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Electric MotorAn Electric Motoris an electromechanical device that converts electricalenergy into mechanical energy.

The interaction between an electric motor's magnetic field andwindingcurrents generates force and useful in the motor. The reverse process, producingelectrical energy from mechanical energy, is done by generators such asan alternatoror a dynamo; some electric motors can also be used as generators,for example, a traction motoron a vehicle may perform both tasks. Electricmachines are traditionally understood to includetransformers as well as electricmotors and generators.

Found in applications as diverse as industrial fans, blowers andpumps, machinetools, household appliances, power tools, and disk drives, electric motors can bepowered by direct current sources, such as from batteries, motor vehicles

orrectifiers, or by alternating current sources, such as from the powergrid, inverters or generators. Small motors may be found in electric wristwatches.General-purpose motors with highly standardized dimensions and characteristicsprovide convenient mechanical power for industrial use. The largest of electricmotors are used for propulsion of ships, pipeline compressors, andwaterpumps with ratings approaching a million watts. Electric motors may be classifiedby electric power source type, internal construction, application, type of motionoutput, and so on.

The principle behind production of mechanical force by the interactions of anelectric current and a magnetic field,Ampre's force law, was discovered

byAndr-Marie Ampre in 1820. Although electric motors were graduallyimproved from 1821 through the end of the 19th century, commercial exploitationof electric motors on a large scale required efficient electricalgenerators and electrical distribution networks. The first commercially successfulmotors, DC motors, were made around 1873 by Znobe Gramme.[1]

Devices such as magneticsolenoids and loudspeakers that convert electricityinto motion but do not generate usable mechanical power are respectivelyreferred to as actuators and transducers. Electric motors are used to producerotary or linear torque or force.
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Background

Faraday's electromagnetic experiment, 1821[3]

The conversion of electrical energy into mechanical energyby electromagnetic means was demonstrated by the British scientist Michael

Faraday in 1821. A free-hanging wire was dipped into a pool ofmercury, onwhich a permanent magnet was placed. When a current was passed through thewire, the wire rotated around the magnet, showing that the current gave rise to aclose circular magnetic field around the wire. [4] This motor is often demonstratedin school physics classes, but brine (salt water) is sometimes used in place of thetoxic mercury. A later refinement is the Barlow's wheel. These weredemonstration devices only, unsuited to practical applications due to theirprimitive construction.[citation needed] These are the simplest forms of a class ofdevices called homopolar motors.

Jedlik's "electromagnetic self-rotor", 1827 (Museum of Applied Arts, Budapest.

The historic motor still works perfectly today. [5]

In 1827, Hungarianphysicistnyos Jedlikstarted experimenting

with electromagnetic coils. After Jedlik solved the technical problems of thecontinuous rotation with the invention of commutator, he called his early devicesas "electromagnetic self-rotors". Although they were used only for instructionalpurposes, in 1828 Jedlik demonstrated the first device to contain the three maincomponents of practical direct current motors: the stator,rotorandcommutator.The device employed no permanent magnets, as the magnetic fields of both the
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stationary and revolving components were produced solely by the currentsflowing through their windings.

The first electric motors

The first commutator-type direct current electric motor capable of turningmachinery was invented by the British scientist William Sturgeon in 1832.[13]Following Sturgeon's work, a commutator-type direct-current electric motormade with the intention of commercial use was built by

Americans Emily andThomas Davenport and patented in 1837. Their motors ranat up to 600 revolutions per minute, and powered machine tools and a printingpress.[14] Due to the high cost of the zinc electrodes required by primary batterypower, the motors were commercially unsuccessful and the Davenports wentbankrupt. Several inventors followed Sturgeon in the development of DC motorsbut all encountered the same cost issues with primary battery power. Noelectricity distribution had been developed at the time. Like Sturgeon's motor,there was no practical commercial market for these motors.[15]

In 1855 Jedlik built a device using similar principles to those used in hiselectromagnetic self-rotors that was capable of useful work. [6][12] He built amodelelectric motor-propelled vehicle that same year.[16]

The modern DC motor was invented by accident in 1873, when ZnobeGramme connected the dynamo he had invented to a second similar unit, drivingit as a motor. The Gramme machinewas the first electric motor that wassuccessful in the industry.[1]

