whose low-power appliances rely upon single-phase electric power. However, many larger industries and organizations still prefer to receive the three-phase power directly because it can be used to drive highly efficient electric motors such as three-phase induction motors.

Transformers play an important role in power transmission because they allow power to be converted to and from higher voltages. This is important because higher voltages suffer less power loss during transmission. This is because higher voltages allow for lower current to deliver the same amount of power as power is the product of the two.

Thus, as the voltage steps up, the current steps down. It is the current flowing through the components that result in both the losses and the subsequent heating. These losses, appearing in the form of heat, are equal to the current squared times the electrical resistance through which the current flows.

For these reasons, electrical substations exist throughout power grids to convert power to higher voltages before transmission and to lower voltages suitable for appliances after transmission.

Components

Power engineering is usually broken into three parts:

Generation

Generation is converting other forms of power into electrical power. The sources of power include fossil fuels such as coal and natural gas, hydropower, nuclear power, solarpower, wind power and other forms.

Transmission

Transmission includes moving power over somewhat long distances, from a power station to near where it is used. Transmission involves high voltages, almost always higher than voltage at which the power is either generated or used. Transmission also includes connecting together power systems owned by various companies and perhaps in different states or countries. Transimission includes long meduim and short lines.

Distribution

Distribution involves taking power from the transmission system to end users, converting it to voltages at which it is ultimately required.

COMPUTER NETWORKS AND THE INTERNET

On September 11, 1940 George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at

Dartmouth College in New Hampshire. This configuration of a centralized computer or mainframe with remote dumb terminals remained popular throughout the 1950s.

However it was not until the 1960s that researchers started to investigate packet switching — a technology that would allow chunks of data to be sent to different computers without first passing through a centralized mainframe. A four-node network emerged on December 5, 1969; this network would become ARPANET, which by 1981 would consist of 213 nodes.

ARPANET’s development centered on the Request for Comment process and on April 7, 1969, RFC 1 was published. This process is important because ARPANET would eventually merge with other networks to form the Internet and many of the protocols the Internet relies upon today were specified through this process. In September 1981, RFC 791 introduced the Internet Protocol v4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) — thus creating the TCP/IP protocol that much of the Internet relies upon today.

However not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the Token Ring protocol was filed by Olof Soderblom on October 29, 1974. And a paper on the Ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM. These protocols are discussed in more detail in the next section.

RADIO AND TELEVISION

In 1832, James Lindsay gave a classroom demonstration of wireless telegraphy to his students. By 1854 he was able to demonstrate a transmission across the Firth of Tay from Dundee to Woodhaven, a distance of two miles, using water as the transmission medium. In December 1901, Guglielmo Marconi established wireless communication between Britain and the United States earning him the Nobel Prize in physics in 1909 (which he shared with Karl Braun).

On March 25, 1925, John Logie Baird was able to demonstrate the transmission of moving pictures at the London department store Selfridges. Baird’s device relied upon the Nipkow disk and thus became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning September 30, 1929. However for most of the twentieth century televisions depended upon the cathode ray tube invented by Karl Braun. The first version of such a television to show promise was produced by Philo Farnsworth and demonstrated to his family on September 7, 1927.

TELEGRAPH AND TELEPHONE

The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke and opened on 9 April 1839. Both Wheatstone and Cooke viewed their device as “an improvement to the [existing] electromagnetic telegraph” not as a new device.

On the other side of the Atlantic Ocean, Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September

1837. Soon after he was joined by Alfred Vail who developed the register — a telegraph terminal that integrated a logging device for recording messages to paper tape. This was demonstrated successfully on 6 January 1838. The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telecommunication for the first time.

The conventional telephone was invented by Alexander Bell in 1876. Although in 1849

Antonio Meucci invented a device that allowed the electrical transmission of voice over a line. Meucci’s device depended upon the electrophonic effect and was of little practical value because it required users to place the receiver in their mouth to “hear” what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London.

