A unipolar generator is an electrical direct current mechanism comprising a conductive disk or cylinder rotating in a plane. It has potentials of different power between the center of the disk and the rim (or the ends of the cylinder) with electric polarity, which depends on the direction of rotation and orientation of the field.
It is also known as the Faraday unipolar generator. The voltage is usually low, of the order of several volts in the case of small demonstration models, but large research machines can generate hundreds of volts, and some systems have several serial generators to obtain even greater voltage. 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.
History of invention
The first homopolar mechanism was developed by Michael Faraday during his experiments in 1831. He is often called the disk or wheel of Faraday in his honor. This was the beginning of modern dynamo machines, that is, electric generators operating in a magnetic field. It was very inefficient and was not used as a practical source of energy, but showed the possibility of generating electricity using magnetism and paved the way for switched dynamo DC sources, and then for alternating current generators.
The disadvantages of the first generator
The Faraday disk was primarily ineffective due to 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 below the magnet, the current circulated in the opposite direction. A counter current limits the output power to the receiving wires and causes unnecessary heating of the copper disk. Later homopolar generators could solve this problem by using a set of magnets located around the perimeter of the disk to maintain a constant field around the circumference and eliminate the areas in which countercurrent can occur.
Further developments
Soon after the original Faraday disk was discredited as a practical generator, a modified version was developed combining a magnet and a disk in one rotating part (rotor), but the idea of an unipolar shock generator was reserved for this configuration. One of the earliest patents for unipolar mechanisms of the general type was obtained by AF Delafield, US patent 278 516.
Studies of outstanding minds
Other early patents for unipolar shock generators were awarded separately to S.Z. De Ferranti and S. Batchelor. Nikola Tesla was interested in the Faraday disk and worked with homopolar mechanisms, and eventually patented an improved version of the device in US patent 406,968.
The Tesla patent "Dynamo Electric Machine" (Tesla's unipolar generator) describes the arrangement of two parallel disks with separate parallel shafts connected, like pulleys, to 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 significantly reduce the friction losses caused by sliding contacts, allowing both electrical sensors to interact with the shafts of the two disks, rather than with the shaft and high-speed rim.
Patents were later awarded to S.P. Steinmets and E. Thomson for their work with unipolar high voltage generators. Dynamo Forbes, designed by Scottish electrical engineer George Forbes, was widely used in the early twentieth century. Most of the developments carried out in homopolar mechanisms were patented by JE 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 a flywheel to store mechanical energy that could be quickly dumped into an experimental apparatus.
An early example of this kind of device was created by Sir Mark Oliphant at the Australian National University's School of Physics and Engineering. It stored up to 500 megajoules of energy, and it was used as an ultrahigh current source for experiments with the synchrotron from 1962 until it was disassembled in 1986. The design of Oliphant was able to supply currents up to 2 megaamperes (MA).
Developed by Parker Kinetic Designs Corporation
These larger devices are designed and manufactured by Parker Kinetic Designs (formerly OIME Research & Development) from Austin. They produced devices for a variety of purposes: from powering railway pistols to linear engines (for space launches) and various weapon designs. 10 MJ designs were 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 about the axis, and the other about the periphery. They were used to generate very high currents at low voltages in areas such as welding, electrolysis, and rail gun research. In applications with pulsed energy, the angular momentum of the rotor is used to accumulate energy for a long period, and then to release it in a short time.
Unlike other types of unipolar oscillators with a switch, the output voltage never changes polarity. The separation of charges is the result of the action of the Lorentz force on free charges in the disk. The movement is azimuthal, and the field is axial, so the electromotive force is radial.
Electrical contacts are usually made through a “brush” or slip ring, resulting in large losses at low voltage generated. Some of these losses can be reduced by using mercury or other easily liquefied metal, or an alloy (gallium, NaK) as a “brush” to provide almost uninterrupted electrical contact.
Modification
A recently proposed modification was the use of a plasma contact equipped with a negative-resistance neon streamer touching the edge of a disk or drum using specialized carbon with a low work function in vertical stripes. This would have the advantage of a very low resistance in the current range, possibly up to thousands of amperes without contact with liquid metal.
