Motor
Content
An electric motor is is an an electric machine that machine that converts electrical energy into mechanical energy. mechanical energy. In normal motoring mode, most electric motors operate through the interaction between an field and andwinding winding currents to currents to generate force within the motor. In electric motor's magnetic field certain applications, applications, such as in the transportation industry with traction motors motors,, electric motors can operate in both motoring and generating or braking modes braking modes to also produce electrical energy from mechanical energy. Found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives, electric motors can be powered by direct current (D! (D! sources, such as from batteries, motor vehicles or rectifiers, or grid, inverters inverters or or generators. byalternating byalternating current (A! sources, (A! sources, such as from the power grid, "mall motors may be found in electric watches. #eneral$purpose motors with highly standardi%ed dimensions and characteristics provide convenient mechanical power for industrial use. &he largest of electric motors are used for ship propulsion, pipeline compression and pumped$storage pumped$storage applications applications with ratings reaching megawatts. )lectric motors may be classified by electric power source type, internal construction, application, type of motion output, and so on. Devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical powe powerr are respectively referred to as actuators and transducers. )lectric motors are used to produce linear force or tor*ue (rotary!.
utaway view through stator of induction motor. Contents
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. )arly moto motors rs
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)merge nce of A motors ./ )mergence 0otor cons construc truction tion
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2 0a5or categories categor ies
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4 "elf$ "elf$comm commutat utated ed motor 6rushe d D motor 4. 6rushed
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4.. 8ermanent magnet D motor 4. )lectronic commutator ()! motor
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4.. )lectrically )lectricall y e7cited D motor
4.. 6rus 4.. 6rushless hless D moto motor r
4.. "witched reluctance motor 9nivers al A$D motor 4./ 9niversal : )7ternally commutated A machine commutat ed A
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:. Indu Inducti ction on motor
:.. age and wound rotor induction motor
:.. motor or :. . &or*ue mot :. "ync "ynchrono hronous us moto motor r
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:./ Doubly fed electric machine ; "peci "pecial al magne magnetic tic motors
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;.. -ydraulic cylinder displacement
;.. Ironless or coreless rotor motor
;../ 8ancake or a7ial rotor motor
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;..2 "ervo o moto motor r ;.. 2 "erv ;.. 4 "tep ;..4 "tepper per motor ;. <inear mot motor or
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&or*ue capabilit . &or*ue capab ilityy of moto motorr types
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-istory+edit
Main article: article: History of the electric motor
Early motors+edit
Faraday's electromagnetic e7periment, = +
8erhaps the first electric motors were simple simple electrostatic electrostatic devices created by the "cottish Andrew #ordon #ordon in in the ;2s.+ &he theoretical principle behind production of monk Andrew monk mechanical force by the interactions of an electric current and a magnetic field, Amp@re's field, Amp@re's force law, law, was discovered later by Andr$0arie by Andr$0arie Amp@re Amp@re in in =. &he conversion of electrical energy into mechanical energy byelectromagnetic byelectromagnetic means means was demonstrated by the 6ritish scientist 0ichael Faraday in Faraday in =. A free$hanging wire was dipped into a pool of mercury, on which a permanent magnet (80! was (80! was placed. 3hen 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.+/ &his motor is often demonstrated in physics e7periments, brine substituting for to7ic mercury. &hough 6arlow's wheel was wheel was an early refinement to this Faraday demonstration, these and similar homopolar motors motors were to remain unsuited to practical application until late in the century.
Bedlik''s Celectromagnetic self$rotorC, =; (0useum of Applied Arts, 6udapest!. &he historic motor still Bedlik works perfectly today. today.+2
In =;, -ungarian -ungarian physicist physicist nyos nyos Bedlik Bedlik started e7perimenting with electromagnetic coils coils.. After Bedlik solved solved the technical p problems roblems of the con continuous tinuous rotation with with the invention of commutator , he called his early devices Celectromagnetic self$rotorsC. Although they were used only for instructional purposes, in == Bedlik demonstrated the first device to contain the three main components of practical D motorsE the stator , rotor and and commutator. &he 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. +4 +:+; +: +;+= +=+>+ +>++ +
Success with DC motors+edit &he first commutator D electric motor capable of turning machinery was invented by the 6ritish scientist scientist 3illiam "turgeon "turgeon in in =/.+Following "turgeon's work, a commutator$typ commutator$type e direct$currentt electric motor made with the intention of commercial use was built by the direct$curren American inventor inventor &homas Davenport Davenport,, which he patented in =/;. &he motors ran at up to : revolutions per minute, and powered machine tools and a printing press.+/ Due to the high cost of primary battery power , the motors were commercially unsuccess unsuccessful ful and Davenport went bankrupt. "everal inventors followed "turgeon in the development of D motors but all encountered the same battery power cost issues. ?o electricity distribution had been developed at the time. <ike "turgeon's motor, there was no practical commercial market for these motors.+2 In =44, Bedlik built a device using similar principles to those used in his electromagnetic +4+ -e built a model electric vehicle vehicle that that same self$rotors that was capable of useful work. +4+ year.+4 &he first commercially successful D motors followed the invention by by nobe #ramme #ramme who who had in =; developed the anchor ring dynamo which solved the double$& armature pulsating D problem. In =;/, #ramme found that this dynamo could be used as a motor,
which he demonstrated to great effect at e7hibitions in Gienna and 8hiladelphia by connecting two such D motors at a distance of up to km away from each other, one as a 3orkaround .! generator .+: ("ee also =;/ E l'e7prience dcisive +Decisive 3orkaround .! "prague invented invented the first practical D motor, a non$sparking motor In ==:, Frank Bulian "prague that maintained relatively constant speed under variable loads. Hther "prague electric inventions about this time greatly improved grid electric distribution (prior work done while employed by by &homas )dison )dison!, !, 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 controls systems for electric operations. &his allowed "prague to use electric motors to invent the first electric trolley system in ==;== in 1ichmond GA, GA, the electric elevator and control system in =>, and the electric subway with independently powered centrally controlled controlled cars, which were ffirst irst installed in => in hicago by the "outh "ide )levated 1ailway where it became popularly known as the C<C. "prague's motor and related inventions led to an e7plosion of interest and use in electric motors for f or industry, wh while ile almost simultaneously another great inventor was developing its primary competitor, whic which h would become much more widespread widespread.. &he developmen developmentt of electric motors of acceptable efficiency was delayed for several decades by failure to recogni%e the e7treme importance of a relatively small air gap between rotor and stator. )fficient designs have a comparatively small air gap.+; +a &he "t. <ouis motor, long used in classrooms to illustrate motor principles, is e7tremely inefficient for the same reason, as well as appearing nothing like a modern motor .+= Application of of electric motors revo revolutioni%ed lutioni%ed iindustry ndustry.. Industrial processes we were re no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead every machine could be e*uipped with its own electric motor, providing providing easy control at the point of use, and improving power transmission efficiency. )lectric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. -ousehold uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. &oday, electric motors stand for more than half of the electric energy consump consumption tion in the 9".+>
