Unipolar machine

A unipolar machine (also known as a unipolar generator , homopolar machine or Faraday generator ) is an electrical machine in a special design for generating a high direct current with a low voltage . The specialty of the unipolar machine is that it generates direct current with the help of unipolar induction . In contrast, direct current generators usually only generate a (pulsating) direct current with the help of commutators (mechanical rectifiers).

Historical unipolar generator, so-called Faraday disk

The unipolar generators are of little practical importance today, since powerful rectifiers are available and the efficiency of alternating voltage supplying generators such as the synchronous generator or asynchronous generator is significantly higher. In the early days of electrical engineering, especially in the first half of the 20th century, unipolar generators were a simple way of generating low, pulse-free DC voltages with low internal generator resistance. At that time, the only technical alternative to generating pulse-free DC voltage was the use of batteries or accumulators .

construction

Schematic structure of a unipolar machine

The main component of the unipolar machine is a rotatably mounted shaft on which there is a disk, preferably made of a material that is a good electrical conductor such as copper . Two sliding contacts , one on the edge of the disc and one on the axis of rotation, lead the electrical connections to the outside. The arrangement is in a constant magnetic field which penetrates the disc perpendicular to its surface, e.g. B. by a permanent magnet in the shape of a horseshoe. It works like a Barlow's wheel in "reverse mode". In the classic view, induction (see also Faraday's paradox ) seems to be contradicting itself and, especially in the case of the N-machine with a rotating magnetic field generator, still leads to disagreement about the exact mechanism today. Only the help of the theory of relativity and its application to the correct transformation of the electromagnetic fields resolve this contradiction.

The generated voltage can then be calculated according to the principles of unipolar induction as follows:

${\displaystyle U_{i}=\int _{R_{i}}^{R_{a}}\omega rB\,dr={\frac {1}{2}}\omega B(R_{a}^{2}-R_{i}^{2})}$

with R _i , R _a as the outer and inner radius of the disk, ω the angular frequency of the disk and B the magnetic flux density .

Such a device was first built and described by Michael Faraday in 1832, the "Unipolar Inductor".

Applications

Unipolar machine from Siemens & Halske

The application of the unipolar machine extends today only to special areas of technology, e.g. B. full round pipe welding, energy storage (up to approx. 500 MJ) for high-current applications (military. Railgun ), since the voltage that can be drawn (up to approx. 500 V) is low and the handling of high currents (up to approx. 300 kA) is difficult. Another application is tachometers .

Unipolar machines experienced a certain renaissance as a source of high pulse power in the 1950s . These machines use a massive metal plate, which serves as a flywheel to store rotational energy in order to be able to briefly emit very high electrical current pulses due to the very low-resistance conductor arrangement with only low internal resistance . One such facility was built by Mark Oliphant at the Australian National University . The disk could store a rotational energy of 500  MJ and emit impulse currents of up to 2  MA , which were used from 1962 to 1986 in experiments with synchrotron particle accelerators .

N machine

One form of the unipolar machine is the arrangement of two axially polarized, cylindrical permanent magnets, designed by Bruce de Palma and designated as the N machine , between which a metal disc with the lowest possible specific resistance is located. When the arrangement rotates, a DC voltage can be detected between the axis and the disk between the two magnets. Since these rotate with them, the field lines are not "cut" according to the classical conception, which is why the machine was assumed to be free of feedback , so that supposedly no mechanical energy is absorbed. This observation apparently contradicts the law of induction , which is expressed and resolved in Faraday's paradox . The induced voltage can be explained by the relative movement of the rotating conductor disk and thus of the B-field to the stationary consumer contact. If you attach the measuring arrangement to the conductor disk and let it rotate with it, no more voltage can be detected. As a result, the conservation of energy is not violated and it is only an apparent perpetual motion machine.

Web links

• The simplest electric motor in the world (PDF file; 289 kB)
• Answer # 218 ( Memento from May 11, 2006 in the Internet Archive )
• N machines and homopolar generators ( Memento from September 21, 1999 in the Internet Archive )
• dePalma's N machine ( Memento from December 30, 2001 in the Internet Archive )
• T. Schneider: The unipolar machine of the second kind . German National Library February 9, 2015, urn : nbn: de: 101: 1-2015020917870 .

Individual evidence

1. ^ JW Blamey, PO Carden, LU Hibbard, EK Inall, RA Marshall and Mark Oliphant: The large homopolar generator at Canberra: Initial Tests , Nature 195, 1962, pp 113-114.
2. ↑ THE HOME OF PRIMORDIAL ENERGY - website for the N machine. (No longer available online.) Archived from the original on March 8, 2017 ; accessed on September 11, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
3. ↑ The N-machine and the homopolar generator