Electric motor

For other kinds of motors, see Motor (disambiguation).For a railroad engine, see Electric locomotive.An electric motor is an electrical machine that converts

Various electric motors, compared to 9 V battery.

electrical energy into mechanical energy. The reverse ofthis would be the conversion of mechanical energy intoelectrical energy and is done by an electric generator.In normal motoring mode, most electric motors oper-ate through the interaction between an electric motorsmagnetic eld and winding currents to generate forcewithin the motor. In certain applications, such as in thetransportation industry with traction motors, electric mo-tors can operate in both motoring and generating or brak-ingmodes to also produce electrical energy frommechan-ical energy.Found in applications as diverse as industrial fans, blow-ers and pumps, machine tools, household appliances,power tools, and disk drives, electric motors can be pow-ered by direct current (DC) sources, such as from bat-teries, motor vehicles or rectiers, or by alternating cur-rent (AC) sources, such as from the power grid, invertersor generators. Small motors may be found in electricwatches. General-purpose motors with highly standard-ized dimensions and characteristics provide convenientmechanical power for industrial use. The largest of elec-tric motors are used for ship propulsion, pipeline com-pression and pumped-storage applications with ratingsreaching 100 megawatts. Electric motors may be classi-

ed by electric power source type, internal construction,application, type of motion output, and so on.Electric motors are used to produce linear or rotary force(torque), and should be distinguished from devices suchas magnetic solenoids and loudspeakers that convert elec-tricity into motion but do not generate usable mechanicalpowers, which are respectively referred to as actuatorsand transducers.

Cutaway view through stator of induction motor.

1 HistoryMain article: History of the electric motor

1.1 Early motorsPerhaps the rst electric motors were simple electrostaticdevices created by the Scottish monk Andrew Gordon inthe 1740s.[2] The theoretical principle behind productionof mechanical force by the interactions of an electric cur-rent and amagnetic eld, Ampres force law, was discov-ered later by Andr-Marie Ampre in 1820. The con-version of electrical energy into mechanical energy byelectromagnetic means was demonstrated by the Britishscientist Michael Faraday in 1821. A free-hanging wirewas dipped into a pool of mercury, on which a permanentmagnet (PM) was placed. When a current was passedthrough the wire, the wire rotated around the magnet,showing that the current gave rise to a close circular mag-netic eld around the wire.[3] This motor is often demon-strated in physics experiments, brine substituting for toxic

1

2 1 HISTORY

Faradays electromagnetic experiment, 1821[1]

mercury. Though Barlows wheel was an early rene-ment to this Faraday demonstration, these and similarhomopolar motors were to remain unsuited to practicalapplication until late in the century.

Jedlik's electromagnetic self-rotor, 1827 (Museum of AppliedArts, Budapest). The historic motor still works perfectly today.[4]

In 1827, Hungarian physicist nyos Jedlik started exper-imenting with electromagnetic coils. After Jedlik solvedthe technical problems of the continuous rotation withthe invention of commutator, he called his early deviceselectromagnetic self-rotors. Although they were usedonly for instructional purposes, in 1828 Jedlik demon-strated the rst device to contain the three main compo-nents of practical DC motors: the stator, rotor and com-mutator. The device employed no permanent magnets,as the magnetic elds of both the stationary and revolvingcomponents were produced solely by the currents owingthrough their windings.[5][6][7][8][9][10][11]

1.2 Success with DC motors

After many other more or less successful attempts withrelatively weak rotating and reciprocating apparatus theGerman-speaking Prussian Moritz von Jacobi created therst real rotating electric motor in May 1834 that actuallydeveloped a remarkable mechanical output power. Hismotor set a world record which was improved only fouryears later in September 1838 by Jacobi himself. His sec-ond motor was powerful enough to drive a boat with 14people across a wide river. It was not until 1839/40 thatother developers worldwide managed to build motors ofsimilar and later also of higher performance.The rst commutator DC electric motor capable ofturning machinery was invented by the British scien-tist William Sturgeon in 1832.[12] Following Sturgeonswork, a commutator-type direct-current electric motormade with the intention of commercial use was builtby the American inventor Thomas Davenport, which hepatented in 1837. Themotors ran at up to 600 revolutionsper minute, and powered machine tools and a printingpress.[13] Due to the high cost of primary battery power,the motors were commercially unsuccessful and Daven-port went bankrupt. Several inventors followed Sturgeonin the development of DC motors but all encountered thesame battery power cost issues. No electricity distribu-tion had been developed at the time. Like Sturgeons mo-tor, there was no practical commercial market for thesemotors.[14]

In 1855, Jedlik built a device using similar principles tothose used in his electromagnetic self-rotors that was ca-pable of useful work.[5][11] He built a model electric ve-hicle that same year.[15]

The rst commercially successful DC motors followedthe invention by Znobe Gramme who had in 1871 de-veloped the anchor ring dynamo which solved the double-T armature pulsating DC problem. In 1873, Grammefound that this dynamo could be used as a motor, whichhe demonstrated to great eect at exhibitions in Viennaand Philadelphia by connecting two such DC motors ata distance of up to 2 km away from each other, one as agenerator.[16] (See also 1873 : l'exprience dcisive [De-cisive Workaround] .)In 1886, Frank Julian Sprague invented the rst practicalDC motor, a non-sparking motor that maintained rela-tively constant speed under variable loads. Other Spragueelectric inventions about this time greatly improved gridelectric distribution (prior work done while employed byThomas Edison), allowed power from electric motors tobe returned to the electric grid, provided for electric dis-tribution to trolleys via overhead wires and the trolleypole, and provided controls systems for electric opera-tions. This allowed Sprague to use electric motors to in-vent the rst electric trolley system in 188788 in Rich-mond VA, the electric elevator and control system in1892, and the electric subway with independently pow-

3ered centrally controlled cars, which were rst installedin 1892 in Chicago by the South Side Elevated Railwaywhere it became popularly known as the L. Spraguesmotor and related inventions led to an explosion of in-terest and use in electric motors for industry, while al-most simultaneously another great inventor was develop-ing its primary competitor, which would become muchmore widespread. The development of electric motors ofacceptable eciency was delayed for several decades byfailure to recognize the extreme importance of a relativelysmall air gap between rotor and stator. Ecient designshave a comparatively small air gap.[17] [lower-alpha 1] The St.Louis motor, long used in classrooms to illustrate motorprinciples, is extremely inecient for the same reason, aswell as appearing nothing like a modern motor.[18]

Application of electric motors revolutionized industry.Industrial processes were no longer limited by powertransmission using line shafts, belts, compressed air orhydraulic pressure. Instead every machine could beequipped with its own electric motor, providing easy con-trol at the point of use, and improving power transmis-sion eciency. Electric motors applied in agricultureeliminated human and animal muscle power from suchtasks as handling grain or pumping water. Householduses of electric motors reduced heavy labor in the homeand made higher standards of convenience, comfort andsafety possible. Today, electric motors stand for morethan half of the electric energy consumption in the US.[19]

