Homopolar generator

A homopolar generator is a DC electrical generator that is made when a magnetic electrically conductive rotating disk has a different magnetic field passing through it (it can be thought of as slicing through the magnetic field). This creates a voltage and current difference between 2 contact points, one in the center of the disk the other on the outside of the disk. For simplicity one contact point can be considered positive +, and the other contact point can be considered ground or negative (- or 0). In general the 2 contact points are linked together as the armature. It has the same polarity at every point, so that the armature that passes through the magnetic field lines of force continually move in the same direction.^[1] The device is electrically symmetrical^[1] (bidirectional), and generates continuous direct current. It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disk. Relatively speaking they can source tremendous electric current (10 to 10000 amperes) but at low potential differences (typically 0.5 to 3 volts). This property is due to the fact that the homopolar generator has very low internal resistance.

The Faraday Disc

The homopolar generator was developed first by Michael Faraday during his 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 Faraday disc was primarily inefficient due to counterflows of current. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field radially from axis to edge, and eliminate areas where counterflow could occur.

Homopolar Generator Development

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.^[2] 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.

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. It is important to note that the "cutting lines of force" model for understanding induction cannot be applied (and therefore is violated) for some configurations of the homopolar generator. This is due to the fact that magnetic fields are uniform. For example, if a wire is stationary and a large magnet is moved over it (like an infinitely large north pole of a permanent magnet) no current will be produced because there is no change in the magnetic field (uniformity). The only place there could be a current produced is at the ends of the large north pole of the magnet where the magnetic field drops off. If viewed from the "lines of force" model, one might think that since the wire would be getting crossed by "lines of force" of the magnet a current would be produced. This is not so.

Disk type 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.^[3]

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.^[4] 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. A conducting drum spun from above in the field of a "speaker" type of magnet that has one pole in the center of the drum and the other pole surrounding the drum could use conducting ball bearings at the top and bottom of the drum to pick up the generated current.

Astrophysical unipolar inductors

Unipolar inductors occur in astrophysics where a conductor rotates through a magnetic field, for example, the movement of the highly conductive plasma in a cosmic body's ionosphere through its magnetic field. In their book, Cosmical Electrodynamics, Hannes Alfvén and Carl-Gunne Fälthammar write:

Unipolar inductors have been associated with the aurorae on Uranus,^[6] binary stars,^[7] ^[8] black holes,^[9] ^[10] galaxies,^[11] the Jupiter Io system,^[12] ^[13] the Moon,^[14] ^[15] the Solar Wind,^[16] sunspots,^[17] ^[18] and in the Venusian magnetic tail.^[19]

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 field 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.

Contents

The Faraday Disc