An electric motor is an

electrical machine In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity ...

that converts electrical energy into

mechanical energy In physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical energy is ...

. Most electric motors operate through the interaction between the motor's

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

and

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

in a wire winding to generate force in the form of

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy. Electric motors can be powered by

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...

(DC) sources, such as from batteries, or rectifiers, or by

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(AC) sources, such as a power grid,

inverters A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...

or electrical generators. Electric motors may be classified by considerations such as power source type, construction, application and type of motion output. They can be powered by AC or DC, be brushed or brushless,

single-phase In electrical engineering, single-phase electric power (abbreviated 1φ) is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when load ...

, two-phase, or

three-phase Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, Electric power transmission, transmission, and Electric power distribution, distribution. It is a type of polyphase system empl ...

, axial or radial flux, and may be air-cooled or liquid-cooled. Standardized motors provide convenient mechanical power for industrial use. The largest are used for ship propulsion, pipeline compression and

pumped-storage Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of gravitational potential ...

applications with output exceeding 100

megawatts The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wat ...

. Applications include industrial fans, blowers and pumps, machine tools, household appliances, power tools, vehicles, and disk drives. Small motors may be found in electric watches. In certain applications, such as in

regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction mo ...

with

traction motors A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles (electric multiple uni ...

, electric motors can be used in reverse as generators to recover energy that might otherwise be lost as heat and friction. Electric motors produce linear or rotary force (

) intended to propel some external mechanism, such as a fan or an elevator. An electric motor is generally designed for continuous rotation, or for linear movement over a significant distance compared to its size. Magnetic

solenoids upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...

are also transducers that convert electrical power to mechanical motion, but can produce motion over only a limited distance.

History

Early motors

Before modern electromagnetic motors, experimental motors that worked by electrostatic force were investigated. The first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter

Benjamin Franklin Benjamin Franklin ( April 17, 1790) was an American polymath who was active as a writer, scientist, inventor, statesman, diplomat, printer, publisher, and political philosopher. Encyclopædia Britannica, Wood, 2021 Among the leading inte ...

in the 1740s.Tom McInally, The Sixth Scottish University. The Scots Colleges Abroad: 1575 to 1799 (Brill, Leiden, 2012) p. 115 The theoretical principle behind them,

Coulomb's law Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventiona ...

, was discovered but not published, by

Henry Cavendish Henry Cavendish ( ; 10 October 1731 – 24 February 1810) was an English natural philosopher and scientist who was an important experimental and theoretical chemist and physicist. He is noted for his discovery of hydrogen, which he termed "infl ...

in 1771. This law was discovered independently by

Charles-Augustin de Coulomb Charles-Augustin de Coulomb (; ; 14 June 1736 – 23 August 1806) was a French officer, engineer, and physicist. He is best known as the eponymous discoverer of what is now called Coulomb's law, the description of the electrostatic force of attrac ...

in 1785, who published it so that it is now known with his name. Due to the difficulty of generating the high voltages they required, electrostatic motors were never used for practical purposes. The invention of the electrochemical battery by

Alessandro Volta Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist and lay Catholic who was a pioneer of electricity and power who is credited as the inventor of the electric battery and the ...

in 1799 made possible the production of persistent electric currents. Hans Christian Ørsted discovered in 1820 that an electric current creates a magnetic field, which can exert a force on a magnet. It only took a few weeks for André-Marie Ampère to develop the first formulation of the electromagnetic interaction and present the

Ampère's force law In magnetostatics, the force of attraction or repulsion between two current-carrying wires (see first figure below) is often called Ampère's force law. The physical origin of this force is that each wire generates a magnetic field, followin ...

, that described the production of mechanical force by the interaction of an electric current and a magnetic field. The first demonstration of the effect with a rotary motion was given by

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic inducti ...

on 3 September 1821 in the basement of the Royal Institution. 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. Faraday published the results of his discovery in the ''

Quarterly Journal of Science ''Quarterly Journal of Science'' was the title of two British scientific periodicals of the 19th century. The first was established in 1816 by William Thomas Brande, as the ''Quarterly Journal of Science, Literature and the Arts''. He edited it w ...

'', and sent copies of his paper along with pocket-sized models of his device to colleagues around the world so they could also witness the phenomenon of electromagnetic rotations. This motor is often demonstrated in physics experiments, substituting

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...

for (toxic) mercury.

Barlow's wheel An 1842 diagram of Barlow's wheel Barlow's wheel was an early demonstration of a homopolar motor, designed and built by English mathematician and physicist, Peter Barlow in 1822. It consists of a star-shaped wheel free to turn suspended over a ...

was an early refinement to this Faraday demonstration, although these and similar homopolar motors remained unsuited to practical application until late in the century. Jedlik_motor

In 1827, Hungarian

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...

Ányos Jedlik Ányos István Jedlik ( hu, Jedlik Ányos István; sk, Štefan Anián Jedlík; in older texts and publications: la, Stephanus Anianus Jedlik; 11 January 1800 – 13 December 1895) was a Hungarian inventor, engineer, physicist, and Bened ...

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 Rotor may refer to: Science and technology Engineering *Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator * Helicopter rotor, the rotary wing(s) of a rotorcraft ...

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.

DC motors

The first commutator capable of turning machinery was invented by British scientist

William Sturgeon William Sturgeon (22 May 1783 – 4 December 1850) was an English physicist and inventor who made the first electromagnets, and invented the first practical British electric motor. Early life Sturgeon was born on 22 May 1783 in Whittington, ...

in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor was built by American inventor Thomas Davenport and Emily Davenport, which he patented in 1837. The motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press. 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. As no

electricity distribution Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...

system was available at the time, no practical commercial market emerged for these motors. After many other more or less successful attempts with relatively weak rotating and reciprocating apparatus Prussian/Russian

Moritz von Jacobi Moritz Hermann or Boris Semyonovich (von) Jacobi (russian: Борис Семёнович Якоби; 21 September 1801, Potsdam – 10 March 1874, Saint Petersburg) was a Prussian and Russian Imperial engineer and physicist of Jewish descent. Jac ...