Emergence of AC motors+edit In =2, the French physicist FranJois Arago Arago formulated formulated the e7istence of rotating magnetic fields,, termed Arago's fields termed Arago's rotations rotations,, which, by manually turning switches on and off, 3alter 6aily ++ ++/ demonstrated in =;> as in effect the first primitive induction motor .++ ++/ In the ==s, many inventors were trying to develop workable A motors +2because A's advantages in long distance high voltage transmission were counterbalanced counterbalanced by the inability to operate motors on A. 8ractical rotating A induction motors were independently invented by #alileo Ferraris and Ferraris and ?ikola &esla &esla,, a working motor model having been demonstrated by the former in ==4 and by the latter in ==;. In ===, the Royal Academy of Science of Turin published Ferraris's research detailing foundations of motor operation while however concluding that Cthe apparatus based onthe t hat that principle could not be of any commercial importance as
+/+4+ +4+:+ :+;+ ;+=+ =+> >+/+ +/+/+ /+/+ /+//+ //+/2 /2+/4+ +/4+/: /: In ===, &esla presented his paper A New System System for motor.C+/ that described three patented Alternating Current Current Motors and Transformers Transformers to the AIEE that two$phase four$stator$pole four$stator$pole motor typesE one with a four$pole rotor forming a non$self$ startingreluctance starting reluctance motor , another with a wound rotor forming a self$starting induction motor , and the third a true true synchronous motor with with separately e7cited D supply to rotor winding. Hne of the patents &esla filed in ==;, however, also described a shorted$winding$rotor
3estinghouse promptly promptly bought &esla's patents, employed &esla to induction motor. #eorge 3estinghouse "cott to to help &esla, &esla leaving for other pursuits in develop them, and assigned . F. "cott +/+/+//+ //+/2+ /2+/4+ /4+/:+ /:+/;+/;+ /;+/;+/=+/=+ /=+/=+/>+ />+2 2 +2+2+ 2/+22 22 ==>..+/+/+ ==> +2+2+2/+ &he constant speed A induction motor was found 3estinghouse ouse engineers successfully adapted it to not to be suitable for street cars +2 but 3estingh +2; +24+2:+2; power a mining operation in &elluride, olorado in =>. +24+2: "teadfast in his promotion of Dolivo$Dobrovolsky invented invented the three$phase cage$rotor three$phase development, 0ikhail Dolivo$Dobrovolsky induction motor in ==> and the three$limb transformer in in =>. &his type of motor is now +2=+2> 2> -owever, he claimed that &esla's used for the vast ma5ority of commercial applications.+2=+ motor was not practical because of two$phase pulsations pulsations,, which prompted him to persist in his three$phase work.+4 Although 3estinghouse achieved achieved its ffirst irst practical induction motor in => and developed a line of polyphas polyphase e : hert% induction motors in =>/, these early 3estinghouse 3estingho use motors were two$phase motors motors with wound rotors until 6. #. +/; <amme <amme developed developed a rotating bar winding rotor. with &he &he #eneral )lectric ompany ompany began began developing three$phase three$phase induction motors in =>.+/; 6y =>:, #eneral )lectric and 3estinghouse 3estingho use signed a cross$licensin cross$licensing g agreement for the bar$winding$rotor design, later +/; called the s*uirrel$cage rotor . Induction motor improvements flowing from these inventions and innovations were such that a horsepower (-8! (-8! induction induction motor currently has the same mounting dimensions as a ;.4 -8 motor in =>;.+/;
0otor construction+edit
)lectric motor rotor (left! and stator (right!
Rotor +edit Main article: article: Rotor (electric)
In an electric motor the moving part is the rotor which which turns the shaft to deliver the mechanical power. power. &he rotor usually has conductors laid into it which carry currents that
interact with the magnetic field of the stator to generate the forces that turn the shaft. -owever, some rotors carry permanent magnets, and the stator holds the conductors.
Stator +edit Main article: article: Stator
&he stationary part is the stator , usually has either windings or permanent magnets. &he stator is the stationary part of the motorKs electromagne electromagnetic tic circuit. &he stator core is made up of many thin metal sheets, called laminations. <amination <aminationss are used to reduce energy loses that would result if a solid core were used.
Air gap+edit In between the rotor and stator is the air gap. &he air gap has important effects, and is generally as small as possible, as a large gap has a strong negative effect on the performance of an electric motor.
Windings+edit Main article: article: Windings
3indings are wires that are laid in coils, usually wrapped around a laminated soft core so so as to form magnetic poles when energi%ed with current. iron magnetic core iron )lectric machines come in two basic magnet field pole configurationsE salient-ole machine and nonsalient-ole machine. In the t he salient$pole machine the pole's magnetic field is produced by a winding wound around the pole below the pole face. In tthe he nonsalient-ole , or distributed field, or round$rotor, machine, machine, the winding is distributed in pole face slots. +4 A shaded$pole motor has has a winding around part of the pole that delays the phase of the magnetic field for that pole. "ome motors have conductors which consist consist of thicker metal, such as bars or sheets of sometimes aluminum aluminum is used. &hese are usually powered metal, usually copper , although sometimes by electromagnetic electromagnetic induction induction..
Commutator +edit Main article: article: !ommutator (electric)
&oy's &oy's small D motor with commutator.
A commutator is is a mechanism used to switch switch the input of certain A and D machines consisting of slip ring ring segments segments insulated from each other and from the electric motor's
brushes in in contact shaft. &he motor's armature current is supplied through the stationary stationary brushes with the revolving commutator, which causes re*uired current reversal and applies power to +4+4/ 4/ the machine in an optimal manner as the rotor rotates rotates from pole to pole.+4+ In absence of such current reversal, the motor would brake to a stop. In light of significant advanc advances es in the past few decades due to improved technologie technologiess in electronic controller controller,, sensorless control, induction motor, and permanent magnet motor fields, electromechanically commutated motors are increasingly being displaced by e7ternally commutated induction and permanent magnet motors.
0otor supply and control+edit Motor supply+edit A DC motor is is usually supplied through slip ring commutator as described above. AC motors' commutation can be either slip ring commutator or e7ternally commutated type, can be fi7ed$speed or variable$speed control type, and can be synchronous or asynchronous type. "ni#ersal motors can run on either A or D.
Motor control+edit Fi7ed$speed controlled A A motors are provided with direct$on$line or soft$start starters. Gariable speed controlled A motors are provided with a range of different power inverter , variable$fre* variable$fre*uency uency drive drive or electronic commutator technologie technologies. s. &he term electronic commutator is usually associated with self$commutated brushless D motor and and switched reluctance motor application applications. s.
0a5or categories+edit )lectric motors operate on three different physical principlesE magnetic principlesE magnetic,, electrostatic electrostatic and and pie%oelectric. pie%oelectric. 6y far the most common is magnetic. In magnetic motors, magnetic fields are formed in both the rotor and the stator. &he product between these two fields gives rise to a force, and thus a tor*ue on the motor shaft. Hne, or both, of these fields must be made to change with the rotation of the motor. &his is done by switching the poles on and off at the right time, or varying the strength of the pole. &he main types are D motors and A motors, the former increasingly being displaced by the latter. A electric motors motors are either asy asynchronous nchronous and synchronous. Hnce started, a synchronou s ynchronouss motor re*uires synchronism with the moving magnetic field's synchronous speed for all normal tor*ue conditions. In synchronous machines, the magnetic field must be provided by means other than induction such as from separately e7cited windings or permanent magnets.
It is usual to distinguish motors' rated output power about the unity horsepower threshold so horsepowerr that integral horsepower refers to motor(s! e*ual to or above, and fractional horsepowe (F-8! refers (F-8! refers to motor(s! below, the threshold. Type of Motor Commutation +42+ +42+44+ 44+4:+ 4:+4;+ 4;+4= 4=+4> +4>
Major Categories y
E"ternally Commutated
Self!Commutated
Electronic! Mechanical!
