A homopolar motor is a direct current electric motor with two magnetic poles, the conductors of which always cut unidirectional lines of

magnetic flux In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ...

by rotating a conductor around a fixed axis so that the conductor is at right angles to a static magnetic field. The resulting force being continuous in one direction, the homopolar motor needs no

commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, ...

but still requires

slip rings Slip or SLIP may refer to: Science and technology Biology * Slip (fish), also known as Black Sole * Slip (horticulture), a small cutting of a plant as a specimen or for grafting * Muscle slip, a branching of a muscle, in anatomy Computing and ...

. The name ''homopolar'' indicates that the electrical polarity of the conductor and the magnetic field poles do not change (i.e., that it does not require commutation).

History

The homopolar motor was the first electrical motor to be built. Its operation was demonstrated by Michael Faraday in 1821 at the Royal Institution in London. In 1821, soon after the Danish physicist and chemist Hans Christian Ørsted discovered the phenomenon of

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...

Humphry Davy Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for the ...

and British scientist

William Hyde Wollaston William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable ingot ...

tried, but failed, to design an electric motor."Archives Biographies: Michael Faraday", The Institution of Engineering and Technology.
Faraday, having been challenged by Davy as a joke, went on to build two devices to produce what he called "electromagnetic rotation". One of these, now known as the ''homopolar motor'', caused a continuous circular motion that was engendered by the circular magnetic force around a wire that extended into a pool of mercury wherein was placed a magnet. The wire would then rotate around the magnet if supplied with current from a chemical battery. These experiments and inventions formed the foundation of modern electromagnetic technology. In his excitement, Faraday published results. This strained his mentor relationship with Davy, due to his mentor's jealousy of Faraday's achievement, and is the reason for Faraday’s assignment to other activities, which consequently prevented his involvement in electromagnetic research for several years. B. G. Lamme described in 1913 a homopolar machine rated 2,000 kW, 260 V, 7,700 A and 1,200 rpm with 16 slip rings operating at a peripheral velocity of 67 m/s. A unipolar generator rated 1,125 kW, 7.5 V 150,000 A, 514 rpm built in 1934 was installed in a U.S. steel mill for pipe welding purposes.

Principle of operation

The homopolar motor is driven by the

Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...

. A conductor with a current flowing through it when placed in a magnetic field which is perpendicular to the current feels a force in the direction perpendicular to both the magnetic field and the current. This force provides a torque around the axis of rotation.See, e.g.,

Richard P. Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superflu ...

, ''The Feynman Lectures on Physics'', vol. II, section 17-2, (Reading, MA: Addison-Wesley, 1964). Because the axis of rotation is parallel to the magnetic field, and the opposing magnetic fields do not change polarity, no commutation is required for the conductor to keep turning. This simplicity is most readily achieved with single turn designs, which makes homopolar motors unsuitable for most practical applications. Like most electro-mechanical machines, a homopolar motor is reversible: if the conductor is turned mechanically, then it will operate as a homopolar generator, producing a direct current voltage between the two terminals of the conductor. The direct current produced is an effect of the homopolar nature of the design. Homopolar generators (HPGs) were extensively researched in the late 20th century as low voltage but very high current DC power supplies and have achieved some success powering experimental

railgun A railgun or rail gun is a linear motor device, typically designed as a weapon, that uses electromagnetic force to launch high velocity projectiles. The projectile normally does not contain explosives, instead relying on the projectile's high s ...

Building a simple homopolar motor

A homopolar motor is very easy to build. A permanent magnet is used to provide the external magnetic field in which the conductor will turn, and a battery causes a current to flow along a conducting wire. It is not necessary for the magnet to move, or even to be in contact with the rest of the motor; its sole purpose is to provide a magnetic field that will interact with the magnetic field induced by the current in the wire. One can attach the magnet to the battery and allow the conducting wire to rotate freely while closing the electric circuit by touching both the top of the battery and the magnet attached to the bottom of the battery. The wire and the battery may become hot if operated continuously. "Amazing Double 'A' Battery Trick"
from ''Youtube'', Mar. 27, 2015

Gallery

File:Motor_homopolar_neutral.png, Homopolar motor 3D File:Motor_homopolar_plane_neutral.png, Homopolar motor 2D File:Motor_homopolar_flux_force_neutral.png, Current, magnetic field lines and Lorentz force on homopolar motor File:Homopolarmotor MAQ03891 smial wp.ogv, Video File:Simple Homopolar Motor.webm, Video

Examples

Railgun A railgun or rail gun is a linear motor device, typically designed as a weapon, that uses electromagnetic force to launch high velocity projectiles. The projectile normally does not contain explosives, instead relying on the projectile's high s ...

Ball bearing motor A ball bearing motor or ball-race motor consists simply of a small ball- bearing assembly with provision for passing current radially between inner and outer tracks to produce circular motion. Explanation A ball bearing motor is an unusual electric ...

References