Homopolar generator

Faraday disk, the first homopolar generator

A homopolar generator is a DC electrical generatorcomprising an electrically conductive disc or cylinder ro-tating in a plane perpendicular to a uniform static mag-netic field. A potential difference is created between thecenter of the disc and the rim (or ends of the cylinder)with an electrical polarity that depends on the direction ofrotation and the orientation of the field. It is also known asa unipolar generator, acyclic generator, disk dynamo,or Faraday disc. The voltage is typically low, on the or-der of a few volts in the case of small demonstration mod-els, but large research generators can produce hundredsof volts, and some systems have multiple generators in se-ries to produce an even larger voltage.[1] They are unusualin that they can source tremendous electric current, somemore than amillion amperes, because the homopolar gen-erator can be made to have very low internal resistance.

1 The Faraday disc

The homopolar generator was developed first by MichaelFaraday during his experiments in 1831. It is frequentlycalled the Faraday disc or Faraday wheel in his honor.It was the beginning of modern dynamos — that is,electrical generators which operate using a magnetic field.It was very inefficient and was not used as a practicalpower source, but it showed the possibility of generat-ing electric power using magnetism, and led the way forcommutated direct current dynamos and then alternatingcurrent alternators.The Faraday disc was primarily inefficient due to coun-terflows of current. While current flow was induced di-rectly underneath the magnet, the current would circulate

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B B

Faraday disc

backwards in regions outside the influence of the mag-netic field. This counterflow limits the power output tothe pickup wires, and induces waste heating of the copperdisc. Later homopolar generators would solve this prob-lem by using an array of magnets arranged around thedisc perimeter to maintain a steady field around the cir-cumference, and eliminate areas where counterflow couldoccur.

2 Homopolar generator develop-ment

The remains of the ANU 500 MJ generator

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2 4 ASTROPHYSICAL UNIPOLAR INDUCTORS

Long after the original Faraday disc had been abandonedas a practical generator, a modified version combining themagnet and disc in a single rotating part (the rotor) wasdeveloped. Sometimes the name homopolar generator isreserved for this configuration. One of the earliest patentson the general type of homopolar generators was attainedby A. F. Delafield, U.S. Patent 278,516. Other earlypatents for homopolar generators were awarded to S. Z.De Ferranti and C. Batchelor separately. Nikola Teslawas interested in the Faraday disc and conducted workwith homopolar generators.[2] He eventually patented animproved version of the device and his US patent (“Dy-namo Electric Machine”) describes an arrangement oftwo parallel discs with separate, parallel shafts, joinedlike pulleys by a metallic belt. Each disc had a field thatwas the opposite of the other, so that the flow of cur-rent was from the one shaft to the disc edge, across thebelt to the other disc edge and to the second shaft. Thiswould have greatly reduced the frictional losses caused bysliding contacts by allowing both electrical pickups to in-terface with the shafts of the two disks rather than at theshaft and a high-speed rim. Later, patents were awardedto C. P. Steinmetz and E. Thomson for their work withhomopolar generators. The Forbes dynamo, developedby the Scottish electrical engineer George Forbes, was inwidespread use during the beginning of the 20th century.Much of the development done in homopolar generatorswas patented by J. E. Noeggerath and R. Eickemeyer.Homopolar generators underwent a renaissance in the1950s as a source of pulsed power storage. These devicesused heavy disks as a form of flywheel to store mechan-ical energy that could be quickly dumped into an experi-mental apparatus. An early example of this sort of devicewas built by Sir Mark Oliphant at the Research School ofPhysical Sciences and Engineering, Australian NationalUniversity. It stored up to 500 megajoules of energy[3]and was used as an extremely high-current source forsynchrotron experimentation from 1962 until it was dis-assembled in 1986. Oliphant’s construction was capableof supplying currents of up to 2 megaamperes (MA).Similar devices of even larger size are designed and builtby Parker Kinetic Designs (formerly OIME Research &Development) of Austin. They have produced devices fora variety of roles, from powering railguns to linear mo-tors (for space launches) to a variety of weapons designs.Industrial designs of 10 MJ were introduced for a varietyof roles, including electrical welding.[4]

3 Description and operation

3.1 Disk-type generator

This device consists of a conducting flywheel rotating ina magnetic field with one electrical contact near the axisand the other near the periphery. It has been used forgenerating very high currents at low voltages in applica-

Basic Faraday disc generator

tions such as welding, electrolysis and railgun research.In pulsed energy applications, the angular momentum ofthe rotor is used to store energy over a long period andthen release it in a short time.In contrast to other types of generators, the output volt-age never changes polarity. The charge separation re-sults from the Lorentz force on the free charges in thedisk. The motion is azimuthal and the field is axial, sothe electromotive force is radial. The electrical contactsare usually made through a "brush" or slip ring, which re-sults in large losses at the low voltages generated. Some ofthese losses can be reduced by usingmercury or other eas-ily liquified metal or alloy (gallium, NaK) as the “brush”,to provide essentially uninterrupted electrical contact.If the magnetic field is provided by a permanent magnet,the generator works regardless of whether the magnetis fixed to the stator or rotates with the disc. Beforethe discovery of the electron and the Lorentz force law,the phenomenon was inexplicable and was known as theFaraday paradox.