1.3 Emergence of AC motors

In 1824, the French physicist Franois Arago formu-lated the existence of rotating magnetic elds, termedAragos rotations, which, by manually turning switcheson and o, Walter Baily demonstrated in 1879 as ineect the rst primitive induction motor.[20][21] [22][23]In the 1880s, many inventors were trying to developworkable AC motors[24] because ACs advantages inlong distance high voltage transmission were counterbal-anced by the inability to operate motors on AC. Prac-tical rotating AC induction motors were independentlyinvented by Galileo Ferraris and Nikola Tesla, a work-ing motor model having been demonstrated by the for-mer in 1885 and by the latter in 1887. In 1888, theRoyal Academy of Science of Turin published Ferrarissresearch detailing the foundations of motor operationwhile however concluding that the apparatus based onthat principle could not be of any commercial impor-tance as motor.[23][25][26][27][28][29][30][31][32][33][34][35][36]In 1888, Tesla presented his paper A New System for Al-ternating Current Motors and Transformers to the AIEEthat described three patented two-phase four-stator-polemotor types: one with a four-pole rotor forming a non-self-starting reluctance motor, another with a wound ro-tor forming a self-starting induction motor, and the thirda true synchronous motor with separately excited DCsupply to rotor winding. One of the patents Tesla led

in 1887, however, also described a shorted-winding-rotor induction motor. George Westinghouse promptlybought Teslas patents, employed Tesla to develop them,and assigned C. F. Scott to help Tesla, Tesla leavingfor other pursuits in 1889.[23][30][33][34][35][36][37][38][39][40][41][42][43][44] The constant speed AC induction motor wasfound not to be suitable for street cars[24] but Westing-house engineers successfully adapted it to power a min-ing operation in Telluride, Colorado in 1891.[45][46][47]Steadfast in his promotion of three-phase development,Mikhail Dolivo-Dobrovolsky invented the three-phasecage-rotor induction motor in 1889 and the three-limbtransformer in 1890. This type of motor is now usedfor the vast majority of commercial applications.[48][49]However, he claimed that Teslas motor was not practi-cal because of two-phase pulsations, which prompted himto persist in his three-phase work.[50] Although Westing-house achieved its rst practical induction motor in 1892and developed a line of polyphase 60 hertz inductionmotors in 1893, these early Westinghouse motors weretwo-phase motors with wound rotors until B. G. Lammedeveloped a rotating bar winding rotor.[37] The GeneralElectric Company began developing three-phase induc-tion motors in 1891.[37] By 1896, General Electric andWestinghouse signed a cross-licensing agreement for thebar-winding-rotor design, later called the squirrel-cagerotor.[37] Induction motor improvements owing fromthese inventions and innovations were such that a 100horsepower (HP) induction motor currently has the samemounting dimensions as a 7.5 HP motor in 1897.[37]

2 Motor construction

Electric motor rotor (left) and stator (right)

2.1 RotorMain article: Rotor (electric)

In an electric motor the moving part is the rotor whichturns the shaft to deliver the mechanical power. The ro-

4 4 MAJOR CATEGORIES

tor usually has conductors laid into it which carry currentsthat interact with the magnetic eld of the stator to gen-erate the forces that turn the shaft. However, some rotorscarry permanent magnets, and the stator holds the con-ductors.

2.2 Stator

Main article: Stator

The stationary part is the stator, usually has either wind-ings or permanent magnets. The stator is the stationarypart of the motors electromagnetic circuit. The statorcore is made up of many thin metal sheets, called lam-inations. Laminations are used to reduce energy lossesthat would result if a solid core were used.

2.3 Air gap

In between the rotor and stator is the air gap. The air gaphas important eects, and is generally as small as pos-sible, as a large gap has a strong negative eect on theperformance of an electric motor.

2.4 Windings

Main article: Windings

Windings are wires that are laid in coils, usually wrappedaround a laminated soft iron magnetic core so as to formmagnetic poles when energized with current.Electric machines come in two basic magnet eld polecongurations: salient-pole machine and nonsalient-polemachine. In the salient-pole machine the poles magneticeld is produced by a winding wound around the pole be-low the pole face. In the nonsalient-pole, or distributedeld, or round-rotor, machine, the winding is distributedin pole face slots.[51] A shaded-pole motor has a wind-ing around part of the pole that delays the phase of themagnetic eld for that pole.Some motors have conductors which consist of thickermetal, such as bars or sheets of metal, usually copper,although sometimes aluminum is used. These are usuallypowered by electromagnetic induction.

2.5 Commutator

Main article: Commutator (electric)A commutator is a mechanism used to switch the input ofmost DC machines and certain AC machines consistingof slip ring segments insulated from each other and fromthe electric motors shaft. The motors armature currentis supplied through the stationary brushes in contact with

A toys small DC motor with its commutator

the revolving commutator, which causes required currentreversal and applies power to the machine in an optimalmanner as the rotor rotates from pole to pole.[52][53] Inabsence of such current reversal, the motor would braketo a stop. In light of signicant advances in the past fewdecades due to improved technologies in electronic con-troller, sensorless control, induction motor, and perma-nent magnet motor elds, electromechanically commu-tated motors are increasingly being displaced by exter-nally commutated induction and permanent-magnet mo-tors.

3 Motor supply and control

3.1 Motor supply

ADCmotor is usually supplied through slip ring commu-tator as described above. AC motors commutation canbe either slip ring commutator or externally commutatedtype, can be xed-speed or variable-speed control type,and can be synchronous or asynchronous type. Universalmotors can run on either AC or DC.

3.2 Motor control

Fixed-speed controlled AC motors are provided withdirect-on-line or soft-start starters.Variable speed controlled AC motors are provided witha range of dierent power inverter, variable-frequencydrive or electronic commutator technologies.The term electronic commutator is usually associatedwith self-commutated brushless DC motor and switchedreluctance motor applications.

4 Major categoriesElectric motors operate on three dierent physical prin-ciples: magnetic, electrostatic and piezoelectric. By farthe most common is magnetic.

5In magnetic motors, magnetic elds are formed in boththe rotor and the stator. The product between these twoelds gives rise to a force, and thus a torque on the mo-tor shaft. One, or both, of these elds must be made tochange with the rotation of the motor. This is done byswitching the poles on and o at the right time, or vary-ing the strength of the pole.The main types are DC motors and AC motors, the for-mer increasingly being displaced by the latter.AC electric motors are either asynchronous or syn-chronous.Once started, a synchronous motor requires synchronismwith the moving magnetic elds synchronous speed forall normal torque conditions.In synchronous machines, the magnetic eld must be pro-vided by means other than induction such as from sepa-rately excited windings or permanent magnets.A fractional horsepower (FHP) motor has a rating belowabout 1 horsepower (0.746 kW), or that is manufacturedwith a standard frame size smaller than a standard 1 HPmotor. Many household and industrial motors are in thefractional horsepower class.Notes:

1. Rotation is independent of the frequency of the ACvoltage.

2. Rotation is equal to synchronous speed (motor statoreld speed).

3. In SCIM xed-speed operation rotation is equal toslip speed (synchronous speed less slip).

4. In non-slip energy recovery systems WRIM is usu-ally used for motor starting but can be used to varyload speed.

5. Variable-speed operation.6. Whereas induction and synchronous motor drives

are typically with either six-step or sinusoidal wave-form output, BLDC motor drives are usually withtrapezoidal current waveform; the behavior of bothsinusoidal and trapezoidal PM machines is howeveridentical in terms of their fundamental aspects.[61]

