A Brief Explanation of Faraday Disk, Homopolar, and " Statorless" Generators
Magnetically Inducing Current in a Wire
In 1832, Michael Faraday discovered that a changing magnetic field could generate an electric current in a wire. This can be accomplished, simply by moving a magnet perpendicular to a wire. In this generator, the wire is the stator, and the magnet is moving. The relative motion between the magnet and the wire creates a voltage across the wire. If there is a closed circuit, the voltage will dissipate into a current. The Prime Minister of England visited Faraday's laboratory to see the world's first electric generator. When asked of what use it was, Faraday replied, "I know not, but I wager that one day your government will tax it"
Faraday Disk Generators
Next, Faraday discovered that if a magnet is placed perpendicular to a rotating conductive disk, there is a voltage created between the inside and the outside of the disk. Imagine sliding a horseshoe magnet over a spinning disk. The horseshoe magnet produces invisible magnetic lines of flux between the poles, which are cut by the spinning disk. In this experiment, the horseshoe magnet is the stator. The stator produces the magnetic field which is cut by the rotor, the disk. The relative motion between the magnetic field, and the rotor, creates an electric potential. This voltage can be taken off the disk, using carbon brushes. The electrical current can be used to power a circuit.
Homopolar Generators
A Homopolar generator is much like a Faraday disk generator, except, the magnet is centered on the axis of rotation of the conductive disk. Imagine a circular magnet, the same diameter as the conductive disk. The disk is aligned very close to the magnet, and on the same center axis. Then the conductive disk is spun. There is still relative motion between the conductive disk and the magnetic field. The disk is cutting the magnetic field lines and creating a voltage or electrical potential between the inside and the outside of the conductive disk.
"Statorless" Generators
A "Statorless" generator is much like a homopolar generator, except nothing stands still. Like the homopolar generator, the magnet is a uniformly round short cylinder, polarized from one end to the other. There is a conductive spinning disk. Now take the stator, or magnet, and attach it to the disk. Spin them both. There is an observed generated voltage. When you spin a cylindrical magnet, about its radial axis, the magnetic field doesn't spin. The field stays stationary, and the disk cuts through it. There is no relative motion between the conductor and the magnetic source, however there is relative motion between the conductor and the magnetic field.
If it is hard to believe that uniform magnetic fields don't spin, try one of these experiments.
1. Build a "Statorless" generator that allows you to spin the magnetic disk independently of the conductive disk. Visit Prototypes and Models for basic designs. Hold the magnetic disk still while spinning the conductive disk at a known speed. Measure the voltage. Now spin them together at the same speed. Measure the voltage. Now, hold the conductive disk still while you spin the magnetic disk. Measure the voltage. Your results will turn out as follows:
Conductive disk spins at speed (A) and magnetic disk is held still. Voltage will equal (X)
Conductive disk spins together with magnetic disk at speed (A). Voltage will equal (X)
Conductive disk is held still while magnetic disk spins at speed (A). Voltage will equal Zero
2. For this experiment you will need a large cylindrical magnet, preferably around .5" thick with a 2" diameter and 20 MGOe (megagauss). Slide the magnet over a nonmagnetic conductive slab of metal with a face of the cylinder against the slab . A square foot of half inch thick aluminum works well, but a frying pan will work if it isn't magnetic. Try to quickly move the magnet across the metal surface. Then tilt the metal surface as much as 85 degrees and slide the magnet down. Next, spin the magnet along its radial axis, on-top of the metal surface. Your observations should include:
The magnet is hard to slide across the metal. Even though the magnet doesn't stick to the metal at rest, it resists movement across it. The harder and faster you pull the magnet across the metal, the harder it resists.
This illustrates "Back Electromotive Force" or Back EMF. Back EMF is caused when you move a magnet across a conductor. The relative motion, between the magnet and the metal slab, induces an electric current in the magnetic slab. This current produces its own magnetic field, which opposes the movement of the magnet. The harder you push and pull the magnet across the conductor, the more current is generated, and the greater the opposing magnetic force. Tilting the metal slab is an excellent example of this phenomena. Especially if you have a thick slab. Even at angles pushing 85 degrees, the magnet will slowly move down the slab, sometimes taking almost 5 seconds to go a foot. Try letting a coin slide down the nearly vertical slab for comparison.
When you spin the magnet, the resistive force is not present.
This is because the magnetic field is standing still. There is no current generated and no back emf. If the magnet were shaped like a top, it could spin like a top, only if it wobbled or tried to move around on the slab, the back emf would slow it to a halt. As long as it spun so the field remained uniform, back emf would have no effect.
Last Updated on 10/13/99
By Gabe Grant
Email: grantgb@purdue.edu
Liquid Glyph, © 2003.