Term Paper on History of AC &; DC

TERM PAPER ON

HISTORY OF AC & DC

SUBMITTED TO:

KHONDOKAR FIDA HASAN

LECTURER,

DEPARTMENT OF

INFORMATION & COMMUNICATION TECHNOLOGY.

COMILLA UNIVERSITY

SUBMITTED BY:

NAME: MD. AL-FAHADID NO:1109031SEMISTER: 01COMILLA UNIVERSITY

NAME: MD.MUJIBUR RAHMANID NO: 1109026SEMISTER: 01 COMILLA UNIVERSITY

DATE OF SUBMISSION: 29-04-2012

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ACKNOWLEDGEMENT:

At first thanks my God who helps to complete term paper successfully.

The special thank goes to my helpful teacher Khondokar

Fida Hasan who gave truly help the progression and

smoothness of the term paper.

Great deals appreciated go to the contribution of my

Department ‘Information & communication Technology’.

Khondokar Fida Hasan.

Lecturer,

Department of

Information & Communication Technology.

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HISTORY OF

AC & DC

Index3

No. Contents page no.1 INTRODUCTION 5

2 HISTORY 5

3 HISTORICAL FACTS 7

4 ALTERNATING CURRENT ANALYSIS 9

5 AC POWER DEVELOPMENT TIMELINE 15

6 PRODUCING SYSTEM OF AC 18

7 WORKING SYSTEM OF AC 19

8 TRANSFER AC IN DC 21

9 DC CURRENT THEORY 23

10 DC CURRENT ANALYSIS 24

11 MEASUREMENT OF DC CURRENT 30

12 RESULT 31

13 CONCLUSION 31

14 BIBLIOGRAPHY 32

Introduction:

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A magnetic field near a wire causes electrons to flow in a single direction along the wire, because they are repelled by the negative side of a magnet and attracted toward the positive side. This is how DC power from a battery was born, primarily attributed to Thomas Edison's work.

AC generators gradually replaced Edison's DC battery system because AC is safer to transfer over the longer city distances and can provide more power. Instead of applying the magnetism along the wire steadily, scientist Nikola Tesla, used a magnet that was rotating. When the magnet was oriented in one direction, the electrons flowed towards the positive, but when the magnet's orientation was flipped, the electrons turned as well.

History:

City lights viewed in a motion blurred exposure. The AC blinking causes the lines to be dotted rather than continuous.

Westinghouse Early AC System 1887

The first alternator to produce alternating current was a dynamo electric generator based on Michael Faraday's principles constructed by the French instrument maker Hippolyte Pixii in 1832.[3] Pixii later added a commutator to his device to produce more commonly used direct current. The earliest recorded practical application of alternating current is by Guillaume Duchenne, inventor and developer of electrotherapy. In 1855, he announced that AC was superior to direct current for electrotherapeutic triggering of muscle contractions.[4]

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A power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, and attracted the interest of Westinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Many of their designs were adapted to the particular laws governing electrical distribution in the UK.[

In 1882, 1884, and 1885 Gaulard and Gibbs applied for patents on their transformer; however, these were overturned due to prior arts of Nikola Tesla and actions initiated by Sebastian Ziani de Ferranti.

Ferranti went into this business in 1882 when he set up a shop in London designing various electrical devices. Ferranti believed in the success of alternating current power distribution early on, and was one of the few experts in this system in the UK. In 1887 the London Electric Supply Corporation (LESCo) hired Ferranti for the design of their power station at Deptford. He designed the building, the generating plant and the distribution system. On its completion in 1891 it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world. Many homes all over the world still electric meters with the Ferranti AC patent stamped on them.

William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early transformer. The AC power system used today developed rapidly after 1886, and includes key concepts by Nikola Tesla, who subsequently sold his patent to George Westinghouse. Lucien Gaulard, John Dixon Gibbs, Carl Wilhelm Siemens and others contributed subsequently to this field. AC systems overcame the limitations of the direct current system used by Thomas Edison to distribute electricity efficiently over long distances even though Edison attempted to discredit alternating current as too dangerous during the War of Currents.

The first commercial power plant in the United States using three-phase alternating current was at the Mill Creek No. 1 Hydroelectric Plant near Redlands, California, in 1893 designed by Almirian Decker. Decker's design incorporated 10,000-volt three-phase transmission and established the standards for the complete system of generation, transmission and motors used today.

The Ames Hydroelectric Generating Plant (spring of 1891) and the original Niagara Falls Adams Power Plant (August 25, 1895) were among the first AC-powered hydroelectric plants.

The Jaruga Hydroelectric Power Plant in Croatia was set in operation on 28 August 1895. The two generators (42 Hz, 550 kW each) and the transformers were produced and installed by the Hungarian company Ganz. The transmission line from the power

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plant to the City of Šibenik was 11.5 kilometers (7.1 mi) long on wooden towers, and the municipal distribution grid 3000 V/110 V included six transforming stations.

