Homopolar motor

DIY simple homopolar motor made with a drywall screw, a bat-tery cell, a wire, and a disk magnet. The magnet is on top ofthe screw head. The screw and magnet make contact with thebottom of the battery cell and are held together by the magnet’sattraction. The screw and magnet spin, with the screw tip actingas a bearing.

A homopolar motor is a direct current electric motorwith two magnetic poles, the conductors of which al-ways cut unidirectional lines of magnetic flux by rotat-ing a conductor around a fixed axis that is parallel to themagnetic field. The resulting EMF (Electromotive Force)being continuous in one direction, the homopolar motorneeds no commutator but still requires slip rings.[1] Thename homopolar indicates that the electrical polarity ofthe conductor and the magnetic field poles do not change(i.e., that it does not require commutation).

1 History

The homopolar motor was the first electrical motor to bebuilt. Its operation was demonstrated byMichael Faradayin 1821 at the Royal Institution in London.[3][4]

In 1821, soon after the Danish physicist and chemistHans Christian Ørsted discovered the phenomenon ofelectromagnetism, Humphry Davy and British scientistWilliam Hyde Wollaston tried, but failed, to design anelectric motor.[5] Faraday, having discussed the problemwith the twomen, went on to build two devices to producewhat he called “electromagnetic rotation”. One of these,now known as the homopolar motor, caused a continu-ous circular motion that was engendered by the circularmagnetic force around a wire that extended into a pool ofmercury wherein was placed a magnet. The wire wouldthen rotate around the magnet if supplied with current

Electromagnetic rotation experiment of Faraday, ca. 1821[2]

from a chemical battery. These experiments and inven-tions formed the foundation of modern electromagnetictechnology. In his excitement, Faraday published resultswithout acknowledging his work with either Wollaston orDavy. The resulting controversy within the Royal Soci-ety strained his mentor relationship with Davy and maywell have contributed to Faraday’s assignment to other ac-tivities, which consequently prevented his involvement inelectromagnetic research, for several years.[6][7]

B. G. Lamme described in 1912 a homopolar machinerated 2,000 kW, 260 V, 7,700 A and 1,200 rpm with 16slip rings operating at a peripheral velocity of 67 m/s. Aunipolar generator rated 1,125 kW, 7.5 V 150,000A, 514rpm built in 1934 was installed in a U.S. steel mill for pipewelding purposes.[8]

2 Principle of operation

The homopolar motor is driven by the Lorentz force: as itmoves through a magnetic field, the conductor is pushedthrough a magnetic field by opposing forces. This forceinduces a torque around the axis of rotation.[9] Becausethe axis of rotation is parallel to the magnetic field, andthe opposing magnetic fields do not change polarity, nocommutation is required for the conductor to keep turn-ing. This simplicity is achieved at the cost of not beingable to have more than one coil turn, which makes this

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2 7 REFERENCES

configured homopolar motor unsuited for most useful ap-plications. Homopolar motors have advantages and dis-advantages and have not been fully developed.Like most electro-mechanical machines, a homopolarmotor is reversible: if the conductor is turned mechan-ically, then it will operate as a homopolar generator, pro-ducing a direct current voltage between the two terminalsof the conductor. The direct current produced is an effectof the homopolar nature of the design.

3 Building a simple homopolarmo-tor

A homopolar motor is very easy to build. A permanentmagnet is used to provide the external magnetic field inwhich the conductor will turn, and a battery causes a cur-rent to flow along a conducting wire. It is not necessaryfor the magnet to move, or even to be in contact with therest of the motor; its sole purpose is to provide a magneticfield that will interact with the magnetic field induced bythe current in the wire. One can attach the magnet tothe battery and allow the conducting wire to rotate freelywhile closing the electric circuit by touching both the topof the battery and the magnet attached to the bottom ofthe battery. The wire and the battery may become hot ifoperated continuously.[10][11]

4 Gallery

• Homopolar motor 3D

• Homopolar motor 2D

• Current, magnetic field lines and Lorentz force onHomopolar motor

• Play media

Video

5 Examples

• Railgun

• Ball bearing motor

6 See also

• Homopolar generators

• Barlow’s wheel

7 References[1] Lynn, L. (1949). Knowlton, A.E., ed. “Standard Hand-

book for Electrical Engineers” (8th ed.). McGraw-Hill.|chapter= ignored (help)

[2] Faraday, Michael (1844). Experimental Researches inElectricity 2. ISBN 0-486-43505-9. See plate 4.

[3] Michael Faraday, “New Electro-Magnetic Apparatus,”Quarterly Journal of Science, Literature and the Arts 12,186-187 (1821). Scanned pages

[4] Michael Faraday, “Description of an Electro-magneticApparatus for the Exhibition of Rotatory Motion,” Quar-terly Journal of Science, Literature and the Arts 12, 283-285 (1821). Scanned pages Original illustration (plate) ofthe homopolar motor, referred to on p. 283 of Faraday’ssecond article. Index of Quarterly Journal for Faraday,Michael

[5] “Archives Biographies: Michael Faraday”, The Institutionof Engineering and Technology.

[6] Hamilton’s A Life of Discovery: Michael Faraday, Giantof the Scientific Revolution (2004) pp. 165–71, 183, 187–90.

[7] Cantor’s Michael Faraday, Sandemanian and Scientist(1991) pp. 231–3.

[8] Lynn, p. 842

[9] See, e.g., Richard P. Feynman, The Feynman Lectures onPhysics, vol. II, section 17-2, (Reading, MA: Addison-Wesley, 1964). ISBN 0-201-02117-X

[10] “How the build the simplest electric motor”, from EvilMad Scientist Laboratories, Aug. 7, 2006

[11] “How tomake a homopolarmotor, fromDangerously Fun,Jan. 31, 2007

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8 Text and image sources, contributors, and licenses

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