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AC motor

INTRODUCTION

An AC motor is an electric motorthat is driven by an alternating current. It

consists of two basic parts, an outside stationary statorhaving coils supplied

with alternating current to produce a rotating magnetic field, and an inside

rotorattached 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. The

first is the synchronous motor, which rotates exactly at the supply frequency

or a submultiple of the supply frequency. The magnetic field on the rotor is

either generated by current delivered through slip rings or by a permanent

magnet.

The second type is the induction motor, which turns slightly slower than the

supply frequency. The magnetic field on the rotor of this motor is created by

an induced current.

History

In 1882, Serbian inventorNikola Tesla identified the rotating magnetic

induction field principle[citation needed] and pioneered the use of this rotating and

inducting electromagnetic field force to generate torque in rotating

machines. He exploited this principle in the design of a poly-phase induction

motor in 1883. In 1885, Galileo Ferraris independently researched the

concept. In 1888, Ferraris published his research in a paper to the Royal

Academy of Sciences in Turin.

Introduction of Tesla's motor from 1888 onwards initiated what is sometimes

referred to as the Second Industrial Revolution, making possible both the

efficient generation and long distance distribution of electrical energy using

the alternating current transmission system, also of Tesla's invention(1888).[1] Before widespread use of Tesla's principle of poly-phase induction

for rotating machines, all motors operated by continually passing a

conductor through a stationary magnetic field (as in homopolar motor).

Initially Tesla suggested that the commutators from a machine could be

removed and the device could operate on a rotary field of electromagnetic

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force. Professor Poeschel, his teacher, stated that would be akin to building a

perpetual motion machine. This was because Tesla's teacher had only

understood one half of Tesla's ideas. Professor Poeschel had realized that the

induced rotating magnetic field would start the rotor of the motor spinning,

but he did not see that the counter electromotive force generated would

gradually bring the machine to a stop.[2] Tesla would later obtain U.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 a three-phase "cage-

rotor" in 1890. This type of motor is now used for the vast majority of

commercial applications.

Three-phase AC induction motors

Three 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 head

traverse. (9 V battery shown, at bottom center, for size comparison.)

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Disassembled 250 W motor from a washing machine. The 12 stator

windings are in the housing on the left. Next to it is the squirrel cage rotor

on its shaft.

Where a polyphase electrical supply is available, the three-phase (orpolyphase) AC induction motoris commonly used, especially for higher-

powered motors. The phase differences between the three phases of the

polyphase electrical supply create a rotating electromagnetic field in the

motor.

Through electromagnetic induction, the time changing and reversing rotating

magnetic field induces a time changing and reversing current in the

conductors in the rotor; this sets up a time changing and opposing moving

electromagnetic field that causes the rotor to turn with the field. Note that

current is induced in the rotor, and hence torque is developed, due to thedifference in rotational speed between the rotor and the magnetic field. As a

result, the rotor will always move slower than the rotating magnetic field

produced by the polyphase electrical supply (see slip below).

Induction motors are the workhorses of industry and motors up to about

500 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, for

pipeline compressors, wind-tunnel drives, and overland conveyor systems.

There are two types of rotors used in induction motors: squirrel cage rotors

and wound rotors.

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Squirrel-cage rotors

Most common AC motors use the squirrel cage rotor, which will be found in

virtually all domestic and light industrial alternating current motors. The

squirrel cage refers to the rotating exercise cage for pet animals. The motor

takes its name from the shape of its rotor "windings"- a ring at either end of

the rotor, with bars connecting the rings running the length of the rotor. It is

typically cast aluminum or copper poured between the iron laminates of the

rotor, and usually only the end rings will be visible. The vast majority of the

rotor currents will flow through the bars rather than the higher-resistance and

usually varnished laminates. Very low voltages at very high currents are

typical in the bars and end rings; high efficiency motors will often use cast

copper in order to reduce the resistance in the rotor.

In operation, the squirrel cage motor may be viewed as a transformerwith arotating secondary. When the rotor is not rotating in sync with the magnetic

field, large rotor currents are induced; the large rotor currents magnetize the

rotor and interact with the stator's magnetic fields to bring the rotor almost

into synchronization with the stator's field. An unloaded squirrel cage motor

at rated no-load speed will consume electrical power only to maintain rotor

speed against friction and resistance losses; as the mechanical load increases,

so will the electrical load - the electrical load is inherently related to the

mechanical load. This is similar to a transformer, where the primary's

electrical load is related to the secondary's electrical load.