Alternating current circuit theory developed rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, James Clerk Maxwell, Oliver Heaviside, and many others. Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918

HISTORICAL FACTS:AC/DC DEBATE, EDISON vs. TESLA, circa 1900Nicola Tesla; "Alternating Current will allow the transmission of electrical power to any point on theplanet, either through wires or through the air, as I have demonstrated."Thomas Edison; "Transmission of AC over long distances requires lethally high voltages, and should beoutlawed. To allow Tesla and Westinghouse to proceed with their proposals is to risk untold deaths byelectricide."Tesla; "How will DC power a 1,000 horsepower electric motor as well as a single light bulb? With AC,the largest as well as the smallest load may be driven from the same line."Edison; "The most efficient and proper electrical supply for every type of device from the light bulb tothe phonograph is Direct Current at low voltage."Tesla; "A few large AC generating plants, such as my hydroelectric station at Niagara Falls, are all youneed: from these, power can be distributed easily wherever it is required."Edison; "Small DC generating plants, as many as are required, should be built according to local needs,after the model of my power station in New York City."

EARLY AC DOMINANCE:

After Edison introduced his DC power stations, the first of their kind in the world, the demand forelectricity became overwhelming. Soon, the need to send power over long distances in rural and suburbanAmerica was paramount. How did the two power systems compare in meeting this need?

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AC - Alternating current could be carried over long distances via a relatively small line given anextremely high transmission voltage of 50,000 volts or above. The high voltage could then be transformeddown to lower levels for residential, office and industrial use.DC - While higher in quality and more efficient than alternating current, DC power could not betransformed or transmitted over distances via small cables without suffering significant losses throughresistance.The Result - AC power became the standard of all public utilities, overshadowing issues of safety andefficiency and forcing manufacturers to produce appliances and motors compatible with the national grid.

The 100 YEAR OLD POWER SCHEME:

With AC power the only option available from power utilities, the world came to rely almost exclusivelyon AC-based motors and other appliances, and the efficiencies and disadvantages of AC power becameaccepted as unavoidable. Nicola Tesla's development of the polyphase induction AC motor was a key stepin the evolution of AC power applications. His discoveries contributed greatly to the development ofdynamos, vacuum bulbs and transformers, strengthening the existing AC power scheme 100 years ago.Compared to DC and Edison's findings, AC power is inefficient because of the energy lost with the rapidreversals of the current's polarity. We often hear these reversals as the familiar 60 cycles per second (60hertz) hum of the appliance. AC power is also prone to harmonic distortion, which occurs when there is adisruption in the ideal AC sinusoidal power wave shape. Since most of today's technological advances ofon-site power devices are DC, there is a need use inverters to produce AC through the system and thenback to DC into the end source of power. These inverters are inefficient; energy is lost (up to 50%) whenthese devices are used. This characteristic is evident in many of today's electronic devices that haveinternal converters, such as fluorescent lighting.

AC/DC 1950 to 2000

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The discovery of semiconductors and the invention of the transistor, along with the growth of theAmerican economy, triggered a quiet but profound revolution in how we use electricity. Changes over thelast half-century have brought the world into the era of electronics with more and more machines andappliances operating internally on DC power and requiring more and more expensive solutions for theconversion and regulation of incoming AC supply:

NEXTEK BENEFITS:Easy conversion of AC lighting fixtures to DC-powered unitsEasy conversion of AC grid power into DC power into lighting systemsHighly efficient management of peak loadsComplete continuity of supply through the seamless integration of rechargeable batteriesComplete continuity of alternative energy sources such as PV, micro turbines and fuel cellsFuture-proof lighting and other systems to be developedA world which Thomas Edison envisioned which is clean, efficient and less costlyThe CYCLE of 100 YEARS NEEDS to be ADDRESSED for a SUSTAINABLE POWER FUTUREThe computer industry alone accounts for 15% of the total power capacity in AmericaFossil Fuel needs to be slowed to prevent global warming concernsOn-site power using DC to the end-source is the most efficient use of power

No conversion losses using DC power for the full potential of alternative energy

Alternating current analysis:

broadly defined, an electric current that changes with time. In engineering, an alternating current is understood to mean a periodic current. For such a current, the average values of the current and the voltage over a period are equal to zero. A period T is defined as the smallest time interval, measured in sec, that separates recurring changes of current and voltage (see Figure 1). Frequency f—the number of periods per sec, or 1/T—is an important characteristic of alternating currents.

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Figure 1. Graph of a periodic alternating current, i(t)

In the electric power systems of the USSR and most countries of the world, the established standard frequency is f = 50 hertz (Hz); in the USA it is 60 Hz. In communications engineering, high-frequency alternating currents are often used, with frequencies ranging from 100 kHz to 30 GHz. Alternating currents of widely differing frequencies, as well as pulsed currents, are used for special purposes in industry, medicine, and other areas of science and engineering.

