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07 November 2003

An Ingenious Electric Motor

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published June, 1975.

During a mathematical investigation of electromagnetic forces that arise locally in various electric circuits, Harry E. Stockman (75 Gray Street, Arlington, Mass. 02174) has discovered a number of novel motors. Some of them, such as the tunnel-diode motor and the parametric motor, have been described in these columns [see "The Amateur Scientist," October, 1965, and January, 1973] Stockman's most recent construction is a variable-speed motor that operates on alternating current. Unlike most variable-speed motors, his version requires no commutator or brushes. Stockman describes the motor.

"A demonstration model of the asynchronous motor includes a fixed electromagnet energized by a potential of about 20 volts at a frequency of 60 hertz. The speed of the motor can be altered by varying the applied potential. The moving element consists of a nonmagnetic rotor that supports at one end a coil of insulated wire connected to a silicon diode. The rotor is balanced mechanically by a counterweight and turns on a vertical shaft, as shown in the accompanying illustration [right].

"A bar magnet is supported near the electromagnet. As the small coil approaches the electromagnet it cuts through the field of the magnet at approximately right angles. The design is not critical. All dimensions can be altered."

An alternating potential applied to the electromagnet causes a force of magnetic repulsion to act on the small coil. Simultaneously the alternating potential induces a unidirectional current in the small coil connected to the rectifying diode. The resulting unidirectional magnetic field of the small coil can interact with the field of a bar magnet to minimize the repulsive force exerted on the coil by the electromagnet.

"By experiment one can find a position of the bar magnet that enables the small coil to approach the electromagnet and move freely into alignment with it. At this position, however, the coil is violently repelled. As the coil recedes from the electromagnet it acquires angular momentum that carries it into the field of the bar magnet in preparation for the next revolution The speed of rotation is a function of the forces and hence of the applied potential."