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A homopolar motor uses a magnetic field and direct current to create motion or generate electricity. It has a permanent magnet, metal disc, and shaft. The Barlow wheel is a variant that uses a rotating metal disc connected to a battery and suspended above mercury. Homopolar motors have practical applications in marine propulsion and linear acceleration technology. They have the advantage of reducing parts and eliminating the need for a commutator.
A homopolar motor is a device that converts a magnetic field and direct electric current (DC) into motion. The main parts are a permanent magnet with an adjacent metal disc and shaft; homopolar means that only one pole or side of the magnet is near the rotor. Direct current is applied to the disc and it rotates due to the current flowing through the magnetic field. Scientist Michael Faraday first demonstrated the effect in 1821 with a wire rotating around a magnet in a bath of mercury.
Electricity can also be generated by a homopolar motor, which acts as a generator by rotating the metal disc through the magnetic field. The spinning disc will create a DC current that can be stored in a battery. While this principle is impractical for large generation plants, a unipolar generator can be useful when DC power is needed.
Another variant of the homopolar engine was the Barlow wheel. Mathematician Peter Barlow developed the wheel in 1822 to demonstrate the Faraday effect. The Barlow wheel uses a rotating metal disc connected to a battery and suspended above a tray or trough of mercury surrounded by a permanent magnet. When the spinning disc is lowered into the mercury and an electrical circuit is completed, the current interacts with the magnetic field and the wheel spins.
One advantage of unipolar motor technology is the reduction of parts. Wire coil stator and rotor motors must also use a commutator for proper operation. A commutator is a device that reverses motor polarity as the rotor turns. This is necessary because the rotor changes position in the magnetic field of the motor and the change in polarity is required to provide torque or rotational force.
There are a variety of practical applications for a homopolar motor. Marine propulsion began using these engines in the 20th century, with electric drives replacing diesel engines attached to propeller shafts that passed through the ship’s hull. Electric generators can produce direct current for systems connected directly to the propellers.
Beginning in the early 21st century, electric drives have been installed in rotating pods under a ship’s keel that can provide thrust in any direction. This technology provides good propulsion efficiency and provides excellent ship control for docking and maneuvering. Pods can be controlled from a ship’s bridge with a joystick and eliminate propeller shafts with their maintenance and potential leakage problems.
One technology studied in various devices since the 1700s is linear acceleration, known in weapon development as a railgun. Linear accelerators take advantage of the principles of the Faraday motor by powering a double rail with electrical energy. A metal sled or projectile rests on top of the rails, with current passing through the sled from rail to rail. The resulting effect is a homopolar motor. Instead of spinning, however, the sled or projectile is propelled at increasing speeds along the track.