AC systems are preferred for the transmission and distribution of electric power because the transformation of alternating voltages is quite simple and almost entirely free of power losses. Three-phase AC systems are widely used. A comparison of AC generators and motors and the corresponding DC machines shows that AC machines are smaller, simpler, more reliable, and less expensive for the same power rating. An alternating current can be rectified, for instance, by semiconductor rectifiers, and then converted by semiconductor inverters to an alternating current of a different controlled frequency. This permits the use of simple and inexpensive asynchronous and synchronous AC motors without commutators in various electric drives that require a continuous range of speeds.

Alternating current is widely used in communication systems, for example, radio, television, and long-distance wire telephony.

An alternating current is generated by an alternating voltage. An alternating electromagnetic field is created in the space surrounding a current-carrying conductor. This field causes energy oscillations in the AC circuit. This energy is periodically either stored in the magnetic or electric field or is returned to the electric power source. These energy oscillations cause reactive currents to flow through the AC circuit, causing an unproductive loading of the conductors and power source and thus leading to additional power losses. This fact is a drawback of power transmission in AC networks.

The magnitude of an alternating current is basically defined by a comparison of the average thermal effect of the alternating current with the thermal effect of a direct current of corresponding magnitude. The value of the current I thus obtained is called the effective value. Mathematically, this is the root-mean-square value of the current during one period. The effective value of the

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alternating voltage U is similarly defined. The effective values of voltage and current are the quantities measured by AC voltmeters and ammeters.

In the case that is both the simplest and the most important for practical applications, the instantaneous value i of the alternating current changes in time according to the sine law: i = Im sin (ωt + α),

Figure 2. Graph of voltage u and current i in an AC circuit with phase shift ϕ

where Im is the amplitude of the current, ω = 2πf is the angular frequency of the current, and α is the phase at t = 0. A sinusoidal (harmonic) current is created by a sinusoidal voltage of the same frequency; that is, u = Um sin (ωt + β), where Um is the amplitude of the voltage and β is the initial phase(see Figure 2). The effective values of the current and the voltage for alternating current are, respectively, and . For sinusoidal currents that satisfy the conditions for being quasi-stationary (the only currents considered in the following discussion), Ohm’s law is valid. (Ohm’s law in its differential form is also valid for nonquasi-stationary currents in linear circuits.) In the general case, the presence of inductance and/or capacitance in an AC circuit causes a phase shift ϕ = β – α. This shift is determined by the parameters of the circuit—the resistance r, the inductance L, and the capacitance C—and by the angular frequency ω. As a result of the phase shift, the average power P of the alternating current measured by a wattmeter is smaller than the product of the effective values of current and voltage: P = IU cos ϕ.

Figure 3. Schematic diagram and graph of voltage u and current i in a circuit containing only the effective resistance r

In a circuit without inductance or capacitance, the current is in phase with the voltage (see Figure 3). In such a circuit, Ohm’s law for the effective values has

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the same form as Ohm’s law for DC circuits: I = U/r. Here r is the resistance of the circuit, determined by the actual powerP consumed in the circuit: r = P/I2.

If an inductance L is present in the circuit, the alternating current will induce in L an electromotive force (emf) of self-induction eL = – L · di/dt = –ωLIM cos (ωt + α) = (ωLIm sin (ωt + α – π/2). The self-induced emf opposes changes of the current; in a circuit that contains only an inductance, the current will lag in phase behind the voltage by one-quarter of a period; that is, ϕ = π/2 (see Figure 4). The effective value of eL is equal to EL = IωL = IxL, where XL = ωL is the inductive reactance of the circuit. Ohm’s law for such a circuit has the form I = U/xL = U/ωL.

Figure 4. Schematic diagram and graph of voltage u and current i in a circuit containing only the inductance L

When a capacitance C is connected to a voltage u, its charge is equal to q = Cu. Periodic changes in the voltage cause periodic changes in the charge and give rise to a capacitance current i = dq/dt = C· du/dt = ωCUm cos (ωt + β) = ωCUm sin (ωt + β + π/2). Thus, the sinusoidal alternating current that flows through the capacitance leads the voltage at the capacitance terminals by one-quarter of a period; that is, ϕ = – π/2 (see Figure 5). The effective values in such a circuit satisfy the relationship I = ωCU = U/xc, where xc = 1/ωC is the capacitive reactance of the circuit.

Figure 5. Schematic diagram and graph of voltage u and current i in a circuit containing only the capacitance C

If an AC circuit consists of r, L, and C connected in series, its total impedance is , where x = xL – xC = ωL – 1/ωC is the reactance of the circuit. Correspondingly, Ohm’s law assumes the form , and the phase shift between the voltage and the current is determined by the ratio of the reactance of the

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circuit to its resistance: tan ϕ = x/r. In such a circuit the frequency ω of the forced oscillations generated by the source of alternating current may coincide with the resonant frequency . In this case, the inductive reactance and the capacitive reactance are equal (ωL = 1/ωC) and fully balance each other out, the magnitude of the current attains a maximum value, and the phenomenon of resonance occurs. Under conditions of resonance, the voltage across the inductance and the capacitance can significantly exceed the voltage at the circuit terminals and can frequently be many times as great.

