Electric Generator

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Electric generator
From Wikipedia, the free encyclopedia

U.S. NRC image of a modern steam turbine generator

Early Ganz Generator in Zwevegem,West Flanders, Belgium

Early 20th century alternator made inBudapest, Hungary, in the power generating hall of a hydroelectric station
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge (usually carried by electrons) to flow through an external electrical circuit. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, aninternal combustion engine, a wind turbine, a hand crank, compressed air, or any other source of mechanical energy. Generators supply almost all of the power for the electric power grids which provide most of the world's electric power.
The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators have many similarities. Many motors can be mechanically driven to generate electricity and frequently make acceptable generators.
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[edit]History
Before the connection between magnetism and electricity was discovered, electrostatic generators were used. They operated on electrostaticprinciples. Such generators generated very high voltage and low current. They operated by using moving electrically charged belts, plates, and disks that carried charge to a high potential electrode. The charge was generated using either of two mechanisms: * Electrostatic induction * The triboelectric effect, where the contact between two insulators leaves them charged.
Because of their inefficiency and the difficulty of insulating machines that produced very high voltages, electrostatic generators had low power ratings, and were never used for generation of commercially significant quantities of electric power. The Wimshurst machine and Van de Graaff generator are examples of these machines that have survived.
In 1827, Hungarian Anyos Jedlik started experimenting with the electromagnetic rotating devices which he called electromagnetic self-rotors, now called the Jedlik's dynamo. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years beforeSiemens and Wheatstone but didn't patent it as he thought he wasn't the first to realize this. In essence the concept is that instead of permanent magnets, two electromagnets opposite to each other induce the magnetic field around the rotor. It was also the discovery of the principle of self-excitation.[1]

Faraday disk, the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center of the disk through the axle.
In the years of 1831–1832, Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux. He also built the first electromagnetic generator, called the Faraday disk, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.
This design was inefficient, due to self-cancelling counterflows of current in regions that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counterflow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

Dynamos are no longer used for power generation due to the size and complexity of the commutator needed for high power applications. This large belt-driven high-current dynamo produced 310 amperes at 7 volts, or 2,170 watts, when spinning at 1400 RPM.

Dynamo Electric Machine [End View, Partly Section] (U.S. Patent 284,110)
The dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic induction to convert mechanical rotation into direct currentthrough the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.
A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.
Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current. The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii.
Faraday developed the "rotating rectangle", whose operation was heteropolar - each active conductor passed successively through regions where the magnetic field was in opposite directions.[2] The first public demonstration of a more robust "alternator system" took place in 1886.[3] Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. Lord Kelvin andSebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15 kHz).[4] After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.[5] Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.[6]
Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution. Before the adoption of AC, very large direct-current dynamos were the only means of power generation and distribution. AC has come to dominate due to the ability of AC to be easily transformed to and from very high voltages to permit low losses over large distances.
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[edit]Electromagnetic generators
[edit]Dynamo
Main article: Dynamo

