Homopolar generator

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A homopolar generator is a DC
electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delive ...
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) with an
electrical polarity Electrical polarity is a term used throughout industries and fields that involve electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves ...
that depends on the direction of rotation and the orientation of the field. 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. They are unusual in that they can source tremendous electric current, some more than a million
amperes The ampere (, ; symbol: A), often Clipping (morphology), shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the SI base unit, base unit of electric current in the International System of ...
, because the homopolar generator can be made to have very low
internal resistance A practical electrical power Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt The watt (symbol: W) is a unit of Power (physics), power or radiant ...

. Also, the homopolar generator is unique in that no other rotary electric machine can produce DC without using rectifiers or commutators.

The first homopolar generator was developed by
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

during his experiments in 1831. It is frequently called the Faraday disc or Faraday wheel in his honor. It was the beginning of modern
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

s — that is,
electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delive ...
s which operate using a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

. 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 for commutated
direct current Direct current (DC) is one-directional flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathe ...
dynamos and then
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
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.

# Homopolar generator development

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 (the ''rotor'') was developed. Sometimes the name ''homopolar generator'' is reserved for this configuration. One of the earliest patents on the general type of homopolar generators was attained by A. F. Delafield, . Other early patents for homopolar generators were awarded to and C. Batchelor separately.
Nikola Tesla Nikola Tesla ( ; sr-cyr, Никола Тесла, ; 10 July 1856 – 7 January 1943) was a Serbian-American inventor, electrical engineer, mechanical engineer, and futurist best known for his contributions to the design of the moder ...

was interested in the Faraday disc and conducted work with homopolar generators, and eventually patented an improved version of the device in . Tesla's "Dynamo Electric Machine" patent describes an arrangement of two parallel discs with separate, parallel shafts, joined like
pulley A pulley is a wheel File:Roue primitive.png, An early wheel made of a solid piece of wood A wheel is a circular component that is intended to rotate on an axle An axle or axletree is a central shaft for a rotating wheel or gear. On ...

s 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 to and E. Thomson for their work with homopolar generators. The Forbes dynamo, developed by the Scottish electrical engineer George Forbes, was in widespread use during the beginning of the 20th century. Much of the development done in homopolar generators was patented by J. E. Noeggerath and . Homopolar generators underwent a renaissance in the 1950s as a source of pulsed power storage. These devices used heavy disks as a form of
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum. It is an important quantit ...

to store mechanical energy that could be quickly dumped into an experimental apparatus. An early example of this sort of device was built by Sir Mark Oliphant at the
Research School of Physical Sciences and Engineering The Research School of Physics (RSPhys) was established with the creation of the Australian National University (ANU) in 1947. Located at the ANU's main campus in Canberra, the school is one of the four founding research schools in the ANU's Austr ...
,
Australian National University The Australian National University (ANU) is a national research university located in Canberra Canberra ( ) is the capital city of Australia. Founded following the Federation of Australia, federation of the colonies of Australia as t ...
. It stored up to 500
megajoule The joule ( ; symbol: J) is a derived unit of energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform W ...
s of energy and was used as an extremely high-current source for
synchrotron A synchrotron is a particular type of cyclic , descended from the , in which the accelerating particle beam travels around a fixed closed-loop path. The which bends the particle beam into its closed path increases with time during the acceler ...

experimentation from 1962 until it was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2
megaampere The ampere (, ; symbol: A), often Clipping (morphology), shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the SI base unit, base unit of electric current in the International System of ...

s (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 powering
railgun A railgun is a linear motor Synchronous linear motors are straightened versions of permanent magnet rotor motors A linear motor is an electric motor that has had its stator and rotor (electric), rotor "unrolled" thus instead of producing a to ...
s to
linear motor Synchronous linear motors are straightened versions of permanent magnet rotor motors A linear motor is an electric motor that has had its stator The stator is the stationary part of a rotary system, found in electric generators, electric mo ...

s (for space launches) to a variety of weapons designs. Industrial designs of 10 MJ were introduced for a variety of roles, including electrical welding.

# Description and operation

## Disc-type generator

This device consists of a
conducting Conducting is the art of directing a musical performance, such as an orchestral or Choir, choral concert. It has been defined as "the art of directing the simultaneous performance of several players or singers by the use of gesture." The primary ...
flywheel A flywheel is a mechanical device which uses the conservation of angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum. It is an important quantit ...

rotating in a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

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 as
welding Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

,
electrolysis In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms ...

and
railgun A railgun is a linear motor Synchronous linear motors are straightened versions of permanent magnet rotor motors A linear motor is an electric motor that has had its stator and rotor (electric), rotor "unrolled" thus instead of producing a to ...
research. In pulsed energy applications, the
angular momentum In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant. In three , the ...