Computations of sinusoidal AC circuits are facilitated by the construction of vector diagrams. It is customary to designate the vectors of sinusoidal currents and voltages by a point placed above the letter (İ, U$ ). The lengths of the vectors are usually made equal to the effective values of I and U (on the scale of the diagram); the angles between the vectors are made equal to the phase shifts between the instantaneous values of the appropriate quantities. The algebraic addition of instantaneous values of any of the sinusoidal quantities then corresponds to the geometric addition of the vectors of these quantities.

Figure 6. Schematic and vector diagrams for an AC circuit with an inductance (L) and effective resistance (r) and a capacitance (C) connected in series

Figure 6 shows the vector diagram for an AC circuit with r, L, and C connected in series. The instantaneous value of the voltage across the terminals of this circuit is equal to the algebraic sum of the voltages across the resistance and across the reactance: u = uL + ur + uC. Therefore, U$ = U$ L + U$ r + U$ C. In constructing the diagram, the current vector is taken as the reference vector, because for a nonbranching circuit the current is the same in all sections of the circuit. Inasmuch as the inductive voltage leads the current in phase by π/2 and the capacitive voltage lags behind the current by π/2, these voltages are in antiphase and will partially cancel each other in the series connection.

Vector diagrams visually illustrate the course of computations and serve as a control on the results of the computations. If requirements of scale are observed, such diagrams yield a graphical determination of the effective circuit voltage U and the phase-shift angle ϕ.

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Calculations for branching circuits with quasi-stationary alternating currents are based on Kirchhoff’s laws. The method of complex quantities (a symbolic method) is usually used in these calculations. This method makes it possible to express in algebraic form the geometric operations performed with the AC vectors and thus to apply all the calculation methods for DC circuits to AC circuits.

In electric power systems, nonsinusoidal operation is usually undesirable, and special measures are taken to avoid such a mode of operation. However, nonsinusoidal conditions are inherent in the workings of electric communication circuits and semiconductor and electronic devices. If the average value of the current over a cycle is not equal to zero, the current must contain a constant component. To facilitate the analysis of circuits operating with nonsinusoidal currents, the current is represented as a sum of simple harmonic components. The frequencies of these harmonics are equal to integral multiples of the fundamental frequency: i = I0 + I1m sin (ωt + α1) + I2m sin (2ωt + α2) + ··· + Ikm sin (kωt + αk), where I0 is the constant component of the current, I1m sin (ωt + α1) is the first harmonic component (the fundamental), and all other terms are higher harmonics.

The computations used for nonsinusoidal currents in linear circuits are based on the superposition principle. Since xL and xC are functions of the frequency, these computations are conducted for each component. Algebraic addition of the results of such calculations yields the instantaneous value of current or voltage for the nonsinusoidal current.

Inventors of alternative current:

Tesla, an American immigrant from Serbia, invented most a.c. technology. Edison invented most d.c. technology. George Westinghouse was the financial backer of Tesla. J.P. Morgan backed Edison. Tesla and Edison were bitter rivals.

Today, all power companies use Tesla's a.c. technology, even though they are commonly named for Edison! And, Tesla's work laid the foundation for radio, which is a very high frequency a.c. phenomenon.

But Edison's contributions are also important. His lab's inventions cover most of the components of the electric power industry not associated with a.c., including such mundane devices as insulated wire, switches, fuses and other practical necessities of electric power distribution, in additon to of course the incandescent light bulb. And, he invented what today we would call the modern corporate research lab, an economic institution just as important to modern industry as Ford's assembly line.

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AC Power Development Timeline:

1835 - Hippolyte Pixii builds the first alternator. Pixii builds a device with a rotating magnet. He doesn't know how to make his creation useful since all the other experimenters of the time were building DC devices. Others like Faraday and Henry were experimenting at the time with primitive electric motors using electromagnets.

1855 - Guillaume Duchenne uses alternating current in electrotherapeutic triggering of muscle contractions. (Paris, France) AC power is not viewed as useful for anything else at the time.

1878 - Ganz Company starts working with single phase AC power systems in Budapest, Austro-Hungary

1879 - London: Walter Baily makes a copper disc rotate using alternating current (this is a weak early AC motor) which was not effective for baring any load.

The 1880s: This decade proved to be an exciting time for the development of electric power, read below to find out some of the major developments by year.

1882 - London, Sabastian Ferranti (Englishman with an Italian parent) works at Siemens Brothers firm in London with Lord Kelvin (William Thompson), and Ince. With the help of Lord Kelvin Ferranti pioneers early AC power technology, including an early transformer. Later on John Gibbs and Lucien Gaulard would base their designs off of Ferranti.

