A unipolar generator is a direct current electrical mechanism containing an electrically conductive disk or cylinder rotating in a plane. It has potentials of different power between the center of the disk and the rim (or ends of the cylinder) with electric polarity, which depends on the direction of rotation and the orientation of the field.
It is also known as the unipolar Faraday oscillator. The voltage is usually low, on the order of a few volts in the case of small demonstration models, but large research machines can generate hundreds of volts, and some systems have multiple series oscillators for even higher voltages. They are unusual in that they can generate an electric current that can exceed a million amperes, since a unipolar generator does not necessarily have a high internal resistance.
Invention Story
The first homopolar mechanism was developed by Michael Faraday during his experiments in 1831. It is often referred to as a Faraday disc or wheel after him. This was the beginning of modern dynamosmachines, that is, electric generators operating on a magnetic field. It was very inefficient and not used as a practical power source, but showed the possibility of generating electricity using magnetism and paved the way for switched DC dynamos and then alternators.
Disadvantages of the first generator
Faraday's disc was primarily inefficient due to the oncoming current flows. The principle of operation of a unipolar generator will be described just by its example. While the current flow was induced directly under the magnet, the current circulated in the opposite direction. The backflow limits the output power for the receiving wires and causes unnecessary heating of the copper disk. Later homopolar generators could solve this problem with a set of magnets placed around the perimeter of the disk to maintain a constant field around the circumference and eliminate areas where backflow could occur.
Further developments
Shortly after the original Faraday disk was discredited as a practical generator, a modified version was developed combining magnet and disk in one rotating part (rotor), but the very idea of an impact unipolar generator was reserved for this configuration. One of the earliest patents for generic unipolar mechanisms was obtained by A. F. Delafield, U. S. Patent 278,516.
Research of outstanding minds
Other early impact unipolar patentsthe generators were awarded separately to S. Z. De Ferranti and S. Batchelor. Nikola Tesla was interested in the Faraday disk and did work with homopolar mechanisms, and eventually patented an improved version of the device in US Patent 406,968.
Tesla's "Dynamo Electric Machine" patent (Tesla's unipolar generator) describes an arrangement of two parallel disks with separate parallel shafts connected, like pulleys, by a metal belt. Each disk had a field opposite to the other, so that the current flow passed from one shaft to the edge of the disk, through the belt to the other edge, and to the second shaft. This would greatly reduce the friction losses caused by the sliding contacts, allowing both electrical sensors to interact with the shafts of the two discs rather than the shaft and high speed rim.
Later patents were awarded to S. P. Steinmetz and E. Thomson for their work on high voltage unipolar generators. The Forbes Dynamo, designed by Scottish electrical engineer George Forbes, was widely used in the early 20th century. Most of the developments made in homopolar mechanisms have been patented by J. E. Noeggerath and R. Eickemeyer.
50s
Homopolar generators experienced a renaissance in the 1950s as a source of pulsed energy storage. These devices used heavy disks as a form of flywheel to store mechanical energy that could be quickly dumped into the experimental apparatus.
An early example of this kind of device was created by Sir Mark Oliphant at the Research SchoolPhysical Sciences and Engineering from the Australian National University. It stored up to 500 megajoules of energy and was used as an ultra-high current source for synchrotron experiments from 1962 until it was dismantled in 1986. Oliphant's design was capable of delivering currents up to 2 megaamperes (MA).
Developed by Parker Kinetic Designs
Even larger devices like this are designed and built by Parker Kinetic Designs (formerly OIME Research & Development) of Austin. They produced devices for a variety of purposes, from powering railroad pistols to linear motors (for space launches) and various weapon designs. 10 MJ industrial designs have been introduced for various roles including electric welding.
These devices consisted of a conductive flywheel, one of which rotated in a magnetic field with one electrical contact near the axis and the other near the periphery. They have been used to generate very high currents at low voltages in areas such as welding, electrolysis, and railgun research. In pulsed energy applications, the angular momentum of the rotor is used to store energy for a long period and then release it in a short time.
Unlike other types of commutated unipolar generators, the output voltage never reverses polarity. The separation of charges is the result of the action of the Lorentz force on the free charges in the disk. The motion is azimuthal and the field is axial, soelectromotive force is radial.
Electrical contacts are usually made through a "brush" or slip ring, resulting in high losses at the low voltages generated. Some of these losses can be reduced by using mercury or another easily liquefied metal or alloy (gallium, NaK) as a "brush" to provide nearly continuous electrical contact.
Modification
A recently proposed modification has been to use a plasma contact fitted with a negative resistance neon streamer touching the edge of the disc or drum using specialized low work function carbon in vertical stripes. This would have the advantage of very low resistance in the current range, possibly up to thousands of amps, without contact with liquid metal.
If the magnetic field is created by a permanent magnet, the generator works regardless of whether the magnet is attached to the stator or rotates with the disk. Before the discovery of the electron and Lorentz's law of force, this phenomenon was inexplicable and was known as Faraday's paradox.