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. His second motor was powerful enough to drive a boat with 14 people across a wide river. It was also in 1839/40 that other developers managed to build motors with similar and then higher performance. In 1855, Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work. He built a model electric vehicle that same year. A major turning point came in 1864, when

Antonio Pacinotti Antonio Pacinotti (17 June 1841 – 24 March 1912) was an Italian physicist, who was Professor of Physics at the University of Pisa. Biography Pacinotti was born in Pisa, where he also died. He was the son of Luigi Pacinotti and Caterina ...

first described the ring armature (although initially conceived in a DC generator, i.e. a dynamo). 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. The first commercially successful DC motors followed the developments by

Zénobe Gramme Zénobe Théophile Gramme (4 April 1826 – 20 January 1901) was a Belgian electrical engineer. He was born at Jehay-Bodegnée on 4 April 1826, the sixth child of Mathieu-Joseph Gramme, and died at Bois-Colombes on 20 January 1901. He invented ...

who, in 1871, reinvented Pacinotti's design and adopted some solutions by

Werner Siemens Ernst Werner Siemens (von Siemens from 1888; ; ; 13 December 1816 – 6 December 1892) was a German electrical engineer, inventor and industrialist. Siemens's name has been adopted as the SI unit of electrical conductance, the siemens. He foun ...

. A benefit to DC machines came from the discovery of the reversibility of the electric machine, which was announced by Siemens in 1867 and observed by Pacinotti in 1869. Gramme accidentally demonstrated it on the occasion of the

1873 Vienna World's Fair ) , building = Rotunda , area = 233 Ha , invent = , visitors = 7,255,000 , organized = , cnt = , org = , biz = , country = Austria-Hungary , city ...

, when he connected two such DC devices up to 2 km from each other, using one of them as a generator and the other as motor. The drum rotor was introduced by

Friedrich von Hefner-Alteneck Friedrich Heinrich Philipp Franz von Hefner-Alteneck (April 27, 1845 in Aschaffenburg – January 6, 1904 in Biesdorf near Berlin) was a German electrical engineer and one of the closest aides of Werner von Siemens. He is largely remembered fo ...

of Siemens & Halske to replace Pacinotti's ring armature in 1872, thus improving the machine efficiency. The laminated rotor was introduced by Siemens & Halske the following year, achieving reduced iron losses and increased induced voltages. In 1880,

Jonas Wenström Jonas Wenström (August 4, 1855 in Hällefors – December 22, 1893 in Västerås) was a Swedish engineer and inventor. the basis for ASEA (later ABB ABB Ltd. is a Swedish- Swiss multinational corporation headquartered in Zürich, Swi ...

provided the rotor with slots for housing the winding, further increasing the efficiency. In 1886,

Frank Julian Sprague Frank Julian Sprague (July 25, 1857 in Milford, Connecticut – October 25, 1934) was an American inventor who contributed to the development of the electric motor, electric railways, and electric elevators. His contributions were especially ...

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 Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventio ...

), 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 (Thus do we reach the stars) , image_map = , mapsize = 250 px , map_caption = Location within Virginia , pushpin_map = Virginia#USA , pushpin_label = Richmond , pushpin_m ...

, 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 The South Side Elevated Railroad (originally Chicago and South Side Rapid Transit Railroad) was the first elevated rapid transit line in Chicago, Illinois. The line ran from downtown Chicago to Jackson Park, with branches to Englewood, Normal Pa ...

, 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. The St. Louis motor, long used in classrooms to illustrate motor principles, is inefficient for the same reason, as well as appearing nothing like a modern motor. 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 (like in washing machines, dishwashers, fans, air conditioners and refrigerators (replacing ice boxes)) 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.

AC motors

In 1824, French physicist François Arago formulated the existence of rotating magnetic fields, termed

Arago's rotations Arago's rotations is an observable magnetic phenomenon that involves the interactions between a magnetized needle and a moving metal disk. The effect was discovered by François Arago in 1824. At the time of their discovery, Arago's rotations were s ...

, which, by manually turning switches on and off, Walter Baily demonstrated in 1879 as in effect the first primitive induction motor. In the 1880s many inventors were trying to develop workable AC motors 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 motor was invented by

Galileo Ferraris Galileo Ferraris (31 October 1847 – 7 February 1897) was an Italian university professor, physicist and electrical engineer, one of the pioneers of AC power system and inventor of the induction motor although he never patented his work. Many ...

in 1885. Ferraris was able to improve his first design by producing more advanced setups in 1886. In 1888, the ''Royal Academy of Science of Turin'' published Ferraris's research detailing the foundations of motor operation, while concluding at that time that "the apparatus based on that principle could not be of any commercial importance as motor." Possible industrial development was envisioned by

Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''

AIEE The American Institute of Electrical Engineers (AIEE) was a United States-based organization of electrical engineers that existed from 1884 through 1962. On January 1, 1963, it merged with the Institute of Radio Engineers (IRE) to form the Institu ...

that described three patented two-phase four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting

reluctance motor A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance. Reluctance motor subtypes include synchro ...

, another with a wound rotor forming a self-starting induction motor, and the third a true

synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Syn ...

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 George Westinghouse Jr. (October 6, 1846 – March 12, 1914) was an American entrepreneur and engineer based in Pennsylvania who created the railway air brake and was a pioneer of the electrical industry, receiving his first patent at the age ...

, who had already acquired rights from Ferraris (US$1,000), promptly bought Tesla's patents (US$60,000 plus US$2.50 per sold hp, paid until 1897), employed Tesla to develop his motors, and assigned C.F. Scott to help Tesla; however, Tesla left for other pursuits in 1889. The constant speed AC induction motor was found not to be suitable for street cars, but Westinghouse engineers successfully adapted it to power a mining operation in Telluride, Colorado in 1891. Westinghouse achieved its first practical induction motor in 1892 and developed a line of polyphase 60 hertz induction motors in 1893, but these early Westinghouse motors were two-phase motors with wound rotors. B.G. Lamme later developed a rotating bar winding rotor. Steadfast in his promotion of three-phase development,

Mikhail Dolivo-Dobrovolsky Mikhail Osipovich Dolivo-Dobrovolsky (russian: Михаи́л О́сипович Доли́во-Доброво́льский; german: Michail von Dolivo-Dobrowolsky or ''Michail Ossipowitsch Doliwo-Dobrowolski''; – ) was a Russian Empire ...

invented the three-phase induction motor in 1889, of both types cage-rotor and wound rotor with a starting rheostat, and the three-limb

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

in 1890. After an agreement between AEG and

Maschinenfabrik Oerlikon Maschinenfabrik Oerlikon was a Swiss engineering company based in the Zürich district of Oerlikon (Zürich), Oerlikon known for the early development of electric locomotives. It was founded in 1876 by the industrialist Peter Emil Huber-Werdmü ...