7. In variable-speed operation WRIM is used in slipenergy recovery and double-fed induction machineapplications.

8. A cage winding is a shorted-circuited squirrel-cagerotor, a wound winding is connected externallythrough slip rings.

9. 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 synchronousmotor

PMSM - Permanent magnet synchronous motor SPMSM - Surface permanent magnet synchronousmotor

SCIM - Squirrel-cage induction motor SRM - Switched reluctance motor SyRM - Synchronous reluctance motor VFD - Variable-frequency drive WRIM - Wound-rotor induction motor WRSM - Wound-rotor synchronous motor

5 Self-commutated motor

5.1 Brushed DC motorMain article: DC motor

All self-commutated DC motors are by denition run onDC electric power. Most DC motors are small PM types.They contain a brushed internal mechanical commutationto reverse motor windings current in synchronism withrotation.[62]

5.1.1 Electrically excited DC motor

Main article: Brushed DC electric motorA commutated DC motor has a set of rotating wind-ings wound on an armature mounted on a rotating shaft.The shaft also carries the commutator, a long-lasting ro-tary electrical switch that periodically reverses the ow ofcurrent in the rotor windings as the shaft rotates. Thus,every brushed DC motor has AC owing through its ro-tating windings. Current ows through one or more pairsof brushes that bear on the commutator; the brushes con-nect an external source of electric power to the rotatingarmature.The rotating armature consists of one or more coils ofwire wound around a laminated, magnetically soft fer-romagnetic core. Current from the brushes ows throughthe commutator and onewinding of the armature, makingit a temporary magnet (an electromagnet). The magneticeld produced by the armature interacts with a stationary

6 5 SELF-COMMUTATED MOTOR

Workings of a brushed electric motor with a two-pole rotor andPM stator. (N and S designate polarities on the inside facesof the magnets; the outside faces have opposite polarities.)

magnetic eld produced by either PMs or another wind-ing a eld coil, as part of the motor frame. The forcebetween the two magnetic elds tends to rotate the motorshaft. The commutator switches power to the coils as therotor turns, keeping the magnetic poles of the rotor fromever fully aligning with the magnetic poles of the statoreld, so that the rotor never stops (like a compass nee-dle does), but rather keeps rotating as long as power isapplied.Many of the limitations of the classic commutator DCmotor are due to the need for brushes to press against thecommutator. This creates friction. Sparks are created bythe brushes making and breaking circuits through the ro-tor coils as the brushes cross the insulating gaps betweencommutator sections. Depending on the commutator de-sign, this may include the brushes shorting together adja-cent sections and hence coil ends momentarily whilecrossing the gaps. Furthermore, the inductance of the ro-tor coils causes the voltage across each to rise when itscircuit is opened, increasing the sparking of the brushes.This sparking limits the maximum speed of the machine,as too-rapid sparking will overheat, erode, or even meltthe commutator. The current density per unit area of thebrushes, in combination with their resistivity, limits theoutput of the motor. The making and breaking of elec-tric contact also generates electrical noise; sparking gen-erates RFI. Brushes eventually wear out and require re-placement, and the commutator itself is subject to wearand maintenance (on larger motors) or replacement (onsmall motors). The commutator assembly on a large mo-tor is a costly element, requiring precision assembly ofmany parts. On small motors, the commutator is usuallypermanently integrated into the rotor, so replacing it usu-ally requires replacing the whole rotor.

While most commutators are cylindrical, some are atdiscs consisting of several segments (typically, at leastthree) mounted on an insulator.Large brushes are desired for a larger brush contact areato maximize motor output, but small brushes are desiredfor low mass to maximize the speed at which the mo-tor can run without the brushes excessively bouncing andsparking. (Small brushes are also desirable for lowercost.) Stier brush springs can also be used to makebrushes of a given mass work at a higher speed, but at thecost of greater friction losses (lower eciency) and accel-erated brush and commutator wear. Therefore, DC mo-tor brush design entails a trade-o between output power,speed, and eciency/wear.DC machines are dened as follows:[63]

Armature circuit - A winding where the load cur-rent is carried, such that can be either stationary orrotating part of motor or generator.

Field circuit - A set of windings that produces amag-netic eld so that the electromagnetic induction cantake place in electric machines.

Commutation: A mechanical technique in whichrectication can be achieved, or from which DC canbe derived, in DC machines.

M M M

A B C

ff f f1 2

A: shunt B: series C: compound f = eld coil

There are ve types of brushed DC motor:

DC shunt-wound motor DC series-wound motor DC compound motor (two congurations):

Cumulative compound Dierentially compounded

PM DC motor (not shown) Separately excited (not shown).

5.1.2 Permanent magnet DC motor

Main article: Permanent-magnet electric motor

5.2 Electronic commutator (EC) motor 7

A PM motor does not have a eld winding on the sta-tor frame, instead relying on PMs to provide the mag-netic eld against which the rotor eld interacts to pro-duce torque. Compensating windings in series with thearmature may be used on large motors to improve com-mutation under load. Because this eld is xed, it cannotbe adjusted for speed control. PM elds (stators) are con-venient in miniature motors to eliminate the power con-sumption of the eld winding. Most larger DCmotors areof the dynamo type, which have stator windings. His-torically, PMs could not bemade to retain high ux if theywere disassembled; eld windings were more practical toobtain the needed amount of ux. However, large PMsare costly, as well as dangerous and dicult to assemble;this favors wound elds for large machines.To minimize overall weight and size, miniature PM mo-tors may use high energy magnets made with neodymiumor other strategic elements; most such are neodymium-iron-boron alloy. With their higher ux density, electricmachines with high-energy PMs are at least competitivewith all optimally designed singly fed synchronous andinduction electric machines. Miniature motors resemblethe structure in the illustration, except that they have atleast three rotor poles (to ensure starting, regardless ofrotor position) and their outer housing is a steel tube thatmagnetically links the exteriors of the curved eld mag-nets.

5.2 Electronic commutator (EC) motor5.2.1 Brushless DC motor

Main article: Brushless DC electric motor

Some of the problems of the brushed DCmotor are elim-inated in the BLDC design. In this motor, the mechanicalrotating switch or commutator is replaced by an exter-nal electronic switch synchronised to the rotors position.BLDC motors are typically 8590% ecient or more.Eciency for a BLDC motor of up to 96.5% have beenreported,[64] whereas DC motors with brushgear are typ-ically 7580% ecient.The BLDC motors characteristic trapezoidal back-emfwaveform is derived partly from the stator windings be-ing evenly distributed, and partly from the placement ofthe rotors PMs. Also known as electronically commu-tated DC or inside out DC motors, the stator windings oftrapezoidal BLDC motors can be with single-phase, two-phase or three-phase and use Hall eect sensors mountedon their windings for rotor position sensing and low costclosed-loop control of the electronic commutator.BLDC motors are commonly used where precise speedcontrol is necessary, as in computer disk drives or invideo cassette recorders, the spindles within CD, CD-ROM (etc.) drives, and mechanisms within oce prod-ucts such as fans, laser printers and photocopiers. They

have several advantages over conventional motors:

Compared to AC fans using shaded-pole motors,they are very ecient, running much cooler than theequivalent AC motors. This cool operation leads tomuch-improved life of the fans bearings.