Commutator
Commutator Motors
#EC$ +4>+b b +4>+
Motors
+:+c c AC+:+
Asynchronous
Synchronous
Machines
Machines%
AC& (
DC
AC(
Three!phase motors. ) WRSM ) *ni+ersal
Electrically
motor
e"cited DC
#AC
motor.
commutator series
) Separately e"cited
motor +4= or
) Series
AC,DC
) Shunt
motor +4;$-
) Compound
) Repulsion motor
Three!phase With /M rotor. ) 01DC motor
motors.
ferromagnetic rotor. ) SRM
5 6 4
! S/MSM ) 8yrid
AC motors.-7 ) Capacitor ) Resistance ) Split ) Shaded!pole
/M DC motor
01A 01AC C moto m otorr+4> ! 2/MSM
) SC2 SC 2M3 4 ) WR2M
With
) /MSM /MSM or or
AC motors.-7 ) /ermanent!split capacitor ) 8ysteresis ) Stepper ) SyRM ) SyRM!/M hyrid
Simple
Rectifier
More elaorate
Most elaorate
linear electronics
transistor#s$
electronics
9:D$ $ when pro+ided electronics #9:D
or DC chopper
?otesE . 1otation is is indepe independent ndent of the fre*ue fre*uency ncy of the A A voltag voltage. e. . 1otation is is e*ual to synchro synchronous nous spee speed d (motor stator field speed!. /. In "I0 fi7ed$speed fi7ed$speed ope operation ration rotatio rotation n is e*ual to slip sp speed eed (synchro (synchronous nous speed less slip!. 2. In non$slip energy reco recovery very systems 31 31I0 I0 is usuall usuallyy used for motor starting but can be used to vary load speed. 4. Gariabl Gariable$sp e$speed eed oper operatio ation. n. :. 3hereas induction induction and synchronou synchronouss motor drive drivess are typical typically ly with ei either ther si7$step o orr sinusoidal waveform output, 6<D motor drives are usually with trape%oidal current waveformL the behavior of both sinusoidal and trape%oidal 80 machines is however identical in terms of their fundamental aspects.+: ;. In variable$speed variable$speed oper operation ation 31I0 iiss used in slip energy recovery an and d double$fed induction machine applications. =. age winding winding refers to shorted$cir shorted$circuited cuited s*ui s*uirrel$cage rrel$cage rotor rotor,, woun wound d winding being connected e7ternally through slip rings. >. 0ostly 0ostly singl single$ph e$phase ase wit with h some thre three$ph e$phase. ase. AbbreviationsE Abbreviation sE •
6<A $ $ 6rushless A
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6<D $ 6rushless D
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6<D0 $ 6rushless D motor
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) $ )lectronic commutator
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80 $ 8ermanent magnet
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I80"0 $ Interior permanent magnet synchronous motor
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80"0 $ 8ermanent magnet synchronous motor
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"80"0 $ "urface permanent magnet synchronous motor
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"I0 $ "*uirrel$cage "*uirrel$cage induction motor
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"10 $ "witched reluctance motor
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"y10 $ "ynchronous reluctance motor
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GFD $ Gariable$fre*uency drive
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31I0 $ 3ound$rotor induction motor
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31"0 $ $ 3ound$ro 3ound$rotor tor synchronou synchronouss motor
"elf$commutated "elf$commut ated motor +edit 0rushed DC motor +edit Main article: article: $! motor
All self$commutated self$commutated D motors a are re by definition ru run n on D elec electric tric power power.. 0ost D mo motors tors brushed internal internal mechanical commutation to reverse are small 80 types. &hey contain a brushed motor windings' current in synchronism with rotation.+: Electrically e"cited DC motor +edit Main article: article: %rushed $! electric motor
3orkings of a brushed electric motor with a two$pole rotor and 80 stator. (C?C and C"C designate polarities on the inside faces of the magnetsL the outside faces have opposite polarities.! po larities.!
A commutated commutated D mo motor tor has a set of rotati rotating ng windings w wound ound on an armature armature mounted mounted on a rotating shaft. &he shaft also carries the commutator, a long$lasting long$lasting rotary electrical switch that periodically reverses the flow of current in the rotor windings as the shaft rotates. &hus, every brushed D motor has A flowing through its rotating windings. urrent flows through
one or more pairs of brushes that bear on the commutatorL the brushes connect an e7ternal source of electric power to the rotating armature. &he rotating armature consists of one or more coils of wire wound around a laminated, magnetically CsoftC ferromagnetic CsoftC ferromagnetic core. urrent from the brushes flows through the commutator and one winding of the t he armature, making it a temporary magnet (an electromagnet electromagnet!. !. &he magnets field produced by the armature interacts with a stationary magnetic field produced by either 80s or another winding winding a field coil, as part of the motor frame. &he force between the two magnetic fields tends to rotate the motor shaft. &he commutator switches power power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops (like a compass needle does!, but rather keeps rotating as long as power is applied. 0any of the limitations of the classic commutator D motor are due to the need for brushes to press against the commutator. commutator. &his creates friction. "parks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together ad5acent sections and hence coil ends momentarily while crossing the gaps. Furthermore, the inductance of inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. &his sparking limits the ma7imum speed of the machine, as too$rapid sparking will overheat, erode, or even melt the commutator. &he current density per unit area of the brushes, in combination with their resistivity, resistivity, limits the output of the motor. &h &he e making and breaking of electric noiseLL sparking generates 1FI 1FI.. 6rushes eventually wear out contact also generates electrical noise and re*uire replacement, and the commutator itself is sub5ect to wear and maintenance (on larger motors! or replacement (on small motors!. &he commutator assembly on a large motor is a costly element, re*uiring precision assembly of many parts. Hn small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually re*uires replacing the whole rotor. 3hile most commutators are cylindrical, some are flat discs consisting of several segments (typically, at least three! mounted on an insulator. <arge brushes are desired for a larger brush contact area to ma7imi%e motor output, but small brushes are desired for low mass to ma7imi%e the speed at which the motor can run without the brushes e7cessively bouncing and sparking. ("mall brushes are also desirable for lower cost.! "tiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses (lower efficiency efficiency!! and accelerated brush and commutator wear. &herefore, D motor brush design entails a trade$off between output power, speed, and efficiencyMwear. D machines are defined as followsE +:/
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Armature circuit circuit $ A winding w where here the load cur current rent is carried carried,, such that can be eithe eitherr stationary or rotating part of motor or generator. Field circuit $ A set of windings that produces a magnetic field so that the electromagnetic electromagn etic induction can take place in electric machines. ommutationE A mechanical techni*ue in which rectification can be achieved, or from which D can be derived, in D machines.
AE shunt 6E series E compound f N field coil
&here are five types of brushed D motorE •
D shunt$wound motor
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D series$wound motor
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D compound motor (two (t wo configurations!E
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umulative compound
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Differentially compounded 80 D motor (not shown!
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"eparately e7cited (not shown!. /ermanent magnet DC motor +edit edit &ermanent-magnet net electric motor Main article: article: &ermanent-mag
A 80 motor motor does not ha have ve a field wind winding ing on the stator fra frame, me, instead rely relying ing on 80s to provide the magnetic field against which the rotor field interacts to produce tor*ue. ompensating ompensatin g windings in series with the armature may be used on large motors to improve commutation under load. 6ecause this field is fi7ed, f i7ed, it cannot be ad5usted for speed control. 80 fields (stators! are convenient in miniature motors to eliminate the power consumption of the field winding. 0ost larger D motors are of the CdynamoC type, which have stator windings. -istorically, -istorically, 80s could not be made to retain high flu7 if they were disassembledL field windings were more practical to obtain the needed amount of flu7. -owever, large 80s are costly, as well as dangerous and difficult to assembleL this favors wound fields for large machines.
