Electric motor

Animation of an Electric motor.

An electric motor is an electrical machine that converts electrical energy into mechanical energy. An electric generator operates in the reverse direction, converting mechanical energy into electrical energy.

Most electric motors operate through the interaction between the motor's magnetic field and winding currents to generate force. In certain applications, such as in regenerative braking with traction motors, electric motors can be used in reverse as generators to recover energy that might otherwise be lost as heat and friction.

Electric motors can be powered by direct current (DC) sources, such as from batteries, motor vehicles or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or generators.

General-purpose motors with standard dimensions and characteristics provide convenient mechanical power for industrial use. The largest electric motors are used for ship propulsion, pipeline compression and pumped-storage applications with ratings reaching 100 megawatts. Electric motors may be classified by considerations such as power source type, internal construction, application and type of motion output.

Electric motors produce linear or rotary force (torque) and can be distinguished from devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical force, which are respectively referred to as actuators and transducers.

Electric motors are found in industrial fans, blowers and pumps, machine tools, household appliances, power tools and disk drives. Small motors may be found in electric watches.

History

Early motors

Faraday's electromagnetic experiment, 1821[1]

The first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the 1740s.[2][3] The theoretical principle behind them, Ampère's force law, was discovered by André-Marie Ampère in 1820. The law described the production of mechanical force by the interactions of an electric current and a magnetic field. The conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by English scientist Michael Faraday in 1821. A free-hanging wire was dipped into a pool of mercury, on which a permanent magnet (PM) was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire.[4] This motor is often demonstrated in physics experiments, substituting brine for (toxic) mercury. Though Barlow's wheel was an early refinement to this Faraday demonstration, these and similar homopolar motors remained unsuited to practical application until late in the century.

Jedlik's "electromagnetic self-rotor", 1827 (Museum of Applied Arts, Budapest). The historic motor still works perfectly today.[5]
An electric motor presented to Kelvin by James Joule in 1842, Hunterian Museum, Glasgow

In 1827, Hungarian physicist Ányos Jedlik started experimenting with electromagnetic coils. After Jedlik solved the technical problems of continuous rotation with the invention of the commutator, he called his early devices "electromagnetic self-rotors". Although they were used only for teaching, in 1828 Jedlik demonstrated the first device to contain the three main components of practical DC motors: the stator, rotor and commutator. The device employed no permanent magnets, as the magnetic fields of both the stationary and revolving components were produced solely by the currents flowing through their windings.[6][7][8][9][10][11][12]

DC motors

After many other more or less successful attempts with relatively weak rotating and reciprocating apparatus Prussian Moritz von Jacobi created the first real rotating electric motor in May 1834. It developed remarkable mechanical output power. His motor set a world record, which Jacobi improved four years later in September 1838.[13] His second motor was powerful enough to drive a boat with 14 people across a wide river. In 1839/40 that other developers managed to build motors with similar and then higher performance.

The first commutator DC electric motor capable of turning machinery was invented by British scientist William Sturgeon in 1832.[14] Following Sturgeon's work, a commutator-type direct-current electric motor made was built by American inventor Thomas Davenport, which he patented in 1837. The motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press.[15] Due to the high cost of primary battery power, the motors were commercially unsuccessful and bankrupted Davenport. Several inventors followed Sturgeon in the development of DC motors, but all encountered the same battery cost issues. No electricity distribution system was available at the time. No practical commercial market emerged for these motors.[16]

In 1855, Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work.[6][12] He built a model electric vehicle that same year.[17]

A major turning point came n 1864, when Antonio Pacinotti first described the ring armature. This featured symmetrically-grouped coils closed upon themselves and connected to the bars of a commutator, the brushes of which delivered practically non-fluctuating current.[18][19] The first commercially-successful DC motors followed the invention by Zénobe Gramme who, in 1871, reinvented Pacinotti's design. In 1873, Gramme showed that his dynamo could be used as a motor, which he demonstrated to great effect at exhibitions in Vienna and Philadelphia by connecting two such DC motors up to 2 km from each other, using one as a generator.[20]

