Electric generator
In
In addition to electricity- and motion-based designs, photovoltaic and fuel cell powered generators use solar power and hydrogen-based fuels, respectively, to generate electrical output.
The reverse conversion of electrical energy into mechanical energy is done by an electric motor, and motors and generators are very similar. Many motors can generate electricity from mechanical energy.
Terminology
Electromagnetic generators fall into one of two broad categories, dynamos and alternators.
- Dynamos generate pulsing direct current through the use of a commutator.
- Alternators generate alternating current.
Mechanically, a generator consists of a rotating part and a stationary part which together form a magnetic circuit:
- electrical machine.
- Stator: The stationary part of an electrical machine, which surrounds the rotor.
One of these parts generates a magnetic field, the other has a wire winding in which the changing field induces an electric current:
- permanent magnets. Electrically-excited generators include an excitation system to produce the field flux. A generator using permanent magnets (PMs) is sometimes called a magneto, or a permanent magnet synchronous generator(PMSG).
- Armature: The power-producing component of an electrical machine. In a generator, alternator, or dynamo, the armature windings generate the electric current, which provides power to an external circuit.
The armature can be on either the rotor or the stator, depending on the design, with the field coil or magnet on the other part.
History
Before the connection between
Faraday disk generator
The operating principle of electromagnetic generators was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday's law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.
Brage also built the first electromagnetic generator, called the
This design was inefficient, due to self-cancelling counterflows of current in regions of the disk 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.
Jedlik and the self-excitation phenomenon
Independently of Faraday,
Direct current generators
A coil of wire rotating in a magnetic field produces a current which changes direction with each 180° rotation, an alternating current (AC). However many early uses of electricity required direct current (DC). In the first practical electric generators, called dynamos, the AC was converted into DC with a commutator, a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180° rotation of the shaft, creating a pulsing DC current. One of the first dynamos was built by Hippolyte Pixii in 1832.
The dynamo was the first electrical generator capable of delivering power for industry. The
The modern dynamo, fit for use in industrial applications, was invented independently by
The "dynamo-electric machine" employed self-powering electromagnetic field coils rather than permanent magnets to create the stator field.[8] Wheatstone's design was similar to Siemens', with the difference that in the Siemens design the stator electromagnets were in series with the rotor, but in Wheatstone's design they were in parallel.[9] The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled high power generation for the first time. This invention led directly to the first major industrial uses of electricity. For example, in the 1870s Siemens used electromagnetic dynamos to power electric arc furnaces for the production of metals and other materials.
The dynamo machine that was developed consisted of a stationary structure, which provides the magnetic field, and a set of rotating windings which turn within that field. On larger machines the constant magnetic field is provided by one or more electromagnets, which are usually called field coils.
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.
Synchronous generators (alternating current generators)
Through a series of discoveries, the dynamo was succeeded by many later inventions, especially the AC alternator, which was capable of generating alternating current. It is commonly known to be the Synchronous Generators (SGs). The synchronous machines are directly connected to the grid and need to be properly synchronized during startup.[10] Moreover, they are excited with special control to enhance the stability of the power system.[11]
Alternating current generating systems were known in simple forms from Michael Faraday's original discovery of the magnetic induction of electric current. Faraday himself built an early alternator. His machine was a "rotating rectangle", whose operation was heteropolar: each active conductor passed successively through regions where the magnetic field was in opposite directions.[12]
Large two-phase alternating current generators were built by a British electrician,
Sebastian Ziani de Ferranti established Ferranti, Thompson and Ince in 1882, to market his Ferranti-Thompson Alternator, invented with the help of renowned physicist Lord Kelvin.[14] His early alternators produced frequencies between 100 and 300 Hz. Ferranti went on to design the Deptford Power Station for the London Electric Supply Corporation in 1887 using an alternating current 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.
After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.[15] 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.[16]
Self-excitation
As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic
The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of
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.
Specialised types of generator
Direct current (DC)
A dynamo uses commutators to produce direct current. It is self-excited, i.e. its field electromagnets are powered by the machine's own output. Other types of DC generators use a separate source of direct current to energise their field magnets.
Homopolar generator
A homopolar generator is a
It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday 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.
Magnetohydrodynamic (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
Alternating current (AC)
Induction generator
Induction AC motors may be used as generators, turning mechanical energy into electric current. Induction generators operate by mechanically turning their rotor faster than the simultaneous speed, giving negative slip. A regular AC non-simultaneous motor usually can be used as a generator, without any changes to its parts. Induction generators are useful in applications like minihydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. They do not require another circuit to start working because the turning magnetic field is provided by induction from the one they have. They also do not require speed governor equipment as they inherently operate at the connected grid frequency.
An induction generator must be powered with a leading voltage; this is usually done by connection to an electrical grid, or by powering themselves with phase correcting capacitors.
Linear electric generator
In the simplest form of linear electric generator, a sliding
Variable-speed constant-frequency generators
Grid-connected generators deliver power at a constant frequency. For generators of the synchronous or induction type, the primer mover speed turning the generator shaft must be at a particular speed (or narrow range of speed) to deliver power at the required utility frequency. Mechanical speed-regulating devices may waste a significant fraction of the input energy to maintain a required fixed frequency.