TELECOMMUNICATION

Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process almost always involves the sending of electromagnetic waves by electronic transmitters but in earlier years it may have involved the use of smoke signals, drums or semaphore. Today, telecommunication is widespread and devices that assist the process such as the television, radio and telephone are common in many parts of the world. There is also a vast array of networks that connect these devices, including computer networks, public telephone networks, radio networks and television networks. Computer communication across the Internet, such as e-mail and instant messaging, is just one of many examples of telecommunication.

Telecommunication systems are generally designed by telecommunication engineers.

Major contributors to the field of telecommunications include Alexander Bell who invented the telephone (as we know it), John Logie Baird who invented the mechanical television and Guglielmo Marconi who first demonstrated transatlantic radio communication. In recent times, optical fiber has radically improved the bandwidth available for inter congenital communication helping to facilitate a faster and richer.

Internet experience and digital television has eliminated effects such as snowy pictures and ghosting. Telecommunication remains an important part of the world economy and the telecommunication industry’s revenue has been placed at just under 3% of the gross world product.

The basic elements of a telecommunication system are:

• A transmitter that takes information and converts it to a signal for transmission

• A transmission medium over which the signal is transmitted

• A receiver that receives and converts the signal back into usable information

For example, consider a radio broadcast. In this case the broadcast tower is the transmitter, the radio is the receiver and the transmission medium is free space. Often telecommunication systems are two-way and devices act as both a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a phone line is called point-to-point communication because it is between one transmitter and one receiver, telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.

Signals can either be analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (e.g. 1’s and 0’s).

A collection of transmitters, receivers or transceivers that communicate with each other is known as a network. Digital networks may consist of one or more routers that route data to the correct user. An analogue network may consist of one or more switches that establish a connection between two or more users. For both types of network, a repeater may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.

A channel is a division in a transmission medium so that it can be used to send multiple independent streams of data. For example, a radio station may broadcast at 96 MHz while another radio station may broadcast at 94.5 MHz In this case the medium has been divided by frequency and each channel received a separate frequency to broadcast on.

Alternatively one could allocate each channel a recurring segment of time over which to broadcast.

The shaping of a signal to convey information is known as modulation. Modulation is a key concept in telecommunications and is frequently used to impose the information of one signal on another. Modulation is used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist these include phase shift keying, amplitude-shift keying and minimum-shift keying. Bluetooth, for example, uses phase shift keying for exchanges between devices.

However, more relevant to earlier discussion, modulation is also used to boost the frequency of analogue signals. This is because a raw signal is often not suitable for transmission over long distances of free space due to its low frequencies. Hence its information must be superimposed on a higher frequency signal (known as a carrier wave) before transmission. There are several different modulation schemes available to achieve this some of the most basic being amplitude modulation and frequency modulation. An example of this process is a DJ’s voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel “96 FM”).

ELECTRIC POWER TRANSMISSION

By Engr. Aneel Kumar January 03, 2014 No comments:

Electric energy is produced at electric power generating stations and transported over high-voltage transmission lines to utilization points. The trend toward using higher voltages is motivated by the increased line capacity while reducing line losses per unit of power transmitted. The reduction in losses is significant and is an important aspect of energy conservation. Better use of land is a benefit of the larger transmission capacity of the lines.

KIRCHHOFF’S CURRENT LAW

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Circuit analysis refers to characterizing the current flowing through and voltage across every circuit element within a given circuit. Some general rules apply when analyzing any circuit with any number of elements. However, before discussing this rules, we need to define other terms that are commonly used in circuit analysis literature: a node and a branch.

A node is the connecting point of two (or more) elements of a circuit.

A branch represents a circuit element that is located between any two nodes in a circuit.

Russian scientist Gustav Robert Kirchhoff (1824–1887) introduced the two laws that now bear his name. These laws allow the calculation of currents and voltages in electric circuits with multiple loops using simple algebraic equations. Kirchhoff’s current law (KCL) states that the net current entering a node in a circuit is zero.

Some currents enter into a node and some leave the node. Thus, based on this law, the sum of the currents entering a node is equal to the sum of the currents leaving that node. KCL results from the law of the conservation of charge.

