Ferraris' motor

Lore & Background

Professor Galileo Ferraris, of Turin, had already in 1885 arrived at the same fundamental ideas as those of Baily and of Deprez, but without knowing of their work. He proposed to produce rotation of a copper conductor by eddy-currents induced by a progressively shifted magnetic field, generated as a true rotating field by combining at right angles two alternate currents differing by a quarter-period. In 1885, Ferraris constructed a motor consisting of two pairs of electromagnets with a common yoke made by winding iron wire around the exterior. Two alternate currents differing in phase were led into these circuits, and a pivoted central body was observed to revolve. His first publication was in March 1888, entitled 'Electrodynamic rotations produced by means of alternate currents.' He suggested a simple way to procure the desired phase-currents by branching an alternate current circuit into two parts: one with a resistance without self-induction, the other with a coil of much self-induction but small resistance. The two windings of the motor were introduced into these branches. Experiments in autumn 1885 used two flat coils set at right angles; a small hollow closed copper cylinder suspended in the central space rotated when both currents were turned on. The sense of rotation could be reversed by changing connections of the second coil. Similar results were found with an iron cylinder and a laminated iron cylinder built of insulated disks.

Reader's Guide

Ferraris' motor demonstrated the principle of the rotating magnetic field produced by two alternating currents 90° apart, a concept that underlies the induction motor. Ferraris himself remarked that a motor thus made could not have any importance as a means of industrial transformation of power, and a larger model with a 10 lb copper cylinder was of but small power. He discussed the elementary theory, noting that inductive action is proportional to slip—the difference between the angular velocity of the magnetic field and that of the rotating cylinder—and that motor power is proportional jointly to slip and velocity. He also suggested measuring instruments for alternate currents based on this principle and succeeded in producing rotation in a mass of mercury placed in a vessel in the rotatory field. In 1894 Ferraris published another discussion of the theory. The motor was exhibited in 1893 at the World's Fair at Chicago. Its legacy lies in establishing the theoretical and experimental foundation for polyphase induction motors, though Ferraris did not pursue industrial application.

Did You Know?

• The rotating part was a small hollow closed cylinder of copper, later a 10 lb copper cylinder of length 18 cm and diameter 8.9 cm.
• Ferraris suggested producing the required phase difference by branching an alternate current circuit into a resistive branch and an inductive branch.