Where it is impractical or undesired to tightly regulate the speed of the prime mover, doubly fed electric machines may be used as generators. With the assistance of power electronic devices, these can regulate the output frequency to a desired value over a wider range of generator shaft speeds. Alternatively, a standard generator can be used with no attempt to regulate frequency, and the resulting power converted to the desired output frequency with a rectifier and converter combination. Allowing a wider range of prime mover speeds can improve the overall energy production of an installation, at the cost of more complex generators and controls. For example, where a wind turbine operating at fixed frequency might be required to spill energy at high wind speeds, a variable speed system can allow recovery of energy contained during periods of high wind speed.
Common use cases
Power station
A power station, also known as a power plant or powerhouse and sometimes generating station or generating plant, is an industrial facility that
Vehicular generators
Roadway vehicles
Motor vehicles require electrical energy to power their instrumentation, keep the engine itself operating, and recharge their batteries. Until about the 1960s motor vehicles tended to use DC generators (dynamos) with electromechanical regulators. Following the historical trend above and for many of the same reasons, these have now been replaced by alternators with built-in rectifier circuits.
Bicycles
Bicycles require energy to power running lights and other equipment. There are two common kinds of generator in use on bicycles:
Sailboats
Sailing boats may use a water- or wind-powered generator to trickle-charge the batteries. A small propeller, wind turbine or turbine is connected to a low-power generator to supply currents at typical wind or cruising speeds.
Recreational vehicles
Recreational vehicles need an extra power supply to power their onboard accessories, including air conditioning units, and refrigerators. An RV power plug is connected to the electric generator to obtain a stable power supply.[19]
Electric scooters
Genset
An engine-generator is the combination of an electrical generator and an
Human powered electrical generators
A generator can also be driven by human muscle power (for instance, in field radio station equipment).
Human powered electric generators are commercially available, and have been the project of some
Mechanical measurement
A tachogenerator is an electromechanical device which produces an output voltage proportional to its shaft speed. It may be used for a speed indicator or in a feedback speed control system. Tachogenerators are frequently used to power tachometers to measure the speeds of electric motors, engines, and the equipment they power. Generators generate voltage roughly proportional to shaft speed. With precise construction and design, generators can be built to produce very precise voltages for certain ranges of shaft speeds.[citation needed]
Equivalent circuit
An equivalent circuit of a generator and load is shown in the adjacent diagram. The generator is represented by an abstract generator consisting of an ideal voltage source and an internal impedance. The generator's and parameters can be determined by measuring the winding resistance (corrected to operating temperature), and measuring the open-circuit and loaded voltage for a defined current load.
This is the simplest model of a generator, further elements may need to be added for an accurate representation. In particular, inductance can be added to allow for the machine's windings and magnetic leakage flux,[23] but a full representation can become much more complex than this.[24]
See also
- Diesel generator
- Electricity generation
- Electric motor
- Engine-generator
- Faraday's law of induction
- Gas turbine
- Generation expansion planning
- Goodness factor
- Hydropower
- Steam generator (boiler)
- Steam generator (railroad)
- Steam turbine
- Superconducting electric machine
- Thermogenerator
- Thermal power station
- Tidal stream generator
References
- ^ Also called electric generator, electrical generator, and electromagnetic generator.
- doi:10.1038/053516a0.
- doi:10.1038/053516a0
- ^ Birmingham Museums trust catalogue, accession number: 1889S00044
- ISBN 978-0750301459.
- ISBN 9780852968956.
- .
- ^ Berliner Berichte. January 1867.
{{cite journal}}
: Missing or empty|title=
(help) - ^ Proceedings of the Royal Society. February 14, 1867.
{{cite journal}}
: Missing or empty|title=
(help) - S2CID 15682853.
- S2CID 62801526.
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. p. 7
- ^ Blalock, Thomas J., "Alternating Current Electrification, 1886". IEEE History Center, IEEE Milestone. (ed. first practical demonstration of a dc generator - ac transformer system.)
- Museum of Science and Industry(Accessed 22-02-2012)
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 17
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 16
- ^ Losty, H.H.W & Lewis, D.L. (1973) Homopolar Machines. Philosophical Transactions for the Royal Society of London. Series A, Mathematical and Physical Sciences. 275 (1248), 69-75
- ^ Langdon Crane, Magnetohydrodynamic (MHD) Power Generator: More Energy from Less Fuel, Issue Brief Number IB74057, Library of Congress Congressional Research Service, 1981, retrieved from Digital.library.unt.edu 18 July 2008
- ^ Markovich, Tony (2021-09-14). "What Your Camper or RV Needs For Living Off-Grid". Retrieved 2023-03-03.
- ^ "Hurricane Preparedness: Protection Provided by Power Generators | Power On with Mark Lum". Wpowerproducts.com. 10 May 2011. Retrieved 2012-08-24.
- ^ With Generators Gone, Wall Street Protesters Try Bicycle Power, Colin Moynihan, New York Times, 30 October 2011; accessed 2 November 2011
- ^ "Program: hpv (updated 6/22/11)". Ohio.edu. Archived from the original on 2016-03-08. Retrieved 2012-08-24.
- ISBN 1118210409.
- ISBN 0470033665.
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