1884

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1884 - Turin, Italy: Lucien Gaulard develops transformers and the power transmission system from Lanzo to Turino. The demonstration of AC power includes a 25 mile trolley with step down transformers that allow low power Edison incandescent lights to light the path along with arc lamps. Galileo Ferraris was head of the Electrical Department. The next year Ferraris would invent the polyphase motor.

18851885 - Ferraris conceives the idea of the first polyphase AC motor: " In the summer of 1885 he conceived the idea that two out-of-phase, but synchronized, currents might be used to produce two magnetic fields that could be combined to produce a rotating field without any need for switching or for moving parts. "

1885 - Elihu Thomson at Thomson-Houston starts experimenting with AC power (the first company in the US to start work on AC)

1885 - George Westinghouse is intrigued by AC power and buys North American rights to Gaulard and Gibbs system for $50,000

1885 - George Westinghouse orders a Siemens alternator (AC generator) and a Gaulard and Gibbs transformer. Stanley begin experimenting with this system.

1886An important year for AC power

1886 - Great Barrington, Massachusetts - the first full AC power system in the world is demonstrated using step up and step down transformers. The system was built by William Stanley and funded by Westinghouse.

1886 - November - Buffalo, New York receives the first commercialAC power system in the USA. This system designed by George Westinghouse, William Stanley, and Oliver B. Shallenberger

1886 - William Stanley designs an improved version of the Siemens single phase alternator

1886 - Fall - Elihu Thomson's AC power system is rejected by the patent office. Westinghouse is already far ahead, having sold its system commercially already.

1886 - Nikola Tesla tries to sell his AC power system to investors in New York City, but it fails to be of interest in a city which is already heavily invested in DC power systems. Other inventors around the world also promoting AC power have similar problems. This is especially due to the fact that no one has yet to invent an AC electric motor which is efficient.

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1886 - Otto Blathy comes to the USA and Thomas Edison buys options on the Z.B.D. Transformer. This would put him in the position to rival Westinghouse that controlled the Gaulard and Gibbs transformer patent. Later Edison decides that it is not worth going into AC and drops his options on the Z.B.D. Transformer.

1887

1887 - C.S. Bradley builds the first AC 3 phase generator. Up until this time Siemens and Westinghouse had been producing single phase AC generators. The 3 phase system would be a great improvement.

1887 - F. Augus Haselwander develops the first AC 3 phase generator in Europe. He is behind Bradley by a couple months and it is generally believed that he built his design independently of Bradley.

1887 - Sabastian Ferranti designs Depford Power Station in London. When it is completed in 1891 it would be an important early site in AC power history.

1888

1888 - Mikhail Dolivo-Dobrovsky in Germany builts his first AC polyphase generator. He works for AEG. (Allgemeine Elektricitäts-Gesellschaft)l

1888 - April - Galileo Ferraris makes public his AC polyphase motor first conceived in 1885. His motor works without a commutator, this development finally makes the AC motor efficient, and therefore competitive with DC motors. The A motor report was first published at the Royal Academy of Sciences in Turin. Westinghouse read the report of Ferraris and saw a chance for AC systems to become much more marketable

1888 - May 15 - Tesla stands before the AIEE showing his polyphase motor. Elihu Thomson was there and some in the group seemed to be impressed. One week later Westinghouse sent out a recruiter to get Tesla to join him. Tesla's progress on the motor is slightly ahead of Oliver Shallenberger's 3 phase electric motor. Shallenberger was already working for Westinghouse.Tesla claims he "dreamed up" the first polyphase motor before Galileo Ferraris. Later at a trial a US court sides with Tesla despite the fact that Tesla has no proof besides witness testimony.Read more here

1889 - Dobrovolsky builds his first transformer and motor to work with his 3 phase AC system

1891 - Frankfurt, Germany: First distance power transmission (for electric power utility) Lauffen to Frankfurt 109 miles. The entire system was designed by Dobrovsky from generator to electric motor. Many important figures of AC

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power were invited to the event, at the Congress Dinner Galileo Ferraris was hailed as “the father of three-phase current.”

1892 - Charles P. Steinmetz goes before the AIEE and presents his paper on hysteresis, or the delay effect in 3 phase AC power. Steinmetz was the first person to understand AC power from a mathematical point of view. After his paper he is hired by General Electric Company and joins forces with Elihu Thomson and William Stanley. Steinmetz would go on to improve and troubleshoot future AC power systems.