, Doliwo-Dobrowolski and

Charles Eugene Lancelot Brown Brown c. 1900 Charles Eugene Lancelot Brown (17 June 1863 – 2 May 1924) was a Swiss businessman and engineer who co-founded Brown, Boveri & Cie (BBC), which later became ASEA Brown Boveri. Biography Brown was born on 17 June 1863 in Winterth ...

developed larger models, namely a 20-hp squirrel cage and a 100-hp wound rotor with a starting rheostat. These were the first three-phase asynchronous motors suitable for practical operation. Since 1889, similar developments of three-phase machinery were started Wenström. At the 1891 Frankfurt International Electrotechnical Exhibition, the first long distance three-phase system was successfully presented. It was rated 15 kV and extended over 175 km from the Lauffen waterfall on the Neckar river. The Lauffen power station included a 240 kW 86 V 40 Hz alternator and a step-up transformer while at the exhibition a step-down transformer fed a 100-hp three-phase induction motor that powered an artificial waterfall, representing the transfer of the original power source. The three-phase induction is now used for the vast majority of commercial applications.

claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work. The

General Electric Company The General Electric Company (GEC) was a major British industrial conglomerate involved in consumer and defence electronics, communications, and engineering. The company was founded in 1886, was Britain's largest private employer with over 250 ...

began developing three-phase induction motors in 1891. By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the

squirrel-cage rotor A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. In operation, the non-rotating stator winding i ...

. Induction motor improvements flowing from these inventions and innovations were such that a 100-

horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the ...

induction motor currently has the same mounting dimensions as a 7.5-horsepower motor in 1897.

Twenty-first century

In 2022, electric motor sales were estimated to be 800 million units, increasing by 10% annually. Electric motors consume ~50% of the world's electricity.

Components

The two mechanical parts of an electric motor are the rotor, which moves, and the stator, which does not. It also includes two electrical parts, a set of magnets and an armature, one of which is attached to the rotor and the other to the stator, together forming a magnetic circuit: Section 1.2.2 * Field magnets - The magnets create a magnetic field that passes through the armature. These can be

electromagnets An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in t ...

permanent magnets A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

. The field magnet is usually on the stator and the armature on the rotor, but in some types of motor these are reversed.

Bearings

The rotor is supported by bearings, which allow the rotor to turn on its axis. The bearings are in turn supported by the motor housing.

Rotor

The rotor is the moving part that delivers the mechanical power. The rotor typically holds conductors that carry currents, which the magnetic field of the stator exerts force on to turn the shaft. Alternatively, some rotors carry permanent magnets, and the stator holds the conductors. Permanent magnets offer high efficiency over a larger operating speed and power range. An air gap between the stator and rotor allows it to turn. The width of the gap has a significant effect on the motor's electrical characteristics. It is generally made as small as possible, as a large gap weakens performance. It is the main source of the low power factor at which motors operate. The

magnetizing current A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

increases and the power factor decreases with the air gap, so narrow gaps are better. Conversely, gaps that are too small may pose mechanical problems in addition to noise and losses. The motor shaft extends through the bearings to the outside of the motor, where the load is applied. Because the forces of the load are exerted beyond the outermost bearing, the load is said to be overhung.

Stator

The stator surrounds the rotor, and usually holds field magnets, which are either

consisting of wire windings around a ferromagnetic iron core or

. These create a

that passes through the rotor armature, exerting force on the windings. The stator core is made up of many thin metal sheets that are insulated from each other, called laminations. These laminations are made using

electrical steel Electrical steel (E-steel, lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is an iron alloy tailored to produce specific magnetic properties: small hysteresis area resulting in low power loss per cycle ...

which has a specified magnetic permeability, hysteresis, and saturation. Laminations are used to reduce losses that would result from induced circulating eddy currents that would flow if a solid core were used. Mains powered AC motors typically immobilize the wires within the windings by impregnating them with varnish in a vacuum. This prevents the wires in the winding from vibrating against each other which would abrade the wire insulation causing it to fail prematurely. Resin-packed motors, used in deep well submersible pumps, washing machines, and air conditioners, encapsulate the stator in plastic

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

to prevent corrosion and/or reduce conducted noise. Salient-pole_rotor

Armature

The armature consists of wire windings on a ferromagnetic core. Electric current passing through the wire causes the

from the field magnet to exert a force ( Lorentz force) on it, turning the rotor, which delivers the mechanical output. Windings are wires that are laid in coils, usually wrapped around a laminated, soft, iron, ferromagnetic core so as to form magnetic poles when energized with current. Electric machines come in salient- and nonsalient-pole configurations. In a salient-pole motor the ferromagnetic cores on the rotor and stator have projections called poles facing each other, with a wire winding around each pole below the pole face, which become north or south poles of the

when current flows through the wire. In a nonsalient-pole (or distributed field or round-rotor) motor, the ferromagnetic core is a smooth cylinder, with the windings distributed evenly in slots about the circumference. Supplying alternating current in the windings creates poles in the core that rotate continuously. in A

shaded-pole motor The shaded-pole motor is the original type of AC single-phase induction motor, dating back to at least as early as 1890. A shaded-pole motor is a small squirrel-cage motor in which the auxiliary winding is composed of a copper ring or bar sur ...

has a winding around part of the pole that delays the phase of the magnetic field for that pole.

Commutator

A commutator is a rotary

electrical switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

that supplies current to the rotor. It periodically reverses the flow of current in the rotor windings as the shaft rotates. It consists of a cylinder composed of multiple metal contact segments on the armature. Two or more

electrical contacts An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they c ...

called "

brushes A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped durin ...

" made of a soft conductive material like

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

press against the commutator. The brushes make sliding contact with successive commutator segments as it rotates, supplying current to the rotor. The windings on the rotor are connected to the commutator segments. The commutator periodically reverses the

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

direction in the rotor windings with each half turn (180°), so the torque applied to the rotor is always in the same direction. Without this current reversal, the direction of torque on each rotor winding would reverse with each half turn, so the rotor would stop. Commutators are inefficient and commutated motors have been mostly replaced by brushless direct current motors, permanent magnet motors, and

induction motors An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction mot ...

Motor supply and control

Motor supply

A DC motor is usually supplied through a split ring commutator as described above. AC motors' commutation can be achieved using either a slip ring commutator or external commutation. It can be fixed-speed or variable-speed control type, and can be synchronous or asynchronous. Universal motors can run on either AC or DC.

Motor control

DC motors can be operated at variable speeds by adjusting the voltage applied to the terminals or by using

pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...

(PWM). AC motors operated at a fixed speed are generally powered directly from the grid or through motor soft starters. AC motors operated at variable speeds are powered with various

power inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...

variable-frequency drive A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or current variat ...

or electronic commutator technologies. The term electronic commutator is usually associated with self-commutated brushless DC motor and switched reluctance motor applications.

Types

Electric motors operate on one of three physical principles:

magnetism Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...

, electrostatics and

piezoelectricity Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...

