AC motorINTRODUCTIONAn AC motor is an electric motor that is driven by an alternating current. Itconsists of two basic parts, an outside stationary stator having coils suppliedwith alternating current to produce a rotating magnetic field, and an insiderotor attached to the output shaft that is given a torque by the rotating field.There are two types of AC motors, depending on the type of rotor used. Thefirst is the synchronous motor, which rotates exactly at the supply frequencyor a submultiple of the supply frequency. The magnetic field on the rotor iseither generated by current delivered through slip rings or by a permanentmagnet.The second type is the induction motor, which turns slightly slower than thesupply frequency. The magnetic field on the rotor of this motor is created byan induced current.HistoryIn 1882, Serbian inventor Nikola Tesla identified the rotating magneticinduction field principle and pioneered the use of this rotating andinducting electromagnetic field force to generate torque in rotatingmachines. He exploited this principle in the design of a poly-phase inductionmotor in 1883. In 1885, Galileo Ferraris independently researched theconcept. In 1888, Ferraris published his research in a paper to the RoyalAcademy of Sciences in Turin.Introduction of Teslas motor from 1888 onwards initiated what is sometimesreferred to as the Second Industrial Revolution, making possible both theefficient generation and long distance distribution of electrical energy usingthe alternating current transmission system, also of Teslas invention(1888). Before widespread use of Teslas principle of poly-phase inductionfor rotating machines, all motors operated by continually passing aconductor through a stationary magnetic field (as in homopolar motor).Initially Tesla suggested that the commutators from a machine could beremoved and the device could operate on a rotary field of electromagnetic
force. Professor Poeschel, his teacher, stated that would be akin to building aperpetual motion machine. This was because Teslas teacher had onlyunderstood one half of Teslas ideas. Professor Poeschel had realized that theinduced rotating magnetic field would start the rotor of the motor spinning,but he did not see that the counter electromotive force generated wouldgradually bring the machine to a stop. Tesla would later obtain U.S. Patent0,416,194, Electric Motor (December 1889), which resembles the motorseen in many of Teslas photos. This classic alternating current electro-magnetic motor was an induction motor.Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor is now used for the vast majority ofcommercial applications.Three-phase AC induction motorsThree phase AC induction motors rated 746 W (1.000 hp) and 25 W (left),with smaller motors from CD player, toy and CD/DVD drive reader headtraverse. (9 V battery shown, at bottom center, for size comparison.)
Disassembled 250 W motor from a washing machine. The 12 statorwindings are in the housing on the left. Next to it is the squirrel cage rotoron its shaft.Where a polyphase electrical supply is available, the three-phase (orpolyphase) AC induction motor is commonly used, especially for higher-powered motors. The phase differences between the three phases of thepolyphase electrical supply create a rotating electromagnetic field in themotor.Through electromagnetic induction, the time changing and reversing rotatingmagnetic field induces a time changing and reversing current in theconductors in the rotor; this sets up a time changing and opposing movingelectromagnetic field that causes the rotor to turn with the field. Note thatcurrent is induced in the rotor, and hence torque is developed, due to thedifference in rotational speed between the rotor and the magnetic field. As aresult, the rotor will always move slower than the rotating magnetic fieldproduced by the polyphase electrical supply (see slip below).Induction motors are the workhorses of industry and motors up to about500 kW (670 hp) in output are produced in highly standardized frame sizes,making them nearly completely interchangeable between manufacturers(although European and North American standard dimensions are different).Very large induction motors are capable of tens of megawatts of output, forpipeline compressors, wind-tunnel drives, and overland conveyor systems.There are two types of rotors used in induction motors: squirrel cage rotorsand wound rotors.