1893 - Redlands Power House - the first commercial installation of 3 phase AC power, 40 hz. 1895 - Folsom Power House - The first installation of modern AC power in the USA: 3 Phase AC at 60 Hz.1895 - Westinghouse builds the power system for the Adams Power Station at Niagara Falls. Benjamin Garver Lamme is the principal engineer of the operation. General Electric builds the 25 mile power transmission system from the Niagara power house to Buffalo, NY which is made operational in 1896.1897 - Mechanicville Power Station - Charles P. Steinmetz experiments with a unique single phase AC power transmission system.1900s - Three Phase AC power is fully established as the principle source of power for the world

Producing System of Alternating Current:

AC is produced in two ways.The most common is with an alternator. The simplest is a bicycle dynamo which consists of a single wire coil and a permanent magnet.By rotating the magnet in the coil an alternating voltage is induced. When connected to a circuit an alternating current will flow (provided there is no rectification in the circuit). Large amounts of power as are used in mains electricity generation use a similar method. Instead of a single coil they use (usually) three coils at 120° to each other and instead of permanent magnets they use an electromagnet.An example of one with a permanent magnet is here

The advantage of the electromagnet is that it can be a much stronger magnet than is available from permanent magnets and it can be adjusted to give the desired voltage output whatever the load (up to maximum output of the alternator.

The alternators in cars use pole pieces on their rotor to increase the frequency of the output. This is so that when it is rectified for the DC that the car requires it has a much smaller AC ripple on the waveform so causes less problems with electrical noise or the need for large smoothing circuits.

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Working System Of Alternating Current:

"Back and forth" is a very poor way of describing alternating electricity and I really do wish that such descriptions would cease to be used. Alternating electricity, or current, or voltage is an electrical wave, for lack of a better term that goes from a zero point in value to a peak positive, back to zero, and again to a negative peak value and then returns to zero within a specified period of time with no interruption. So long as there is something generating this phenomena, as well as something to conduct it away from it's generating source, the wave will continue to move away from the generating source until it is cut off, used up or the potential drops to zero because of resistance in the material transporting it. If you consider a loop, of wire passing through a magnetic field, when the loop is at a right angle (90 degrees) to the magnetic filed, no current is induced, so there is a zero potential in the wire loop. As this loop moves forward, it cuts through magnetic lines of force, which induces a current flow to a peak value 90 degrees later, to another zero point which would now be 180 degrees of rotation, to another peak at 270 degrees, and finally back to zero again. Older alternators used what are known as slip rings to conduct the generated voltage away from the alternator. Now, it is the generating coils that are stationary and a magnet coil spins inside the alternator to create the electrical energy. The difference between the DC generator, and the AC alternator was the commutator in the DC machine. Each coil end, ended at a contact , one for each end of the coil wire and spaced so that the brushes would make contact for each coil as the rotor spun inside the generator. As for the music part of the question, the music wave, or electrical voltage is rapidly changing from a zero point to positive peak, to zero and to a negative peak, as just described in the same time rate of time as the music is being played. This alternating voltage flows through the wires to the speakers, and into the coils that are wrapped on a paper sleeve, around a magnet in the speaker. As the sound voltage, as we will call it, makes a magnetic field, the magnet causes the movable sleeve the coils are wound on to move, or vibrate, in time with the music wave from the amplifier. You could call the speaker a type of linear motor, since it does move that sleeve, which makes the cone it is attached to vibrate. This vibrates the air around it, and you hear the vibrations as sound waves. I call a speaker an electro-mechanical transducer, which is exactly what it does.

Alternatng current is a current that flows in both directions. For some interval of time it will flow in one direction followed by an interval of flow in the opposite direction.

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The total of these 2 intervals is called the frequency of the AC. Now this assumes that the rate of alternation is constant and amplitude is symmetrical in both directions. This is certainly true for the AC that is generated by your local power company or from a conventional AC generator. This conventional AC can be represented as a sine wave, which is so because of the configuration of the generator.

Contrast this with DC (direct current) where the current flow is always in the same direction, and usually constant

. Advantages of alternating current:

The main advantage offered by using alternating current is that it is easy to achieve the transmission of large amounts of power over very long distances and to do it much more cheaply than by using direct current.

The most efficient way to transmit energy by wire is to make it low current and high voltage. AC voltage can be transformed very easily with transformers (called stepping-up and stepping-down), and transformers only work with AC.

Powerplants produce AC by default, so it would take additional effort to convert it to DC.

It is much easier and cheaper to convert AC to DC than to convert DC to AC.

Domestic Uses Of Alternating Current:

Alternating current is easier to distribute than direct current, because alternating current can be raised or lowered in voltage by a transformer. This allows very high voltages to be used in long distance transfer of electricity, which is then stepped down several times before being supplied to the end user. As the voltage is increased in a circuit, the current, as measured in amperes, decreases for a given load. This allows the use of lighter wire to transmit large amounts of electricity.