. In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft. One, or both, of these fields must change with the rotation of the rotor. This is done by switching the poles on and off at the right time, or varying the strength of the pole. The main types are DC motors and AC motors, with the latter replacing the former. AC electric motors are either asynchronous or synchronous. Once started, a synchronous motor requires synchrony with the moving magnetic field's speed for all normal torque conditions. In synchronous machines, the magnetic field must be provided by means other than induction, such as from separately excited windings or permanent magnets. A

fractional-horsepower motor A fractional-horsepower motor (FHP) is an electric motor with a rated output power of less than (the term 'fractional' indicates less than one unit). There is no defined minimum output, however, it is generally accepted that a motor with a frame ...

either has a rating below about 1 horsepower (0.746 kW), or is manufactured with a standard-frame size smaller than a standard 1 HP motor. Many household and industrial motors are in the fractional-horsepower class. Notes: * Rotation is independent of the frequency of the AC voltage. * Rotation is equal to synchronous speed (motor-stator-field speed). * In SCIM, fixed-speed operation rotation is equal to synchronous speed, less slip speed. * In non-slip energy-recovery systems, WRIM is usually used for motor-starting but can be used to vary load speed. * Variable-speed operation. * Whereas induction- and synchronous-motor drives are typically with either six-step or sinusoidal-waveform output, BLDC-motor drives are usually with trapezoidal-current waveform; the behavior of both sinusoidal and trapezoidal PM machines is, however, identical in terms of their fundamental aspects. * In variable-speed operation, WRIM is used in slip-energy recovery and double-fed induction-machine applications. * A cage winding is a shorted-circuited squirrel-cage rotor, a wound winding is connected externally through slip rings. * Mostly single-phase with some three-phase. Abbreviations: * BLAC – Brushless AC * BLDC – Brushless DC * BLDM – Brushless DC motor * EC – Electronic commutator * PM – Permanent magnet * IPMSM – Interior permanent-magnet synchronous motor * PMSM –

Permanent magnet synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Sync ...

* SPMSM – Surface permanent magnet synchronous motor * SCIM – Squirrel-cage induction motor * SRM – Switched reluctance motor * SyRM – Synchronous reluctance motor * VFD –

Variable-frequency drive A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or current variat ...

* WRIM – Wound-rotor induction motor * WRSM – Wound-rotor synchronous motor * LRA – Locked-rotor amps: The current you can expect under starting conditions when you apply full voltage. It occurs instantly during start-up. * RLA – Rated-load amps: The maximum current a motor should draw under any operating conditions. Often mistakenly called running-load amps, which leads people to believe, incorrectly, that the motor should always pull these amps. * FLA – Full-load amps: Changed in 1976 to "RLA – rated-load amps".

Self-commutated motor

Brushed DC motor

Most DC motors are small permanent magnet (PM) types. They contain a brushed internal mechanical commutation to reverse motor windings' current in synchronism with rotation. in

Electrically excited DC motor

A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator. Thus, every brushed DC motor has AC flowing through its windings. Current flows through one or more pairs of brushes that touch the commutator; the brushes connect an external source of electric power to the rotating armature. The rotating armature consists of one or more wire coils wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet (an

electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...

). The magnetic field produced interacts with a stationary magnetic field produced by either PMs or another winding (a field coil), as part of the motor frame. The force between the two magnetic fields rotates the shaft. The commutator switches power to the coils as the rotor turns, keeping the poles from ever fully aligning with the magnetic poles of the stator field, so that the rotor keeps turning as long as power is applied. Many of the limitations of the classic commutator DC motor are due to the need for brushes to maintain contact with the commutator, creating friction. The brushes create sparks while crossing the insulating gaps between commutator sections. Depending on the commutator design, the brushes may create short circuits between adjacent sections—and hence coil ends. Furthermore, the rotor coils' inductance causes the voltage across each to rise when its circuit opens, increasing the sparking. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their

resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

, limits the motor's output. Crossing the gaps also generates

electrical noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...

; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance or replacement. The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the rotor. While most commutators are cylindrical, some are flat, segmented discs mounted on an insulator. Large brushes create a large contact area, which maximizes motor output, while small brushes have low mass to maximize the speed at which the motor can run without excessive sparking. (Small brushes are desirable for their lower cost.) Stiffer brush springs can be used to make brushes of a given mass work at a higher speed, despite greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear. DC machines are defined as follows: * Armature circuit – A winding that carries the load, either stationary or rotating. * Field circuit – A set of windings that produces a magnetic field. * Commutation: A mechanical technique in which rectification can be achieved, or from which DC can be derived. Serie_Shunt_Coumpound

The five types of brushed DC motor are: * Shunt-wound * Series-wound * Compound (two configurations): ** Cumulative compound ** Differentially compounded * Permanent magnet (not shown) * Separately excited (not shown).

Permanent magnet

A permanent magnet (PM) motor does not have a field winding on the stator frame, relying instead on PMs to provide the magnetic field. Compensating windings in series with the armature may be used on large motors to improve commutation under load. This field is fixed and cannot be adjusted for speed control. PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines. To minimize overall weight and size, miniature PM motors may use high energy magnets made with

neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishe ...

; most are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly-fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles (to ensure starting, regardless of rotor position) and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.

Electronic commutator (EC)

Brushless DC

Some of the problems of the brushed DC motor are eliminated in the BLDC design. In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. BLDC motors are typically 85%+ efficient, reaching up to 96.5%, while brushed DC motors are typically 75–80% efficient. The BLDC motor's characteristic trapezoidal

counter-electromotive force Counter-electromotive force (counter EMF, CEMF, back EMF),Graf, "counterelectromotive force", Dictionary of Electronics is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF c ...

(CEMF) waveform is derived partly from the stator windings being evenly distributed, and partly from the placement of the rotor's permanent magnets. Also known as electronically commutated DC or inside-out DC motors, the stator windings of trapezoidal BLDC motors can be single-phase, two-phase or three-phase and use

Hall effect sensors A Hall effect sensor (or simply Hall sensor) is a type of sensor which detects the presence and magnitude of a magnetic field using the Hall effect. The output voltage of a Hall sensor is directly proportional to the strength of the field. I ...

mounted on their windings for rotor position sensing and low cost closed-loop commutator control. BLDC motors are commonly used where precise speed control is necessary, as in computer disk drives or video cassette recorders. The spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products, such as fans, laser printers and photocopiers. They have several advantages over conventional motors: * They are more efficient than AC fans using shaded-pole motors, running much cooler than the AC equivalents. This cool operation leads to much-improved life of the fan's bearings. * Without a commutator, the life of a BLDC motor can be significantly longer compared to a brushed DC motor with a commutator. Commutation tends to cause electrical and RF noise; without a commutator or brushes, a BLDC motor may be used in electrically sensitive devices like audio equipment or computers. * The same

Hall effect The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was dis ...

sensors that provide the commutation can provide a convenient

tachometer A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analo ...

signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal as well as provide running speed feedback. * The motor can be synchronized to an internal or external clock, providing precise speed control. * BLDC motors do not spark, making them better suited to environments with volatile chemicals and fuels. Sparking also generates ozone, which can accumulate in poorly ventilated buildings. * BLDC motors are usually used in small equipment such as computers and are generally used in fans to remove heat. * They make little noise, which is an advantage in equipment that is affected by vibrations. Modern BLDC motors range in power from a fraction of a watt to many kilowatts. Larger BLDC motors rated up to about 100 kW are used in electric vehicles. They also find use in electric

model aircraft A model aircraft is a small unmanned aircraft. Many are replicas of real aircraft. Model aircraft are divided into two basic groups: flying and non-flying. Non-flying models are also termed static, display, or shelf models. Aircraft manufactur ...

