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Robotic Arm

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 A robotic arm is a robot  manipulator , usuallyprogrammable, with similar functions to a human arm.The links of such a manipulator are connected by joints

allowing either rotational motion (such as in anarticulated robot) or translational (linear) displacement.The links of the manipulator can be considered to forma kinematic chain. The business end of the kinematicchain of the manipulator is called the end effector andit is analogous to the human hand. The end effector can be designed to perform any desired task such aswelding, gripping, spinning etc., depending on the

application. For example robot arms in automotive assembly lines perform a variety of tasks such aswelding and parts rotation and placement duringassembly.

In space the Space Shuttle Remote Manipulator System also known as Canadarm or  SRMS and its

successor Canadarm are examples of multi degree of  freedom robotic arms that have been used to perform avariety of tasks such as inspections of the SpaceShuttle using a specially deployed boom with camerasand sensors attached at the end effector and satellite deployment and retrieval manoeuvres from the cargo bay of the Space Shuttle.

The robot arms can be autonomous or controlledmanually and can be used to perform a variety of taskswith great accuracy.

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The robotic arm can be fixed or mobile (i.e. wheeled)and can be designed for  industrial or homeapplications.

The European Robotic Arm (ERA) is a robotic arm tobe attached to the Russian Segment of theInternational Space Station. It will be the first robot-armthat is able to work on Russian space station segments

and is an additional robotic system to the two RussianStrela cargo cranes that are already installed on Pirs.

 

Major features and tasks

The intelligent space robot has several interesting

features. Most prominent are its ability to 'walk' aroundthe exterior of the station under its own control, hand-over-hand between pre-fixed basepoints and its abilityto perform many tasks automatically or semi-automatically, thereby freeing its operators to do other work. Specific tasks of ERA include:

• Installation and deployment of solar arrays• Replacement of solar arrays• Inspection of the station• Handling of (external) payloads• Support of astronauts during space walks 

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The International Space Station already features onerobotic arm, the Canadarm2 but because of thedifferent types of basepoints and payload mounting

units that arm cannot be used on the Russian part of the ISS. The European arm is smaller, and lesspowerful than Canadarm2. There is no "hand" or Special Purpose Dexterous Manipulator planned for it.

Developed for the European Space Agency (ESA) bythe European space industry with Dutch Space asprime contractor and subcontractors in 8 countries, the

robot arm will be launched by a Russian Proton rocket to be put to work in space by the ISS crew. During thelaunch, ERA is attached to the MultipurposeLaboratory Module (MLM). This Russian module willalso serve as home base for ERA during operationswith the robot arm. Originally, it was going to beattached to the Science Power Platform.

Control of ERA

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Control and data interfaces of ERA

 Astronauts can control the robot from both inside aswell as outside the space station. Control from insidethe space station (Intra Vehicular Activity-Man MachineInterface (IVA-MMI)) uses a laptop which shows amodel of the ERA and its surroundings. Control fromoutside the space station (Extra Vehicular Activity-ManMachine Interface (EVA-MMI)) uses a speciallydesigned interface that can be used while in aspacesuit.

Arm components

in-orbit replaceable units (ERUs) of ERA

• Two approximately 5 metres long, symmetricalarm sections made of carbon fibre ('limbs')

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• Two identical gripper mechanisms (End Effectors'EE') also capable of transferring data, power or mechanical actuation to payloads

•

Two wrists with three joints each• One elbow joint• One central control computer within the arm

('ECC')• Four camera and lighting units ('CLU')

Electric motor 

Electric motors

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 An electric motor uses electrical energy to producemechanical energy. The reverse process, that of usingmechanical energy to produce electrical energy, is

accomplished by a generator  or dynamo. Traction motors used on locomotives and some electric andhybrid automobiles often perform both tasks if thevehicle is equipped with dynamic brakes. Electricmotors are found in household appliances such asfans, refrigerators, washing machines, pool pumps,floor vacuums, and fan-forced ovens. They are alsofound in many other devices such as computer 

equipment, in its disk drives, printers, and fans; and insome sound and video playing and recordingequipment as DVD/CD players and recorders, tapeplayers and recorders, and record players. Electricmotors are also found in several kinds of toys such assome kinds of vehicles and robotic toys.

History and development

Jedlik's electric car of 1828.