In 1886 Frank Julian Sprague invented the first practical DC motor, a non-sparking motor that maintained relatively constant speed under variable loads.Other Sprague electric inventions about this time greatly improved grid electric

distribution (prior work done while employed by Thomas Edison), allowed powerfrom electric motors to be returned to the electric grid, provided for electricdistribution to trolleys via overhead wires and the trolley pole, and providedcontrols systems for electric operations. This allowed Sprague to use electricmotors to invent the first electric trolley system in 188788 in Richmond VA, theelectric elevator and control system in 1892, and the electric subway withindependently powered centrally controlled cars, which were first installed in1892 in Chicago by the South Side Elevated Railway where it became popularlyknown as the "L". Sprague's motor and related inventions led to an explosion ofinterest and use in electric motors for industry, while almost simultaneouslyanother great inventor was developing its primary competitor, which wouldbecome much more widespread. The development of electric motors ofacceptable efficiency was delayed for several decades by failure to recognize theextreme importance of a relatively small air gap between rotor and stator.Efficient designs have a comparatively small air gap. [17] The St. Louis motor, longused in classrooms to illustrate motor principles, is extremely inefficient for thesame reason, as well as appearing nothing like a modern motor.
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Terminology

In an electric motor the moving part is called the rotorand the stationary part iscalled the stator. Magnetic fields are produced onpoles, and these canbe salient poles where they are driven bywindings of electrical wire. A shaded-

pole motorhas a winding around part of the pole that delays the phase of themagnetic field for that pole.

A commutatorswitches the current flow to the rotor windings depending on therotor angle.

A DC motoris powered by direct current, although there is almost always aninternal mechanism (such as a commutator) converting DC to AC for part of themotor. AnAC motoris supplied withalternating current, often avoiding the needfor a commutator. A synchronous motoris an AC motor that runs at a speed fixedto a fraction of the power supply frequency, and an asynchronous motoris an ACmotor, usually an induction motor, whose speed slows with increasing torque to

slightly less than synchronous speed. Universal motorscan run on either AC orDC, though the maximum frequency of the AC supply may be limited.

Major categories

At least 3 different operating principles are used to make electric motors:magnetic, electrostatic and piezoelectric. By far the most common is magnetic.

Nearly all electric motors are based around magnetism (exceptionsinclude piezoelectric motors and ultrasonic motors). In these motors, magneticfields are formed in both the rotor and the stator. The product between these twofields gives rise to a force, and thus a torque on the motor shaft. One, or both, of

these fields must be made to change with the rotation of the motor. This is doneby switching the poles on and off at the right time, or varying the strength of thepole.

The main types are DC motors and AC motors, although the ongoing trendtoward electronic control somewhat softens the distinction, [citation needed][dubiousdiscuss] as modern drivers have moved the commutator out of the motor shellfor some types of DC motors.

Considering all rotating (or linear) electric motors require synchronism between amoving magnetic field and a moving current sheet for average torque production,there is a clear distinction between an asynchronous

motorand synchronous types. An asynchronous motor requires slip - relativemovement between the magnetic field (generated by the stator) and a windingset (the rotor) to induce current in the rotor by mutual inductance. The mostubiquitous example of asynchronous motors is the common ACinductionmotorwhich must slip to generate torque.
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In the synchronous types, induction (or slip) is not a requisite for magnetic field orcurrent production (e.g. permanent magnet motors, synchronous brush-lesswound-rotor doubly fed electric machine).

Specifying an electric motor

When specifying what type of electric motor is needed, the mechanical power

available at the shaft is used. This means that users can predict the torque andspeed of the motor without having to know the mechanical losses associated withthe motor. Example: 10 kW induction motor.

Power

The power output of a rotary electric motor is:

Where P is in horsepower, rpm is the shaft speed in revolutions per minuteand T is the torque in foot pounds. The equivalent equation with SI-units is:

Where P is the powerin watts, rpm is the shaft speed in revolutions perminute and T is the torque in Nm.[52]

And for a linear motor:

Where P is the power in watts, and F is in Newtons and v is the speedin metres per second.

Efficiency

To calculate a motor's efficiency, the mechanical output power is

divided by the electrical input power: , where is energyconversion efficiency, is electrical input power, and ismechanical output power.

In simplest case , and , where is inputvoltage, is input current, is output torque, and is output angularvelocity. It is possible to derive analytically the point of maximum

efficiency. It is typically at less than 1/2 the stall torque.Torque capability of motor types

When optimally designed within a given core saturation constraint andfor a given active current (i.e., torque current), voltage, pole-pairnumber, excitation frequency (i.e., synchronous speed), and air-gapflux density, all categories of electric motors or generators will exhibitvirtually the same maximum continuous shaft torque (i.e., operating
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torque) within a given air-gap area with winding slots and back-irondepth, which determines the physical size of electromagnetic core.Some applications require bursts of torque beyond the maximumoperating torque, such as short bursts of torque to accelerate anelectric vehicle from standstill. Always limited by magnetic coresaturation or safe operating temperature rise and voltage, the capacityfor torque bursts beyond the maximum operating torque differs

significantly between categories of electric motors or generators.Capacity for bursts of torque should not be confused with fieldweakening capability. Field weakening allows an electric machine tooperate beyond the designed frequency of excitation.