Squirrel-cage rotorsMost common AC motors use the squirrel cage rotor, which will be found invirtually all domestic and light industrial alternating current motors. Thesquirrel cage refers to the rotating exercise cage for pet animals. The motortakes its name from the shape of its rotor "windings"- a ring at either end ofthe rotor, with bars connecting the rings running the length of the rotor. It istypically cast aluminum or copper poured between the iron laminates of therotor, and usually only the end rings will be visible. The vast majority of therotor currents will flow through the bars rather than the higher-resistance andusually varnished laminates. Very low voltages at very high currents aretypical in the bars and end rings; high efficiency motors will often use castcopper in order to reduce the resistance in the rotor.In operation, the squirrel cage motor may be viewed as a transformer with arotating secondary. When the rotor is not rotating in sync with the magneticfield, large rotor currents are induced; the large rotor currents magnetize therotor and interact with the stators magnetic fields to bring the rotor almostinto synchronization with the stators field. An unloaded squirrel cage motorat rated no-load speed will consume electrical power only to maintain rotorspeed against friction and resistance losses; as the mechanical load increases,so will the electrical load - the electrical load is inherently related to themechanical load. This is similar to a transformer, where the primaryselectrical load is related to the secondarys electrical load.This is why, for example, a squirrel cage blower motor may cause the lightsin a home to dim as it starts, but doesnt dim the lights on startup when itsfan belt (and therefore mechanical load) is removed. Furthermore, a stalledsquirrel cage motor (overloaded or with a jammed shaft) will consumecurrent limited only by circuit resistance as it attempts to start. Unlesssomething else limits the current (or cuts it off completely) overheating anddestruction of the winding insulation is the likely outcome.In order to prevent the currents induced in the squirrel cage fromsuperimposing itself back onto the supply, the squirrel cage is generallyconstructed with a prime number of bars, or at least a small multiple of aprime number (rarely more than 2). There is an optimum number of bars inany design, and increasing the number of bars beyond that point merelyserves to increase the losses of the motor particularly when starting.
Virtually every washing machine, dishwasher, standalone fan, record player,etc. uses some variant of a squirrel cage motor.SlipIf the rotor of a squirrel-cage motor were to run at synchronous speed, theflux in the rotor at any given place on the rotor would not change, and nocurrent would be created in the squirrel cage. For this reason, ordinarysquirrel-cage motors run at some tens of rpm slower than synchronousspeed, even at no load. Because the rotating field (or equivalent pulsatingfield) actually or effectively rotates faster than the rotor, it could be said toslip past the surface of the rotor. The difference between synchronous speedand actual speed is called slip, and loading the motor increases the amountof slip as the motor slows down slightly.Two-phase AC servo motorsA typical two-phase AC servo-motor has a squirrel cage rotor and a fieldconsisting of two windings: 1. a constant-voltage (AC) main winding. 2. a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. Reversing phase makes the motor reverse.An AC servo amplifier, a linear power amplifier, feeds the control winding.The electrical resistance of the rotor is made high intentionally so that thespeed/torque curve is fairly linear. Two-phase servo motors are inherentlyhigh-speed, low-torque devices, heavily geared down to drive the load. Inthe World War II Ford Instrument Company naval analog fire-controlcomputers, these motors had identical windings and an associated phase-shift capacitor. AC power was fed through tungsten contacts arranged in avery simple bridge-topology circuit to develop reversible torque.Single-phase AC induction motorsThree-phase motors produce a rotating magnetic field. However, when onlysingle-phase power is available, the rotating magnetic field must beproduced using other means. Several methods are commonly used:
Shaded-pole motorA common single-phase motor is the shaded-pole motor, and is used indevices requiring low starting torque, such as electric fans or other smallhousehold appliances. In this motor, small single-turn copper "shading coils"create the moving magnetic field. Part of each pole is encircled by a coppercoil or strap; the induced current in the strap opposes the change of fluxthrough the coil. This causes a time lag in the flux passing through theshading coil, so that the maximum field intensity moves across the pole faceon each cycle. This produces a low level rotating magnetic field which islarge enough to turn both the rotor and its attached load. As the rotor picksup speed the torque builds up to its full level as the principal magnetic fieldis rotating relative to the rotating rotor.A reversible shaded-pole motor was made by Barber-Colman severaldecades ago. It had a single field coil, and two principal poles, each splithalfway to create two pairs of poles. Each of these four "half-poles" carrieda coil, and the coils of diagonally-opposite half-poles were connected to apair of terminals. One terminal of each pair was common, so only threeterminals were needed in all.The motor would not start with the terminals open; connecting the commonto one other made the motor run one way, and connecting common to theother made it run the other way. These motors were used in industrial andscientific devices.An unusual, adjustable-speed, low-torque shaded-pole motor could befound in traffic-light and advertising-lighting controllers. The pole faceswere parallel and relatively close to each other, with the disc centeredbetween them, something like the disc in a watthour meter. Each pole facewas split, and had a shading coil on one part; the shading coils were on theparts that faced each other. Both shading coils were probably closer to themain coil; they could have both been farther away, without affecting theoperating principle, just the direction of rotation.Applying AC to the coil created a field that progressed in the gap betweenthe poles. The plane of the stator core was approximately tangential to animaginary circle on the disc, so the traveling magnetic field dragged the discand made it rotate.