&o minimi%e overall weight and si%e, miniature 80 motors may use high energy magnets neodymium or or other strategic elementsL most such are neodymium$iron$boro neodymium$iron$boron n made with neodymium alloy. 3ith their higher flu7 density, electric machines with high$energy 80s are at least competitive with all optimally designed designed singly fed synchronous fed synchronous and induction electric machines. 0iniature motors resemble the structure in the illustration, e7cept that they have at least three rotor poles (to ensure starting, regardless of rotor position! and their outer housing is a steel tube that magnetically links the e7teriors of tthe he curved field magnets.
Electronic commutator #EC$ motor +edit 0rushless DC motor +edit edit Main article: article: %rushless $! electric motor
"ome of the problems of the brushed D motor are eliminated in the 6<D design. In this motor, the mechanical Crotating switchC or commutator is replaced by an e7ternal electronic switch synchronised to the rotor's position. 6<D motors are typically =4>O efficien efficientt or more. )fficiency for a 6<D motor of up to >:.4O have been reported,+:2whereas D motors with brushgear are typically ;4=O efficient. &he 6<D motor's characteristic trape%oidal back$emf waveform is derived partly from stator the stator windings being evenly distributed, partly from the placement of the rotor's 80s. Also known as electronically electronically commutated D o orr inside out D m motors, otors, the stator w windings indings of trape%oidal 6<D motors can be with single$phase, two$phase or three$phase and use -all effect sensors sensors mounted on their windings for rotor position sensing and crude, low closed$loop p control control of of the electronic commutator. cost closed$loo 6<D motors are commonly used where precise speed control is necessary, as in computer disk drives or in video cassette recorders, the spindles within D, D$1H0 (etc.! drives, and mechanisms within office products such as fans, laser printers and photocopie photocopiers. rs. &hey have several advantages over conventiona conventionall motorsE •
ompared to A fans using shaded$pol shaded$pole e motors, they are very efficient, running much cooler than the e*uivalent A motors. &his cool operation leads to much$improved life of the fan's bearings.
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3ithout a commutator to wear out, the life of a 6<D motor can be significantly longer compared to a D motor using brushes and a commutator. ommutation also tends to cause a great deal of electrical and 1F noiseL without a commutator or brushes, a 6<D motor may be used in electrically sensitive devices like audio e*uipment or computers. &he same -all effect sensors that provide the commutation can also provide a convenient tachometer signal convenient signal for closed$loop control (servo$controlled! applications. applications. In fans, the tachometer signal can be used to derive a Cfan HPC signal as well as provide running speed feedback.
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&he motor can be easily synchroni%ed to an internal or e7ternal clock, leading to precise speed control. 6<D motors have no chance of sparking, unlike brushed motors, making them better suited to environme environments nts with volatile chemicals and fuels. Also, sparking generates o%one which can accumulate in poorly ventilated buildings risking harm to occupants' health.
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6<D motors are usually used in small e*uipment such as computers and are generally used in fans to get rid of unwanted heat. &hey are also acoustically very *uiet motors which is an advantage if being used in e*uipment that is affected by vibrations.
0odern 6<D motors range in power from a fraction of a watt to many kilowatts. <arger 6<D motors up to about k3 rating are used in electric vehicles. &hey also find significant use in high$performance high$performance electric model aircraft.
edit Switched reluctance motor +edit
:M2 pole switched reluctance motor Main article: article: S'itched reluctance motor
&he "10 has no brushes or 80s, and the rotor has no electric currents. Instead, tor*ue comes from a slight misalignment of poles on the rotor with poles on the stator. &he rotor aligns itself with the magnetic field of the stator, while the stator field stator windings are se*uentially energi%ed energi%ed to rotate the stator field. &he magnetic flu7 created by the t he field windings follows the path of least magnetic reluctance, meaning the flu7 will flow through poles of the rotor that are closest to the energi%ed poles of the stator, thereby magneti%ing magneti%ing those poles of the rotor and creating tor*ue. As the rotor turns, different windings will be energi%ed, keeping the rotor turning. "10s are now being used in some appliances.+:4
*ni+ersal AC!DC motor +edit Main article: article: "ni#ersal motor
0odern low$cost universal motor, from a vacuum cleaner. Field windings are dark copper$colored, toward the back, on both sides. &he rotor's laminated core is gray metallic, with dark slots for winding the coils. &he commutator (partly hidden! has become dark from useL it is toward the front. &he large brown molded$ plastic piece in the foreground supports the brush guides and brushes (both sides!, as well as the front motor bearing.
A commutated commutated electri electrically cally e7cited se series ries or parall parallel el wound motor iiss referred to as a un universal iversal motor because it can be designed to operate on both A and D power. A universal motor can operate well on A because the current in both the field and the armature coils (and hence the resultant magnetic fields! will alternate (reverse polarity! in synchronism, and hence the resulting mechanical force will occur in a constant direction of rotation. re*uenciess, universal motors are often found in a range less Hperating at normal power line ffre*uencie than watts. 9niversal motors also formed the basis of the traditional railway traction motor in electric railways railways.. In this application, the use of A to power a motor originally designed to run on D would lead to efficiency losses due to to eddy current current heating heating of their magnetic components, components, particularly the motor field pole$pieces that, for D, would have used solid (un$laminated! iron and they are now rarely used. An advantage of of the universal motor motor is that A A supplies m may ay be used on moto motors rs which have some characteristics more common in D motors, specifically high starting tor*ue and very compact design if high running speeds are used. &he negative aspect is the maintenance and short life problems caused by the commutator. "uch motors are used in devices such as food mi7ers and power tools which are used only intermittently, and often have high starting$ tor*ue demands. 0ultiple taps on the field f ield coil provide (imprecise! stepped speed control. -ousehold blenders blenders that advertise many speeds fre*uently 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$wave rectified A!. A!. 9niversal motors also lend themselves to electronic speed control and, control and, as such, are an ideal choice for devices like domestic washing machines. &he motor can be used to agitate the drum (both forwards and in reverse! by switching the field winding with respect to the armature.
3hereas "I0s cannot turn a shaft faster than allowed by the power line fre*uency, universal motors can run at much higher speeds. &his makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable. &hey are also commonly used in portable power tools, such as drills, sanders, circular and 5ig saws, where the motor's characteristics work well. 0any vacuum cleaner and weed trimmer motors e7ceed , rpm, while many similar miniature grinders e7ceed /, rpm.
)7ternally commutated A machine+edit Main article: article: A! A! motor
&he design of A induction and synchronous motors is optimi%ed for operation on single$ phase or polyphase sinusoidal or *uasi$sinusoidal waveform power such as supplied for fi7ed$speed application from the A power grid or for variable$speed application from GFD controllers. An A motor has two partsE a stationary stator having coils supplied with A to produce a rotating magnetic field, and a rotor attached to the output shaft that is given a tor*ue by the rotating field. f ield.
