Homopolar generator

# Homopolar generator

Overview
A homopolar generator is a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

comprising an electrically conductive disc 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, the electrical polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...

depending 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, because the homopolar generator can be made to have very low internal resistance.
Discussion

Encyclopedia
A homopolar generator is a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

comprising an electrically conductive disc 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, the electrical polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...

depending 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, because the homopolar generator can be made to have very low internal resistance.

The homopolar generator was developed first by Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....

during his experiments in 1831. It is frequently called the Faraday disc in his honor. It was the beginning of modern dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...

s — that is, electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

s which operate using a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. 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
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...

direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

dynamos and then alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

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 radially from axis to edge, 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 S. Z. De Ferranti
Sebastian Ziani de Ferranti
Sebastian Pietro Innocenzo Adhemar Ziani de Ferranti was an electrical engineer and inventor.-Personal life:...

and C. Batchelor
Charles Batchelor
Charles W. Batchelor was an inventor and close associate of American inventor Thomas Alva Edison during much of Edison’s career. He was involved in some of the greatest inventions and technological developments in history....

separately. Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

was interested in the Faraday disc and conducted work with homopolar generators. He eventually patented an improved version of the device and his US patent ("Dynamo Electric Machine") describes an arrangement of two parallel discs with separate, parallel shafts, joined like pulley
Pulley
A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

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 C. P. Steinmetz
Charles Proteus Steinmetz
Charles Proteus Steinmetz was a German-American mathematician and electrical engineer. He fostered the development of alternating current that made possible the expansion of the electric power industry in the United States, formulating mathematical theories for engineers...

and E. Thomson
Elihu Thomson
Elihu Thomson was an American engineer and inventor who was instrumental in the founding of major electrical companies in the United States, the United Kingdom and France.-Early life:...

for their work with homopolar generators. The Forbes dynamo, developed by the Scottish electrical engineer George Forbes
George Forbes (scientist)
George Forbes FRS was an electrical engineer, astronomer, explorer, author and inventor, some of whose inventions are still in use.-Early life:...

, 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 R. Eickemeyer.

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
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

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
Mark Oliphant
Sir Marcus 'Mark' Laurence Elwin Oliphant, AC, KBE, FRS was an Australian physicist and humanitarian who played a fundamental role in the first experimental demonstration of nuclear fusion and also the development of the atomic bomb.During his retirement, Oliphant was appointed as the Governor of...

at the Research School of Physical Sciences and Engineering
Research School of Physical Sciences and Engineering
The Research School of Physics and Engineering was established with the creation of the Australian National University 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 Institute of Advanced Studies.As part of the Institute...

, Australian National University
Australian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...

. It stored up to 500 megajoules of energy and was used as an extremely high-current source for synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...

experimentation from 1962 until it was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2 megaampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

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
Railgun
A railgun is an entirely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor. Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts...

s to linear motor
Linear motor
A linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque it produces a linear force along its length...

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.

### Disk-type generator

This device consists of a conducting flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

rotating in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

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
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

, electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

and railgun
Railgun
A railgun is an entirely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor. Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts...

research. In pulsed energy applications, the angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

of the rotor is used to store 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
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...

on the free charges in the disk. The motion is azimuthal and the field is axial, so the electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

Brush (electric)
A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electric motors, alternators and electric generators.-Etymology:...

" or slip ring
Slip ring
A slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...

, which results in large losses at the low voltages generated. Some of these losses can be reduced by using mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

or other easily liquified metal or alloy (gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...

, NaK
NaK
NaK, or sodium-potassium alloy, an alloy, of potassium , and sodium , is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions...

) as the "brush", to provide essentially uninterrupted electrical contact.

If the magnetic field is provided by a permanent magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...

, the generator works regardless of whether the magnet is fixed to the stator or rotates with the disc. Before the discovery of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

and the Lorentz force law, the phenomenon
Phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

The Faraday paradox is an experiment that illustrates Michael Faraday's law of electromagnetic induction. Faraday deduced this law in 1831, after inventing the first electromagnetic generator or dynamo, but was never satisfied with his own explanation of the paradox.-The equipment:The experiment...

.

### Drum-type generator

A drum-type HPG 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 "speaker" 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
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

in a cosmic body's ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

through its magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. In their book, Cosmical Electrodynamics, Hannes Alfvén
Hannes Alfvén
Hannes Olof Gösta Alfvén was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics . He described the class of MHD waves now known as Alfvén waves...

and Carl-Gunne Fälthammar
Carl-Gunne Fälthammar
Carl-Gunne Fälthammar is Professor Emeritus at the Royal Institute of Technology in Stockholm, Sweden, specialising in space and plasma physics in the School of Electrical Engineering...

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
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

, binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

s, black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

s, galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, the Jupiter Io system, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, the Solar Wind, sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

s, and in the Venusian magnetic tail
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

.

## Physics

Like all dynamos, the Faraday disc converts kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

to electrical energy. This machine can be analysed using Faraday's own law of electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

. This law (in its modern form) states that an electric current is induced in a closed electrical circuit when the magnetic flux enclosed by the circuit changes (in either magnitude or direction). For the Faraday disk it is necessary, however, to consider that the circuit(s) consist of each radial "spoke" of the disk connected to the rim and center and then through the external circuit.

The Lorentz force law is more easily used to explain the machine's behaviour. This law, discovered thirty years after Faraday's death, states that the force on an electron is proportional to the cross product
Cross product
In mathematics, the cross product, vector product, or Gibbs vector product is a binary operation on two vectors in three-dimensional space. It results in a vector which is perpendicular to both of the vectors being multiplied and normal to the plane containing them...

of its velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

and the magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

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.

• Barlow's wheel
Barlow's Wheel
thumb|An 1842 diagram of Barlow's wheelBarlow's wheel was an early demonstration of a homopolar motor, designed and built by English mathematician and physicist, Peter Barlow in 1822. An electric current passes through the hub of the wheel to a mercury contact on the rim; this is contained in a...

• Electric generator
• Electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

• List of homopolar generator patents
• Homopolar motor
Homopolar motor
A homopolar motor is an electric motor that works without the need for a commutator, by rotating along a fixed axis that is parallel to the external magnetic field produced by a permanent magnet. The name homopolar indicates that the electrical polarity of the motor does not change...

The Faraday paradox is an experiment that illustrates Michael Faraday's law of electromagnetic induction. Faraday deduced this law in 1831, after inventing the first electromagnetic generator or dynamo, but was never satisfied with his own explanation of the paradox.-The equipment:The experiment...

Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...

• Single axially rotating flux generator system
Single axially rotating flux generator system
The Single Axially Rotating Flux generator system uses fewer parts, and has a significantly less weight, when compared to convention dual magnet electrical generators. With the use of super strong neodymium rare earth magnets, there is no need to waste energy electrifying the field...

## External references

• http://www.andrijar.com/homopolar/index.html
• http://www.andrijar.com/nt/index.html

### 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, Wiley, 3rd ed. 1998, ISBN 0-471-30932-X
• 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, ISBN 0-19-850594-9
• 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. ISBN 0-85800-048-2. (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. ISBN 0-9641070-1-5