The stator was mounted on a pivot so it could be positioned for the desiredspeed and then clamped in position. Keeping in mind that the effective speedof the traveling magnetic field in the gap was constant, placing the polesnearer to the center of the disc made it run relatively faster, and toward theedge, slower.Its possible that these motors are still in use in some older installations.Split-phase induction motorAnother common single-phase AC motor is the split-phase inductionmotor, commonly used in major appliances such as washing machines andclothes dryers. Compared to the shaded pole motor, these motors cangenerally provide much greater starting torque by using a special startupwinding in conjunction with a centrifugal switch.In the split-phase motor, the startup winding is designed with a higherresistance than the running winding. This creates an LR circuit whichslightly shifts the phase of the current in the startup winding. When themotor is starting, the startup winding is connected to the power source via aset of spring-loaded contacts pressed upon by the stationary centrifugalswitch. The starting winding is wound with fewer turns of smaller wire thanthe main winding, so it has a lower inductance (L) and higher resistance (R).The lower L/R ratio creates a small phase shift, not more than about 30degrees, between the flux due to the main winding and the flux of thestarting winding. The starting direction of rotation may be reversed simplyby exchanging the connections of the startup winding relative to the runningwinding..The phase of the magnetic field in this startup winding is shifted from thephase of the mains power, allowing the creation of a moving magnetic fieldwhich starts the motor. Once the motor reaches near design operating speed,the centrifugal switch activates, opening the contacts and disconnecting thestartup winding from the power source. The motor then operates solely onthe running winding. The starting winding must be disconnected since itwould increase the losses in the motor.Capacitor start motorA capacitor start motor is a split-phase induction motor with a startingcapacitor inserted in series with the startup winding, creating an LC circuit
which is capable of a much greater phase shift (and so, a much greaterstarting torque). The capacitor naturally adds expense to such motors.Resistance start motorA resistance start motor is a split-phase induction motor with a starterinserted in series with the startup winding, creating capacitance. This addedstarter provides assistance in the starting and initial direction of rotation.Permanent-split capacitor motorAnother variation is the permanent-split capacitor (PSC) motor (also knownas a capacitor start and run motor). This motor operates similarly to thecapacitor-start motor described above, but there is no centrifugal startingswitch, and what correspond to the the start windings (second windings)are permanently connected to the power source (through a capacitor), alongwith the run windings. PSC motors are frequently used in air handlers,blowers, and fans (including ceiling fans) and other cases where a variablespeed is desired.A capacitor ranging from 3 to 25 microfarads is connected in series with the"start" windings and remains in the circuit during the run cycle. The "start"windings and run windings are identical in this motor, and reverse motioncan be achieved by reversing the wiring of the 2 windings, with thecapacitor connected to the other windings as "start" windings. By changingtaps on the running winding but keeping the load constant, the motor can bemade to run at different speeds. Also, provided all 6 winding connections areavailable separately, a 3 phase motor can be converted to a capacitor startand run motor by commoning two of the windings and connecting the thirdvia a capacitor to act as a start winding. hiWound rotorsAn alternate design, called the wound rotor, is used when variable speed isrequired. In this case, the rotor has the same number of poles as the statorand the windings are made of wire, connected to slip rings on the shaft.Carbon brushes connect the slip rings to an external controller such as avariable resistor that allows changing the motors slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy iscaptured, rectified and returned to the power supply through an inverter.