Use of transformers with Alternating Current (AC)

Another difference between AC and DC involves the amount of energy it can carry. Each battery is designed to produce only one voltage, and that voltage of DC cannot travel very far until it begins to lose energy. But AC's voltage from a generator, in a power plant, can be bumped up or down in strength by another mechanism called a

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transformer. Transformers are located on the electrical pole on the street, not at the power plant. They change very high voltage into a lower voltage appropriate for your home appliances, like lamps and refrigerato

Transfer ac current in dc:

Alternating Current is safer and less expensive to generate. Direct current in a house can be very dangerous and deadly. I work on elevators for a living and work with high voltage DC all the time. Believe me, you don't want that in your house. I watched a mechanic cut a set of wires that were 440VDC... Blew him straight across the room and broke his arm. If you cut 120/220 VAC in your house you'll just get a loud bang and a spark. Plus nothing in a household requires a constant current like DC.

AC/DC current transfer:

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AC/DC transfer links quantum DC voltage traceability to a wide range of ac frequencies using the heating effect of electrical current. Decades of technology have refined this simple concept to give a measurement uncertainties at the ppm level for ac voltage and current beyond the audio frequency range.

Alternating currents and voltages are usually related to their direct counterparts using ac/dc transfer devices, that is devices which will respond to both ac and dc in a known way. The most common variety used in precision measurements is the thermal ac/dc transfer standard. The heart of such a standard is a thermal element – a short heater wire with a thermocouple attached and enclosed in an evacuated glass envelope. The ac and dc signals are applied in turn to the heater and the corresponding temperature rise is indicated by the electrical output from the thermocouple.

In a perfect device an equal temperature rise would indicate that the root–mean–square (rms) value of the ac waveform was equal to the value of the dc signal. For a more complicated device, the multi–junction thermal converter (MJTC), first developed at NPL in 1965. Unfortunately in the case of the more common single–junction thermal converter (SJTC) anomalous temperature distributions caused by Peltier and Thomson effects result in there being a difference in outputs even when the same dc and rms ac are applied to the input.

The ac/dc transfer difference is the difference between the ac and the dc signals that are required to give the same output. It is usually expressed as parts–per–million (ppm) of the total dc signal.

In use, the range of these simple SJTCs is extended from currents of typically 1 mA to currents of 20 A by the addition of resistive shunts. For voltages from 0.1 V to 1000 V series ranging resistors are added. For voltages between 1 mV and 0.1 V, resistive voltage dividers are used.

As MJTCs are only available over a limited current/voltage range, a build-up technique is performed on sets of working standards to relate their transfer differences to the known small errors of the MJTCs.

The major difficulty with such thermal transfer devices is that the output voltage generated by the thermocouple is very small (in the region of 7 mV) and is very prone to drifting. Measurements of ac/dc transfer difference at NPL have been made under computer control. In this way the switching between ac and dc and the reading of the devices can take place at regular, precise intervals. NPL uses a third order curve fitting techniques to remove the thermal drifts from the measurements.

By making a DC measurement using an electronic voltage standard, the ac/dc transfer system has been adapted for the direct calibration of ac digital voltmeters. These measurements are important for the highest grade instruments and using this method uncertainties of 7 ppm can be achieved at a 95% confidence level.

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UKAS accredited service for AC/DC transfer. NPL is able to offer measurements over an extensive range of voltages/frequencies. Best measurement uncertainty is ± 5ppm. AC/DC Voltage: range from 1mV to 1000V, 10Hz to 1MHz (100 kHz max above 20V); AC/DC Current: range from 1mA to 20A 10Hz to 100kHz; AC/DC Voltage (High Frequency): range from 0.5V to 1V up to 100MHz AC Voltage: range from 0.5V to 1000V, 10Hz to 1 MHz (100kHz max above 20V).

DC current theory:

1:

currents used in battery-charging systems must be regulated so as to not overcharge the battery(ies) they are connected to. Here is a crude, relay-based voltage regulator for a DCcurrent:

Simple electromechanical relay circuits such as this one were very common in automotive electrical systems during the 1950's, 1960's, and 1970's. The fundamental principle upon which their operation is based is called negative feedback: where a system takes action to oppose any change in a certain variable. In this case, the variable is generator output voltage. Explain how the relay works to prevent the generator from overcharging the battery with excessive voltage.

2:

A mechanic has an idea for upgrading the electrical system in an automobile originally designed for 6 volt operation. He wants to upgrade the 6 volt headlights, starter motor, battery, etc, to 12 volts, but wishes to retain the original 6-volt generator and regulator. Shown here is the original 6-volt electrical system:

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The mechanic's plan is to replace all the 6-volt loads with 12-volt loads, and use two 6-volt batteries connected in series, with the original (6-volt) regulator sensing voltage across only one of those batteries:

DC current analysis

1:

Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way. You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the änswers" instead of a book or another person. For successful circuit-building exercises, follow these steps:

1.Carefully measure and record all component values prior to circuit construction.

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2.Draw the schematic diagram for the circuit to be analyzed.

3.Carefully build this circuit on a breadboard or other convenient medium.

4.Check the accuracy of the circuit's construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.