2nduction motor +edit Main article: article: Induction motor
Cage and wound rotor induction motor +edit edit
An induction motor motor is an asynchronous asynchronous A A motor where power is transfe transferred rred to the rotor by electromagnetic electromagne tic induction, much like transformer action. An induction motor resembles a rotating transformer, transformer, because the stator (stationary part! is essentially the primary side of the transformer and the rotor (rotating part! is the secondary side. 8olyphase induction motors are widely used in industry. Induction motors may be further divided into "I0s and 31I0s. "I0s have a heavy winding made up of solid bars, usually aluminum or copper, 5oined by rings at the ends of the rotor. 3hen one considers only the bars and rings as a whole, they are much like an animal's rotating e7ercise cage, hence the name. urrents induced into this winding provide the rotor magnetic field. &he shape of the rotor bars determines the speed$tor*ue characteristics. characteristics. At low speeds, the current induced in the s*uirrel cage is nearly at line fre*uency and tends to be in the outer parts of the rotor cage. As the motor accelerates, accelerates, the slip fre* fre*uency uency become becomess lower, and more curren currentt is in the interior of the winding. 6y shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit. -owever, the ma5ority of such motors have uniform bars. In a 31I0, the rotor winding is made of many turns of insulated wire and is connected to slip rings on rings on the motor shaft. An e7ternal resistor or other control devices can be connected in the rotor circuit. 1esistors allow control of the motor speed, although significant power is dissipated in the e7ternal resistance. A converter converter can be fed from the rotor circuit and return
the slip$fre*uency power that would otherwise be wasted back into the power system through an inverter or separate motor$genera motor$generator. tor. &he 31I0 is used primarily to start a high inertia load or a load that re*uires a very high starting tor*ue across the full speed range. 6y correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce ma7imum tor*ue at a relatively low supply current from %ero speed to full speed. &his type of motor also offers controllable speed. 0otor speed can be changed because the tor*ue curve of the motor is effectively modified by the amount of resistance connected to the rotor circuit. Increasing the value of resistance will move the speed of ma7imum tor*ue down. If the resistance connected to the rotor is increased beyond the point where the ma7imum tor*ue occurs at %ero speed, the tor*ue will be further reduced. 3hen used with a load that has a tor*ue curve that increases with speed, the motor will operate at the speed where the tor*ue developed by the motor is e*ual to the load tor*ue. 1educing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor ttor*ue or*ue are e*ual. Hperated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. &he speed regulation and net efficiency is also very poor.
edit Tor;ue motor +edit Main article: article: Torue motor
A tor*ue tor*ue motor is a sp speciali%ed eciali%ed form of of electric motor w which hich can operate indefinitely w while hile stalled, that is, with the rotor blocked from tturning, urning, without incurring damage damage.. In this mode of operation, the motor will apply a steady tor*ue to the load (hence the name!. A common common applica application tion of a tor*ue mo motor tor would be the ssupply$ upply$ and take$up take$up reel motors iin na tape drive. In this t his application, driven from a low voltage, the characteristic characteristicss of these motors allow a relatively constant light tension to be applied to the tape whether or not the capstan is feeding tape past the t he tape heads. Driven from a higher voltage, (and so delivering a higher tor*ue!, the tor*ue motors can also achieve fast$forward and rewind operation without re*uiring any additional mechanics mechanics such as gears or clutches. In the computer gaming world, tor*ue motors are used in force feedback steering wheels. Another common application application iiss the control of the thro throttle ttle of an internal combustion en engine gine in con5unction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. &he latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small ad5ustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more tor*ue, pulling against the return spring and opening the throttle. "hould the engine run too fast, the governor will reduce the
current being applied to the motor, causing the return spring to pull back and close the throttle.
Synchronous motor +edit Main article: article: Synchronous motor
A synchronous synchronous ele electric ctric motor is an A motor d distinguished istinguished by a rotor spinni spinning ng with coils passing magnets at the same rate as the A and resulting magnetic field which drives it. Another way of saying saying this is that it has has %ero slip under under usual opera operating ting condition conditions. s. ontrast this with an induction motor, which must slip to produce tor*ue. Hne type of synchronous motor is like an induction motor e7cept the rotor is e7cited by a D field. f ield. "lip rings and brushes are used to conduct current to the rotor. &he rotor poles connect to each other and move at the same speed hence the name synchronous motor. Another type, for low load tor*ue, has flats ground onto a conventional s*uirrel$cage rotor to create discrete poles. Qet another, such as made by -ammond for its pre$3orld 3ar II clocks, and in the older -ammond organs, has no rotor windings and discrete poles. It is not self$starting. &he clock re*uires manual starting by a small knob on the back, while the older -ammond organs had an au7iliary starting motor connected by a spring$load spring$loaded ed manually operated switch. Finally, hysteresis synchronous motors typically are (essentially! two$phase motors with a phase$shifting capacitor capacitor for one phase. &hey start like induction motors, but when slip rate decreases sufficiently, sufficiently, the rotor (a smooth ccylinder! ylinder! becomes temporarily magneti%ed magneti%ed.. Its distributed poles make it act like a 80"0. &he rotor material, like that of a common nail, will stay magneti%ed, but can also be demagneti%ed with little difficulty. difficulty. Hnce running, the rotor poles stay in placeL they do not drift. <ow$power synchronous synchronous timing motors (such as those for traditional electric clocks! may have multi$pole 80 e7ternal cup rotors, and use shading coils to provide starting tor*ue. Telechron clock motors have shaded poles for starting tor*ue, and a two$spoke ring rotor that performs like a discrete two$pole rotor.
+
Douly fed electric machine edit Main article: article: $ouly fed electric machine
Doubly fed electric motors 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. &wo independent multiphase winding sets (i.e., dual armature! are the ma7imum provided in a single package without topology duplication. duplication. Doubly fed electric motors are machines with an effective constant tor*ue speed range that is twice synchronous speed for a given fre*uency of e7citation. &his is twice the constant tor*ue t or*ue speed range as as singly fed electric machines, machines, which have only one active winding set.
A doubly doubly fed motor al allows lows for a small smaller er electronic con converter verter but the cos costt of the rotor windin winding g and slip rings may offset the saving in the power electronics componen components. ts. Difficulties with +:: controlling speed near synchronou synchronouss speed limit applications.
"pecial magnetic motors+edit Rotary+edit 8ydraulic cylinder displacement +edit edit +:;+:= := )lectric motors are replacing hydraulic cylinders in airplanes and military e*uipment.+:;+
2ronless or coreless rotor motor +edit edit
A 0iniature oreless 0otor
?othing in the principle of any of the motors described above re*uires that the iron (steel! portions of the rotor actually rotate. If the soft magnetic material of the rotor is made in the form of a cylinder cylinder,, then (e7cept for tthe he effect of hysteresis! tor*ue is e7erted only on the windings of the electromagnets. &aking advantage of this fact is the coreless or ironless $! motor , a speciali%ed form of a 80 D motor .+: Hptimi%ed for rapid acceleration acceleration,, these motors have a rotor that is constructed without any iron core. &he rotor can take the form of a winding$filled cylinder, or a self$supporting structure comprising only the magnet wire and the bonding material. &he rotor can fit inside the stator magnetsL a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flu7. A second arrangement arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the housing for the motor, and likewise provides a return path for the flu7. 6ecause the rotor is much lighter in weight (mass! than a conventiona conventionall rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical mechanical time constant constant under under one ms. &his is especially true if the windings use aluminum rather than the heavier copper. 6ut because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air. Hverheating might be an issue for coreless D motor designs. Among these types types are the disc$ro disc$rotor tor types, describe described d in more detail in in the ne7t sectio section. n.