Switched reluctance motor

The switched reluctance motor (SRM) has no brushes or permanent magnets, and the rotor has no electric currents. Torque comes from a slight misalignment of poles on the rotor with poles on the stator. The rotor aligns itself with the magnetic field of the stator, while the stator field windings are sequentially energized to rotate the stator field. The magnetic flux created by the field windings follows the path of least magnetic sending the flux through rotor poles that are closest to the energized poles of the stator, thereby magnetizing those poles of the rotor and creating torque. As the rotor turns, different windings are energized, keeping the rotor turning. SRMs are used in some appliances and vehicles.

Universal AC/DC motor

A commutated, electrically excited, series or parallel wound motor is referred to as a universal motor because it can be designed to operate on either AC or DC power. A universal motor can operate well on AC because the current in both the field and the armature coils (and hence the resultant magnetic fields) synchronously reverse polarity, and hence the resulting mechanical force occurs in a constant direction of rotation. Operating at normal power line frequencies, universal motors are often used in sub-kilowatt applications. Universal motors formed the basis of the traditional railway traction motor in

electric railways A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), e ...

. In this application, using AC power on a motor designed to run on DC would experience efficiency losses due to

eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...

heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid (un-laminated) iron. They are now rarely used. An advantage is that AC power may be used on motors that specifically have high starting torque and compact design if high running speeds are used. By contrast, maintenance is higher and lifetimes are shortened. Such motors are used in devices that are not heavily used, and have high starting-torque demands. Multiple taps on the field coil provide (imprecise) stepped speed control. Household blenders that advertise many speeds typically combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC). Universal motors also lend themselves to

electronic speed control An electronic speed control (ESC) is an electronic circuit that controls and regulates the speed of an electric motor. It may also provide reversing of the motor and dynamic braking. Miniature electronic speed controls are used in electrically p ...

and, as such, are a choice for devices such as domestic washing machines. The motor can agitate the drum (both forwards and in reverse) by switching the field winding with respect to the armature. Whereas SCIMs cannot turn a shaft faster than allowed by the power line frequency, universal motors can run at much higher speeds. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable. They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed , while miniature grinders may exceed .

Externally commutated AC machine

AC induction and synchronous motors are optimized for operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power such as supplied for fixed-speed applications by the AC power grid or for variable-speed application from

(VFD) controllers.

Induction motor

An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action. An induction motor resembles a rotating transformer, because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Polyphase induction motors are widely used in industry. Elliott-Motor

Cage and wound rotor

Induction motors may be divided into Squirrel Cage Induction Motors (SCIM) and Wound Rotor Induction Motors (WRIM). SCIMs have a heavy winding made up of solid bars, usually aluminum or copper, electrically connected by rings at the ends of the rotor. The bars and rings as a whole are much like an animal's rotating exercise cage. Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars determines the speed-torque characteristics. At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to stay in the outer parts of the cage. As the motor accelerates, the slip frequency becomes lower, and more current reaches the interior. By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, a variable resistance is effectively inserted in the rotor circuit. However, most such motors employ uniform bars. In a WRIM, the rotor winding is made of many turns of insulated wire and is connected to

slip rings Slip or SLIP may refer to: Science and technology Biology * Slip (fish), also known as Black Sole * Slip (horticulture), a small cutting of a plant as a specimen or for grafting * Muscle slip, a branching of a muscle, in anatomy Computing and t ...

on the motor shaft. An external resistor or other control device can be connected in the rotor circuit. Resistors allow control of the motor speed, although dissipating significant power. A converter can be fed from the rotor circuit and return the slip-frequency power that would otherwise be wasted into the power system through an inverter or separate motor-generator. WRIMs are used primarily to start a high inertia load or a load that requires high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero speed to full speed. Motor speed can be changed because the motor's torque curve is effectively modified by the amount of resistance connected to the rotor circuit. Increasing resistance lowers the speed of maximum torque. If the resistance is increased beyond the point where the maximum torque occurs at zero speed, the torque is further reduced. When used with a load that has a torque curve that increases with speed, the motor operates at the speed where the torque developed by the motor is equal to the load torque. Reducing the load causes the motor to speed up, while increasing the load causes the motor to slow down until the load and motor torque are again equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be significant. The speed regulation and net efficiency is poor.

Torque motor

A torque motor can operate indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor applies a steady torque to the load. A common application is the supply- and take-up reel motors in a tape drive. In this application, driven by a low voltage, the characteristics of these motors apply a steady light tension to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage (delivering a higher torque), torque motors can achieve fast-forward and rewind operation without requiring additional mechanics such as gears or clutches. In the computer gaming world, torque motors are used in force feedback steering wheels. Another common application is to control the throttle of an internal combustion engine with an electronic governor. The motor works against a return spring to move the throttle in accord with the governor output. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and depending on the speed, makes small adjustments to the amount of current. If the engine slows down relative to the desired speed, the current increases, producing more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor reduces the current, allowing the return spring to pull back and reduce the throttle.

Synchronous motor

A synchronous electric motor is an AC motor. It includes a rotor spinning with coils passing magnets at the same frequency as the AC and produces a magnetic field to drive it. It has zero slip under typical operating conditions. By contrast induction motors must slip to produce torque. One type of synchronous motor is like an induction motor except that the rotor is excited by a DC field. Slip rings and brushes conduct current to the rotor. The rotor poles connect to each other and move at the same speed. Another type, for low load torque, has flats ground onto a conventional squirrel-cage rotor to create discrete poles. Yet another, as made by Hammond for its pre-World War II clocks, and in older

Hammond organs Hammond may refer to: People * Hammond Innes (1913–1998), English novelist * Hammond (surname) * Justice Hammond (disambiguation) Places Antarctica * Hammond Glacier, Antarctica Australia *Hammond, South Australia, a small settlement in South ...