Without a commutator to wear out, the life of aBLDC motor can be signicantly longer comparedto a DC motor using brushes and a commutator.Commutation also tends to cause a great deal ofelectrical and RF noise; without a commutator orbrushes, a BLDC motor may be used in electricallysensitive devices like audio equipment or comput-ers.

The same Hall eect sensors that provide the com-mutation can also provide a convenient tachometersignal for closed-loop control (servo-controlled) ap-plications. In fans, the tachometer signal can be usedto derive a fan OK signal as well as provide run-ning speed feedback.

The motor can be easily synchronized to an internalor external clock, leading to precise speed control.

BLDC motors have no chance of sparking, unlikebrushed motors, making them better suited to en-vironments with volatile chemicals and fuels. Also,sparking generates ozone which can accumulate inpoorly ventilated buildings risking harm to occu-pants health.

BLDC motors are usually used in small equipmentsuch as computers and are generally used in fans toget rid of unwanted heat.

They are also acoustically very quiet motors whichis an advantage if being used in equipment that isaected by vibrations.

Modern BLDC motors range in power from a fractionof a watt to many kilowatts. Larger BLDC motors upto about 100 kW rating are used in electric vehicles.They also nd signicant use in high-performance elec-tric model aircraft.

5.2.2 Switched reluctance motor

Main article: Switched reluctance motor

The SRM has no brushes or PMs, and the rotor has noelectric currents. Instead, torque comes from a slight mis-alignment of poles on the rotor with poles on the stator.The rotor aligns itself with the magnetic eld of the sta-tor, while the stator eld stator windings are sequentiallyenergized to rotate the stator eld.The magnetic ux created by the eld windings followsthe path of least magnetic reluctance, meaning the ux

8 6 EXTERNALLY COMMUTATED AC MACHINE

6/4 pole switched reluctance motor

will ow through poles of the rotor that are closest to theenergized poles of the stator, thereby magnetizing thosepoles of the rotor and creating torque. As the rotor turns,dierent windings will be energized, keeping the rotorturning.SRMs are now being used in some appliances.[65]

5.3 Universal AC-DC motor

Main article: Universal motorA commutated electrically excited series or parallel

Modern low-cost universal motor, from a vacuum cleaner. Fieldwindings are dark copper-colored, toward the back, on bothsides. The rotors laminated core is gray metallic, with dark slotsfor winding the coils. The commutator (partly hidden) has be-come dark from use; it is toward the front. The large brownmolded-plastic piece in the foreground supports the brush guidesand brushes (both sides), as well as the front motor bearing.

woundmotor is referred to as a universal motor because itcan be designed to operate on both AC and DC power. A

universal motor can operate well on AC because the cur-rent in both the eld and the armature coils (and hence theresultant magnetic elds) will alternate (reverse polarity)in synchronism, and hence the resulting mechanical forcewill occur in a constant direction of rotation.Operating at normal power line frequencies, universalmotors are often found in a range less than 1000 watts.Universal motors also formed the basis of the traditionalrailway traction motor in electric railways. In this ap-plication, the use of AC to power a motor originally de-signed to run on DC would lead to eciency losses due toeddy current heating of their magnetic components, par-ticularly the motor eld pole-pieces that, for DC, wouldhave used solid (un-laminated) iron and they are nowrarely used.An advantage of the universal motor is that AC suppliesmay be used on motors which have some characteristicsmore common in DC motors, specically high startingtorque and very compact design if high running speeds areused. The negative aspect is the maintenance and shortlife problems caused by the commutator. Suchmotors areused in devices such as foodmixers and power tools whichare used only intermittently, and often have high starting-torque demands. Multiple taps on the eld coil provide(imprecise) stepped speed control. Household blendersthat advertise many speeds frequently combine a eld coilwith several taps and a diode that can be inserted in serieswith the motor (causing the motor to run on half-waverectied AC). Universal motors also lend themselves toelectronic speed control and, as such, are an ideal choicefor devices like domestic washing machines. The motorcan be used to agitate the drum (both forwards and inreverse) by switching the eld winding with respect to thearmature.Whereas SCIMs cannot turn a shaft faster than allowedby the power line frequency, universal motors can run atmuch higher speeds. This makes them useful for appli-ances such as blenders, vacuum cleaners, and hair dryerswhere high speed and light weight are desirable. Theyare also commonly used in portable power tools, suchas drills, sanders, circular and jig saws, where the mo-tors characteristics work well. Many vacuum cleaner andweed trimmer motors exceed 10,000 rpm, while manysimilar miniature grinders exceed 30,000 rpm.

6 Externally commutated AC ma-chine

Main article: AC motor

The design of AC induction and synchronous motors isoptimized for operation on single-phase or polyphase si-nusoidal or quasi-sinusoidal waveform power such as sup-plied for xed-speed application from the AC power grid

6.1 Induction motor 9

or for variable-speed application from VFD controllers.An AC motor has two parts: a stationary stator havingcoils supplied with AC to produce a rotating magneticeld, and a rotor attached to the output shaft that is givena torque by the rotating eld.

6.1 Induction motor

Main article: Induction motor

6.1.1 Cage and wound rotor induction motor

An induction motor is an asynchronous AC motor wherepower is transferred to the rotor by electromagnetic in-duction, much like transformer action. An induction mo-tor resembles a rotating transformer, because the stator(stationary part) is essentially the primary side of thetransformer and the rotor (rotating part) is the secondaryside. Polyphase induction motors are widely used in in-dustry.Induction motors may be further divided into SquirrelCage Induction Motors and Wound Rotor Induction Mo-tors. SCIMs have a heavy winding made up of solid bars,usually aluminum or copper, joined by rings at the ends ofthe rotor. When one considers only the bars and rings asa whole, they are much like an animals rotating exercisecage, hence the name.Currents induced into this winding provide the rotor mag-netic eld. The shape of the rotor bars determines thespeed-torque characteristics. At low speeds, the currentinduced in the squirrel cage is nearly at line frequencyand tends to be in the outer parts of the rotor cage. Asthe motor accelerates, the slip frequency becomes lower,and more current is in the interior of the winding. Byshaping the bars to change the resistance of the windingportions in the interior and outer parts of the cage, eec-tively a variable resistance is inserted in the rotor circuit.However, the majority of such motors have uniform bars.In a WRIM, the rotor winding is made of many turns ofinsulated wire and is connected to slip rings on the motorshaft. An external resistor or other control devices can beconnected in the rotor circuit. Resistors allow control ofthe motor speed, although signicant power is dissipatedin the external resistance. A converter can be fed fromthe rotor circuit and return the slip-frequency power thatwould otherwise be wasted back into the power systemthrough an inverter or separate motor-generator.The WRIM is used primarily to start a high inertia loador a load that requires a very high starting torque acrossthe full speed range. By correctly selecting the resistorsused in the secondary resistance or slip ring starter, themotor is able to produce maximum torque at a relativelylow supply current from zero speed to full speed. Thistype of motor also oers controllable speed.