The principle of conversion of electrical energy intomechanical energy by electromagnetic means was

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demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dippinginto a pool of  mercury. A permanent magnet was

placed in the middle of the pool of mercury. When acurrent was passed through the wire, the wire rotatedaround the magnet, showing that the current gave riseto a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, butbrine (salt water) is sometimes used in place of thetoxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later 

refinement is the Barlow's Wheel. These weredemonstration devices, unsuited to practicalapplications due to limited power.

The first real electric motor, using electromagnets for both stationary and rotating parts, was demonstratedby Ányos Jedlik in 1828 Hungary. He built an electric-

motor propelled vehicle in 1828.The first English commutator -type direct-currentelectric motor capable of a practical application wasinvented by the British scientist William Sturgeon in1832. Following Sturgeon's work, a commutator-typedirect-current electric motor made with the intention of commercial use was built by the American Thomas 

Davenport and patented in 1837. Although several of these motors were built and used to operate equipmentsuch as a printing press, due to the high cost of primary battery power , the motors were commerciallyunsuccessful and Davenport went bankrupt. Several

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inventors followed Sturgeon in the development of DCmotors but all encountered the same cost issues withprimary battery power. No electricity distribution had

been developed at the time. Like Sturgeon's motor,there was no practical commercial market for thesemotors.

The modern DC motor was invented by accident in1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as amotor. The Gramme machine was the first electric

motor that was successful in the industry.In 1888 Nikola Tesla invented the first practicable AC motor  and with it the polyphase power transmissionsystem. Tesla continued his work on the AC motor inthe years to follow at the Westinghouse company.

Categorization of electric motors

The classic division of electric motors has been that of Direct Current (DC) types vs  Alternating Current (AC)types. This is more a de facto convention, rather than arigid distinction. For example, many classic DC motorsrun on AC power, these motors being referred to asuniversal motors.

The ongoing trend toward electronic control further muddles the distinction, as modern drivers have movedthe commutator out of the motor shell. For this newbreed of motor, driver circuits are relied upon togenerate sinusoidal AC drive currents, or some

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approximation of. The two best examples are: thebrushless DC motor  and the stepping motor , bothbeing poly-phase AC motors requiring external

electronic control.Considering all rotating (or linear) electric motorsrequire synchronism between a moving magnetic fieldand a moving current sheet for average torqueproduction, there is a clearer distinction between anasynchronous motor  and synchronous types. Anasynchronous motor requires slip between the moving

magnetic field and a winding set to induce current inthe winding set by mutual inductance; the mostubiquitous example being the common AC induction motor  which must slip in order to generate torque. Inthe synchronous types, induction (or slip) is not arequisite for magnetic field or current production (eg.permanent magnet motors, synchronous brush-less 

wound-rotor doubly-fed electric machine).Comparison of motor types

Type Advantages DisadvantagesTypical

 ApplicationTypicalDrive

 AC Induction(ShadedPole)

LeastexpensiveLong lifehigh power 

Rotation slipsfrom frequencyLow startingtorque

Fans Uni/Polyphase A

 AC InductionHigh powerRotation slips Appliances Uni/Poly

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(split-phasecapacitor)

high startingtorque

from frequency phase A

 ACSynchronous

Rotation in-sync withfreqlong-life(alternator)

Moreexpensive

Clocks Audioturntablestape drives

Uni/Polyphase A

Stepper DC

PrecisionpositioningHigh holdingtorque

Slow speedRequires acontroller 

Positioningin printersand floppydrives

MultiphaDC

Brushless

DC electric motor 

Longlifespanlow

maintenanceHighefficiency

High initial cost

Requires acontroller 

HarddrivesCD/DVD

playerselectricvehicles

Multipha

DC

Brushed DC electricmotor 

Low initialcostSimple

speedcontrol(Dynamo)

Highmaintenance

(brushes)Low lifespan

Treadmillexercisers

automotivestarters

Direct

(PWM)

 Torque capability of motor types

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When optimally designed for a given active current(i.e., torque current), voltage, pole-pair number,excitation frequency (i.e., synchronous speed), and

core flux density, all categories of electric motors or generators will exhibit virtually the same maximumcontinuous shaft torque (i.e., operating torque) within agiven physical size of electromagnetic core. Someapplications require bursts of torque beyond themaximum operating torque, such as short bursts of torque to accelerate an electric vehicle from standstill.