3.2 Drum-type generator

A drum-type homopolar generator has a magnetic field(B) that radiates radially from the center of the drum andinduces voltage (V) down the length of the drum. A con-ducting drum spun from above in the field of a “loud-speaker” type of magnet that has one pole in the center ofthe drum and the other pole surrounding the drum coulduse conducting ball bearings at the top and bottom of thedrum to pick up the generated current.

4 Astrophysical unipolar inductors

Unipolar inductors occur in astrophysics where a con-ductor rotates through a magnetic field, for example, themovement of the highly conductive plasma in a cos-mic body’s ionosphere through its magnetic field. In

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their book, Cosmical Electrodynamics, Hannes Alfvénand Carl-Gunne Fälthammar write:

“Since cosmical clouds of ionized gas are gen-erally magnetized, their motion produces in-duced electric fields [..] For example the mo-tion of the magnetized interplanetary plasmaproduces electric fields that are essential for theproduction of aurora and magnetic storms” [..]".. the rotation of a conductor in a magneticfield produces an electric field in the system atrest. This phenomenon is well known from lab-oratory experiments and is usually called 'ho-mopolar ' or 'unipolar' induction.[5]

Unipolar inductors have been associated with the auro-rae on Uranus,[6] binary stars,[7][8] black holes,[9][10][11]galaxies,[12] the Jupiter Io system,[13][14] the Moon,[15][16]the Solar Wind,[17] sunspots,[18][19] and in the Venusianmagnetic tail.[20]

5 Physics

Like all dynamos, the Faraday disc converts kinetic en-ergy to electrical energy. This machine can not be anal-ysed using Faraday’s own law of electromagnetic induc-tion. This law (in its modern form) states that an electriccurrent is induced in a closed electrical circuit when themagnetic flux enclosed by the circuit changes. In Fara-day’s law, EMF is the time-derivative of flux, so a DCEMF is only possible if the magnetic flux is getting uni-formly larger and larger perpetually. But in the genera-tor, the magnetic field is constant and the disc stays in thesame position, so no magnetic fluxes are growing largerand larger. So this example cannot be analyzed directlywith Faraday’s law.The Lorentz force law is more easily used to explain themachine’s behaviour. This law, formulated thirty yearsafter Faraday’s death, states that the force on an electronis proportional to the cross product of its velocity and themagnetic flux vector. In geometrical terms, this meansthat the force is at right-angles to both the velocity (az-imuthal) and the magnetic flux (axial), which is thereforein a radial direction. The radial movement of the elec-trons in the disc produces a charge separation betweenthe center of the disc and its rim, and if the circuit iscompleted an electric current will be produced.[21]

6 See also• Barlow’s wheel

• Electric generator

• Electric motor

• Homopolar motor

• Faraday paradox

• Faraday’s law of induction

7 References[1] Losty, H.H.W & Lewis, D.L. (1973) Homopolar Ma-

chines. Philosophical Transactions for the Royal Soci-ety of London. Series A, Mathematical and Physical Sci-ences. 275 (1248), 69-75

[2] Nikola Tesla, "Notes on a Unipolar Dynamo". The Elec-trical Engineer, N.Y., Sept. 2, 1891. (Also available attesla.hu, Article 18910902)

[3] J.W. Blamey, P.O. Carden, L.U. Hibbard, E.K. Inall,R.A. Marshall and Sir Mark Oliphant, 'The large ho-mopolar generator at Canberra: initial tests’, Nature, 195(1962), pp. 113–114.

[4] Thomas Valone, “The Homopolar Handbook”, IntegrityResearch Institute, 1994 , pg. 45

[5] Hannes Alfvén and Carl-Gunne Fälthammar, CosmicalElectrodynamics (1963) 2nd Edition, Oxford UniversityPress. See sec. 1.3.1. Induced electric field in uniformlymoving matter.

[6] Hill, T. W.; Dessler, A. J.; Rassbach, M. E., "Auroraon Uranus – A Faraday disc dynamo mechanism" (1983)Planetary and Space Science (ISSN 0032-0633), vol. 31,Oct. 1983, p. 1187–1198

[7] Hannes Alfvén, "Sur l'origine de la radiation cosmique"(On the origin of cosmic radiation)" Comptes Rendus,204, pp.1180–1181 (1937)

[8] Hakala, Pasi et al., "Spin up in RX J0806+15: the shortestperiod binary" (2003) Monthly Notice of the Royal Astro-nomical Society, Volume 343, Issue 1, pp. L10–L14

[9] Lovelace, R.V.E. "Dynamo Model of Double RadioSources"

[10] Burns, M. L.; Lovelace, R. V. E., "Theory of electron-positron showers in double radio sources" (1982) Astro-physical Journal, Part 1, vol. 262, Nov. 1, 1982, p. 87–99

[11] Shatskii, A. A., "Unipolar Induction of a Magnetized Ac-cretion Disk around a Black Hole", (2003)Astronomy Let-ters, vol. 29, p. 153–157

[12] Per Carlqvist, "Cosmic electric currents and the general-ized Bennett relation" (1988) Astrophysics and Space Sci-ence (ISSN 0004-640X), vol. 144, no. 1–2, May 1988,pp. 73–84.