Compared to squirrel cage rotors, wound rotor motors are expensive andrequire maintenance of the slip rings and brushes, but they were the standardform for variable speed control before the advent of compact powerelectronic devices. Transistorized inverters with variable-frequency drivecan now be used for speed control, and wound rotor motors are becomingless common.Several methods of starting a polyphase motor are used. Where the largeinrush current and high starting torque can be permitted, the motor can bestarted across the line, by applying full line voltage to the terminals (Direct-on-line, DOL). Where it is necessary to limit the starting inrush current(where the motor is large compared with the short-circuit capacity of thesupply), reduced voltage starting using either series inductors, anautotransformer, thyristors, or other devices are used. A techniquesometimes used is (Star-Delta, YΔ) starting, where the motor coils areinitially connected in star for acceleration of the load, then switched to deltawhen the load is up to speed. This technique is more common in Europe thanin North America. Transistorized drives can directly vary the applied voltageas required by the starting characteristics of the motor and load.This type of motor is becoming more common in traction applications suchas locomotives, where it is known as the asynchronous traction motor.The speed of the AC motor is determined primarily by the frequency of theAC supply and the number of poles in the stator winding, according to therelation: Ns = 120F / pwhere Ns = Synchronous speed, in revolutions per minute F = AC power frequency p = Number of poles per phase windingActual RPM for an induction motor will be less than this calculatedsynchronous speed by an amount known as slip, that increases with thetorque produced. With no load, the speed will be very close to synchronous.When loaded, standard motors have between 2-3% slip, special motors mayhave up to 7% slip, and a class of motors known as torque motors are ratedto operate at 100% slip (0 RPM/full stall).
The slip of the AC motor is calculated by: S = (Ns − Nr) / Nswhere Nr = Rotational speed, in revolutions per minute. S = Normalised Slip, 0 to 1.As an example, a typical four-pole motor running on 60 Hz might have anameplate rating of 1725 RPM at full load, while its calculated speed is1800 RPM.The speed in this type of motor has traditionally been altered by havingadditional sets of coils or poles in the motor that can be switched on and offto change the speed of magnetic field rotation. However, developments inpower electronics mean that the frequency of the power supply can also nowbe varied to provide a smoother control of the motor speed.Three-phase AC synchronous motorsIf connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field(or if the rotor consists of a permanent magnet), the result is called asynchronous motor because the rotor will rotate synchronously with therotating magnetic field produced by the polyphase electrical supply.The synchronous motor can also be used as an alternator.Nowadays, synchronous motors are frequently driven by transistorizedvariable-frequency drives. This greatly eases the problem of starting themassive rotor of a large synchronous motor. They may also be started asinduction motors using a squirrel-cage winding that shares the commonrotor: once the motor reaches synchronous speed, no current is induced inthe squirrel-cage winding so it has little effect on the synchronous operationof the motor, aside from stabilizing the motor speed on load changes.Synchronous motors are occasionally used as traction motors; the TGV maybe the best-known example of such use.
One use for this type of motor is its use in a power factor correction scheme.They are referred to as synchronous condensers. This exploits a feature ofthe machine where it consumes power at a leading power factor when itsrotor is over excited. It thus appears to the supply to be a capacitor, andcould thus be used to correct the lagging power factor that is usuallypresented to the electric supply by inductive loads. The excitation is adjusteduntil a near unity power factor is obtained (often automatically). Machinesused for this purpose are easily identified as they have no shaft extensions.Synchronous motors are valued in any case because their power factor ismuch better than that of induction motors, making them preferred for veryhigh power applications.Some of the largest AC motors are pumped-storage hydroelectricitygenerators that are operated as synchronous motors to pump water to areservoir at a higher elevation for later use to generate electricity using thesame machinery. Six 350-megawatt generators are installed in the BathCounty Pumped Storage Station in Virginia, USA. When pumping, each unitcan produce 563,400 horsepower (420 megawatts).Repulsion motorRepulsion motors are wound-rotor single-phase AC motors that are similarto universal motors. In a repulsion motor, the armature brushes are shortedtogether rather than connected in series with the field. By transformer action,the stator induces currents in the rotor, which create torque by repulsioninstead of attraction as in other motors. Several types of repulsion motorshave been manufactured, but the repulsion-start induction-run (RS-IR)motor has been used most frequently. The RS-IR motor has a centrifugalswitch that shorts all segments of the commutator so that the motor operatesas an induction motor once it has been accelerated to full speed. Some ofthese motors also lift the brushes out of contact with the commutator oncethe commutator is shorted. RS-IR motors have been used to provide highstarting torque per ampere under conditions of cold operating temperaturesand poor source voltage regulation. Few repulsion motors of any type aresold as of 2005.