 Always limited by magnetic core saturation or safe

operating temperature rise and voltage, the capacityfor torque bursts beyond the maximum operatingtorque differs significantly between categories of electric motors or generators.

Note: Capacity for bursts of torque should not beconfused with Field Weakening capability inherent in

fully electromagnetic electric machines (PermanentMagnet (PM) electric machine are excluded). FieldWeakening, which is not readily available with PMelectric machines, allows an electric machine tooperate beyond the designed frequency of excitationwithout electrical damage.

Electric machines without a transformer circuit

topology, such as Field-Wound (i.e., electromagnet) or Permanent Magnet (PM) Synchronous electricmachines cannot realize bursts of torque higher thanthe maximum designed torque without saturating themagnetic core and rendering any increase in current

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(i.e., torque) as useless. Furthermore, the permanentmagnet assembly of PM synchronous electricmachines can be irreparably damaged, if bursts of 

torque exceeding the maximum operating torque ratingare attempted.

Electric machines with a transformer circuit topology,such Induction (i.e., asynchronous) electric machines,Induction Doubly-Fed electric machines, and Inductionor Synchronous Wound-Rotor Doubly-Fed (WRDF)electric machines, exhibit very high bursts of torque

because the active current (i.e., Magneto-Motive-Forceor the product of current and winding-turns) induced oneither side of the transformer oppose each other andas a result, the active current contributes nothing to thetransformer coupled magnetic core flux density, whichwould otherwise lead to core saturation.

Electric machines that rely on Induction or 

 Asynchronous principles short-circuit one port of thetransformer circuit and as a result, the reactiveimpedance of the transformer circuit becomesdominant as slip increases, which limits the magnitudeof active (i.e., real) current. Still, bursts of torque thatare two to three times higher than the maximum designtorque are realizable.

The Synchronous WRDF electric machine is the onlyelectric machine with a truly dual ported transformer circuit topology (i.e., both ports independently excitedwith no short-circuited port). The dual portedtransformer circuit topology is known to be unstable

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and requires a multiphase slip-ring-brush assembly topropagate limited power to the rotor winding set. If aprecision means were available to instantaneously

control torque angle and slip for synchronous operationduring motoring or generating while simultaneouslyproviding brushless power to the rotor winding set (seeBrushless wound-rotor doubly-fed electric machine),the active current of the Synchronous WRDF electricmachine would be independent of the reactiveimpedance of the transformer circuit and bursts of torque significantly higher than the maximum operating

torque and far beyond the practical capability of anyother type of electric machine would be realizable.Torque bursts greater than eight times operatingtorque have been calculated.

DC Motors

 A DC motor is designed to run on DC electric power.

Two examples of pure DC designs are Michael Faraday's homopolar motor (which is uncommon), andthe ball bearing motor , which is (so far) a novelty. Byfar the most common DC motor types are the brushedand brushless types, which use internal and externalcommutation respectively to create an oscillating ACcurrent from the DC source -- so they are not purely

DC machines in a strict sense.Brushed DC motors

The classic DC motor design generates an oscillatingcurrent in a wound rotor with a split ring commutator ,

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and either a wound or permanent magnet stator. Arotor consists of a coil wound around a rotor which isthen powered by any type of battery.

Many of the limitations of the classic commutator  DCmotor are due to the need for brushes to press againstthe commutator. This creates friction. At higher speeds, brushes have increasing difficulty inmaintaining contact. Brushes may bounce off theirregularities in the commutator surface, creatingsparks. This limits the maximum speed of the machine.

The current density per unit area of the brushes limitsthe output of the motor. The imperfect electric contactalso causes electrical noise. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance. The commutator assembly on a large machine is a costly element,requiring precision assembly of many parts. There are

three types of DC motor:1. DC series motor 2. DC shunt motor 3. DC compound motor - these are also two types:

1. cumulative compound2. differentially compounded

Brushless DC motors

Some of the problems of the brushed DC motor areeliminated in the brushless design. In this motor, the

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mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switchsynchronised to the rotor's position. Brushless motors

are typically 85-90% efficient, whereas DC motors withbrushgear are typically 75-80% efficient.