Motor speed can be changed because the torque curve ofthe motor is eectively modied by the amount of resis-tance connected to the rotor circuit. Increasing the valueof resistance will move the speed of maximum torquedown. If the resistance connected to the rotor is increasedbeyond the point where the maximum torque occurs atzero speed, the torque will be further reduced.When used with a load that has a torque curve that in-creases with speed, the motor will operate at the speedwhere the torque developed by the motor is equal to theload torque. Reducing the load will cause the motor tospeed up, and increasing the load will cause the motorto slow down until the load and motor torque are equal.Operated in this manner, the slip losses are dissipated inthe secondary resistors and can be very signicant. Thespeed regulation and net eciency is also very poor.

6.1.2 Torque motor

Main article: Torque motor

A torque motor is a specialized form of electric motorwhich can operate indenitely while stalled, that is, withthe rotor blocked from turning, without incurring dam-age. In this mode of operation, the motor will apply asteady torque to the load (hence the name).A common application of a torque motor would be thesupply- and take-up reel motors in a tape drive. In thisapplication, driven from a low voltage, the characteristicsof these motors allow a relatively constant light tensionto be applied to the tape whether or not the capstan isfeeding tape past the tape heads. Driven from a highervoltage, (and so delivering a higher torque), the torquemotors can also achieve fast-forward and rewind opera-tion without requiring any additional mechanics such asgears or clutches. In the computer gaming world, torquemotors are used in force feedback steering wheels.Another common application is the control of the throt-tle of an internal combustion engine in conjunction withan electronic governor. In this usage, the motor worksagainst a return spring to move the throttle in accordancewith the output of the governor. The latter monitors en-gine speed by counting electrical pulses from the ignitionsystem or from a magnetic pickup and, depending on thespeed, makes small adjustments to the amount of currentapplied to themotor. If the engine starts to slow down rel-ative to the desired speed, the current will be increased,the motor will develop more torque, pulling against thereturn spring and opening the throttle. Should the enginerun too fast, the governor will reduce the current being ap-plied to the motor, causing the return spring to pull backand close the throttle.

10 7 SPECIAL MAGNETIC MOTORS

6.2 Synchronous motorMain article: Synchronous motor

A synchronous electric motor is an AC motor distin-guished by a rotor spinning with coils passing magnetsat the same rate as the AC and resulting magnetic eldwhich drives it. Another way of saying this is that ithas zero slip under usual operating conditions. Contrastthis with an induction motor, which must slip to producetorque. One type of synchronous motor is like an induc-tion motor except the rotor is excited by a DC eld. Sliprings and brushes are used to conduct current to the ro-tor. The rotor poles connect to each other and move atthe same speed hence the name synchronous motor. An-other type, for low load torque, has ats ground onto aconventional squirrel-cage rotor to create discrete poles.Yet another, such as made by Hammond for its pre-WorldWar II clocks, and in the older Hammond organs, has norotor windings and discrete poles. It is not self-starting.The clock requires manual starting by a small knob on theback, while the older Hammond organs had an auxiliarystartingmotor connected by a spring-loadedmanually op-erated switch.Finally, hysteresis synchronous motors typically are (es-sentially) two-phase motors with a phase-shifting capac-itor for one phase. They start like induction motors,but when slip rate decreases suciently, the rotor (asmooth cylinder) becomes temporarily magnetized. Itsdistributed poles make it act like a PMSM. The rotor ma-terial, like that of a common nail, will stay magnetized,but can also be demagnetized with little diculty. Oncerunning, the rotor poles stay in place; they do not drift.Low-power synchronous timing motors (such as those fortraditional electric clocks) may have multi-pole PM ex-ternal cup rotors, and use shading coils to provide start-ing torque. Telechron clock motors have shaded poles forstarting torque, and a two-spoke ring rotor that performslike a discrete two-pole rotor.

6.3 Doubly fed electric machineMain article: Doubly fed electric machine

Doubly fed electric motors have two independent multi-phase winding sets, which contribute active (i.e., work-ing) power to the energy conversion process, with at leastone of the winding sets electronically controlled for vari-able speed operation. Two independent multiphase wind-ing sets (i.e., dual armature) are the maximum providedin a single package without topology duplication. Doublyfed electric motors are machines with an eective con-stant torque speed range that is twice synchronous speedfor a given frequency of excitation. This is twice the con-stant torque speed range as singly fed electric machines,which have only one active winding set.

A doubly fed motor allows for a smaller electronic con-verter but the cost of the rotor winding and slip ringsmay oset the saving in the power electronics com-ponents. Diculties with controlling speed near syn-chronous speed limit applications.[66]

7 Special magnetic motors

7.1 Rotary

7.1.1 Ironless or coreless rotor motor

A Miniature Coreless Motor

Nothing in the principle of any of the motors describedabove requires that the iron (steel) portions of the rotoractually rotate. If the soft magnetic material of the rotoris made in the form of a cylinder, then (except for theeect of hysteresis) torque is exerted only on the wind-ings of the electromagnets. Taking advantage of this factis the coreless or ironless DC motor, a specialized formof a PM DC motor.[62] Optimized for rapid acceleration,these motors have a rotor that is constructed without anyiron core. The rotor can take the form of a winding-lledcylinder, or a self-supporting structure comprising onlythe magnet wire and the bonding material. The rotor cant inside the stator magnets; a magnetically soft station-ary cylinder inside the rotor provides a return path for thestator magnetic ux. A second arrangement has the rotorwinding basket surrounding the stator magnets. In thatdesign, the rotor ts inside a magnetically soft cylinderthat can serve as the housing for the motor, and likewiseprovides a return path for the ux.Because the rotor is much lighter in weight (mass) than aconventional rotor formed from copper windings on steellaminations, the rotor can accelerate much more rapidly,often achieving a mechanical time constant under onems. This is especially true if the windings use aluminumrather than the heavier copper. But because there is nometal mass in the rotor to act as a heat sink, even smallcoreless motors must often be cooled by forced air. Over-

7.1 Rotary 11

heating might be an issue for coreless DC motor designs.Among these types are the disc-rotor types, described inmore detail in the next section.Vibrator motors for cellular phones are sometimes tinycylindrical PM eld types, but there are also disc-shapedtypes which have a thin multipolar disc eld magnet, andan intentionally unbalanced molded-plastic rotor struc-ture with two bonded coreless coils. Metal brushes anda at commutator switch power to the rotor coils.Related limited-travel actuators have no core and abonded coil placed between the poles of high-ux thinPMs. These are the fast head positioners for rigid-disk(hard disk) drives. Although the contemporary de-sign diers considerably from that of loudspeakers, it isstill loosely (and incorrectly) referred to as a voice coilstructure, because some earlier rigid-disk-drive headsmoved in straight lines, and had a drive structure muchlike that of a loudspeaker.