If a magnetic field is created by a permanent magnet, the generator operates whether the magnet is attached to the stator or rotates with the disk. Before the discovery of the electron and the law of Lorentz force, this phenomenon was inexplicable and was known as the Faraday paradox.
"Drum type"
The drum type homopolar generator has a magnetic field (B), which is radially radiated 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 located in the center and the other surrounding it, can use conductive ball bearings in its upper and lower parts to capture the generated current.
In nature
Unipolar inductors are found in astrophysics, where a conductor rotates through a magnetic field, for example, when a highly conductive plasma moves in the ionosphere of a cosmic body through its magnetic field.
Unipolar inductors were associated with shining on Uranus, binary stars, black holes, galaxies, Jupiter’s moon Io, Moon, Solar wind, sunspots, and Venusian magnetic tail.
Mechanism features
Like all the aforementioned cosmic 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 the magnetic flux through a closed circuit induces an electromotive force in it, which, in turn, excites an electric current.
The surface integral, which determines the magnetic flux, can be rewritten as linear around the circuit. Although the integrand of the integral of the line does not depend on time, since the Faraday disk, which is part of the boundary of the linear integral, moves, the derivative of the total time is not equal to 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 axis.
The law of Lorentz force is easier to use to explain the behavior of the machine. This law, formulated thirty years after the death of Faraday, states that the force on the electron is proportional to the cross product of its speed and the magnetic flux vector.
In geometric terms, this means that the force is directed at right angles to both speed (azimuthal) and magnetic flux (axial), which is therefore in the radial direction. The radial movement of electrons in the disk causes the separation of charges between its center and rim, and if the circuit closes, an electric current occurs.
Electric motor
A unipolar electric motor is a direct current device with two magnetic poles, the conductors of which always cross the 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 switch, 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 Institute in London.
Invention
In 1821, shortly after Danish physicist and chemist Hans Christian Oersted discovered the phenomenon of electromagnetism, Humphrey Davy and British scientist William Hyde Wollaston tried, but could not, develop an electric motor. Faraday, whom Humphrey disputed as a joke, continued to create two devices for creating the so-called "electromagnetic rotation." One of them, now known as a homopolar motor, created a continuous circular motion. It was caused by a circular magnetic force around a wire laid in a pool of mercury in which a magnet was placed. The wire would rotate around the magnet if it were supplied with current from a chemical battery.
These experiments and inventions formed the basis of modern electromagnetic technology. Soon Faraday published the results. This aggravated relations with Davy because of his jealousy of Faraday's achievements and caused the latter to engage in other affairs, which as a result prevented his participation in electromagnetic research for several years.
In 1912, B. G. Lamm described a homopolar machine with a capacity of 2000 kW, 260 V, 7700 A and 1200 rpm with 16 slip rings operating at a peripheral speed of 67 m / s. The unipolar generator with a capacity of 1125 kW, 7.5 V, 150 000 A, 514 rpm, built in 1934, was installed at an American steel mill for pipe welding.
The same law of Lorentz
The operation of this engine is similar to the principle of operation of a shock unipolar generator. A unipolar motor is driven by Lorentz force. A conductor with a current flowing through it, when it is placed in a magnetic field and is perpendicular to it, senses a force in a direction 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 opposite magnetic fields do not change polarity, switching is not required to continue the rotation of the conductor. This simplicity is easiest to achieve with single-turn designs, which makes homopolar motors unsuitable for most practical applications.
Like most electromechanical machines (like the Neggerath unipolar generator), a homopolar motor is reversible: if the conductor rotates mechanically, it will work like a homopolar generator, creating a DC voltage between the two leads of the conductor.
Direct current is a consequence of the homopolar nature of the design. Homopolar generators (HPGs) were carefully researched at the end of the 20th century as low voltage DC sources, but with very high current, and achieved some success in feeding experimental rail guns.
Structure
Making a unipolar generator with your own hands is quite simple. The unipolar motor is also very easy to assemble. A 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.
There is no need for the magnet to move or even come in contact with the rest of the motor; its sole purpose is to create a magnetic field that will interact with a similar field induced by the current in the wire. You can attach a magnet to the battery and allow the conductor to rotate freely when the electrical circuit is closed, touching both the top of the battery and the magnet attached to its bottom. The wire and battery may become warm during continuous operation.