Midway between ordinary DC motors and stepper  motors lies the realm of the brushless DC motor . Builtin a fashion very similar to stepper motors, these oftenuse a permanent magnet external rotor, three phasesof driving coils, one or more Hall effect sensors to

sense the position of the rotor, and the associateddrive electronics. The coils are activated, one phaseafter the other, by the drive electronics as cued by thesignals from the Hall effect sensors. In effect, they actas three-phase synchronous motors containing their own variable-frequency drive electronics. A specializedclass of brushless DC motor controllers utilize EMF

feedback through the main phase connections insteadof Hall effect sensors to determine position andvelocity. These motors are used extensively in electricradio-controlled vehicles. When configured with themagnets on the outside, these are referred to bymodelists as outrunner motors.

Brushless DC motors are commonly used where

precise speed control is necessary, as in computer disk drives or in video cassette recorders, the spindleswithin CD, CD-ROM (etc.) drives, and mechanismswithin office products such as fans, laser printers and

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photocopiers. They have several advantages over conventional motors:

• Compared to AC fans using shaded-pole motors,they are very efficient, running much cooler thanthe equivalent AC motors. This cool operationleads to much-improved life of the fan's bearings.

• Without a commutator to wear out, the life of a DCbrushless motor can be significantly longer compared to a DC motor using brushes and acommutator. Commutation also tends to cause a

great deal of electrical and RF noise; without acommutator or brushes, a brushless motor may beused in electrically sensitive devices like audioequipment or computers.

• The same Hall effect sensors that provide thecommutation can also provide a convenienttachometer  signal for closed-loop control (servo-

controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal.• The motor can be easily synchronized to an

internal or external clock, leading to precise speedcontrol.

• Brushless motors have no chance of sparking,unlike brushed motors, making them better suitedto environments with volatile chemicals and fuels.

 Also, sparking generates ozone which canaccumulate in poorly ventilated buildings riskingharm to occupants' health.

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• Brushless motors are usually used in smallequipment such as computers and are generallyused to get rid of unwanted heat.

•

They are also very quiet motors which is anadvantage if being used in equipment that isaffected by vibrations.

Modern DC brushless motors range in power from afraction of a watt to many kilowatts. Larger brushlessmotors up to about 100 kW rating are used in electric vehicles. They also find significant use in high-

performance electric model aircraft.Coreless or Ironless DC motors

Nothing in the design of any of the motors describedabove requires that the iron (steel) portions of the rotor actually rotate; torque is exerted only on the windingsof the electromagnets. Taking advantage of this fact is

the coreless or ironless DC motor, a specialized formof a brush or brushless DC motor. Optimized for rapidacceleration, these motors have a rotor that isconstructed without any iron core. The rotor can takethe form of a winding-filled cylinder inside the stator  magnets, a basket surrounding the stator magnets, or a flat  pancake (possibly formed on a printed wiring board) running between upper and lower stator magnets. The windings are typically stabilized by beingimpregnated with Electrical epoxy potting systems.Filled epoxies that have moderate mixed viscosity anda long gel time. These systems are highlighted by lowshrinkage and low exotherm. Typically UL 1446

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recognized as a potting compound for use up to 180C(Class H) UL File No. E 210549.

Because the rotor is much lighter in weight (mass) thana conventional rotor formed from copper  windings onsteel laminations, the rotor can accelerate much morerapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings usealuminum rather than the heavier copper. But becausethere is no metal mass in the rotor to act as a heatsink, even small coreless motors must often be cooled

by forced air.These motors were commonly used to drive thecapstan(s) of magnetic tape drives and are still widelyused in high-performance servo-controlled systems,like radio-controlled vehicles/aircraft, humanoid robotic systems, industrial automation, medical devices, etc.

Universal motors

 A variant of the wound field DC motor is the universalmotor. The name derives from the fact that it may use

 AC or DC supply current, although in practice they arenearly always used with AC supplies. The principle isthat in a wound field DC motor the current in both thefield and the armature (and hence the resultant

magnetic fields) will alternate (reverse polarity) at thesame time, and hence the mechanical force generatedis always in the same direction. In practice, the motor must be specially designed to cope with the AC(impedance must be taken into account, as must the

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pulsating force), and the resultant motor is generallyless efficient than an equivalent pure DC motor.

Operating at normal power line frequencies, themaximum output of universal motors is limited andmotors exceeding one kilowatt (about 1.3 horsepower )are rare. But universal motors also form the basis of the traditional railway traction motor in electric railways.In this application, to keep their electrical efficiencyhigh, they were operated from very low frequency ACsupplies, with 25 and 16.7 hertz (Hz) operation being

common. Because they are universal motors,locomotives using this design were also commonlycapable of operating from a third rail powered by DC.

