Electric motor
In the basement of the Royal Institution, a free-hanging wire dipped into a pool of mercury rotated around a permanent magnet on the 3rd of September 1821. This simple demonstration revealed that an electric current creates a close circular magnetic field capable of generating torque. Modern motors rely on this same interaction between a magnetic field and electric current in a wire winding to produce mechanical force. The motor contains two primary mechanical parts: a rotor that moves and a stator that remains stationary. The rotor typically holds conductors carrying currents while the stator surrounds it with field magnets. These magnets can be electromagnets made from wire windings around a ferromagnetic iron core or permanent magnets. The stator core consists of many thin metal sheets called laminations insulated from each other. Laminations reduce losses caused by induced circulating eddy currents that would flow if a solid core were used. An air gap exists between the stator and rotor allowing rotation. The width of this gap significantly affects electrical characteristics as a large gap weakens performance while gaps too small create friction and noise. Bearings support the rotor transferring axial and radial loads from the shaft to the motor housing.
Experimental electrostatic devices described by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin appeared in the 1740s before practical electromagnetic motors existed. Henry Cavendish discovered Coulomb's law in 1771 but did not publish it until Charles-Augustin de Coulomb published his findings independently in 1785. Alessandro Volta invented the electrochemical battery in 1799 making persistent electric current production possible. Hans Christian Ørsted discovered in 1820 that an electric current creates a magnetic field exerting force on a magnet. André-Marie Ampère developed the first formulation of electromagnetic interaction within weeks presenting what became known as Ampère's force law. Michael Faraday gave the first demonstration of rotary motion on the 3rd of September 1821 showing how current gives rise to circular magnetic fields around wire. Barlow's wheel refined Faraday's demonstration though homopolar motors remained unsuited for practical application until late in the century. Hungarian physicist Ányos Jedlik started experimenting with electromagnetic coils in 1827 solving technical problems of continuous rotation with the invention of the commutator. He called these early devices electromagnetic self-rotors demonstrating the first device containing stator rotor and commutator components in 1828. The historic motor still works perfectly today housed in the Museum of Applied Arts in Budapest. English scientist William Sturgeon invented the first commutator capable of turning machinery in 1832. American inventors Thomas Davenport and Emily Davenport built a commutator-type direct-current electric motor patented in 1837 running at up to 600 revolutions per minute. These motors powered machine tools and printing presses but high battery costs caused commercial failure bankrupting the Davenports. German-Russian Moritz von Jacobi created the first real useful rotating electric motor in May 1834 developing remarkable mechanical output power setting world records improved four years later in September 1838.
French physicist François Arago formulated rotating magnetic fields termed Arago's rotations in 1824 demonstrated by Walter Baily in 1879 as primitive induction motors. Inventors struggled throughout the 1880s to develop workable AC motors because alternating current advantages in long-distance transmission were offset by inability to operate motors on AC. Galileo Ferraris invented the first alternating-current commutatorless induction motor in 1885 improving his design with advanced setups in 1886. The Royal Academy of Science of Turin published Ferraris's research concluding that apparatus based on that principle could not be of any commercial importance as motor. Nikola Tesla independently invented his induction motor in 1887 obtaining a patent in May 1888. Tesla presented paper A New System of Alternate Current Motors and Transformers to the AIEE describing three patented two-phase four-stator-pole motor types including reluctance synchronous and true synchronous motors. George Westinghouse acquired rights from Ferraris for US$1,000 then bought Tesla's patents for US$60,000 plus US$2.50 per sold horsepower paid until 1897. Westinghouse employed Tesla to develop motors though Tesla left for other pursuits in 1889. Mikhail Dolivo-Dobrovolsky invented the three-phase induction motor in 1889 of both cage-rotor and wound rotor types with starting rheostat developing the three-limb transformer in 1890. At the 1891 Frankfurt International Electrotechnical Exhibition the first long distance three-phase system successfully presented rated 15 kV extended over 175 km from Lauffen waterfall on Neckar river. A 100-horsepower three-phase induction motor powered an artificial waterfall representing transfer of original power source. By 1896 General Electric and Westinghouse signed cross-licensing agreement for bar-winding-rotor design later called squirrel-cage rotor. Induction motor improvements meant a 100-horsepower induction motor currently has same mounting dimensions as 7.5-horsepower motor in 1897.
Brushless DC motors eliminate mechanical rotating switches replacing them with external electronic switches synchronized to rotor position. BLDC motors typically achieve efficiency above 85% reaching up to 96.5% while brushed DC motors operate at 75, 80% efficiency. The characteristic trapezoidal counter-electromotive force waveform derives partly from stator windings evenly distributed and partly from placement of rotor permanent magnets. Hall effect sensors mounted on windings provide rotor position sensing enabling low cost closed-loop commutator control. These motors commonly appear in computer disk drives video cassette recorders CD-ROM mechanisms fans laser printers and photocopiers. Without a commutator the life of a BLDC motor significantly exceeds that of brushed DC motors with commutators. Commutation causes electrical and RF noise so BLDC motors suit electrically sensitive devices like audio equipment or computers. Tachometer signals derived from Hall effect sensors provide running speed feedback allowing synchronization to internal or external clocks for precise speed control. BLDC motors do not spark making them better suited to environments with volatile chemicals and fuels where sparking generates ozone accumulating in poorly ventilated buildings. Modern BLDC motors range power from fraction of watt to many kilowatts. Larger BLDC motors rated up to about 100 kW serve electric vehicles and electric model aircraft. Electronic commutators enable variable-speed operation through pulse-width modulation or electronic switching technologies. Universal motors can run on either AC or DC power synchronously reversing polarity in field and armature coils ensuring constant direction rotation. Universal motors often used in sub-kilowatt applications form basis of traditional railway traction motors though now rarely used due to efficiency losses from eddy current heating.