Drum Type
A drum-type homopolar generator has a magnetic field (V) that radiates radially from the center of the drum and induces a voltage (V) along its entire length. A conductive drum rotating from above in the region of a "loudspeaker" type magnet with one pole in the center and the other surrounding it, may use conductive ball bearings in its top andlower parts to capture the generated current.
In nature
Unipolar inductors are found in astrophysics, where the conductor rotates through a magnetic field, for example, when a highly conductive plasma in the ionosphere of a space body moves through its magnetic field.
Unipolar inductors have been associated with the Uranian aurora, binary stars, black holes, galaxies, Jupiter's moon Io, the Moon, the solar wind, sunspots, and the Venusian magnetic tail.
Mechanism features
Like all the above-mentioned space objects, the Faraday disk converts kinetic energy into electrical energy. This machine can be analyzed using Faraday's own law of electromagnetic induction.
This law in its modern form states that the constant derivative of magnetic flux through a closed circuit induces an electromotive force in it, which in turn excites an electric current.
The surface integral that defines the magnetic flux can be rewritten as a linear one around the circuit. Although the integrand of the line integral does not depend on time, since the Faraday disk that is part of the boundary of the line integral moves, the derivative of the total time is not zero and returns the correct value for calculating the electromotive force. Alternatively, the disk can be reduced to a conductive ring around its circumference with a single metal spoke connecting the ring to the axle.
Lorentz force law lighterbe used to explain the behavior of the machine. 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 geometric terms, this means that the force is directed at right angles to both the velocity (azimuth) and the magnetic flux (axial), which is therefore in the radial direction. The radial movement of electrons in the disk causes a separation of charges between its center and rim, and if the circuit is completed, an electric current is generated.
Electric motor
A unipolar motor is a DC device with two magnetic poles, whose conductors always cross unidirectional magnetic flux lines, rotating the conductor around a fixed axis so that it is at right angles to the static magnetic field. The resulting EMF (electromotive force), which is continuous in one direction, to a homopolar motor does not require a commutator, but still requires slip rings. The name "homopolar" indicates that the electrical polarity of the conductor and the poles of the magnetic field do not change (that is, that it does not require switching).
The unipolar motor was the first electric motor to be built. Its action was demonstrated by Michael Faraday in 1821 at the Royal Institution in London.
Invention
In 1821, shortly after the Danish physicist and chemist Hans Christian Oersted discoveredphenomenon of electromagnetism, Humphry Davy and British scientist William Hyde Wollaston tried, but failed, to develop an electric motor. Faraday, disputed as a joke by Humphrey, went on to create two devices to create what he called "electromagnetic rotation". One of them, now known as the homopolar drive, created a continuous circular motion. It was caused by a circular magnetic force around a wire placed in a pool of mercury in which the magnet was placed. The wire would revolve around the magnet if it were powered by a chemical battery.
These experiments and inventions formed the basis of modern electromagnetic technologies. Soon Faraday published the results. This strained relations with Davy due to his jealousy of Faraday's achievements and caused the latter to turn to other things, which as a result prevented him from participating in electromagnetic research for several years.
B. G. Lamm described in 1912 a homopolar machine with a power of 2000 kW, 260 V, 7700 A and 1200 rpm with 16 slip rings operating at a peripheral speed of 67 m/s. A 1125kW, 7.5V, 150,000A, 514rpm unipolar generator built in 1934 was installed in an American steel mill for pipe welding.
The same Lorentz law
The operation of this engine is similar to that of a shock unipolar generator. The unipolar motor is driven by the Lorentz force. A conductor with a current flowing through it, when placed in a magnetic field and perpendicular to it, feels a force indirection perpendicular to both the magnetic field and the current. This force provides a turning moment around the axis of rotation.
Since the latter is parallel to the magnetic field, and opposing magnetic fields do not change polarity, switching is not required to continue rotating the conductor. This simplicity is most easily achieved with single-turn designs, making homopolar motors unsuitable for most practical applications.
Like most electromechanical machines (like Neggerath's unipolar generator), the homopolar motor is reversible: if the conductor is turned mechanically, it will operate as a homopolar generator, creating a DC voltage between the two terminals of the conductor.
The constant current is a consequence of the homopolar nature of the design. Homopolar generators (HPGs) were extensively explored in the late 20th century as sources of low voltage but very high current direct current, and achieved some success in powering experimental railguns.
Building
Making a unipolar generator with your own hands is quite simple. The unipolar motor is also very easy to assemble. The permanent magnet is used to create an external magnetic field in which the conductor will rotate, and the battery causes current to flow along the conductive wire.
It is not necessary for the magnet to move or even come into contact with the rest of the motor; its only purpose is to create a magnetic field that willinteract with a similar field induced by current in the wire. It is possible to attach a magnet to a battery and allow the conductor to rotate freely as the electrical circuit is completed, touching both the top of the battery and the magnet attached to the bottom of the battery. The wire and battery may become warm during continuous use.