The advantage of the universal motor is that ACsupplies may be used on motors which have the typicalcharacteristics of DC motors, specifically high startingtorque and very compact design if high running speeds

are used. The negative aspect is the maintenance andshort life problems caused by the commutator . As aresult such motors are usually used in AC devicessuch as food mixers and power tools which are usedonly intermittently. Continuous speed control of auniversal motor running on AC is easily obtained byuse of a thyristor  circuit, while stepped speed control

can be accomplished using multiple taps on the fieldcoil. Household blenders that advertise many speedsfrequently combine a field coil with several taps and adiode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC).

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Universal motors generally run at high speeds, makingthem useful for appliances such as blenders, vacuum cleaners, and hair dryers where high RPM operation is

desirable. They are also commonly used in portablepower tools, such as drills, circular and jig saws, wherethe motor's characteristics work well. Many vacuumcleaner and weed trimmer  motors exceed 10,000RPM, while Dremel and other similar miniaturegrinders will often exceed 30,000 RPM.

Motor damage may occur due to overspeeding

(running at an RPM in excess of design limits) if theunit is operated with no significant load. On larger motors, sudden loss of load is to be avoided, and thepossibility of such an occurrence is incorporated intothe motor's protection and control schemes. In smaller applications, a fan blade attached to the shaft oftenacts as an artificial load to limit the motor speed to a

safe value, as well as a means to circulate coolingairflow over the armature and field windings.

With the very low cost of  semiconductor   rectifiers,some applications that would have previously used auniversal motor now use a pure DC motor, sometimeswith a permanent magnet field.

AC motors

In 1882, Nikola Tesla invented the rotating magnetic field, and pioneered the use of a rotary field of force to

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operate machines. He exploited the principle to designa unique two-phase induction motor in 1883. In 1885,Galileo Ferraris independently researched the concept.

In 1888, Ferraris published his research in a paper tothe Royal Academy of Sciences in Turin.

Introduction of Tesla's motor from 1888 onwardsinitiated what is sometimes referred to as the Second  Industrial Revolution, making possible the efficientgeneration and long distance distribution of electricalenergy using the alternating current transmission

system, also of Tesla's invention (1888).[2]

Before theinvention of the rotating magnetic field, motorsoperated by continually passing a conductor through astationary magnetic field (as in homopolar motors).

Tesla had suggested that the commutators from amachine could be removed and the device couldoperate on a rotary field of force. Professor Poeschel,

his teacher, stated that would be akin to building aperpetual motion machine.[3] Tesla would later attainU.S. Patent 0,416,194 , Electric Motor  (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.

Michail Osipovich Dolivo-Dobrovolsky later invented athree-phase "cage-rotor" in 1890. This type of motor isnow used for the vast majority of commercialapplications.

Components

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 A typical AC motor consists of two parts:

1. An outside stationary stator having coils suppliedwith AC current to produce a rotating magneticfield, and;

2. An inside rotor attached to the output shaft that isgiven a torque by the rotating field.

Torque motors

 A torque motor (also known as a limited torque motor)is a specialized form of  induction motor  which is

capable of operating indefinitely while stalled, that is,with the rotor  blocked from turning, without incurringdamage. In this mode of operation, the motor will applya steady torque to the load (hence the name).

 A common application of a torque motor would be thesupply- and take-up reel motors in a tape drive. In this

application, driven from a low voltage, thecharacteristics 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 tape heads.Driven from a higher voltage, (and so delivering ahigher torque), the torque motors can also achievefast-forward and rewind operation without requiring anyadditional mechanics such as gears or clutches. In the

computer world, torque motors are used with force feedback steering wheels.

 Another common application is the control of thethrottle of an internal combustion engine in conjunction

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with an electronic  governor . In this usage, the motor works against a return spring to move the throttle inaccordance with the output of the governor. The latter 

monitors engine speed by counting electrical pulsesfrom the ignition system or from a magnetic pickup and, depending on the speed, makes smalladjustments to the amount of  current applied to themotor. If the engine starts to slow down relative to thedesired speed, the current will be increased, the motor will develop more torque, pulling against the returnspring and opening the throttle. Should the engine run

too fast, the governor will reduce the current beingapplied to the motor, causing the return spring to pullback and close the throttle.