Other types of rotorsSingle-phase AC synchronous motorsSmall single-phase AC motors can also be designed with magnetized rotors(or several variations on that idea; see "Hysteresis synchronous motors"below). The rotors in these motors do not require any induced current sothey do not slip backward against the mains frequency. Instead, they rotatesynchronously with the mains frequency. Because of their highly accuratespeed, such motors are usually used to power mechanical clocks, audioturntables, and tape drives; formerly they were also much used in accuratetiming instruments such as strip-chart recorders or telescope drivemechanisms. The shaded-pole synchronous motor is one version.If a conventional squirrel-cage rotor has flats ground on it to create salientpoles and increase reluctance, it will start conventionally, but will runsynchronously, although it can provide only a modest torque at synchronousspeed.Because inertia makes it difficult to instantly accelerate the rotor fromstopped to synchronous speed, these motors normally require some sort ofspecial feature to get started. Some include a squirrel-cage structure to bringthe rotor close to synchronous speed. Various other designs use a smallinduction motor (which may share the same field coils and rotor as thesynchronous motor) or a very light rotor with a one-way mechanism (toensure that the rotor starts in the "forward" direction). In the latter instance,applying AC power creates chaotic (or seemingly chaotic) jumpingmovement back and forth; such a motor will always start, but lacking theanti-reversal mechanism, the direction it runs is unpredictable. TheHammond organ tone generator used a non-self-starting synchronous motor(until comparatively recently), and had an auxiliary conventional shaded-pole starting motor. A spring-loaded auxiliary manual starting switchconnected power to this second motor for a few seconds.Hysteresis synchronous motorsThese motors are relatively costly, and are used where exact speed(assuming an exact-frequency AC source) as well as rotation with a very
small amount of fast variations in speed (called flutter" in audio recordings)is essential. Applications included tape recorder capstan drives (the motorshaft could be the capstan). Their distinguishing feature is their rotor, whichis a smooth cylinder of a magnetic alloy that stays magnetized, but can bedemagnetized fairly easily as well as re-magnetized with poles in a newlocation. Hysteresis refers to how the magnetic flux in the metal lags behindthe external magnetizing force; for instance, to demagnetize such a material,one could apply a magnetizing field of opposite polarity to that whichoriginally magnetized the material.These motors have a stator like those of capacitor-run squirrel-cageinduction motors. On startup, when slip decreases sufficiently, the rotorbecomes magnetized by the stators field, and the poles stay in place. Themotor then runs at synchronous speed as if the rotor were a permanentmagnet. When stopped and re-started, the poles are likely to form at differentlocations.For a given design, torque at synchronous speed is only relatively modest,and the motor can run at below synchronous speed.Electronically commutated motorsSuch motors have an external rotor with a cup-shaped housing and a radiallymagnetized permanent magnet connected in the cup-shaped housing. Aninterior stator is positioned in the cup-shaped housing. The interior stator hasa laminated core having grooves. Windings are provided within the grooves.The windings have first end turns proximal to a bottom of the cup-shapedhousing and second end turns positioned distal to the bottom. The first andsecond end turns electrically connect the windings to one another. Thepermanent magnet has an end face rom the bottom of the cup-shapedhousing. At least one galvano-magnetic rotor position sensor is arrangedopposite the end face of the permanent magnet so as to be located within amagnetic leakage of the permanent magnet and within a magnetic leakage ofthe interior stator. The at least one rotor position sensor is designed tocontrol current within at least a portion of the windings. A magnetic leakageflux concentrator is arranged at the interior stator at the second end turns at aside of the second end turns facing away from the laminated core andpositioned at least within an angular area of the interior stator in which the atleast one rotor position sensor is located.