7.1.2 Pancake or axial rotor motor

A rather unusual motor design, the printed armature orpancake motor has the windings shaped as a disc runningbetween arrays of high-uxmagnets. Themagnets are ar-ranged in a circle facing the rotor with space in between toform an axial air gap.[67] This design is commonly knownas the pancake motor because of its extremely at prole,although the technology has had many brand names sinceits inception, such as ServoDisc.The printed armature (originally formed on a printed cir-cuit board) in a printed armature motor is made frompunched copper sheets that are laminated together us-ing advanced composites to form a thin rigid disc. Theprinted armature has a unique construction in the brushedmotor world in that it does not have a separate ring com-mutator. The brushes run directly on the armature surfacemaking the whole design very compact.An alternative manufacturing method is to use woundcopper wire laid at with a central conventional com-mutator, in a ower and petal shape. The windings aretypically stabilized by being impregnated with electricalepoxy potting systems. These are lled epoxies that havemoderate mixed viscosity and a long gel time. They arehighlighted by low shrinkage and low exotherm, and aretypically UL 1446 recognized as a potting compound in-sulated with 180 C, Class H rating.The unique advantage of ironless DC motors is that thereis no cogging (torque variations caused by changing at-traction between the iron and the magnets). Parasiticeddy currents cannot form in the rotor as it is totally iron-less, although iron rotors are laminated. This can greatlyimprove eciency, but variable-speed controllers mustuse a higher switching rate (>40 kHz) or DC because ofthe decreased electromagnetic induction.These motors were originally invented to drive the cap-

stan(s) of magnetic tape drives in the burgeoning com-puter industry, where minimal time to reach operatingspeed and minimal stopping distance were critical. Pan-cake motors are still widely used in high-performanceservo-controlled systems, robotic systems, industrial au-tomation and medical devices. Due to the variety of con-structions now available, the technology is used in appli-cations from high temperature military to low cost pumpand basic servos.

7.1.3 Servo motor

Main article: Servo motor

A servomotor is a motor, very often sold as a completemodule, which is used within a position-control or speed-control feedback control system mainly control valves,such as motor operated control valves. Servomotors areused in applications such as machine tools, pen plotters,and other process systems. Motors intended for use in aservomechanism must have well-documented character-istics for speed, torque, and power. The speed vs. torquecurve is quite important and is high ratio for a servo mo-tor. Dynamic response characteristics such as windinginductance and rotor inertia are also important; these fac-tors limit the overall performance of the servomecha-nism loop. Large, powerful, but slow-responding servoloops may use conventional AC or DC motors and drivesystems with position or speed feedback on the motor.As dynamic response requirements increase, more spe-cialized motor designs such as coreless motors are used.ACmotors superior power density and acceleration char-acteristics compared to that of DC motors tends to fa-vor PM synchronous, BLDC, induction, and SRM driveapplications.[67]

A servo system diers from some stepper motor applica-tions in that the position feedback is continuous while themotor is running; a stepper system relies on the motor notto miss steps for short term accuracy, although a step-per system may include a home switch or other elementto provide long-term stability of control.[68] For instance,when a typical dot matrix computer printer starts up, itscontroller makes the print head stepper motor drive to itsleft-hand limit, where a position sensor denes home po-sition and stops stepping. As long as power is on, a bidi-rectional counter in the printers microprocessor keepstrack of print-head position.

7.1.4 Stepper motor

Main article: Stepper motorStepper motors are a type of motor frequently used whenprecise rotations are required. In a steppermotor an inter-nal rotor containing PMs or a magnetically soft rotor withsalient poles is controlled by a set of external magnets thatare switched electronically. A stepper motor may also be

12 9 ELECTROMAGNETISM

A B

A stepper motor with a soft iron rotor, with active windingsshown. In 'A' the active windings tend to hold the rotor in po-sition. In 'B' a dierent set of windings are carrying a current,which generates torque and rotation.

thought of as a cross between a DC electric motor anda rotary solenoid. As each coil is energized in turn, therotor aligns itself with the magnetic eld produced by theenergized eld winding. Unlike a synchronous motor, inits application, the stepper motor may not rotate contin-uously; instead, it stepsstarts and then quickly stopsagainfrom one position to the next as eld windings areenergized and de-energized in sequence. Depending onthe sequence, the rotor may turn forwards or backwards,and it may change direction, stop, speed up or slow downarbitrarily at any time.Simple stepper motor drivers entirely energize or en-tirely de-energize the eld windings, leading the rotor tocog to a limited number of positions; more sophisti-cated drivers can proportionally control the power to theeld windings, allowing the rotors to position between thecog points and thereby rotate extremely smoothly. Thismode of operation is often called microstepping. Com-puter controlled stepper motors are one of themost versa-tile forms of positioning systems, particularly when partof a digital servo-controlled system.Stepper motors can be rotated to a specic angle indiscrete steps with ease, and hence stepper motors areused for read/write head positioning in computer oppydiskette drives. They were used for the same purpose inpre-gigabyte era computer disk drives, where the preci-sion and speed they oered was adequate for the correctpositioning of the read/write head of a hard disk drive.As drive density increased, the precision and speed lim-itations of stepper motors made them obsolete for harddrivesthe precision limitation made them unusable,and the speed limitation made them uncompetitivethusnewer hard disk drives use voice coil-based head actua-tor systems. (The term voice coil in this connection ishistoric; it refers to the structure in a typical (cone type)loudspeaker. This structure was used for a while to posi-tion the heads. Modern drives have a pivoted coil mount;the coil swings back and forth, something like a blade ofa rotating fan. Nevertheless, like a voice coil, modernactuator coil conductors (the magnet wire) move perpen-

dicular to the magnetic lines of force.)Stepper motors were and still are often used in com-puter printers, optical scanners, and digital photocopiersto move the optical scanning element, the print head car-riage (of dot matrix and inkjet printers), and the platenor feed rollers. Likewise, many computer plotters (whichsince the early 1990s have been replaced with large-format inkjet and laser printers) used rotary stepper mo-tors for pen and platen movement; the typical alternativeshere were either linear stepper motors or servomotorswith closed-loop analog control systems.So-called quartz analog wristwatches contain the smallestcommonplace stepping motors; they have one coil, drawvery little power, and have a PM rotor. The same kindof motor drives battery-powered quartz clocks. Some ofthese watches, such as chronographs, contain more thanone stepping motor.Closely related in design to three-phase AC synchronousmotors, stepper motors and SRMs are classied as vari-able reluctance motor type.[69] Stepper motors were andstill are often used in computer printers, optical scanners,and computer numerical control (CNC) machines such asrouters, plasma cutters and CNC lathes.