5.Mathematically analyze the circuit, solving for all values of voltage, current, etc.

6.Carefully measure those quantities, to verify the accuracy of your analysis.

7.If there are any substantial errors (greater than a few percent), carefully check your circuit's construction against the diagram, then carefully re-calculate the values and re-measure.

Avoid very high and very low resistor values, to avoid measurement errors caused by meter "loading". I recommend resistors between 1 kΩ and 100 kΩ, unless, of course, the purpose of the circuit is to illustrate the effects of meter loading! One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won't have to measure any component's value more than once. 2:

A transistor is a semiconductor device that acts as a constant-current regulator. For the sake of analysis, transistors are often considered as constant-current sources:

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Suppose we needed to calculate the amount of current drawn from the 6-volt source in this dual-source transistor circuit:

We know the combined currents from the two voltage sources must add up to 5 mA, because Kirchhoff's Current Law tells us that currents add algebraically at any node. Based on this knowledge, we may label the current through the 6-volt battery as "I", and the current through the 7.2 volt battery as "5 mA − I":

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Kirchhoff's Voltage Law tells us that the algebraic sum of voltage drops around any "loop" in a circuit must equal zero. Based on all this data, calculate the value of I:

3:

This transistor circuit is powered by two different voltage sources, one that outputs 6 volts, and the other that is variable.

Transistors naturally act as current-regulating devices, and are often analyzed as though they were current sources. Suppose that this transistor happened to be regulating current at a value of 3.5 mA:

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How high does the voltage of the variable source have to be adjusted, until no current is drawn from the 6-volt battery? Hint: simultaneous equations are not needed to solve this problem!

4:

Describe, step-by-step, the steps required to calculate all currents and voltage drops in a DC network using the Branch Current Method.

5:

While the "Branch Current" method may be used to analyze an unbalanced bridge circuit, it requires a lot of calculation! In this circuit, determine how many variables are needed to solve for all currents:

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6:

Re-draw the circuit shown here into schematic form, and solve for the voltage drops across the two resistors using the "Branch Current" method:

7:

Calculate the amount of charging current through battery #1 using the Branch Current method of analysis, given the open-circuit voltages and resistances of the components in this circuit. Disregard any wire resistance:

Measurement OF DC current:

Multimeters

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Seeing as how a common meter movement can be made to function as a voltmeter, ammeter, or ohmmeter simply by connecting it to different external resistor networks, it should make sense that a multi-purpose meter ("multimeter") could be designed in one unit with the appropriate switch(es) and resistors.

For general purpose electronics work, the multimeter reigns supreme as the instrument of choice. No other device is able to do so much with so little an investment in parts and elegant simplicity of operation. As with most things in the world of electronics, the advent of solid-state components like transistors has revolutionized the way things are done, and multimeter design is no exception to this rule. However, in keeping with this chapter's emphasis on analog ("old-fashioned") meter technology.

Some advantages of DC current :

DC is simpler to use it to power most electronics, which tend to require DC to operate. This is why it's used some rack systems use it - actually following a practice that the phone company has used for a very long time with their central office equipment. DC power also makes it easier to integrate batteries and UPS systems.

On a larger scale, High-voltage DC is seeing increasing use for long lines because it has less reactance loss than AC and it avoids grid synchronization problems. The wire can also be sized to the DC current rather than for the peak AC current. Link below explains this further.

Result:

Alternating Current vs Direct Current:

Electricity flows in two ways; either in alternating current (AC) or in direct current (DC). Electricity or 'current' is nothing more than moving electrons along a conductor, like a wire, that have been harnessed for energy. Therefore, the difference between AC and DC has to do with the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards."

Comparison chart

Alternating Current Direct CurrentTypes: Sinusoidal, Trapezoidal,

Triangular, SquarePure and pulsating

Current: It is the current of It is the current of

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magnitude varying with time constant magnitudeDirection: It reverses its direction while

flowing in a circuitIt flows in one direction in the circuit

Obtained from: A.C Generator and mains Cell or BatteryAmount of energy that can be carried:

Safer to transfer over longer city distances and can provide more power

Voltage of DC cannot travel very far until it begins to lose energy

Flow of Electrons: Electrons keep switching directions - forward and backward

Electrons move steadily in one direction or 'forward'

Cause of the direction of flow of electrons:

Rotating magnet along the wire

Steady magnetism along the wire

CONCLUSION:

DC stands for direct current. It is current that starts at one place and flows in one direction to the end destination, hence the name Direct.AC stands for Alternating Current. The current flows in one direction for a period of time and then switches direction, going the opposite way. It switches diretion over and over again continuously. In the united states the AC current in power lines goes switches direction, forward to backward, then backward to forward, 60 times each second. This is a frequency of 60 Hertz and is called 60Hz AC electricity.

Bibliography:

1.www.yahoo.com

2.www.ask.com

3.www.physicsforum.com

4.www.google.com

5.www.answer.com

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