Slip ring

The slip ring or wound rotor motor is an inductionmachine where the rotor comprises a set of coils thatare terminated in slip rings to which externalimpedances can be connected. The stator is the sameas is used with a standard squirrel cage motor.

By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can

be altered.

The slip ring motor is used primarily to start a highinertia load or a load that requires a very high startingtorque across the full speed range. By correctly

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selecting the resistors used in the secondaryresistance or slip ring starter, the motor is able toproduce maximum torque at a relatively low current

from zero speed to full speed. A secondary use of theslip ring motor is to provide a means of speed control.Because the torque curve of the motor is effectivelymodified by the resistance connected to the rotor circuit, the speed of the motor can be altered.Increasing the value of resistance on the rotor circuitwill move the speed of maximum torque down. If theresistance connected to the rotor is increased beyond

the point where the maximum torque occurs at zerospeed, the torque will be further reduced.

When used with a load that has a torque curve thatincreases with speed, the motor will operate at thespeed where the torque developed by the motor isequal to the load torque. Reducing the load will cause

the motor to speed up, and increasing the load willcause the motor to slow down until the load and motor torque are equal. Operated in this manner, the sliplosses are dissipated in the secondary resistors andcan be very significant. The speed regulation is alsovery poor.

Stepper motors

Closely related in design to three-phase ACsynchronous motors are stepper motors, where aninternal rotor containing permanent magnets or a large

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iron core with salient poles is controlled by a set of external magnets that are switched electronically. Astepper motor may also be thought of as a cross

between a DC electric motor and a solenoid. As eachcoil is energized in turn, the rotor aligns itself with themagnetic field produced by the energized field winding.Unlike a synchronous motor, in its application, themotor may not rotate continuously; instead, it "steps"from one position to the next as field windings areenergized and de-energized in sequence. Dependingon the sequence, the rotor may turn forwards or 

backwards.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading therotor to "cog" to a limited number of positions; moresophisticated drivers can proportionally control thepower to the field windings, allowing the rotors to

position between the cog points and thereby rotateextremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digitalservo-controlled system.

Stepper motors can be rotated to a specific angle withease, and hence stepper motors are used in pre-

gigabyte era computer disk drives, where the precisionthey offered was adequate for the correct positioning of the read/write head of a hard disk drive. As drivedensity increased, the precision limitations of stepper motors made them obsolete for hard drives, thus

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newer hard disk drives use read/write head controlsystems based on voice coils.

Stepper motors were upscaled to be used in electricvehicles under the term SRM (switched reluctancemachine).

Linear motors

 A linear motor is essentially an electric motor that has

been "unrolled" so that, instead of producing a torque (rotation), it produces a linear force along its length bysetting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in amaglev (Transrapid) train, where the train "flies" over the ground, and in many roller-coasters where the

rapid motion of the motorless railcar is controlled bythe rail.

Doubly-fed electric motor 

Doubly-fed electric motors have two independentmultiphase windings that actively participate in theenergy conversion process with at least one of the

winding sets electronically controlled for variable speedoperation. Two is the most active multiphase windingsets possible without duplicating singly-fed or doubly-fed categories in the same package. As a result,doubly-fed electric motors are machines with an

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effective constant torque speed range that is twicesynchronous speed for a given frequency of excitation.This is twice the constant torque speed range as

singly-fed electric machines, which have only oneactive winding set.

 A doubly-fed motor allows for a smaller electronicconverter but the cost of the rotor winding and sliprings may offset the saving in the power electronicscomponents. Difficulties with controlling speed near synchronous speed limit applications.[4]

Singly-fed electric motor 

Singly-fed electric machines incorporate a singlemultiphase winding set that is connected to a power supply. Singly-fed electric machines may be either induction or synchronous. The active winding set canbe electronically controlled. Induction machines

develop starting torque at zero speed and can operateas standalone machines. Synchronous machines musthave auxiliary means for startup, such as a startinginduction squirrel-cage winding or an electroniccontroller. Singly-fed electric machines have aneffective constant torque speed range up tosynchronous speed for a given excitation frequency.

The induction (asynchronous) motors (i.e., squirrelcage rotor or wound rotor), synchronous motors (i.e.,field-excited, permanent magnet or brushless DCmotors, reluctance motors, etc.), which are discussedon the this page, are examples of singly-fed motors. By

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far, singly-fed motors are the predominantly installedtype of motors.