Electric machines without a transformer circuit topology, such asField-Wound (i.e., electromagnet) or Permanent Magnet (PM)Synchronous electric machines cannot realize bursts of torque higherthan the maximum designed torque without saturating the magneticcore and rendering any increase in current as useless. Furthermore,the permanent magnet assembly of PM synchronous electric

machines can be irreparably damaged, if bursts of torque exceedingthe maximum operating torque rating are attempted.

Electric machines with a transformer circuit topology, such asInduction (i.e., asynchronous) electric machines, Induction Doubly Fedelectric machines, and Induction or Synchronous Wound-Rotor DoublyFed (WRDF) electric machines, exhibit very high bursts of torquebecause the active current (i.e., Magneto-Motive-Force or the productof current and winding-turns) induced on either side of the transformeroppose each other and as a result, the active current contributesnothing to the transformer coupled magnetic core flux density, which

would otherwise lead to core saturation.Electric machines that rely on Induction or Asynchronous principlesshort-circuit one port of the transformer circuit and as a result, thereactive impedance of the transformer circuit becomes dominant asslip increases, which limits the magnitude of active (i.e., real) current.Still, bursts of torque that are two to three times higher than themaximum design torque are realizable.

The Synchronous WRDF electric machine is the only electric machinewith a truly dual ported transformer circuit topology (i.e., both portsindependently excited with no short-circuited port). The dual ported

transformer circuit topology is known to be unstable and requires amultiphase slip-ring-brush assembly to propagate limited power to therotor winding set. If a precision means were available toinstantaneously control torque angle and slip for synchronousoperation during motoring or generating while simultaneouslyproviding brushless power to the rotor winding set (see Brushlesswound-rotor doubly fed electric machine), the active current of the
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Synchronous WRDF electric machine would be independent of thereactive impedance of the transformer circuit and bursts of torquesignificantly higher than the maximum operating torque and farbeyond the practical capability of any other type of electric machinewould be realizable. Torque bursts greater than eight times operatingtorque have been calculated.

[edit]Continuous torque density

The continuous torque density of conventional electric machines isdetermined by the size of the air-gap area and the back-iron depth,which are determined by the power rating of the armature winding set,the speed of the machine, and the achievable air-gap flux densitybefore core saturation. Despite the high coercivity of neodymium orsamarium-cobalt permanent magnets, continuous torque density isvirtually the same amongst electric machines with optimally designedarmature winding sets. Continuous torque density should never beconfused with peak torque density, which comes with themanufacturer's chosen method of cooling, which is available to all, or

period of operation before destruction by overheating of windings oreven permanent magnet damage.

Continuous power density

The continuous power density is determined by the product of the continuous

torque density and the constant torque speed range of the electric

machine.tandards

The following are major design and manufacturing standards coveringelectric motors:

International Electrotechnical Commission: IEC 60034Rotating Electrical Machines National Electrical Manufacturers Association: MG-1 Motorsand Generators Underwriters Laboratories: UL 1004 - Standard for ElectricMotors

DC motor

Main article: DC motor

A DC motor is designed to run on DC electric power. Two examples ofpure DC designs are Michael Faraday's homopolar motor(which isuncommon), and the ball bearing motor, which is (so far) a novelty. By
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far the most common DC motor types are the brushed and brushlesstypes, which use internal and external commutation respectively toreverse the current in the windings in synchronism with rotation.

Equivalent circuit

Equivalent circuit of generator and load.

G = generator

VG=generator open-circuit voltage

RG=generator internal resistanceVL=generator on-load voltage

RL=load resistance

The equivalent circuit of a generator and load is shown in the diagram to the right. The

generator's and parameters can be determined by measuring the windingresistance (corrected to operating temperature), and measuring the open-circuit and

loaded voltage for a defined current load.

Vehicle-mounted generators

Early motor vehicles until about the 1960s tended to use DC generators withelectromechanical regulators. These have now been replaced byalternators withbuilt-in rectifiercircuits, which are less costly and lighter for equivalent output.Moreover, the power output of a DC generator is proportional to rotational speed,whereas the power output of an alternator is independent of rotational speed. Asa result, the charging output of an alternator at engine idle speed can be muchgreater than that of a DC generator. Automotive alternators power the electricalsystems on the vehicle and recharge the battery after starting. Rated output willtypically be in the range 50-100 A at 12 V, depending on the designed electricalload within the vehicle. Some cars now have electrically powered steeringassistance and air conditioning, which places a high load on the electricalsystem. Large commercial vehicles are more likely to use 24 V to give sufficientpower at the starter motorto turn over a large diesel engine. Vehicle alternatorsdo not use permanent magnets and are typically only 50-60% efficient over awide speed range.[10] Motorcycle alternators often use permanentmagnet statorsmade with rare earth magnets, since they can be made smallerand lighter than other types. See also hybrid vehicle.
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A magneto, like a dynamo, uses permanent magnets but generates alternatingcurrent like an alternator. Because of the limited field strength of permanentmagnets, magnetos are not used for high-power production applications, but arevery reliable. This reliability is part of why they are used in aviation pistonengines.