This is why, for example, a squirrel cage blower motor may cause the lights

in a home to dim as it starts, but doesn't dim the lights on startup when its

fan belt (and therefore mechanical load) is removed. Furthermore, a stalled

squirrel cage motor (overloaded or with a jammed shaft) will consume

current limited only by circuit resistance as it attempts to start. Unless

something else limits the current (or cuts it off completely) overheating and

destruction of the winding insulation is the likely outcome.

In order to prevent the currents induced in the squirrel cage from

superimposing itself back onto the supply, the squirrel cage is generally

constructed with a prime number of bars, or at least a small multiple of a

prime number (rarely more than 2). There is an optimum number of bars in

any design, and increasing the number of bars beyond that point merely

serves to increase the losses of the motor particularly when starting.

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Virtually every washing machine, dishwasher, standalone fan, record player,

etc. uses some variant of a squirrel cage motor.

Slip

If 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 no

current would be created in the squirrel cage. For this reason, ordinary

squirrel-cage motors run at some tens of rpm slower than synchronous

speed, even at no load. Because the rotating field (or equivalent pulsating

field) actually or effectively rotates faster than the rotor, it could be said to

slip past the surface of the rotor. The difference between synchronous speed

and actual speed is called slip, and loading the motor increases the amount

of slip as the motor slows down slightly.

Two-phase AC servo motors

A typical two-phase AC servo-motor has a squirrel cage rotor and a field

consisting 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 the

speed/torque curve is fairly linear. Two-phase servo motors are inherently

high-speed, low-torque devices, heavily geared down to drive the load. In

the World War II Ford Instrument Company naval analog fire-control

computers, these motors had identical windings and an associated phase-

shift capacitor. AC power was fed through tungsten contacts arranged in a

very simple bridge-topology circuit to develop reversible torque.

Single-phase AC induction motors

Three-phase motors produce a rotating magnetic field. However, when only

single-phase power is available, the rotating magnetic field must be

produced using other means. Several methods are commonly used:

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Shaded-pole motor

A common single-phase motor is the shaded-pole motor, and is used in

devices requiring low starting torque, such as electric fans or other small

household appliances. In this motor, small single-turn copper "shading coils"

create the moving magnetic field. Part of each pole is encircled by a copper

coil or strap; the induced current in the strap opposes the change of flux

through the coil. This causes a time lag in the flux passing through the

shading coil, so that the maximum field intensity moves across the pole face

on each cycle. This produces a low level rotating magnetic field which is

large enough to turn both the rotor and its attached load. As the rotor picks

up speed the torque builds up to its full level as the principal magnetic field

is 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 split

halfway to create two pairs of poles. Each of these four "half-poles" carried

a coil, and the coils of diagonally-opposite half-poles were connected to a

pair of terminals. One terminal of each pair was common, so only three

terminals were needed in all.

The motor would not start with the terminals open; connecting the common

to one other made the motor run one way, and connecting common to the

other made it run the other way. These motors were used in industrial and

scientific devices.

An unusual, adjustable-speed, low-torque shaded-pole motor could be

found in traffic-light and advertising-lighting controllers. The pole faces

were parallel and relatively close to each other, with the disc centered

between them, something like the disc in a watthour meter. Each pole face

was split, and had a shading coil on one part; the shading coils were on the

parts that faced each other. Both shading coils were probably closer to the

main coil; they could have both been farther away, without affecting the

operating principle, just the direction of rotation.

Applying AC to the coil created a field that progressed in the gap between

the poles. The plane of the stator core was approximately tangential to an

imaginary circle on the disc, so the traveling magnetic field dragged the disc

and made it rotate.

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The stator was mounted on a pivot so it could be positioned for the desired

speed and then clamped in position. Keeping in mind that the effective speed

of the traveling magnetic field in the gap was constant, placing the poles

nearer to the center of the disc made it run relatively faster, and toward the

edge, slower.

It's possible that these motors are still in use in some older installations.

Split-phase induction motor

Another common single-phase AC motor is thesplit-phase induction

motor,[3] commonly used in major appliances such as washing machines and

clothes dryers. Compared to the shaded pole motor, these motors can

generally provide much greater starting torque by using a special startup

winding in conjunction with a centrifugal switch.