7.2 Linear motor

Main article: Linear motor

A linear motor is essentially any electric motor that hasbeen unrolled so that, instead of producing a torque (ro-tation), it produces a straight-line force along its length.Linear motors are most commonly induction motors orstepper motors. Linear motors are commonly found inmany roller-coasters where the rapid motion of the mo-torless railcar is controlled by the rail. They are also usedin maglev trains, where the train ies over the ground.On a smaller scale, the 1978 era HP 7225A pen plotterused two linear stepper motors to move the pen along theX and Y axes.[70]

8 Comparison by major categories

9 Electromagnetism

9.1 Force and torque

The fundamental purpose of the vast majority of theworlds electric motors is to electromagnetically inducerelative movement in an air gap between a stator and ro-tor to produce useful torque or linear force.According Lorentz force law the force of a winding con-ductor can be given simply by:

9.4 Losses 13

F = I` B

or more generally, to handle conductors with any geom-etry:

F = J B

The most general approaches to calculating the forces inmotors use tensors.[79]

9.2 PowerWhere rpm is shaft speed and T is torque, a motors me-chanical power output P is given by,[80]

in British units with T expressed in foot-pounds,

Pem =rpm T5252

in SI units with shaft speed expressed in radians per sec-ond, and T expressed in newton-meters,

Pem = speed T

For a linear motor, with force F expressed in newtons andvelocity v expressed in meters per second,

Pem = F v

In an asynchronous or induction motor, the relationshipbetweenmotor speed and air gap power is, neglecting skineect, given by the following:

Pairgap =Rrs I2r , where

R - rotor resistanceI2 - square of current induced inthe rotors - motor slip; ie, dierence be-tween synchronous speed and slipspeed, which provides the relativemovement needed for current in-duction in the rotor.

9.3 Back emfMain article: Electromotive force

Since the armature windings of a direct-current motorare moving through a magnetic eld, they have a voltage

induced in them. This voltage tends to oppose the mo-tor supply voltage and so is called "back electromotiveforce (emf)". The voltage is proportional to the runningspeed of the motor. The back emf of the motor, plus thevoltage drop across the winding internal resistance andbrushes, must equal the voltage at the brushes. This pro-vides the fundamental mechanism of speed regulation ina DC motor. If the mechanical load increases, the motorslows down; a lower back emf results, and more current isdrawn from the supply. This increased current providesthe additional torque to balance the new load.[81]

In ACmachines, it is sometimes useful to consider a backemf source within the machine; this is of particular con-cern for close speed regulation of induction motors onVFDs, for example.[81]

9.4 Losses

Motor losses are mainly due to resistive losses in wind-ings, core losses and mechanical losses in bearings, andaerodynamic losses, particularly where cooling fans arepresent, also occur.Losses also occur in commutation, mechanical commuta-tors spark, and electronic commutators and also dissipateheat.

9.5 Eciency

To calculate a motors eciency, the mechanical outputpower is divided by the electrical input power:

=PmPe

where is energy conversion eciency, Pe is electricalinput power, and Pm is mechanical output power:

Pe = IV

Pm = T!

where V is input voltage, I is input current, T is outputtorque, and ! is output angular velocity. It is possible toderive analytically the point of maximum eciency. It istypically at less than 1/2 the stall torque.Various regulatory authorities in many countries have in-troduced and implemented legislation to encourage themanufacture and use of higher eciency electric motors.

9.6 Goodness factorMain article: Goodness factor

14 10 PERFORMANCE PARAMETERS

Professor Eric Laithwaite[82] proposed a metric to de-termine the 'goodness of an electric motor:[83] G =

!resistancereluctance =

!AmAelmle

Where:

G

Am; Ae

lm; le

!

From this, he showed that the most ecient motors arelikely to have relatively large magnetic poles. However,the equation only directly relates to non PM motors.

10 Performance parameters

10.1 Torque capability of motor types

All the electromagnetic motors, and that includes thetypes mentioned here derive the torque from the vec-tor product of the interacting elds. For calculating thetorque it is necessary to know the elds in the air gap .Once these have been established by mathematical anyl-ysis using FEA or other tools the torque may be calcu-lated as the integral of all the vectors of force multipliedby the radius of each vector. The current owing in thewindings is producing the elds and for a motor usinga magnetic material the eld is not linearilly proprtionalto the current. This makes the calculation dicult buta computer can do the many calculations needed. Oncethis is done a gure relating the current to the torque canbe used as a useful parameter for motor selection. Themaximum torque for a motor will depend on the max-imum current although this will usually be .only usableuntil thermal considerations take precedence. When op-timally designed within a given core saturation constraintand for a given active current (i.e., torque current), volt-age, pole-pair number, excitation frequency (i.e., syn-chronous speed), and air-gap ux density, all categoriesof electric motors or generators will exhibit virtually thesame maximum continuous shaft torque (i.e., operatingtorque) within a given air-gap area with winding slots andback-iron depth, which determines the physical size ofelectromagnetic core. Some applications require burstsof torque beyond the maximum operating torque, suchas short bursts of torque to accelerate an electric vehiclefrom standstill. Always limited by magnetic core satura-tion or safe operating temperature rise and voltage, thecapacity for torque bursts beyond the maximum oper-ating torque diers signicantly between categories ofelectric motors or generators.

Capacity for bursts of torque should not be confusedwith eld weakening capability. Field weakening allowsan electric machine to operate beyond the designed fre-quency of excitation. Field weakening is done when themaximum speed cannot be reached by increasing the ap-plied voltage. This applies to only motors with currentcontrolled elds and therefore cannot be achieved withPM motors.Electric machines without a transformer circuit topology,such as that of WRSMs or PMSMs, cannot realize burstsof torque higher than themaximum designed torque with-out saturating the magnetic core and rendering any in-crease in current as useless. Furthermore, the PM assem-bly of PMSMs can be irreparably damaged, if bursts oftorque exceeding the maximum operating torque ratingare attempted.Electric machines with a transformer circuit topology,such as induction machines, induction doubly fed elec-tric machines, and induction or synchronous wound-rotordoubly fed (WRDF)machines, exhibit very high bursts oftorque because the emf-induced active current on eitherside of the transformer oppose each other and thus con-tribute nothing to the transformer coupled magnetic coreux density, which would otherwise lead to core satura-tion.Electric machines that rely on induction or asynchronousprinciples short-circuit one port of the transformer circuitand as a result, the reactive impedance of the transformercircuit becomes dominant as slip increases, which limitsthe magnitude of active (i.e., real) current. Still, bursts oftorque that are two to three times higher than the maxi-mum design torque are realizable.The brushless wound-rotor synchronous doubly fed(BWRSDF) machine is the only electric machine witha truly dual ported transformer circuit topology (i.e.,both ports independently excited with no short-circuitedport).[84] The dual ported transformer circuit topology isknown to be unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the rotorwinding set. If a precisionmeans were available to instan-taneously control torque angle and slip for synchronousoperation during motoring or generating while simultane-ously providing brushless power to the rotor winding set,the active current of the BWRSDF machine would be in-dependent of the reactive impedance of the transformercircuit and bursts of torque signicantly higher than themaximum operating torque and far beyond the practicalcapability of any other type of electric machine would berealizable. Torque bursts greater than eight times operat-ing torque have been calculated.

10.2 Continuous torque densityThe continuous torque density of conventional electricmachines is determined by the size of the air-gap area andthe back-iron depth, which are determined by the power

15

rating of the armature winding set, the speed of the ma-chine, and the achievable air-gap ux density before coresaturation. Despite the high coercivity of neodymium orsamarium-cobalt PMs, continuous torque density is vir-tually the same amongst electric machines with optimallydesigned armature winding sets. Continuous torque den-sity relates to method of cooling and permissible periodof operation before destruction by overheating of wind-ings or PM damage.