ECM motors are increasingly being found in forced-air furnaces and HVACsystems to save on electricity costs as modern HVAC systems are runningtheir fans for longer periods of time (duty cycle). The cost effectiveness ofusing ECM motors in HVAC systems is questionable, given that the repair(replacement) costs are likely to equal or exceed the savings realized byusing such a motor.Watthour-meter motorsThese are essentially two-phase induction motors with permanent magnetsthat retard rotor speed, so their speed is quite accurately proportional towattage of the power passing through the meter. The rotor is an aluminum-alloy disc, and currents induced into it react with the field from the stator.One phase of the stator is a coil with many turns and a high inductance,which causes its magnetic field to lag almost 90 degrees with respect to theapplied (line/mains) voltage. The other phase of the stator is a pair of coilswith very few turns of heavy-gauge wire, hence quite-low inductance. Thesecoils are in series with the load.The core structure, seen face-on, is akin to a cartoon mouth with one toothabove and two below. Surrounding the poles ("teeth") is the common fluxreturn path. The upper pole (high-inductance winding) is centered, and thelower ones equidistant. Because the lower coils are wound in opposition, thethree poles cooperate to create a "sidewise" traveling flux. The disc isbetween the upper and lower poles, but with its shaft definitely in front ofthe field, so the tangential flux movement makes it rotate.Slow-speed synchronous timing motorsRepresentative are low-torque synchronous motors with a multi-pole hollow cylindricalmagnet (internal poles) surrounding the stator structure. An aluminum cup supports themagnet. The stator has one coil, coaxial with the shaft. At each end of the coil are a pairof circular plates with rectangular teeth on their edges, formed so they are parallel withthe shaft. They are the stator poles. One of the pair of discs distributes the coils fluxdirectly, while the other receives flux that has passed through a common shading coil.The poles are rather narrow, and between the poles leading from one end of the coil arean identical set leading from the other end. In all, this creates a repeating sequence of fourpoles, unshaded alternating with shaded, that creates a circumferential traveling field towhich the rotors magnetic poles rapidly synchronize. Some stepping motors have asimilar structu
Motor controllerA motor controller is a device or group of devices that serves to govern in somepredetermined manner the performance of an electric motor. A motor controller mightinclude a manual or automatic means for starting and stopping the motor, selectingforward or reverse rotation, selecting and regulating the speed, regulating or limiting thetorque, and protecting against overloads and faults.Scope of motor controller applicationsThe scope of motor control technology must be very wide to accommodate the widevariety of motor applications in various fields.Domestic applicationsElectric motors are used domestically in personal care products, small and largeappliances, and residential heating and cooling equipment. In most domestic applications,the motor controller functions are built into the product. In some cases, such as bathroomventilation fans, the motor is controlled by a switch on the wall. Some appliances haveprovisions for controlling the speed of the motor. Built-in circuit breakers protect someappliance motors, but most are unprotected except that the household fuse or circuitbreaker panel disconnects the motor if it fails.Office equipment, medical equipment etc.There is a wide variety of motorized office equipment such as personal computers,computer peripherals, copy machines and fax machines as well as smaller items such aselectric pencil sharpeners. Motor controllers for these types of equipment are built intothe equipment. Some quite sophisticated motor controllers are used to control the motorsin computer disc drives. Medical equipment may include very sophisticated motorcontrollers.Commercial applicationsCommercial buildings have larger heating ventilation and air conditioning (HVAC)equipment than that found in individual residences. In addition, motors are used forelevators, escalators and other applications. In commercial applications, the motor controlfunctions are sometimes built into the motor-driven equipment and sometimes installedseparately.Industrial applications