ELECTRICAL ENGINEERING AND A SUCCESSFUL CAREER

By Engr. Aneel Kumar January 03, 2014 No comments:

As a practicing engineer, you will work on projects that require a wide range of different engineers and engineering disciplines. Communication among those engineers will be vital to the successful completion of the project. You will be in a better position to communicate with the engineers working on electrical systems of all sorts if you have a basic background in EE. Certainly through this course alone you will not be able to design complicated electrical systems, but you will be able to get a feel of how the system works and be better able to discuss the implications of areas where the non-EE system you are designing and the electrical system overlap. For example, mechanical engineers often design packages for electronic systems where heat dissipation due to electronic components can be a major problem. In this instance, the non-EE engineer should be able to help the EE with component placement for optimum heat dissipation. In short, no engineer works in isolation and the more you can communicate with other engineers the better.

The company that hires you out of engineering school understands how important communication is. Thus, they will most likely have training programs that help their engineers learn more about the specific engineering that they will perform as well as other engineering disciplines with which they will be associated. If you have taken EE as an engineering student, then you will have a good foundation for learning EE topics specific to that company, which will make your on-the-job training easier and, consequently, less expensive for your employer. Saving money for your employer is always a good thing.

So, by having taken an EE course, you will be a more promising hire for many companies.

In addition, there will be instances in your engineering career where you will be working directly with electrical or electronic components that you need to understand in some depth. For example, many engineers work in manufacturing processing and will need to work with products that have electrical/electronic content. Likewise, engineers often work with systems that used to be mechanical, but are now electronic (e.g., electronic fuel injection, electronic gas pedals). In the course of your work, you may also need to perform tests in which the test apparatus uses a Wheatstone bridge. If that is the case, then you need to know how a Wheatstone bridge, which is an electric circuit, works to use the equipment adequately. In addition, most mechanical measurements involve converting the mechanical quantity to an electrical signal. Finally, if you need to purchase electrical components and equipment you will need a fundamental background in EE to talk to the vendor in an intelligent manner and get the type of equipment that your company needs. Thus, by knowing some EE, you will be better able to obtain and use electrical components and electrical equipment.

Another reason for learning the principles and practices of EE is that you may be able to make connections between your engineering discipline and EE that lead to creative problem solutions or even inventions. For instance, maybe your job will require you to monitor a system on a regular basis that requires you to perform a significant number of tedious by-hand techniques.

Your familiarity with the monitoring process, combined with your background in EE, might allow you to teach yourself enough in-depth EE to design and build a prototype monitoring system that is faster and less hands-on. This type of invention could lead to a patent or could lead to a significant savings in monitoring costs for your company. In this scenario, you would have been able to do the work yourself and would thus gain ownership of your work and ideas, that is, the design and fabrication of a prototype monitoring system. Learning EE (as well as other engineering fundamentals outside your specific discipline) may allow you to make connections that could lead to creative solutions to certain types of engineering problems.

In conclusion, studying EE will not only help you pass the FE exam, but it will make you more marketable, give you capabilities that will enhance your engineering career, and increase your self-confidence, all of which may allow you to solve problems in ways you cannot now imagine.

CAUSES OF SHORT CIRCUITS

Degradation of insulating materials due to

Degradation in surface quality (pollution).
Excessive temperature.
Partial discharge in the vacuoles (micro pockets) inside the insulating materials.
Accidental reduction in electrical insulation (presence of animals, tree branches, tools left by carelessness on a bus bar, etc.).
Destruction due to external causes (hit by a shovel, etc.).
Over voltages causing a breakdown in equipment insulation (switching surges or lightning strike).

DIFFERENT TYPES OF SHORT CIRCUITS

SHORT CIRCUIT CHARACTERISTICS

Short-circuits can be defined according to three main characteristics:

– Their origin:

They may be mechanical: breakdown of conductors or accidental electrical connection between two conductors via a foreign body such as a tool or animal.

They may be electrical: following the degradation of the insulation between phases, or between phase and frame or earth, or resulting from internal over voltages (switching surges) or atmospheric overvoltage (stroke of lightning).