Gibrator motors for cellular phones are sometimes tiny cylindrical 80 field types, but there are also disc$shaped types which have a thin multipolar disc field magnet, and an intentionally unbalanced unbalanced molded$plastic rotor structure with two bonded coreless coils. 0etal brushes and a flat commutator switch power to the rotor coils. 1elated limited$travel actuators have no core and a bonded coil placed between the poles of high$flu7 thin 80s. &hese are the fast head positioners for rigid$disk (Chard diskC! drives. Although the contemporary contemporary desig design n differs consi considerably derably from tha thatt of loudspeakers loudspeakers,, it is still loosely (and incorrectly! referred to as a Cvoice coilC structure, because some earlier rigid$ disk$drive heads moved in straight straight lines, and had a drive structure much like that of a loudspeaker. /anca<e or a"ial rotor motor +edit edit
A rather rather unusual mo motor tor design, the prin printed ted armature or pancake pancake motor ha hass the winding windingss shaped as a disc running between arrays of high$flu7 magnets. &he magnets are arranged in a circle facing the rotor with space in between to form an a7ial air gap. +:> &his design is commonly known as the pancake motor because of its e7tremely flat profile, although the technology has had many brand names since its inception, such as "ervoDisc. &he printed armature (originally formed on a printed circuit board! in a printed armature motor is made from punched copper sheets that are laminated together using advanced composites to form a thin rigid disc. &he printed armature has a uni*ue construction in the brushed motor world in that it does not have a separate ring commutator. &he brushes run directly on the armature surface making the whole design very compact. An alternative manufacturing manufacturing me method thod is to use w wound ound copper wi wire re laid flat with a ce central ntral conventional commutator, in a flower and petal shape. &he windings are typically stabili%ed by being impregnated with electrical epo7y potting systems. &hese are filled epo7ies that have moderate mi7ed viscosity and a long gel time. &hey are highlighted by low shrinkage and low e7otherm, and are typically 9< 22: recogni%ed as a potting compound insulated with =R, lass - rating. &he uni*ue advantage of ironless D motors is that there is no cogging (tor*ue cogging (tor*ue variations caused by changing attraction between the iron and the magnets!. 8arasitic eddy currents cannot form in the rotor as it is totally ironless, although iron rotors are laminated laminated.. &his can greatly improve efficiency, but variable$speed controllers must use a higher switching rate (S2 k-%! or D because of the decreased decreased electromagnetic induction. induction. &hese motors were originally invented to drive the capstan(s! of magnetic tape drives in the burgeoning computer industry, where minimal time to reach operating speed and minimal stopping distance were critical. 8ancake motors are still widely used in high$performance servo$controlled servo$controll ed 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. Ser+o motor +edit edit
Main article: article: Ser#o motor
A servomotor servomotor is a mo motor, tor, very often sold as a comp complete lete module, module, which is used w within ithin a position$control position$con trol or speed$contro speed$controll feedback control system mainly control valves, such as motor operated control valves. "ervomotors are used in applications such as machine tools, pen plotters, and other process systems. 0otors intended for use in a servomechanism must have well$documented characteristics for speed, tor*ue, and power. &he speed vs. tor*ue curve is *uite important and is high ratio for a servo motor. Dynamic response characteristics such as winding inductance and rotor inertia are also importantL these factors limit the overall performance of the servomechani servomechanism sm loop. <arge, powerful, but slow$responding servo loops may use conventional A or D motors and drive systems with position or speed feedback on the motor. As dynamic response re*uirements increase, more speciali%ed motor designs such as coreless motors are used. A motors' superior power density and acceleration acceleration characteristics compared compared to that of D motors tends to favor 80 synchronous, 6<D, induction, and "10 drive applications.+:> A servo servo system diffe differs rs from some steppe stepperr motor applica applications tions in that the po position sition feedback iiss continuous while the motor is runningL a stepper system relies on the motor not to Cmiss stepsC for short term accuracy, although a stepper system may include a ChomeC switch or other element to provide long$term stability of control.+; For instance, when a typical dot matri7 computer printer starts up, its controller makes the print head stepper motor drive to its left$hand limit, where a position sensor defines home position and stops stepping. As long as power is on, a bidirectional counter in the printer's microprocessor keeps track of print$ head position. Stepper motor +edit edit Main article: article: Steer motor
A stepper motor with a soft iron rotor, rotor, with active windings shown. In 'A' the active windings tend to hold the rotor in position. In '6' a different set of windings are carrying a current, which generates tor*ue and rotation.
"tepper motors are a type of motor fre*uently used when precise rotations are re*uired. In a stepper motor an internal rotor containing 80s or a magnetically soft rotor with salient poles is controlled by a set of e7ternal magnets that are switched electronically. A stepper motor may also be thought of as a cross between a D electric motor and a rotary solenoid. A Ass each coil is energi%ed in turn, t urn, the rotor aligns itself with the magnetic field produced by the
energi%ed field winding. 9nlike a synchronou s ynchronouss motor, in its application application,, the stepper motor may not rotate continuouslyL instead, it CstepsCTstarts and then *uickly stops againTfrom one position to the ne7t as field windings are energi%ed and de$energi%ed in se*uence. Depending on the se*uence, the rotor may turn t urn forwards or backwards, and it may change direction, stop, speed up or slow down arbitrarily at any time. "imple stepper motor drivers entirely energi%e or entirely de$energi%e the field windings, leading the rotor to CcogC to a limited number of positionsL more sophisticated drivers can proportionally proportional ly control the power to the field windings, allowing the rotors to position between the cog points and thereby rotate e7tremely smoothly. &his mode of operation is often microstepping.. omputer controlled stepper motors are one of the most versatile called microstepping servo$controlled system. system. forms of positioning systems, particularly when part of a digital digitalservo$controlled "tepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for readMwrite head positioning in computer floppy diskette drives. &hey were used for the t he same purpose in pre$gigabyte era computer disk drives, where the precision and speed they offered was ade*uate for the correct positioning of the readMwrite head of a hard disk drive. As drive density increased, the precision and speed limitations of stepper motors made them obsolete for hard drivesTthe precision limitation limitation made them unusable, and the speed limitation made them uncompetitiveTthus newer hard disk drives use voice coil$based head actuator systems. (&he term Cvoice coilC in this connection is historicL it refers to the structure in a typical (cone type! loudspeaker. loudspeaker. &his structure was used for a while to position the heads. 0odern drives have a pivoted coil mountL the coil swings back and forth, something like a blade of a rotating fan. ?evertheless, like a voice coil, modern actuator coil conductors (the magnet wire! move perpendicu perpendicular lar to the magnetic lines of force.! "tepper motors were and still are often used in computer printers, optical scanners, and digital photocopiers to move the optical scanning element, the print head carriage (of dot matri7 and ink5et printers!, and the platen or feed rollers. <ikewise, many computer plotters (whichrotary sincestepper the early >>sforhave replaced with large$format ink5et and laserhere printers! used motors penbeen and platen movementL the typical alternatives were either linear stepper motors or servomotors with closed$loop analog control systems. s ystems. "o$called *uart% analog wristwatches contain the smallest commonplac commonplace e stepping motorsL they have one coil, draw very little power, and have a 80 rotor. &he same kind of motor drives battery$powered battery$powered *uart% clocks. "ome of these watches, such as chronograp chronographs, hs, contain more than one stepping motor. losely related in design to three$phase A synchronous motors, stepper motors and "10s are classified as variable reluctance motor type.+; "tepper motors were and still are often used in computer printers, optical scanners, and computer numerical control (?! machines (?! machines such as routers, plasma cutters and ? lathes.