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 the
Lorentz force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...

on the free charges in the disk. The motion is azimuthal and the field is axial, so the
electromotive force In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...
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" or
slip ring A slip ring is an electromechanical In engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. ...
, which results in large losses at the low voltages generated. Some of these losses can be reduced by using mercury (element), mercury or other easily liquefied metal or alloy (gallium, NaK) as the "brush", to provide essentially uninterrupted electrical contact. A recent suggested modification is to use a plasma (physics), plasma contact supplied by a negative resistance neon streamer touching the edge of the disk or drum, using specialized low work function carbon in vertical strips. This would have the advantage of very low resistance within a current range possibly up to thousands of amps without the liquid metal contact. If the magnetic field is provided by a permanent magnet, the generator works regardless of whether the magnet is fixed to the stator or rotates with the disc. Before the discovery of the electron and the Lorentz force law, the phenomenon was inexplicable and was known as the Faraday paradox.

## Drum-type generator

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.

# Astrophysical unipolar inductors

Unipolar inductors occur in astrophysics where a conductor rotates through a magnetic field, for example, the movement of the highly conductive Plasma (physics), plasma in a cosmic body's ionosphere through its
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

. In their book, ''Cosmical Electrodynamics'', Hannes Alfvén and Carl-Gunne Fälthammar write: :"Since cosmical clouds of ionized gas are generally magnetized, their motion produces induced electric fields [..] For example the motion of the magnetized interplanetary plasma produces electric fields that are essential for the production of aurora and magnetic storms" [..] :".. the rotation of a conductor in a magnetic field produces an electric field in the system at rest. This phenomenon is well known from laboratory experiments and is usually called 'homopolar ' or 'unipolar' induction. Unipolar inductors have been associated with the aurorae on Uranus, binary stars, black holes, galaxy, galaxies, the Io (moon)#Interaction with Jupiter's magnetosphere, Jupiter Io system, the Moon, the Solar Wind, sunspots, and in the Venus, Venusian magnetic tail.

# Physics

Like all Electrical generator#Dynamo, dynamos, the Faraday disc converts kinetic energy to electrical energy. This machine can be analysed using Faraday's own law of electromagnetic induction. This law, in its modern form, states that the full-time derivative of the magnetic flux through a closed circuit induces an
electromotive force In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is c ...
in the circuit, which in turn drives an electric current. The surface integral that defines the magnetic flux can be rewritten as a line 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. 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. The Lorentz 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 the cross product of its velocity and the magnetic 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.''Electromagnetic Field Theory''
2nd ed. by Bo Thidé, Department of Physics and Astronomy, Uppsala University, Sweden

* Barlow's wheel * Electric generator * Electric motor * Homopolar motor * Faraday paradox * Faraday's law of induction

# References

## General references

* Don Lancaster, "
Shattering the homopolar myths
'". Tech Musings, October, 1997. (PDF) * Don Lancaster, "
'". Tech Musings, October, 1997. (PDF) * John David Jackson, ''Classical Electrodynamics'', Wiley, 3rd ed. 1998, * Arthur I. Miller, "Unipolar Induction: A Case Study of the Interaction between Science and Technology," Annals of Science, Volume 38, pp. 155–189 (1981). * Olivier Darrigol, ''Electrodynamics from Ampere to Einstein'', Oxford University Press, 2000, * Trevor Ophel and John Jenkin, (1996)
Fire in the belly
: the first 50 years of the pioneer school at the ANU'' Canberra : Research School of Physical Sciences and Engineering, Australian National University. . (PDF) * Thomas Valone, ''The Homopolar Handbook : A Definitive Guide to Faraday Disk and N-Machine Technologies''. Washington, DC, U.S.A.: Integrity Research Institute, 2001.

* Richard A. Marshall and William F. Weldon, "''Parameter Selection for Homopolar Generators Used as Pulsed Energy Stores''", Center for Electromechanics, University of Texas, Austin, Jul. 1980. (also published in: Electrical Machines and Electromechanics, 6:109–127, 1981.)

* Popular Science Monthly, ''Construction of Unipolar Dynamos'', April 1916, pp. 624–626, Scanned article available via Google Books: https://books.google.com/books?id=hCYDAAAAMBAJ&pg=PA624 * Robert Hebner, "

'". Homopolar Welding, UT-CEM. * "

'". physics.umd.edu. * Richard E. Berg and Carroll O. Alley, "
The Unipolar Generator: A Demonstration of Special Relativity
'", Department of Physics, University of Maryland, 2005. (PDF) * Richard Fitzpatrick, "''Magnetohydrodynamic theory ''",

'. farside.ph.utexas.edu, 2006-02-16. * "

Lecture Demonstrations.''". physics.brown.edu * William J. Beaty, "