Linear motors unrolled from rotary designs produce straight-line force along length instead of torque. Linear motors commonly found roller-coasters where rapid motion of motorless railcar controlled by rail also used maglev trains flying over ground. The 1978 era HP 7225A pen plotter utilized two linear stepper motors moving pen along X and Y axes. Stepper motors typically provide precise rotations using internal rotor containing permanent magnets or magnetically soft rotor with salient poles controlled by electronically switched external magnets. Unlike synchronous motors stepper motors may not rotate continuously but move in steps advancing position as field windings energize de-energize sequence. Simple stepper motor drivers entirely energize or de-energize field windings leading rotor cog limited number positions while microstepping drivers proportionally control power allowing rotors position between cog points rotating smoothly. Computer-controlled stepper motors one most versatile positioning systems particularly part digital servo-controlled system used read/write head positioning early disk drives precision speed limitations made them obsolete hard drives newer drives use voice coil-based head actuator systems. So-called quartz analog wristwatches contain smallest commonplace stepping motors drawing little power having one coil permanent magnet rotor. Piezoelectric motors based change shape piezoelectric material when electric field applied make use converse piezoelectric effect producing acoustic ultrasonic vibrations creating linear rotary motion. One mechanism elongation single plane used series stretches position holds similar way caterpillar moves. Electrostatic motors based attraction repulsion electric charge usually require high-voltage power supply though small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction repulsion requiring high current low voltages. Electrostatic motors find frequent use micro-electro-mechanical systems where drive voltages below 100 volts moving charged plates far easier fabricate coils iron cores.
Electric motors consume approximately half world's electricity with sales estimated at 800 million units in 2022 increasing by 10% annually. Motor efficiency ranges from around 15%-20% for shaded pole motors up to 98% for permanent magnet motors depending on load. Peak efficiency typically occurs at less than half stall torque often at 75% rated load example 10 HP motor most efficient driving load requiring 7.5 HP. Efficiency also depends motor size larger motors tend more efficient. Some motors cannot operate continually more specified period time like hour per run. Eric Laithwaite proposed goodness factor metric determining efficiency of electric motor involving cross sectional areas lengths permeability angular frequency. Most efficient motors likely have relatively large magnetic poles equation directly relates non PM motors. Continuous torque density determined size air-gap area back-iron depth determined power rating armature winding set speed machine achievable air-gap flux density before core saturation. Torque density approximately four times greater liquid cooled motors compared those air cooled. Electric machines without transformer circuit topology such WRSMs or PMSMs cannot provide torque bursts saturating magnetic core permanently damaging permanent magnet assembly. Electric machines with transformer circuit topology induction machines doubly fed electric machines permit torque bursts two three times higher maximum design torque realizable. Brushless wound-rotor synchronous doubly fed machine only electric machine truly dual ported transformer circuit topology permitting bursts torque significantly higher eight times operating torque calculated.
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Common questions
When did Michael Faraday demonstrate the first electric motor?
Michael Faraday gave the first demonstration of rotary motion on the 3rd of September 1821 showing how current gives rise to circular magnetic fields around wire. This event took place in the basement of the Royal Institution where a free-hanging wire dipped into a pool of mercury rotated around a permanent magnet.
Who invented the first practical rotating electric motor and when was it created?
German-Russian Moritz von Jacobi created the first real useful rotating electric motor in May 1834 developing remarkable mechanical output power setting world records improved four years later in September 1838. This device represented a significant advancement over earlier experimental motors like Barlow's wheel which remained unsuited for practical application until late in the century.
What are the main components inside an electric motor rotor and stator?
The motor contains two primary mechanical parts: a rotor that moves and a stator that remains stationary. The rotor typically holds conductors carrying currents while the stator surrounds it with field magnets or electromagnets made from wire windings around a ferromagnetic iron core.
How efficient are brushless DC motors compared to brushed DC motors?
BLDC motors typically achieve efficiency above 85% reaching up to 96.5% while brushed DC motors operate at 75, 80% efficiency. These motors eliminate mechanical rotating switches replacing them with external electronic switches synchronized to rotor position to significantly exceed the life of brushed DC motors with commutators.
When did Nikola Tesla patent his induction motor and what was its significance?
Nikola Tesla independently invented his induction motor in 1887 obtaining a patent in May 1888. He presented paper A New System of Alternate Current Motors and Transformers to the AIEE describing three patented two-phase four-stator-pole motor types including reluctance synchronous and true synchronous motors which enabled long distance power transmission systems.