1inear motor +edit
Main article: article: *inear motor
A linear linear motor is ess essentially entially any electric electric motor that ha hass been Cunroll CunrolledC edC so that, instead o off producing a a tor*ue tor*ue (rotation!, (rotation!, it produces a straight$line force along its length. motors or or stepper motors. <inear motors are <inear motors are most commonly commonly induction motors commonly found in many roller$coasters where the rapid motion of the motorless railcar is trains,, where the train CfliesC over the controlled by the rail. &hey are also used in in maglev trains ground. Hn a smaller scale, the >;= era -8 ;4A pen plotter used two linear stepper motors to move the pen along the U and Q a7es. +;
omparison by ma5or categories+edit Comparison of motor types
Type
Ad+antages
Disad+antages
Typical application
Typical dri+e output
Self!commutated Self!commutate d motors
6rushed D
"teel mills 8aper 0aintenance making 1ectifier, linear (brushes! machines "imple speed transistor(s! or D 0edium lifespan &readmill control chopper controller .+;/ ostly commutator e7ercisers and brushes Automotive accessories
6rushless D motor (6<D! or (6<D0!
<ong lifespan <ow maintenance -igh efficiency
-igher initial cost 1e*uires ) controller with closed$loop closed$loo p control
1igid (ChardC! disk drives DMDGD players )lectric vehicles 1 Gehicles 9AGs
"ynchronousL single$ "ynchronousL phase or three$phase with 80 rotor and trape%oidall stator trape%oida windingL GFD typically G" G" 830 830 inver inver ;2 +:>+;/+;2 ter type.+:>+;/+
"witched reluctance motor ("10!
9niversal motor
<ong lifespan <ow maintenance -igh efficiency ?o permanent magnets <ow cost "imple construction
0echanical resonan ce possible Appliances 830 and various other -igh iron losses )lectric drive types, which tend ?ot possibleE Gehicles to be used in very V Hpen or vector &e7tile &e7tile m mills ills speciali%ed M M H)0 H)0 appli appli control Aircraft +;+;4 cations. +;+;4 V 8arallel operation applications 1e*uires ) controller +;
0aintenance (brushes! -igh starting "horter lifespan tor*ue, 9sually acoustically compact, high noisy speed. Hnly small ratings are economical
-andheld Gariable single phase power tools, A, half$wave half$wave or full$ blenders, wave phase$angle vacuum control with triac(s!L cleaners, control insulation closed$loop +;/ optional.. optional blowers
AC asynchronous motors
A polyphase polyphase s*uirrel$cage or wound$rotor
"elf$starting <ow cost 1obust 1eliable
induction motor 1atings to W 03 ("I0! "tandardi%ed or types. (31I0!
-igh starting current <ower efficiency due to need for magneti%ation magneti%ation..
Fi7ed$ speed, traditionally, "I0 the
Fi7ed$speed, low performance applications of all types.
world's workhorse especially in low performanc e applications of all types Gariable$ speed, traditionally,
Gariable$speed, traditionally, 31I0 drives or fi7ed$speed GM-%$controlled GM-%$controll ed G"Ds. Gariable$speed, increasingly, vector$ controlled G"Ds controlled G"Ds displacing displacin g D, 31I0 and single$phase A induction motor drives.
low$ performanc
e variable$ tor*ue pumps, fans, blowers and compressor s. Gariable$ speed, increasingly, other high$ performanc e constant$ tor*ue and constant$ power or dynamic loads.
A "I0 -igh power split$phase high starting capacitor$start tor*ue
"peed slightly below Appliances synchronous "tationary "tarting switch or 8ower &ools relay re*uired
0oderate power A "I0 split$phase capacitor$run
-igh starting "peed slightly below tor*ue synchronous ?o starting "lightly more costly switch omparatively long life
A "I0 split$phase, au7iliary
0oderate power <ow starting
"peed slightly below Appliances "tationary synchronous power tools "tarting switch or
start winding
tor*ue
relay re*uired
Industrial blowers Industrial machinery
Fi7ed or variable single$phase A, variable speed being derived, typically, by full$wave phase$angle control with triac(s!L closed$loop control optional..+;/ optional
A inductionshade induction shade <ow cost <ong life d$pole motor
"peed slightly below synchronous <ow starting tor*ue "mall ratings low efficiency
Fans, appliances, record players
AC synchronous motors
3ound$rotor synchronous motor (31"0!
-ysteresis motor
"ynchronous speed Inherently more efficient induction motor, low power factor
Accurate speed control <ow noise ?o vibration -igh starting tor*ue
0ore costly
Industrial motors
Fi7ed or variable speed, three$phaseL GFD typically si7$ step" step " load$ load$ commutated inverter type or G" G" 830 +;/+;2 +;/+;2
inverter type. type.
Gery low efficiency
locks, timers, sound "ingle$phase A, two$ producing phase capacitor$start, or recording capacitor run motor +;: e*uipment, +;; hard drive, capstan drive
"ynchronous reluctance motor ("y10!
)*uivalent to )*uivalent "I0 e7cept more robust, more efficient, runs cooler, smaller footprint ompetes with 80 synchronous
1e*uires a controller ?ot widely available -igh cost
Appliances )lectric vehicles &e7tile &e7tile m mills ills Aircraft applications
GFD can be D& type type standard D& G" inverter inverter 830 or G" type.+;=
motor without demagneti%ati on issues
Speciality motors
8ancake or a7ial rotor moto mo tors rs +:>
ompact design 0edium cost "imple speed 0edium lifespan control
Hffice )*uip FansM8ump Drives can typically be s, fast brushed or brushless industrial D type.+:> and military servos
8ositioning "tepper motor
8recision positioning -igh holding tor*ue
in printers and floppy ?ot a GFD. "tepper "ome can be costly disc drivesL position is determined 1e*uire a controller +;>+= = by pulse counting. counting.+;>+ industrial machine tools
)lectromagnetism +edit &his section re*uires re*uires e7pansion. e7pansion. (March +,.)
:orce and tor;ue+edit &he fundamental purpose of the vast ma5ority of the world's electric motors is to electromagnetically electromagne tically induce relative movement in an air gap between a stator and rotor to produce useful tor*ue or linear force. According <orent% force law law the the force of a winding conductor can be given simply byE
or more generally, to handle conductors with any geometryE
&he most general approaches to calculating the forces in motors use tensors.+=
/ower +edit 3here rpm is rpm is shaft speed and & is is tor*ue tor*ue,, a motor's mechanical power output 8em is given by, by,+= in 6ritish units with & e7pressed in foot$pounds, (horsepower!, and,
in "I units with units with shaft speed e7pressed in radians per second, and & e7pressed in newton$meters, (watts!.
For a linear motor, with force F and velocity v e7pressed in newtons and meters per second, (watts!.
In an asynchronous or induction motor, the relationship between motor speed and air gap power is, neglecting neglecting skin effect effect,, given by the followingE , where
1r $ $ rotor resistance Ir $ s*uare of current induced in the rotor s $ motor slipL ie, difference between synchronous speed and slip speed, which provides the relative movement needed for current induction in the rotor.
