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Homopolar generator

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A homopolar generator, also known as a unipolar generator, acyclic generator, or disk dynamo, is a DC electrical generator in which the magnetic field has the same polarity at every point, so that the armature passes through the magnetic field lines of force continually in the same direction.[1] The device is electrically symmetrical[2], and generates continuous direct current. Some of these devices also have "homopolar magnets", which have pole pieces arranged around a common centre.

History and development

The homopolar generator was developed first by Michael Faraday during his memorable experiments in 1831. It is sometimes called the Faraday disc 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 practical power source, but it showed the possibility of generating electric power using magnetism, and led the way for commutated direct current dynamos and then alternating current alternators.

The remains of the ANU 500MJ generator

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 Charles E. Ball (US238631; March 1881). Other early patents for homopolar generators were awarded to S. Z. De Ferranti and C. Batchelor separately. Nikola Tesla was interested in the Faraday disc and conducted work with homopolar generators [3]. He eventually patented an improved version of the device and his U.S. patent 406,968 ("Dynamo Electric Machine") describes an arrangement of two parallel discs on separate, parallel axles, and joined like pulleys by a metallic belt. This would have greatly reduced the frictional losses caused by sliding contacts. Later, patents were awarded to C. P. Steinmetz 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 R. Eickemeyer.

One of the larger homopolar generators that was produced by Parker Kinetic Designs via the collaboration of Richard Marshall, William Weldon, and Herb Woodson. Parker Kinetic Designs have produced devices which can produce five amperes. Another large homopolar generator was built by Sir Mark Oliphant at the Research School of Physical Sciences and Engineering, Australian National University. It produced 500 megajoules and was used as an extremely high-current source for experimentation from 1962. It was disassembled in 1986. Oliphant's construction was capable of supplying currents of up to 2 megaamperes.

Description and operation

How magnetism makes current

Magnetism can make, or induce, electric current only when the conductor that carries the current and the lines of magnetic force move so that they cut across each other. The direction in which current flows in the moving conductor can be learned with Fleming's right hand rule for generators.

Disk type generator

Basic Faraday disc generator

The device consists of a conducting flywheel rotating in a magnetic 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 as welding, electrolysis and railgun research. In pulsed energy applications, the angular momentum 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 on the free charges in the disk. The motion is azimuthal and the field is axial, so the electromotive force is radial. The electrical contacts must be made through a "brush" or slip ring, which results in large losses at the low voltages generated. Some of these losses can be reduced by using mercury or other easily liquified metal (gallium) as the "brush", and a vertical-axis generator could allow the entire rim of a disk to be "brushed" by liquid metal. The table below shows the results of different relative motions of the parts of a circular homopolar generator [4].

Magnetic source Disk Indicator Voltage generated
Stationary Stationary Stationary No
Stationary Moved Stationary Yes
Stationary Stationary Moved Yes
Stationary Moved Moved Undetermined
Moved Stationary Stationary No
Moved Moved Stationary Yes
Moved Stationary Moved Yes
Moved Moved Moved Undetermined

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. The voltage is "undetermined" when both the indicator and the disk are rotated, regardless of whether the magnet is moving. There is an EMF and a nonuniform charge density, but no reaction of the indicator[5]. An electric field is generated, but no voltage is brought out for display. This differs from the different relative motions of the parts of a rectilinear homopolar generator (where the answer would be "No").

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.

Physics

Like all dynamos, the Faraday disc converts kinetic energy to electrical energy. However, unlike all other dynamos, this machine cannot be analysed using Faraday's own law of electromagnetic induction. 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). However, the circuit in the Faraday disc is parallel to the magnetic flux vector and therefore encloses no magnetic flux. Therefore, Faraday's law does not apply to this machine.

Instead, the Lorentz force law is 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 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 electron then creates an electric current between the centre of the disc and its rim.

There is a subtle difficulty in this explanation, which often leads to a misunderstanding of how the machine works. The key word in the preceding paragraph is velocity, which prompts the question, "velocity relative to what?". If the velocity relative to the magnet is assumed as the cause of the Lorentz force, then the explanation contradicts special relativity, which states that it is impossible to tell whether a uniform magnetic field is moving or stationary. This assumption would also imply that rotating the magnet and not the disc would cause a current to flow, which is not what experimenters have found.

The correct interpretation of the velocity of the electron is that it is relative to the static parts of the machine, which are the sliding contacts and the circuit to which they are connected. In the language of special relativity, these objects act as the 'observer'. It is the velocity of the electron relative to these components that causes it to experience the Lorentz force.

See also

References and further reading

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
  • Olivier Darrigol, Electrodynamics from Ampere to Einstein, Oxford University Press, 2000, ISBN 0-19-850594-9
  • 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
  • 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)
Citations
  • ^ Rudolf F. Graf, "Dictionary of Electronics; Radio Shack, 1974-75". Fort Worth, Texas.
    • ^ Ibid.
  • ^ Thomas Valone, "Harnessing the Wheelwork of Nature : Tesla's Science of Energy", The Homopolar Generator: Tesla's Contribution. ISBN 1-931882-04-5 (ed. originally presented in the Proceedings of International Tesla Symposium, 1986, p. 6-29)
    • ^ Ibid.
  • ^ Nikola Tesla, "Notes on a Unipolar Dynamo". The Electrical Engineer, N.Y., Sept. 2, 1891. (Also available at tesla.hu, Article 18910902)
Further readings
  • 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.)

External links and other articles

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