, has no rotor windings and discrete poles. It is not self-starting. The clock requires manual starting by a small knob on the back, while the older Hammond organs had an auxiliary starting motor connected by a spring-loaded manually operated switch. Hysteresis synchronous motors typically are (essentially) two-phase motors with a phase-shifting capacitor for one phase. They start like induction motors, but when slip rate decreases sufficiently, the rotor (a smooth cylinder) becomes temporarily magnetized. Its distributed poles make it act like a permanent magnet synchronous motor. The rotor material, like that of a common nail, stays magnetized, but can be demagnetized with little difficulty. Once running, the rotor poles stay in place; they do not drift. Low-power synchronous timing motors (such as those for traditional electric clocks) may have multi-pole permanent magnet external cup rotors, and use shading coils to provide starting torque. ''

Telechron Telechron is the name of a U.S. company that manufactured electric clocks between 1912 and 1992. "Telechron" is derived from the Greek word''tele'' meaning "far off," an''chronos'' "time," thus referring to the transmission of time over long dista ...

'' clock motors have shaded poles for starting torque, and a two-spoke ring rotor that performs like a discrete two-pole rotor.

Doubly-fed electric machine

Doubly fed electric motors have two independent multiphase winding sets, which contribute active (i.e., working) power to the energy conversion process, with at least one of the winding sets electronically controlled for variable speed operation. Two independent multiphase winding sets (i.e., dual armature) are the maximum provided in a single package without topology duplication. Doubly-fed electric motors have an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as singly-fed electric machines, which have only one active winding set. A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings may offset the saving in the power electronics components. Difficulties affect controlling speed near synchronous speed limit applications.

Special magnetic motors

Rotary

Ironless or coreless rotor motor

The coreless or ironless DC motor is a specialized permanent magnet DC motor. Optimized for rapid

acceleration In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...

, the rotor is constructed without an iron core. The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only wire and bonding material. The rotor can fit inside the stator magnets; a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux. A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the motor housing, and provides a return path for the flux. Because the rotor is much lower mass than a conventional rotor, it can accelerate much more rapidly, often achieving a mechanical

time constant In physics and engineering, the time constant, usually denoted by the Greek letter (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system.Concretely, a first-order LTI system is a s ...

under one millisecond. This is especially true if the windings use aluminium rather than (heavier) copper. The rotor has no metal mass to act as a heat sink; even small motors must be cooled. Overheating can be an issue for these designs. The

vibrating alert A vibrating alert is a feature of communications devices to notify the user of an incoming connection or message. It is particularly common on mobile phones and pagers and usually supplements the ring tone. Most 21st-century mobile phones are fit ...

of cellular phones can be generated by cylindrical permanent-magnet motors, or disc-shaped types that have a thin multipolar disc field magnet, and an intentionally unbalanced molded-plastic rotor structure with two bonded coreless coils. Metal brushes and a flat commutator switch power to the rotor coils. Related limited-travel actuators have no core and a bonded coil placed between the poles of high-flux thin permanent magnets. These are the fast head positioners for rigid-disk ("hard disk") drives. Although the contemporary design differs considerably from that of loudspeakers, it is still loosely (and incorrectly) referred to as a "voice coil" structure, because some earlier rigid-disk-drive heads moved in straight lines, and had a drive structure much like that of a loudspeaker.

Pancake or axial rotor motor

The printed armature or pancake motor has windings shaped as a disc running between arrays of high-flux magnets. The magnets are arranged in a circle facing the rotor spaced to form an axial air gap. This design is commonly known as the pancake motor because of its flat profile. The armature (originally formed on a printed circuit board) is made from punched copper sheets that are laminated together using advanced composites to form a thin, rigid disc. The armature does not have a separate ring commutator. The brushes move directly on the armature surface making the whole design compact. An alternative design is to use wound copper wire laid flat with a central conventional commutator, in a flower and petal shape. The windings are typically stabilized with electrical epoxy potting systems. These are filled epoxies that have moderate, mixed viscosity and a long gel time. They are highlighted by low shrinkage and low exotherm, and are typically UL 1446 recognized as a potting compound insulated with , Class H rating. The unique advantage of ironless DC motors is the absence of cogging (torque variations caused by changing attraction between the iron and the magnets). Parasitic eddy currents cannot form in the rotor as it is totally ironless, although iron rotors are laminated. This can greatly improve efficiency, but variable-speed controllers must use a higher switching rate (>40 kHz) or DC because of decreased

electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Clerk ...

. These motors were invented to drive the capstan(s) of magnetic tape drives, where minimal time to reach operating speed and minimal stopping distance were critical. Pancake motors are widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices. Due to the variety of constructions now available, the technology is used in applications from high temperature military to low cost pump and basic servos. Another approach (Magnax) is to use a single stator sandwiched between two rotors. One such design has produced peak power of 15 kW/kg, sustained power around 7.5 kW/kg. This yokeless axial flux motor offers a shorter flux path, keeping the magnets further from the axis. The design allows zero winding overhang; 100 percent of the windings are active. This is enhanced with the use of rectangular-crosssection copper wire. The motors can be stacked to work in parallel. Instabilities are minimized by ensuring that the two rotor discs put equal and opposing forces onto the stator disc. The rotors are connected directly to one another via a shaft ring, cancelling out the magnetic forces. Magnax motors range in size from in diameter.

Servo motor

servo motor A servomotor (or servo motor) is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also r ...

is a motor that is used within a position-control or speed-control feedback system. Servomotors are used in applications such as machine tools, pen plotters, and other process systems. Motors intended for use in a servomechanism must have predictable characteristics for speed, torque, and power. The speed/torque curve is important and is high ratio for a servo motor. Dynamic response characteristics such as winding inductance and rotor inertia are important; these factors limit performance. Large, powerful, but slow-responding servo loops may use conventional AC or DC motors and drive systems with position or speed feedback. As dynamic response requirements increase, more specialized motor designs such as coreless motors are used. AC motors' superior power density and acceleration characteristics tends to favor permanent magnet synchronous, BLDC, induction, and SRM drive approaches. A servo system differs from some stepper motor applications in that position feedback is continuous while the motor is running. A stepper system inherently operates open-loop—relying on the motor not to "miss steps" for short term accuracy—with any feedback such as a "home" switch or position encoder external to the motor system.

Stepper motor

Stepper motors are typically used to provide precise rotations. An internal rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of electronically switched external magnets. A stepper motor may also be thought of as a cross between a DC electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, the stepper motor may not rotate continuously; instead, it moves in steps—starting and then stopping—advancing from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards, and it may change direction, stop, speed up or slow down at any time. Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions.