"Dynamo Electric Machine" (end view, partly section, U.S. Patent 284,110)
A dynamo is an electrical generator that produces direct current with the use of a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting alternating to direct current using power rectification devices (vacuum tube or more recently solid state) is effective and usually economic.
[edit]Alternator
Main article: Alternator
Without a commutator, a dynamo becomes an alternator, which is a synchronous singly fed generator. Alternators produce alternating current with a frequency that is based on the rotational speed of the rotor and the number of magnetic poles.
Automotive alternators produce a varying frequency that changes with engine speed, which is then converted by a rectifier to DC. By comparison, alternators used to feed an electric power grid are generally operated at a speed very close to a specific frequency, for the benefit of AC devices that regulate their speed and performance based on grid frequency. Some devices such as incandescent lamps and ballast-operated fluorescent lamps do not require a constant frequency, but synchronous motors such as in electric wall clocks do require a constant grid frequency.
When attached to a larger electric grid with other alternators, an alternator will dynamically interact with the frequency already present on the grid, and operate at a speed that matches the grid frequency. If no driving power is applied, the alternator will continue to spin at a constant speed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary for an alternator to be accelerated up to the correct speed and phase alignment before connecting to the grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.
Typical alternators use a rotating field winding excited with direct current, and a stationary (stator) winding that produces alternating current. Since the rotor field only requires a tiny fraction of the power generated by the machine, the brushes for the field contact can be relatively small. In the case of a brushless exciter, no brushes are used at all and the rotor shaft carries rectifiers to excite the main field winding.
[edit]Induction generator
Main article: induction generator
An induction generator or asynchronous generator is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.
To operate an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self excited by using phase correcting capacitors.
[edit]MHD generator
Main article: MHD generator
A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The U.S. government funded substantial development, culminating in a 25 MW demonstration plant in 1987. In the Soviet Union from 1972 until the late 1980s, the MHD plant U 25 was in regular commercial operation on the Moscow power system with a rating of 25 MW, the largest MHD plant rating in the world at that time.[7] MHD generators operated as a topping cycle are currently (2007) less efficient than combined cycle gas turbines.
[edit]Other rotating electromagnetic generators
Other types of generators, such as the asynchronous or induction singly fed generator, the doubly fed generator, or the brushless wound-rotor doubly fed generator, do not incorporate permanent magnets or field windings that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or otherrenewable energy technologies.
The full output performance of any generator can be optimized with electronic control but only the doubly fed generators or the brushless wound-rotor doubly fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, a feature which, by itself, offers cost, reliability and efficiency benefits.
[edit]Homopolar generator
Main article: Homopolar generator

Faraday disk, the first homopolar generator
A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polaritydepending on the direction of rotation and the orientation of the field. It is also known as a unipolar generator, acyclic generator, disk dynamo, orFaraday disc. The voltage is typically low, on the order of a few volts in the case of small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even larger voltage.[8] They are unusual in that they can source tremendous electric current, some more than a million amperes, because the homopolar generator can be made to have very low internal resistance.
[edit]Excitation

A small early 1900s 75 KVA direct-driven power station AC alternator, with a separate belt-driven exciter generator.
Main article: Excitation (magnetic)
An electric generator or electric motor that uses field coils rather than permanent magnets requires a current to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all.
Smaller generators are sometimes self-excited, which means the field coils are powered by the current produced by the generator itself. The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to saturation and the generator reaches a steady state power output.
Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.[9]
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[edit]Electrostatic generator
Main article: electrostatic generator

A Van de Graaff generator, for class room demonstrations
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and lowcontinuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th Century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity. Electrostatic generators operate by using manual (or other) power to transform mechanical work into electric energy. Electrostatic generators develop electrostatic charges of opposite signs rendered to two conductors, using only electric forces, and work by using moving plates, drums, or belts to carry electric charge to a high potentialelectrode. The charge is generated by one of two methods: either the triboelectric effect (friction) or electrostatic induction.
[edit]Wimshurst machine
Main article: Wimshurst machine Wimshurst machine with two Leyden jars.Suppose that the conditions are as in the figure, with the segment A1 positive and the segment B1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A1 and neighboring sectors comes opposite the segment B2 of the B plate, which is now in contact with the brush Y, they will cause a displacement of electricity along the conductor between Y and Y1 bringing a negative charge, larger than the positive charge in A1 alone, on Y and sending a positive charge to the segment touching Y1. As A1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X, has a new charge, this time negative, driven into it by induction from B2 and neighboring sectors. As the machine turns, the process causes exponential increases in the voltages on all positions, until sparking occurs limiting the increase. |
The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by Britishinventor James Wimshurst (1832–1903).
It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane, two crossed bars with metallic brushes, and a spark gap formed by two metal spheres.
[edit]Van de Graaff generator
Main article: Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved in modern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitorand a very large electrical resistance, so it can produce a visible electrical discharge to a nearby grounding surface which can potentially cause a "spark" depending on the voltage.