What Is a Homopolar Motor?

A homopolar motor is a device that converts a magnetic field and a direct electrical current (DC) into motion. The main parts are a permanent magnet with an adjacent metal disc and shaft; homopolar means only one pole or side of the magnet is next to the rotor. Direct current is applied to the disc and it rotates due to the effects of the current passing through the magnetic field. Scientist Michael Faraday first demonstrated the effect in 1821 with a wire rotating around a magnet in a mercury bath.

Electricity can also be generated by a homopolar motor, which acts as a generator by rotating the metal disc through the magnetic field. The rotating disc will create a DC current that can be stored in a battery. Although this principle is not practical for large generation facilities, a single pole generator can be useful when DC current is needed.

Another variation of the homopolar motor was Barlow's wheel. Mathematician Peter Barlow developed the wheel in 1822 to demonstrate the Faraday effect. Barlow's 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 rotating disc is lowered into the mercury and an electrical circuit is completed, the current interacts with the magnetic field and the wheel turns.

One advantage of single pole motor technology is a reduction in parts. Motors with a wire coil stator and rotor must also use a commutator for proper operation. A commutator is a device that reverses the motor polarity as the rotor turns. This is necessary because the rotor changes position in the motor magnetic field, and the polarity change is needed to provide torque, or rotational force.

There are a variety of practical applications for a homopolar motor. Ship propulsion began using these motors in the 20th century, with electric drives replacing diesel engines connected to drive shafts passing through the ship's hull. Electric generators can produce DC power for systems connected directly to propellers.

Beginning in the early 21st century, electric drives were installed in rotating pods beneath a ship's keel that can provide thrust in any direction. This technology provides good efficiency of propulsion, and provides excellent ship control for docking and maneuvering. The pods can be controlled from a ship's bridge with a joystick, and eliminate drive shafts with their maintenance and potential leakage issues.

A technology studied in various devices since the 1700s is linear acceleration, known in weapons development as a rail gun. Linear accelerators take advantage of Faraday's motor principles by energizing a dual rail with electric power. A metal sled or projectile rests on top of the rails, with the current passing through the sled from one rail to the other. The resulting effect is a homopolar motor. Rather than rotating, however, the sled or projectile is propelled at increasing speeds along the rail.