Microstepping A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any Rota ...

drivers can proportionally control the power to the field windings, allowing the rotors to position between cog points and rotate smoothly. Computer-controlled stepper motors are one of the most versatile positioning systems, particularly as part of a digital servo-controlled system. Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for read/write head positioning in early

disk drives Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA and DNA are consi ...

, where the precision and speed they offered could correctly position the read/write head. As drive density increased, precision and speed limitations made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it refers to the structure in a cone-type

loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...

.) Stepper motors are often used in computer printers, optical scanners, and digital photocopiers to move the active element, the print head carriage (

inkjet printers Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensi ...

), and the

platen A platen (or platten) is a flat platform with a variety of roles in printing or manufacturing. It can be a flat metal (or earlier, wooden) plate pressed against a medium (such as paper) to cause an impression in letterpress printing. Platen m ...

or feed rollers. So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one coil, draw little power, and have a permanent magnet rotor. The same kind of motor drives battery-powered quartz clocks. Some of these watches, such as

chronographs A chronograph is a specific type of watch that is used as a stopwatch combined with a display watch. A basic chronograph has an independent sweep second hand and a minute sub-dial; it can be started, stopped, and returned to zero by successive ...

, contain more than one stepper motor. Closely related in design to three-phase AC synchronous motors, stepper motors and SRMs are classified as variable reluctance motor type.

Linear motor

A linear motor is essentially any electric motor that has been "unrolled" so that, instead of producing

(rotation), it produces a straight-line force along its length. Linear motors are most commonly

or stepper motors. Linear motors are commonly found in roller-coasters where the rapid motion of the motorless railcar is controlled by the rail. They are also used in maglev trains, where the train "flies" over the ground. On a smaller scale, the 1978 era HP 7225A pen plotter used two linear stepper motors to move the pen along the X and Y axes.

Comparison by major categories

Electromagnetism

Force and torque

An electric motor converts electrical energy to mechanical energy through the force between two opposed magnetic fields. At least one of the two magnetic fields must be created by an electromagnet through the magnetic field caused by an electrical current. The force between a current

I

in a conductor of length

\ell

perpendicular to a magnetic field

\mathbf

may be calculated using the Lorentz force law: :

\mathbf = I \ell \times \mathbf \,\!

Note: X denotes vector

cross product In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and is ...

. The most general approaches to calculating the forces in motors use tensor notation.

Power

Where

\omega_

is shaft speed in

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

and T is torque, a motor's mechanical power output P_em is given by, in

Imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed th ...

with T expressed in foot-pounds, :

P_ = \frac

(horsepower), and, in

SI units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

with shaft

angular speed Angular may refer to: Anatomy * Angular artery, the terminal part of the facial artery * Angular bone, a large bone in the lower jaw of amphibians and reptiles * Angular incisure, a small anatomical notch on the stomach * Angular gyrus, a regio ...

\omega

, expressed in radians per second, and T expressed in newton-meters, :

P_ = \omega T

(watts). For a linear motor, with force F expressed in newtons and velocity v expressed in meters per second, :

P_ = F

(watts). In an asynchronous or induction motor, the relationship between motor speed and air gap power is given by the following: :

P_=\frac I_r^

, where :: R_r – rotor resistance :: I_r² – square of current induced in the rotor :: s – motor slip; i.e., difference between synchronous speed and slip speed, which provides the relative movement needed for current induction in the rotor.

Back EMF

The movement of armature windings of a direct-current or universal motor through a magnetic field, induce a voltage in them. This voltage tends to oppose the motor supply voltage and so is called " back electromotive force (EMF)". The voltage is proportional to the running speed of the motor. The back EMF of the motor, plus the voltage drop across the winding internal resistance and brushes, must equal the voltage at the brushes. This provides the fundamental mechanism of speed regulation in a DC motor. If the mechanical load increases, the motor slows down; a lower back EMF results, and more current is drawn from the supply. This increased current provides the additional torque to balance the load. in In AC machines, it is sometimes useful to consider a back EMF source within the machine; this is of particular concern for close speed regulation of induction motors on VFDs.

Losses

Motor losses are mainly due to resistive losses in windings, core losses and mechanical losses in bearings, and aerodynamic losses, particularly where cooling fans are present, also occur. Losses also occur in commutation, mechanical commutators spark; electronic commutators and also dissipate heat.

Efficiency

To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power: :

\eta = \frac

, where

\eta

is energy conversion efficiency,

P_e

is electrical input power, and

P_m

is mechanical output power: :

P_e = I V

P_m = T \omega

where

V

is input voltage,

I

is input current,

T

is output torque, and

\omega

is output angular velocity. It is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the stall torque. Various national regulatory authorities have enacted legislation to encourage the manufacture and use of higher-efficiency motors. Electric motors have efficiencies ranging from at least 15% for shaded pole motors, up to 98% for permanent magnet motors, with efficiency also dependent on load. Peak efficiency is usually at 75% of the rated load. So (as an example) a 10 HP motor is most efficient when driving a load that requires 7.5 HP. Efficiency also depends on motor size; larger motors tend to be more efficient. Some motors can not operate continually for more than a specified period of time (e.g. for more than an hour per run)

Goodness factor

Eric Laithwaite Eric Roberts Laithwaite (14 June 1921 – 27 November 1997) was a British electrical engineer, known as the "Father of Maglev" for his development of the linear induction motor and maglev rail system. Biography Eric Roberts Laithwaite wa ...

proposed a metric to determine the 'goodness' of an electric motor:

G = \frac   = \frac

Where: :

G

is the goodness factor (factors above 1 are likely to be efficient) :

A_m, A_e

are the cross sectional areas of the magnetic and electric circuit :

l_m, l_e

are the lengths of the magnetic and electric circuits :

\mu

is the permeability of the core :

\omega

is the angular frequency the motor is driven at From this, he showed that the most efficient motors are likely to have relatively large magnetic poles. However, the equation only directly relates to non PM motors.