They may be due to an operating error: earthling of a phase, connection between two different voltage supplies or different phases or closing of a switching device by mistake.

– Their location:

The short-circuit may be generated inside equipment (cable, motor, transformer, switchboard, etc.) and it generally leads to deterioration.

The short circuit may be generated outside equipment (cable, motor, transformer, switchboard, etc.). The consequences are limited to disturbances which may, in the course of time, lead to deterioration of the equipment in question and thereby cause an internal fault.

– Their duration:

Self Extinguishing: the fault disappears on its own.

Fugitive: the fault disappears due to the action of protective devices and does not reappear when the equipment is started up again (the fault is “burnt out” after re energization);

Permanent: these faults require de-energization of a cable, machine, etc., and intervention by the operating personnel.

MOTORING OPERATION OF A DC MACHINE

In the motoring operation the d.c. machine is made to work from a d.c. source and absorb electrical power. This power is converted into the mechanical form. This is briefly discussed here. If the armature of the d.c. machine which is at rest is connected to a dc source then, a current flows into the armature conductors. If the field is already excited then these current carrying conductors experience a force as per the law of interaction discussed above and the armature experiences a torque. If the restraining torque could be neglected the armature starts rotating in the direction of the force. The conductors now move under the field and cut the magnetic flux and hence an induced emf appears in them. The polarity of the induced emf is such as to oppose the cause of the current which in the present case is the applied voltage. Thus a ’back emf’ appears and tries to reduce the current. As the induced emf and the current act in opposing sense the machine acts like a sink to the electrical power which the source supplies. This absorbed electrical power gets converted into mechanical form. Thus the same electrical machine works as a generator of electrical power or the absorber of electrical power depending upon the operating condition. The absorbed power gets converted into electrical or mechanical power.

HETEROPOLAR DC GENERATORS

In the case of a hetero-polar generator the induced emf in a conductor goes through a cyclic change in voltage as it passes under north and south pole polarity alternately. The induced emf in the conductor therefore is not a constant but alternates in magnitude. For a constant velocity of sweep the induced emf is directly proportional to the flux density under which it is moving. If the flux density variation is sinusoidal in space, then a sine wave voltage is generated. This principle is used in the a.c generators. In the case of dc generators our aim is to get a steady d.c. voltage at the terminals of the winding and not the shape of the emf in the conductors. This is achieved by employing an external element, which is called a commutator, with the winding.

Fig. 5 shows an elementary hetero-polar, 2-pole machine and one-coil arma- ture. The ends of the coil are connected to a split ring which acts like a commutator. As the polarity of the induced voltages changes the connection to the brush also gets switched so that the voltage seen at the brushes has a unidirectional polarity. This idea is further developed in the modern day machines with the use of commutators. The brushes are placed on the commutator. Connection to the winding is made through the commutator only. The idea of a commutator is an ingenious one. Even though the instantaneous value of the induced emf in each conductor varies as a function of the flux density under which it is moving, the value of this emf is a constant at any given position of the conductor as the field is stationary. Similarly the sum of a set of coils also remains a constant. This thought is the one which gave birth to the commutator. The coils connected between the two brushes must be ”similarly located” with respect to the poles irrespective of the actual position of the rotor.

This can be termed as the condition of symmetry. If a winding satisfies this condition then it is suitable for use as an armature winding of a d.c. machine. The ring winding due to Gramme is one such. It is easy to follow the action of the d.c. machine using a ring winding, hence it is taken up here for explanation.

Fig. 6 shows a 2-pole, 12 coil, and ring wound armature of a machine. The 12 coils are placed at uniform spacing around the rotor. The junction of each coil with its neighbor is connected to a commutator segment. Each commutator segment is insulated from its neighbor by a mica separator. Two brushes A and B are placed on the commutator which looks like a cylinder. If one traces the connection from brush A to brush B one finds that there are two paths. In each path a set of voltages get added up. The sum of the emfs is constant (nearly). The constancy of this magnitude is altered by a small value corresponding to the coil short circuited by the brush. As we wish to have a maximum value for the output voltage, the choice of position for the brushes would be at the neutral axis of the field. If the armature is turned by a distance of one slot pitch the sum of emfs is seen to be constant even though a different set of coils participate in the addition. The coil which gets short circuited has nearly zero voltage induced in the same and hence the sum does not change substantially. This variation in the output voltage is called the ’ripple’. More the number of coils participating in the sum lesser would be the ’percentage’ ripple.