Many industrial applications are dependent upon motors (or machines), which range fromthe size of ones thumb to the size of a railroad locomotive. The motor controllers can bebuilt into the driven equipment, installed separately, installed in an enclosure along withother machine control equipment or installed in motor control centers. Motor controlcenters are multi-compartment steel enclosures designed to enclose many motorcontrollers. It is also common for more than one motor controller to operate a number ofmotors in the same application. In this case the controllers communicate with each otherso they can work the motors together as a team.Types of motor controllersMotor controllers can be manually, remotely or automatically operated. They may includeonly the means for starting and stopping the motor or they may include otherfunctions.An electric motor controller can be classified by the type of motor it is to drive such aspermanent magnet, servo, series, separately excited, and alternating current.A motor controller is connected to a power source such as a battery pack or power supply,and control circuitry in the form of analog or digital input signals.Motor startersA small motor can be started by simply plugging it into an electrical receptacle or byusing a switch or circuit breaker. A larger motor requires a specialized switching unitcalled a motor starter or motor contactor. When energized, a direct on line (DOL) starterimmediately connects the motor terminals directly to the power supply. A motor softstarter connects the motor to the power supply through a voltage reduction device andincreases the applied voltage gradually or in steps.Adjustable-speed drivesAn adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnectedcombination of equipment that provides a means of driving and adjusting the operatingspeed of a mechanical load. An electrical adjustable-speed drive consists of an electricmotor and a speed controller or power converter plus auxiliary devices and equipment. Incommon usage, the term “drive” is often applied to just the controller.Speed controls for AC induction motorsRecent developments in drive electronics have allowed efficient andconvenient speed control of these motors, where this has not traditionallybeen the case. The newest advancements allow for torque generation downto zero speed. This allows the polyphase AC induction motor to compete in
areas where DC motors have long dominated, and Variable frequencydrivesPhase vector drivesPhase vector drives (or simply vector drives) are an improvement over variablefrequency drives (VFDs) in that they separate the calculations of magnetizing current andtorque generating current. These quantities are represented by phase vectors, and arecombined to produce the driving phase vector which in turn is decomposed into thedriving components of the output stage. These calculations need a fast microprocessor,typically a DSP device.Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor positionand phase currents. Rotor position can be obtained through an encoder, but is oftensensed by the reverse EMF generated on the motor leads.In some configurations, a vector drive may be able to generate full rated motor torque atzero speed.Main article: Vector control (motor)Direct torque control drivesDirect torque control has better torque control dynamics than the PI-current controllerbased vector control. Thus it suits better to servo control applications. However, it hassome advantage over other control methods in other applications as well because due tothe faster control it has better capabilities to damp mechanical resonances and thus extendthe life of the mechanical system.Servo controllersMain article: Servo driveMain article: ServomechanismServo controllers is a wide category of motor control. Common features are: • precise closed loop position control • fast acceleration rates • precise speed controlServo motors may be made from several motor types, the most common being • brushed DC motor • brushless DC motors • AC servo motors
Servo controllers use position feedback to close the control loop. This is commonlyimplemented with encoders, resolvers, and Hall effect sensors to directly measure therotors position. Others measure the back EMF in the undriven coils to infer the rotorposition, and therefore are often called "sensorless" controllers.A servo may be controlled using pulse-width modulation (PWM). How long the pulseremains high (typically between 1 and 2 milliseconds) determines where the motor willtry to position itself. Another control method is pulse and direction.Stepper motor controllersA stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphasemotor. Control is usually, but not exclusively, done open loop, ie. the rotor position isassumed to follow a controlled rotating field. Because of this, precise positioning withsteppers is simpler and cheaper than closed loop controls.Modern stepper controllers drive the motor with much higher voltages than the motornameplate rated voltage, and limit current through chopping. The usual setup is to have apositioning controller, known as an indexer, sending step and direction pulses to aseparate higher voltage drive circuit which is responsible for commutation and currentlimiting.Relevant circuits to motor controlH-bridgeMain article: H-bridgeDC motors are typically controlled by using a transistor configuration called an "H-bridge". This consists of a minimum of four mechanical or solid-state switches, such astwo NPN and two PNP transistors. One NPN and one PNP transistor are activated at atime. Both NPN or PNP transistors can be activated to cause a short across the motorterminals, which can be useful for slowing down the motor from the back EMF it creates.