Performance parameters

Torque

Electromagnetic motors derive torque from the vector product of the interacting fields. Calculating torque requires knowledge of the fields in the air gap. Once these have been established, the torque is the integral of all the force vectors multiplied by the vector's radius. The current flowing in the winding produces the fields. For a motor using a magnetic material the field is not proportional to the current. A figure relating the current to the torque can inform motor selection. The maximum torque for a motor depends on the maximum current, absent thermal considerations. When optimally designed within a given core saturation constraint and for a given active current (i.e., torque current), voltage, pole-pair number, excitation frequency (i.e., synchronous speed), and air-gap flux density, all categories of electric motors/generators exhibit virtually the same maximum continuous shaft torque (i.e., operating torque) within a given air-gap area with winding slots and back-iron depth, which determines the physical size of electromagnetic core. Some applications require bursts of torque beyond the maximum, such as bursts to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or safe

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

rise and voltage, the capacity for torque bursts beyond the maximum differs significantly across motor/generator types. Electric machines without a transformer circuit topology, such as that of WRSMs or PMSMs, cannot provide torque bursts without saturating the magnetic core. At that point, additional current cannot increase torque. Furthermore, the permanent magnet assembly of PMSMs can be irreparably damaged. Electric machines with a transformer circuit topology, such as induction machines, induction doubly-fed electric machines, and induction or synchronous wound-rotor doubly-fed (WRDF) machines, permit torque bursts because the EMF-induced active current on either side of the transformer oppose each other and thus contribute nothing to the transformer coupled magnetic core flux density, avoiding core saturation. Electric machines that rely on induction or asynchronous principles short-circuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active (i.e., real) current. Torque bursts two to three times higher than the maximum design torque are realizable. The brushless wound-rotor synchronous doubly-fed (BWRSDF) machine is the only electric machine with a truly dual ported transformer circuit topology (i.e., both ports independently excited with no short-circuited port). The dual ported transformer circuit topology is known to be unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the rotor winding set. If a precision means were available to instantaneously control torque angle and slip for synchronous operation during operation while simultaneously providing brushless power to the rotor winding set, the active current of the BWRSDF machine would be independent of the reactive impedance of the transformer circuit and bursts of torque significantly higher than the maximum operating torque and far beyond the practical capability of any other type of electric machine would be realizable. Torque bursts greater than eight times operating torque have been calculated.

Continuous torque density

The continuous torque density of conventional electric machines is determined by the size of the air-gap area and the back-iron depth, which are determined by the power rating of the armature winding set, the speed of the machine, and the achievable air-gap flux density before core saturation. Despite the high coercivity of neodymium or

samarium Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...

-cobalt permanent magnets, continuous torque density is virtually the same amongst electric machines with optimally designed armature winding sets. Continuous torque density relates to method of cooling and permissible operation period before destruction by overheating of windings or permanent magnet damage. Other sources state that various e-machine topologies have differing torque density. One source shows the following: where—specific torque density is normalized to 1.0 for the surface permanent magnet (SPM)—brushless ac, 180° current conduction. Torque density is approximately four times greater for liquid cooled motors, compared to those which are air cooled. A source comparing direct current, induction motors (IM), PMSM and SRM showed: Another source notes that PMSM up to 1 MW have considerably higher torque density than induction machines.

Continuous power density

The continuous power density is determined by the product of the continuous torque density and the constant torque speed range. Electric motors can achieve densities of up to 20KW/KG, meaning 20 Kilowatts of output power per kilogram.

Acoustic noise and vibrations

Acoustic noise and vibrations are usually classified in three sources: * mechanical sources (e.g. due to bearings) * aerodynamic sources (e.g. due to shaft-mounted

fans Fan commonly refers to: * Fan (machine), a machine for producing airflow, often used for cooling ** Hand fan, an implement held and waved by hand to move air for cooling * Fan (person), short for fanatic; an enthusiast or supporter, especially wit ...

) * magnetic sources (e.g. due to magnetic forces such as Maxwell and

magnetostriction Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field chan ...

forces acting on stator and rotor structures) The latter source, which can be responsible for the "whining noise" of electric motors, is called

electromagnetically induced acoustic noise Electromagnetically induced acoustic noise (and vibration), electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forc ...

Standards

The following are major design, manufacturing, and testing standards covering electric motors: *

American Petroleum Institute The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in production, refinement, distribution, and many other aspects of the pet ...

: API 541 Form-Wound Squirrel Cage Induction Motors – 375 kW (500 Horsepower) and Larger *

: API 546 Brushless Synchronous Machines – 500 kVA and Larger *

: API 547 General-purpose Form-Wound Squirrel Cage Induction Motors – 250 Hp and Larger * Institute of Electrical and Electronics Engineers: IEEE Std 112 Standard Test Procedure for Polyphase Induction Motors and Generators * Institute of Electrical and Electronics Engineers: IEEE Std 115 Guide for Test Procedures for Synchronous Machines * Institute of Electrical and Electronics Engineers: IEEE Std 841 Standard for Petroleum and Chemical Industry – Premium Efficiency Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors – Up to and Including 370 kW (500 Hp) *

International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...

: IEC 60034 Rotating Electrical Machines *

: IEC 60072 Dimensions and output series for rotating electrical machines *

National Electrical Manufacturers Association The National Electrical Manufacturers Association (NEMA) is the largest trade association of electrical equipment manufacturers in the United States. Founded in 1926, it advocates for the industry, and publishes standards for electrical product ...

MG-1 Motors and Generators
*

Underwriters Laboratories The UL enterprise is a global safety science company headquartered in Northbrook, Illinois, composed of three organizations, UL Research Institutes, UL Standards & Engagement and UL Solutions. Established in 1894, the UL enterprise was founded a ...

: UL 1004 – Standard for Electric Motors *

Indian Standard The Bureau of Indian Standards (BIS) is the National Standards Body of India under Department of Consumer affairs, Ministry of Consumer Affairs, Food & Public Distribution, Government of India. It is established by the Bureau of Indian Standar ...

: IS:12615-2018 – Line Operated Three Phase a.c. Motors (IE CODE) “Efficiency Classes and Performance Specification” (Third Revision)

Non-magnetic motors

An electrostatic motor is based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high-voltage power supply, although small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon. Electrostatic motors find frequent use in micro-electro-mechanical systems (

MEMS Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...

) where their drive voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. The molecular machinery that runs living cells is often based on linear and rotary electrostatic motors. A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a

piezoelectric material Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word '' ...

when an

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

is applied. Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or

ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

vibrations to produce linear or rotary motion. In one mechanism, the elongation in a single plane is used to make a series of stretches and position holds, similar to the way a caterpillar moves. An electrically powered spacecraft propulsion system uses electric motor technology to propel spacecraft in outer space. Most systems are based on electrically accelerating propellant to high speed, while some systems are based on electrodynamic tethers principles of propulsion to the magnetosphere.

Notes

References

Bibliography

* * Houston, Edwin J.; Kennelly, Arthur
''Recent Types of Dynamo-Electric Machinery''
American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902 * * * * *

External links

SparkMuseum: Early Electric Motors

The Invention of the Electric Motor 1800 to 1893
hosted by Karlsrushe Institute of Technology's Martin Doppelbauer

slow motion gifs and oscillograms for many kinds of motors. {{Portalbar, Engineering, Physics, Trains, Energy, Electronics, Technology, Telecommunication} Electromagnetic components Energy conversion British inventions Magnetic propulsion devices Hungarian inventions