Another important observation from the working principle of a heterogeneous generator is that the actual shape of the flux density curve does not matter as long as the integral of the flux entering the rotor is held constant; which means that for a given flux per pole the voltage will be constant even if the shape of this flux density curve changes (speed and other conditions remaining unaltered). This is one reason why an average flux density over the entire pole pitch is taken and flux density curve is assumed to be rectangular.

A rectangular flux density wave form has some advantages in the derivation of the voltage between the brushes. Due to this form of the flux density curve, the induced emf in each turn of the armature becomes constant and equal to each other. With this back ground the emf induced between the brushes can be derived. The value of the induced in one conductor is given by

Ec = Bav.L.v Volts ---------------- 7

where

Bav = Average flux density over a pole pitch, Tesla.
L = Length of the active conductor, m.
v = Velocity of sweep of conductor, m/sec.

If there are Z conductors on the armature and they form b pairs of parallel circuits between the brushes by virtue of their connections, then number of conductors in a series path is Z/2b. The induced emf between the brushes is

E = Ec ( Z/2B ) -------------------- 8

E = Bav . L . v ( Z/2B ) Volts ----- 9

But v = (2p).Y.n where p is the pairs of poles Y is the pole pitch, in meters, and n is the number of revolutions made by the armature per second.

Also Bav can be written in terms of pole pitch Y , core length L, and flux per pole as

Bav = phi / (L.Y) Tesla -------- 10

Substituting in equation Eqn. 9,

E = [phi / (L.Y)] . L . ( 2p . Y .n ) . ( Z/2b )

E= phi . p . Zn / b Volts ---------------- 11

The number of pairs of parallel paths is a function of the type of the winding chosen.

HOMOPOLAR MACHINES

Homopolar generators

Even though the magnetic poles occur in pairs, in a homopolar generator the conductors are arranged in such a manner that they always move under one polarity. Either North Pole or South Pole could be used for this purpose. Since the conductor encounters the magnetic flux of the same polarity everywhere it is called a homopolar generator. A cylindrically symmetric geometry is chosen. The conductor can be situated on the surface of the rotor with one slip-ring at each end of the conductor. A simple structure where there is only one cylindrical conductor with ring brushes situated at the ends is shown in Fig. 4. The excitation coil produces a field which enters the inner member from outside all along the periphery. The conductor thus sees only one pole polarity or the flux directed in one sense.

A steady voltage now appears across the brushes at any given speed of rotation. The polarity of the induced voltage can be reversed by reversing either the excitation or the direction of rotation but not both.

The voltage induced would be very low but the currents of very large amplitudes can be supplied by such machines. Such sources are used in some applications like pulse-current and MHD generators, liquid metal pumps or plasma rockets. The steady field can also be produced using a permanent magnet of ring shape which is radially magnetized. If higher voltages are required one is forced to connect many conductors in series.

This series connection has to be done externally. Many conductors must be situated on the rotating structure each connected to a pair of slip rings. However, this modification introduces parasitic air-gaps and makes the mechanical structure very complex. The magnitude of the induced emf in a conductor 10 cm long kept on a rotor of 10 cm radius rotating at 3000 rpm, with the field flux density being 1 Tesla everywhere in the air gap, is given by

The voltage drops at the brushes become very significant at this level bringing down the efficiency of power conversion. Even though homopolar machines are d.c. generators in a strict sense that they ’generate’ steady voltages, they are not quite useful for day to day use.

A more practical converters can be found in the d.c. machine family called ”hetero-polar” machines.

PRINCIPLES OF DC MACHINES