Some of the smallest generators commonly found powerbicycle lights. Calleda bottle dynamo these tend to be 0.5 ampere, permanent-magnet alternatorssupplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at apremium, so these may incorporate rare-earth magnets and are designed andmanufactured with great precision. Nevertheless, the maximum efficiency is onlyaround 80% for the best of these generators60% is more typicaldue in partto the rolling friction at the tiregenerator interface from poor alignment, the smallsize of the generator, bearing losses and cheap design. The use of permanentmagnets means that efficiency falls even further at high speeds because themagnetic field strength cannot be controlled in any way. Hub dynamos remedymany of these flaws since they are internal to the bicycle hub and do not requirean interface between the generator and tire. Until recently, these generators have

been expensive and hard to find. Major bicycle component manufacturerslike Shimano andSRAM have only just[when?] entered this market. However,significant gains can be expected in future as cycling becomes more mainstreamtransportation and LED technology allows brighter lighting at the reduced currentthese generators are capable of providing.

Sailing yachts may use a water or wind powered generator to trickle-charge thebatteries. A small propeller, wind turbine orimpelleris connected to a low-poweralternator and rectifier to supply currents of up to 12 A at typical cruising speeds.

Still smaller generators are used in micro powerapplications.

Engine-generator

Main article: Engine-generator

The Caterpillar 3512C Genset is an example of the engine-generator package.

This unit produces 1225 kilowatts of electric power.

An engine-generatoris the combination of an electrical generator andan engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gasturbines can also be used. And there are even hybrid diesel-gas units, called
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dual-fuel units. Many different versions of engine-generators are available -ranging from very small portable petrol powered sets to large turbineinstallations. The primary advantage of engine-generators is the ability toindependently supply electricity, allowing the units to serve as backup powersolutions.

Human powered electrical generate

Protesters at Occupy Wall Street using bicycles connected to a motor and one-

way diode to charge batteries for their electronics[12]

A generator can also be driven by human muscle power (for instance, in fieldradio station equipment).

Human powered direct current generators are commercially available, and havebeen the project of some DIY enthusiasts. Typically operated by means of pedalpower, a converted bicycle trainer, or a foot pump, such generators can be

practically used to charge batteries, and in some cases are designed with anintegral inverter. The average adult could generate about 125-200 watts on apedal powered generator, but at a power of 200 W, a typical healthy human willreach complete exhaustion and fail to produce any more power afterapproximately 1.3 hours.[13] Portable radio receivers with a crank are made toreduce battery purchase requirements, see clockwork radio. During the mid 20thcentury, pedal powered radios were used throughout theAustralian outback, toprovide schooling (School of the Air), medical and other needs in remote stationsand towns.

Linear electric generator

Main article: Linear alternator

In the simplest form of linear electric generator, a slidingmagnetmoves back and forththrough asolenoid- a spool of copper wire. Analternating currentis induced in theloops of wire byFaraday's law of inductioneach time the magnet slides through. Thistype of generator is used in theFaraday flashlight. Larger linear electricity generatorsare used inwave powerschemes.

Tachogenerator

Tachogenerators are frequently used to powertachometersto measure the speeds ofelectric motors, engines, and the equipment they power. Generators generate voltage

roughly proportional to shaft speed. With precise construction and design, generators can

be built to produce very precise voltages for certain ranges of shaft speeds.
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BIBLIOGRAPHY

Fink, Donald G.; Beaty, H. Wayne, Standard Handbook for Electrical

Engineers, `14th ed., McGraw-Hill, 1999, ISBN 0070220050.

Houston, Edwin J.; Kennelly, Arthur, Recent Types of Dynamo-ElectricMachinery, American Technical Book Company 1897, published by P.F.

Collier and Sons New York, 1902

Kuphaldt, Tony R. (20002006). "Chapter 13 AC MOTORS". Lessons In

Electric CircuitsVolume II. Retrieved 2006-04-11.

Rosenblatt, Jack; Friedman, M. Harold, Direct and Alternating Current

Machinery, 2nd ed., McGraw-Hill, 1963
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