0ac< emf +edit Main article: article: Electromoti#e force
"ince the armature windings of a direct$curren direct$currentt motor are moving through a magnetic field, they have a voltage induced in them. &his voltage tends to oppose the motor supply voltage and so is called (emf!C. C. &he voltage is proportional to the Cback electromotive force (emf! running speed of the motor. &he back emf of the motor, plus the voltage drop across the t he winding internal resistance and brushes, must e*ual the voltage at the brushes. &his provides the fundamental mechanism of speed regulation in a D motor. If the mechanical load increases, increases, the motor slows downL a lower back emf results, and more current is drawn from the supply. &his increased +=/
current provides the additional tor*ue to balance the new load.
In A machines, it is sometimes useful to consider a back emf source within the machineL this is of particular concern for close speed regulation of induction motors on GFDs, for e7ample.+=/
1osses+edit 0otor losses are mainly due to resistive losses in losses in windings, core losses and mechanical losses in bearings, and aerodynamic losses, particularly where cooling fans are present, also occur. <osses also occur in commutation, mechanical commutators spark, and electronic commutators and also dissipate heat.
Efficiency+edit &o calculate a motor's efficiency, the mechanical output power is divided by the electrical input powerE , where
is energy conversion efficiency, efficiency,
electrical input power, and
where current,
is
is mechanical output powerE
is input voltage, is output tor*ue, and
is input is output
angular velocity. It is possible to derive analytically the point of ma7imum efficiency. It is typically at less than M tor*ue..+citation needed the stall tor*ue Garious regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher efficiency electric motors. &here is e7isting and forthcoming legislation regarding the future mandatory use of premium$efficiency induction$type motors in defined e*uipment. /or more information0 see:&remium see:&remium efficiency and and !oer in energy efficient motors1 motors1
=oodness factor +edit Main article: 2oodness factor
8rofessor )ric <ait <aithwait hwaite e+=2 proposed a metric to determine the 'goodness' of an electric motorE +=4
3hereE is the goodness factor (factors above are likely to be efficient! are the cross sections of the magnetic and electric circuit are the lengths of the magnetic and electric circuits is the permeability of the core is the angular fre*uency the motor is driven at
From this, he showed that the most efficient motors are likely to have relatively large magnetic poles. -owever, the e*uation only directly relates to non 80 motors.
8erformance parameters+edit Tor;ue capaility of motor types+edit &his se
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3hen optimally designed within a given core saturation constraint and for a given active current (i.e., tor*ue current!, voltage, pole$pair number, e7citation fre*uency (i.e., synchronous speed!, and air$gap flu7 density, all categories of electric motors or generators will e7hibit virtually the same ma7imum continuous shaft tor*ue (i.e., operating tor*ue! within a given air$ gap area with winding slots and back$iron depth, which determines the physical si%e of electromagnetic electromagnetic core. "ome applications re*uire bursts of tor*ue beyond the ma7imum operating tor*ue, such as short bursts of tor*ue to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or saturation or safeoperating safeoperating temperature rise temperature rise and voltage, the capacity for tor*ue bursts beyond the ma7imum operating tor*ue differs significantly between categories of electric motors or generators. apacity for bursts of tor*ue should not be confused with field weakening capability. Field weakening allows an electric machine to operate beyond the designed fre*uency of e7citation. )lectric machines without a transformer circuit topology, such as that of 31"0s or 80"0s, cannot reali%e bursts of tor*ue higher than the ma7imum designed tor*ue without saturating the magnetic core and rendering any increase in current as useless. Furthermore, the 80 assembly of 80"0s can be irreparably damaged, damaged, if bursts of
tor*ue e7ceeding the ma7imum operating tor*ue rating are attempted. )lectric machines with a transformer circuit topology, such as induction machines, induction doubly fed electric machines, and induction or synchronous wound$rotor doubly fed (31DF! machines, e7hibit very high bursts of tor*ue because the emf$ induced active current on either side of the transformer t ransformer oppose each other and thus contribute nothing to the transformer coupled magnetic core flu7 density, which would otherwise lead to core saturation. )lectric machines that rely on induction or asynchronou asynchronouss principles short$circuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active (i.e., real! current. "till, bursts of tor*ue that are two to three times higher than the ma7imum design tor*ue are reali%able. wound$rotorr &he brushless wound$roto synchonous doubly fed (631"DF! machine (631"DF! machine is the only electric machine with a truly dual ported transformer circuit topology (i.e., both ports independently e7cited with no short$circuited port!. +=: &he dual ported transformer circuit topology is known to be unstable and re*uires a multiphase slip$ring$bru slip$ring$brush sh assembly to propagate limited power to the rotor winding set. If a precision
means were available to instantaneously control tor*ue angle and slip for f or synchronous operation during motoring or generating while simultaneouslyy providin simultaneousl providing g brushless power to the rotor winding set, the active current of the 631"DF machine would be independent of the reactive impedance of the transformer circuit and bursts of tor*ue significantly higher than the ma7imum operating tor*ue and far beyond the practical capability of any other type of electric machine would be reali%able. &or*ue bursts greater than eight times operating tor*ue have been calculated calculated..
Continuous tor;ue density+edit &he continuous tor*ue density of conventional electric machines is determined by the si%e 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$gap flu7 density before core saturation. Despite the high coercivity of neodymium or samarium$cobalt samarium$cob alt 80s, continuous tor*ue density is virtually the same amongst electric machines with optimally designed armature winding sets. ontinuous tor*ue density relates to method of cooling and permissible period of operation before destruction by overheating of windings or 80 damage.
Continuous power density+edit
&he continuous power density is determined by the product of the continuous tor*ue density and the constant tor*ue speed range of the electric machine.
"tandards+edit
&he following are ma5or design and manufacturing standards covering electric motorsE •
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International )lectrotechnic )lectrotechnical al ommissionEE I) :/2 1otating ommission )lectrical 0achines ?ational )lectrical 0anufacturers AssociationEE 0#$ 0otors and Association #enerators
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9nderwriters <aboratoriesE <aboratoriesE 9< 2 $ "tandard for )lectric 0otors
?on$magnetic motors+edit Main articles: Electrostatic motor 0 &ie3oelectric motor 0 and Electrically o'ered sacecraft roulsion
An electrostatic motor motor is based on the attraction and repulsion of electric charge. 9sually, electrostatic motors are the dual of conventional coil$based motors. &hey typically re*uire a high voltage power supply, although very small motors employ lower voltages. onventional electric motors instead employ magnetic attraction and repulsion, and re*uire high current at low voltages.motors In the ;4s, the first electrostatic
were developed by 6en5amin Franklin and Andrew #ordon. &oday the electrostatic motor finds fre*uent use in micro$electro$m micro$electro$mechanical echanical systems (0)0" (0)0"!! where their drive voltages are below volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. Also, the molecular machinery which runs living cells is often based on linear and rotary electrostatic motors.+citation needed A pie%oelectric pie%oelectric moto motorr or pie%o motor is a type of electric motor based upon the change in shape of material when a pie%oelectric material an electric field field is applied. 8ie%oelectric motors make use of the converse pie%oelectric effect whereby the material produces acoustic or ultrasonic ultrasonic vibrations in order to produce a linear or rotary motion. In one mechanism, the elongation in a single plane is used to make a series stretches and position holds, similar to the way a caterpillar moves.+citation needed An electrically powered powered space spacecraft craft propulsion system uses electric motor technology to propel spacecraft in outer space, most systems being based on electrically powering propellant to high speed, with some systems being based electrodynamicc tethers tethers principles principles on electrodynami of propulsion to the magnetosphe magnetosphere re