10.3 Continuous power densityThe continuous power density is determined by the prod-uct of the continuous torque density and the constanttorque speed range of the electric machine.

11 StandardsThe following are major design and manufacturing stan-dards covering electric motors:

International Electrotechnical Commission: IEC60034 Rotating Electrical Machines

National Electrical Manufacturers Association:MG-1 Motors and Generators

Underwriters Laboratories: UL 1004 - Standard forElectric Motors

12 Non-magnetic motorsMain articles: Electrostatic motor, Piezoelectric motorand Electrically powered spacecraft propulsion

An electrostatic motor is based on the attraction and re-pulsion of electric charge. Usually, electrostatic motorsare the dual of conventional coil-based motors. Theytypically require a high voltage power supply, althoughvery small motors employ lower voltages. Conventionalelectric motors instead employ magnetic attraction andrepulsion, and require high current at low voltages. Inthe 1750s, the rst electrostatic motors were developedby Benjamin Franklin and Andrew Gordon. Today theelectrostatic motor nds frequent use in micro-electro-mechanical systems (MEMS) where their drive voltagesare below 100 volts, and where moving, charged platesare far easier to fabricate than coils and iron cores. Also,the molecular machinery which runs living cells is oftenbased on linear and rotary electrostatic motors.A piezoelectric motor or piezo motor is a type of electricmotor based upon the change in shape of a piezoelectricmaterial when an electric eld is applied. Piezoelec-tric motors make use of the converse piezoelectric eect

whereby the material produces acoustic or ultrasonic vi-brations in order to produce a linear or rotary motion. Inone mechanism, the elongation in a single plane is usedto make a series stretches and position holds, similar tothe way a caterpillar moves.An electrically powered spacecraft propulsion systemuses electric motor technology to propel spacecraft inouter space, most systems being based on electricallypowering propellant to high speed, with some systems be-ing based on electrodynamic tethers principles of propul-sion to the magnetosphere.[85]

13 See also Electric generator Goodness factor Motor capacitor

14 Notes[1] Ganot provides a superb illustration of one such early elec-

tric motor designed by Froment.[17]

[2] The term 'electronic commutator motor' (ECM) is iden-tied with the heating, ventilation and air-conditioning(HVAC) industry, the distinction between BLDC andBLAC being in this context seen as a function of degreeof ECM drive complexity with BLDC drives typicallybeing with simple single-phase scalar-controlled voltage-regulated trapezoidal current waveform output involvingsurface PM motor construction and BLAC drives tend-ing towards more complex three-phase vector-controlledcurrent-regulated sinusoidal waveform involving interiorPM motor construction.[59]

[3] The universal and repulsion motors are part of aclass of motors known as AC commutator motors,which also includes the following now largely obso-lete motor types: Single-phase - straight and compen-sated series motors, railway motor; three-phase - vari-ous repulsion motor types, brush-shifting series motor,brush-shifting polyphase shunt or Schrage motor, Fynn-Weichsel motor.[60]

15 References[1] Faraday, Michael (1822). On Some New Electro-

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[2] Tom McInally, The Sixth Scottish University. The ScotsColleges Abroad: 1575 to 1799 (Brill, Leiden, 2012) p.115

16 15 REFERENCES

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[4] The rst dinamo?". travelhungary.com. Retrieved 12February 2013.

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[6] Heller, Augustus (April 1896). Anianus Jed-lik. Nature (Norman Lockyer) 53 (1379): 516.Bibcode:1896Natur..53..516H. doi:10.1038/053516a0.

[7] Blundel, Stephen J. (2012). Magnetism A Very Short In-troduction. Oxford University Press. p. 36. ISBN 978-0-19-960120-2.

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[9] Elektrische Chronologie. Elektrisiermaschinen im 18.und 19. Jahrhundert Ein kleines Lexikon (Electricalmachinery in the 18th and 19th centuries a small the-saurus) (in German). University of Regensburg. March31, 2004. Retrieved August 23, 2010.

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[16] Znobe Thophile Gramme. Invent Now, Inc. Hall ofFame prole. Retrieved 2012-09-19.

[17] Ganot, Adolphe; Trans. and ed. from the French by E.Atkinson (1881). Elementatry Treatise in Physics (14thed.). William Wood and Co. pp. 907908, sec. 899.

[18] Photo of a traditional form of the St. Louis motor.

[19] Buying an Energy-Ecient Electric Motor - Fact Sheet.USDoE.

[20] Babbage, C.; Herschel, J. F. W. (Jan 1825). Account ofthe Repetition of M. Aragos Experiments on the Mag-netism Manifested by Various Substances during the Actof Rotation. Philosophical Transactions of the Royal So-ciety 115 (0): 467496. doi:10.1098/rstl.1825.0023. Re-trieved 2 December 2012.

[21] Thompson, Silvanus Phillips (1895). Polyphase ElectricCurrents and Alternate-Current Motors (1st ed.). London:E. & F.N. Spon. p. 261. Retrieved 2 December 2012.

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17

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16 Bibliography Fink, Donald G.; Beaty, H. Wayne, Standard Hand-

book for Electrical Engineers, '14th ed., McGraw-Hill, 1999, ISBN 0-07-022005-0.

Houston, Edwin J.; Kennelly, Arthur, Recent Typesof Dynamo-Electric Machinery, American Techni-cal Book Company 1897, published by P.F. Collierand Sons New York, 1902

Kuphaldt, Tony R. (20002006). Chapter 13 ACMOTORS. Lessons In Electric CircuitsVolume II.Retrieved 2006-04-11.

Rosenblatt, Jack; Friedman, M. Harold, Direct andAlternating Current Machinery, 2nd ed., McGraw-Hill, 1963

17 Further reading Bedford, B.D.; Hoft, R.G. (1964). Principles of In-

verter Circuits. New York: Wiley. ISBN 0-471-06134-4.

Bose, Bimal K. (2006). Power Electronics and Mo-tor Drives : Advances and Trends. Academic Press.ISBN 978-0-12-088405-6.

Chiasson, John (2005). Modeling and High-Performance Control of Electric Machines (Onlineed.). Wiley. ISBN 0-471-68449-X.

Fitzgerald, A.E.; Kingsley, Charles , Jr.; Umans,Stephen D. (2003). Electric Machinery (6th ed.).McGraw-Hill. pp. 688 pages. ISBN 978-0-07-366009-7.

Pelly, B.R. (1971). Thyristor Phase-Controlled Con-verters and Cycloconverters : Operation, Control,and Performance. Wiley-Interscience. ISBN 978-0-471-67790-1.

Stlting, H. D.; Kallenbach, E.; Amrhein, W. (eds.)(2008). Handbook of Fractional-Horsepower Drives(Online ed.). Springer. ISBN 978-3-540-73128-3.

18 External links SparkMuseum: Early Electric Motors The Invention of the Electric Motor 1800 to 1893,hosted by Karlsrushe Institute of Technologys Mar-tin Doppelbauer

Electric Motors and Generators, a U. of NSWPhysclips multimedia resource

IEA 4E - Ecient Electrical End-Use Equipment. iPES Rotating Magnetic Field, animation

19

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