In 1886, Frank Julian Sprague invented the first practical DC motor, a non-sparking device that maintained relatively constant speed under variable loads. Other Sprague electric inventions about this time greatly improved grid electric distribution (prior work done while employed by Thomas Edison), allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided control systems for electric operations. This allowed Sprague to use electric motors to invent the first electric trolley system in 1887–88 in Richmond Virginia, the electric elevator and control system in 1892, and the electric subway with independently-powered centrally-controlled cars. The latter were first installed in 1892 in Chicago by the South Side Elevated Railroad, where it became popularly known as the "L". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry. The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of an air gap between the rotor and stator. Efficient designs have a comparatively small air gap.[21][a] The St. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor.[22]

Electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead, every machine could be equipped with its own power source, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of the electric energy produced in the US.[23]

AC motors

In 1824 French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago's rotations, which, by manually turning switches on and off, Walter Baily demonstrated in 1879 as in effect the first primitive induction motor.[24][25][26][27]I n the 1880s many inventors were trying to develop workable AC motors[28] because AC's advantages in long-distance high-voltage transmission were offset by the inability to operate motors on AC. The first alternating-current commutatorless induction motors were independently invented by Galileo Ferraris and Nikola Tesla, in 1885 and 1887, respectively. In 1888, the Royal Academy of Science of Turin published Ferraris's research detailing the foundations of motor operation, while concluding that "the apparatus based on that principle could not be of any commercial importance as motor."[27][29][30][31][32][33][34][35][36][37][38][39][40]

In 1888, Tesla presented his paper A New System for Alternating Current Motors and Transformers to the AIEE that described three patented two-phase four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting reluctance motor, another with a wound rotor forming a self-starting induction motor, and the third a true synchronous motor with separately excited DC supply to rotor winding.

One of the patents Tesla filed in 1887, however, also described a shorted-winding-rotor induction motor. George Westinghouse promptly bought Tesla's patents, employed Tesla to develop them, and assigned C. F. Scott to help Tesla; however, Tesla left for other pursuits in 1889.[27][34][37][38][39][40][41][42][43][44][45][46][47][48] The constant speed AC induction motor was found not to be suitable for street cars,[28] but Westinghouse engineers successfully adapted it to power a mining operation in Telluride, Colorado in 1891.[49][50][51]

Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase cage-rotor induction motor in 1889 and the three-limb transformer in 1890. This type of motor is now used for the vast majority of commercial applications.[52][53] However, he claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work.[54] Although Westinghouse achieved its first practical induction motor in 1892 and developed a line of polyphase 60 hertz induction motors in 1893, these early Westinghouse motors were two-phase motors with wound rotors. B. G. Lamme later developed a rotating bar winding rotor.[41]

The General Electric Company began developing three-phase induction motors in 1891.[41] By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor.[41] Induction motor improvements flowing from these inventions and innovations were such that a 100-horsepower (HP) induction motor currently has the same mounting dimensions as a 7.5 HP motor in 1897.[41]

Other Languages
Afrikaans: Elektriese motor
العربية: محرك كهربائي
asturianu: Motor llétricu
azərbaycanca: Elektrik mühərriki
Bân-lâm-gú: Tiān-khì mò͘-tah
Boarisch: Elektromotoa
bosanski: Elektromotor
čeština: Elektromotor
Cymraeg: Modur trydan
Deutsch: Elektromotor
Esperanto: Elektromotoro
客家語/Hak-kâ-ngî: Mô-tá
한국어: 전동기
hrvatski: Elektromotor
Bahasa Indonesia: Motor listrik
Kreyòl ayisyen: Motè elektrik
latviešu: Elektrodzinējs
magyar: Villanymotor
македонски: Електромотор
Bahasa Melayu: Motor elektrik
မြန်မာဘာသာ: လျှပ်စစ်မိုတာ
Nederlands: Elektromotor
日本語: 電動機
norsk nynorsk: Elektrisk motor
oʻzbekcha/ўзбекча: Elektr dvigateli
português: Motor elétrico
Qaraqalpaqsha: Elektrik dvigateli
română: Motor electric
Simple English: Electric motor
slovenčina: Elektromotor
slovenščina: Elektromotor
српски / srpski: Електромотор
srpskohrvatski / српскохрватски: Elektromotor
Türkçe: Elektrik motoru
Türkmençe: Elektrik dwigateli
українська: Електродвигун
Tiếng Việt: Động cơ điện
粵語: 電摩打
中文: 电动机