[13] Goldreich, P.; Lynden-Bell, D., "Io, a jovian unipolar in-ductor" (1969) Astrophys. J., vol. 156, pp. 59–78 (1969).

[14] Strobel, Darrell F.; et al., "Hubble Space Telescope SpaceTelescope Imaging Spectrograph Search for an Atmo-sphere on Callisto: A Jovian Unipolar Inductor" (2002)The Astrophysical Journal, Volume 581, Issue 1, pp. L51–L54

4 8 EXTERNAL LINKS

[15] “Sonett, C. P.; Colburn, D. S., "Establishment of a LunarUnipolar Generator and Associated Shock and Wake bythe Solar Wind" (1967) Nature, vol. 216, 340–343.

[16] Schwartz, K.; Sonett, C. P.; Colburn, D. S., "Unipolar In-duction in the Moon and a Lunar Limb Shock Mecha-nism" in The Moon, Vol. 1, p.7

[17] Srnka, L. J., "Sheath-limited unipolar induction in the so-lar wind" (1975) Astrophysics and Space Science', vol. 36,Aug. 1975, pp. 177–204.

[18] Yang, Hai-Shou, "A force – free field theory of solarflares I. Unipolar sunspots" Chinese Astronomy and As-trophysics, Volume 5, Issue 1, pp. 77–83.

[19] Osherovich, V. A.; Garcia, H. A., "Electric current in aunipolar sunspot with an untwisted field" (1990)Geophys-ical Research Letters (ISSN 0094-8276), vol. 17, Nov.1990, pp. 2273–2276.

[20] Eroshenko, E. G., "Unipolar induction effects in theVenusian magnetic tail" (1979) Kosmicheskie Issle-dovaniia, vol. 17, Jan.–Feb. 1979, pp. 93–10

[21] Electromagnetic Field Theory, 2nd ed. by Bo Thidé, De-partment of Physics and Astronomy, Uppsala University,Sweden

7.1 General references

• Don Lancaster, "Shattering the homopolar myths".Tech Musings, October, 1997. (PDF)

• Don Lancaster, "Understanding Faraday’s Disk".Tech Musings, October, 1997. (PDF)

• John David Jackson, Classical Electrodynamics, Wi-ley, 3rd ed. 1998, ISBN 0-471-30932-X

• Arthur I. Miller, “Unipolar Induction: A Case Studyof the Interaction between Science and Technol-ogy,” Annals of Science, Volume 38, pp. 155–189(1981).

• Olivier Darrigol, Electrodynamics from Ampere toEinstein, Oxford University Press, 2000, ISBN 0-19-850594-9

• Trevor Ophel and John Jenkin, (1996) Fire in thebelly : the first 50 years of the pioneer school at theANU Canberra : Research School of Physical Sci-ences and Engineering, Australian National Univer-sity. ISBN 0-85800-048-2. (PDF)

• Thomas Valone, The Homopolar Handbook : ADefinitive Guide to Faraday Disk and N-MachineTechnologies. Washington, DC, U.S.A.: IntegrityResearch Institute, 2001. ISBN 0-964107-0-1-5

7.2 Further reading

• Richard A. Marshall and William F. Weldon, "Pa-rameter Selection for Homopolar Generators Used asPulsed Energy Stores", Center for Electromechanics,University of Texas, Austin, Jul. 1980. (also pub-lished in: Electrical Machines and Electromechan-ics, 6:109–127, 1981.)

8 External links• Popular Science Monthly, Construction of UnipolarDynamos, April 1916, pp. 624–626, Scanned arti-cle available via Google Books: http://books.google.com/books?id=hCYDAAAAMBAJ&pg=PA624

• Robert Hebner, "Homopolar Generator". Homopo-lar Welding, UT-CEM.

• "K2-64: Unipolar generator". physics.umd.edu.

• Richard E. Berg and Carroll O. Alley, "The Unipo-lar Generator: A Demonstration of Special Relativ-ity", Department of Physics, University of Mary-land, 2005. (PDF)

• Richard Fitzpatrick, "Magnetohydrodynamic theory", The homopolar generator. farside.ph.utexas.edu,2006-02-16.

• "5K10.80 Homopolar Generator; Lecture Demon-strations.". physics.brown.edu

• William J. Beaty, "Untried Homopolar GeneratorExperiments". 1996.

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