Ahomopolar generator is aDCelectrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniformstatic magnetic field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder) with anelectrical polarity that depends on the direction of rotation and the orientation of the field. It is also known as aunipolar 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.[1] They are unusual in that they can source tremendous electric current, some more than a millionamperes, because the homopolar generator can be made to have very lowinternal resistance. Also, the homopolar generator is unique in that no other rotary electric machine can produce DC without using rectifiers or commutators.[2]
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The first homopolar generator was developed byMichael Faraday during his experiments in 1831. It is frequently called theFaraday disc orFaraday wheel in his honor. It was the beginning of moderndynamos — that is,electrical generators which operate using amagnetic field. It was very inefficient and was not used as a practical power source, but it showed the possibility of generating electric power using magnetism, and led the way forcommutateddirect current dynamos and thenalternating current alternators.
The Faraday disc was primarily inefficient due to counterflows of current. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces 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 around the circumference, and eliminate areas where counterflow could occur.
Long after the original Faraday disc had been abandoned as a practical generator, a modified version combining the magnet and disc in a single rotating part (therotor) was developed. Sometimes the namehomopolar generator is reserved for this configuration. One of the earliest patents on the general type of homopolar generators was attained by A. F. Delafield,U.S. patent 278,516. Other early patents for homopolar generators were awarded toS. Z. De Ferranti andC. Batchelor separately.Nikola Tesla was interested in the Faraday disc and conducted work with homopolar generators,[3] and eventually patented an improved version of the device inU.S. patent 406,968. Tesla's "Dynamo Electric Machine" patent describes an arrangement of two parallel discs with separate, parallel shafts, joined likepulleys by a metallic belt. Each disc had a field that was the opposite of the other, so that the flow of current was from the one shaft to the disc edge, across the belt to the other disc edge and to the second shaft. This would have greatly reduced the frictional losses caused by sliding contacts by allowing both electrical pickups to interface with the shafts of the two disks rather than at the shaft and a high-speed rim. Later, patents were awarded toC. P. Steinmetz andE. Thomson for their work with homopolar generators. TheForbes dynamo, developed by the Scottish electrical engineerGeorge Forbes, was in widespread use during the beginning of the 20th century. Much of the development done in homopolar generators was patented byJ. E. Noeggerath andR. Eickemeyer.
Homopolar generators underwent a renaissance in the 1950s as a source of pulsed power storage. These devices used heavy disks as a form offlywheel to store mechanical energy that could be quickly dumped into an experimental apparatus. An early example of this sort of device was built bySir Mark Oliphant at theResearch School of Physical Sciences and Engineering,Australian National University (ANU). It stored up to 500megajoules of energy[4] and was used as an extremely high-current source forsynchrotron experimentation from 1962 until it was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2megaamperes (MA).
Similar devices of even larger size are designed and built by Parker Kinetic Designs (formerly OIME Research & Development) of Austin. They have produced devices for a variety of roles, from poweringrailguns tolinear motors (for space launches) to a variety of weapons designs. Industrial designs of 10 MJ were introduced for a variety of roles, including electrical welding.[5][6]
This device consists of aconductingflywheel rotating in amagnetic field with one electrical contact near the axis and the other near the periphery. It has been used for generating very high currents at low voltages in applications such aswelding,electrolysis andrailgun research. In pulsed energy applications, theangular momentum of the rotor is used to accumulate energy over a long period and then release it in a short time.
In contrast to other types of generators, the output voltage never changes polarity. The charge separation results from theLorentz force on the free charges in the disk. The motion is azimuthal and the field is axial, so theelectromotive force is radial. The electrical contacts are usually made through a "brush" orslip ring, which results in large losses at the low voltages generated. Some of these losses can be reduced by usingmercury or other easily liquefied metal or alloy (gallium,NaK) as the "brush", to provide essentially uninterrupted electrical contact.
If the magnetic field is provided by a permanentmagnet, the generator works regardless of whether the magnet is fixed to the stator or rotates with the disc. Before the discovery of theelectron and theLorentz force law, thephenomenon was inexplicable and was known as theFaraday paradox.
A drum-type homopolar generator has a magnetic field (B) that radiates radially from the center of the drum and induces voltage (V) down the length of the drum.A conducting drum spun from above in the field of a "loudspeaker" type of magnet that has one pole in the center of the drum and the other pole surrounding the drum could use conducting ball bearings at the top and bottom of the drum to pick up the generated current.
Unipolar inductors occur in astrophysics where a conductor rotates through a magnetic field, for example, the movement of the highly conductiveplasma in a cosmic body'sionosphere through itsmagnetic field. In their book,Cosmical Electrodynamics,Hannes Alfvén andCarl-Gunne Fälthammar write:
Unipolar inductors have been associated with the aurorae onUranus,[8]binary stars,[9][10]black holes,[11][12][13]galaxies,[14] theJupiter Io system,[15][16] theMoon,[17][18] the Solar Wind,[19]sunspots,[20][21] and in theVenusian magnetic tail.[22]
Like alldynamos, the Faraday disc convertskinetic energy toelectrical energy. This machine can be analysed usingFaraday's own law ofelectromagnetic induction. This law, in its modern form, states that the full-time derivative of themagnetic flux through a closed circuit induces anelectromotive force in the circuit, which in turn drives an electric current. Thesurface integral that defines the magnetic flux can be rewritten as aline integral around the circuit. Although the integrand of the line integral is time-independent, because the Faraday disc that forms part of the boundary of line integral is moving, the full-time derivative is non-zero and returns the correct value for calculating the electromotive force.[23][24] Alternatively, the disc can be reduced to a conductive ring along the disc's circumference with a single metal spoke connecting the ring to the axle.[25]
TheLorentz force law is more easily used to explain the machine's behaviour. This law, formulated thirty years after Faraday's death, states that the force on an electron is proportional to thecross product of itsvelocity and themagnetic flux vector. In geometrical terms, this means that the force is at right-angles to both the velocity (azimuthal) and the magnetic flux (axial), which is therefore in a radial direction. The radial movement of the electrons in the disc produces a charge separation between the center of the disc and its rim, and if the circuit is completed an electric current will be produced.[26]
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