In the split-phase motor, the startup winding is designed with a higher

resistance than the running winding. This creates an LR circuit which

slightly shifts the phase of the current in the startup winding. When the

motor is starting, the startup winding is connected to the power source via a

set of spring-loaded contacts pressed upon by the stationary centrifugal

switch. The starting winding is wound with fewer turns of smaller wire than

the main winding, so it has a lower inductance (L) and higher resistance (R).

The lowerL/R ratio creates a small phase shift, not more than about 30

degrees, between the flux due to the main winding and the flux of thestarting winding. The starting direction of rotation may be reversed simply

by exchanging the connections of the startup winding relative to the running

winding..

The phase of the magnetic field in this startup winding is shifted from the

phase of the mains power, allowing the creation of a moving magnetic field

which starts the motor. Once the motor reaches near design operating speed,

the centrifugal switch activates, opening the contacts and disconnecting the

startup winding from the power source. The motor then operates solely on

the running winding. The starting winding must be disconnected since it

would increase the losses in the motor.

Capacitor start motor

A capacitor start motor is a split-phase induction motor with a starting

capacitorinserted in series with the startup winding, creating an LC circuit

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which is capable of a much greater phase shift (and so, a much greater

starting torque). The capacitor naturally adds expense to such motors.

Resistance start motor

A resistance start motor is a split-phase induction motor with a starterinserted in series with the startup winding, creating capacitance. This added

starter provides assistance in the starting and initial direction of rotation.

Permanent-split capacitor motor

Another variation is thepermanent-split capacitor (PSC) motor(also known

as a capacitor start and run motor).[4] This motor operates similarly to the

capacitor-start motor described above, but there is no centrifugal starting

switch,[4] and what correspond to the the start windings (second windings)

are permanently connected to the power source (through a capacitor), along

with the run windings.[4] PSC motors are frequently used in air handlers,

blowers, and fans (including ceiling fans) and other cases where a variable

speed 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.[4] The "start"

windings and run windings are identical in this motor,[4] and reverse motion

can be achieved by reversing the wiring of the 2 windings,[4] with the

capacitor connected to the other windings as "start" windings. By changingtaps on the running winding but keeping the load constant, the motor can be

made to run at different speeds. Also, provided all 6 winding connections are

available separately, a 3 phase motor can be converted to a capacitor start

and run motor by commoning two of the windings and connecting the third

via a capacitor to act as a start winding. hi

Wound rotors

An alternate design, called the wound rotor, is used when variable speed is

required. 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 a

variable resistor that allows changing the motor's slip rate. In certain high-

power variable speed wound-rotor drives, the slip-frequency energy is

captured, rectified and returned to the power supply through an inverter.

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Comparedto squirrel cage rotors, wound rotor motors are expensive and

require maintenance of the slip rings and brushes, but they were the standard

form for variable speed control before the advent of compact power

electronic devices. Transistorized inverters with variable-frequency drive

can now be used for speed control, and wound rotor motors are becoming

less common.

Several methods of starting a polyphase motor are used. Where the large

inrush current and high starting torque can be permitted, the motor can be

started 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 the

supply), reduced voltage starting using either series inductors, an

autotransformer, thyristors, or other devices are used. A technique

sometimes used is (Star-Delta, Y) starting, where the motor coils areinitially connected in star for acceleration of the load, then switched to delta

when the load is up to speed. This technique is more common in Europe than

in North America. Transistorized drives can directly vary the applied voltage

as required by the starting characteristics of the motor and load.

This type of motor is becoming more common in traction applications such

as locomotives, where it is known as the asynchronous traction motor.

The speed of the AC motor is determined primarily by the frequency of the

AC supply and the number of poles in the stator winding, according to therelation:

Ns = 120F/p

where

Ns = Synchronous speed, in revolutions per minute

F= AC power frequency

p = Number of poles per phase winding

Actual RPM for an induction motor will be less than this calculated

synchronous speed by an amount known asslip, that increases with the

torque produced. With no load, the speed will be very close to synchronous.

When loaded, standard motors have between 2-3% slip, special motors may

have up to 7% slip, and a class of motors known as torque motors are rated

to operate at 100% slip (0 RPM/full stall).

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The slip of the AC motor is calculated by:

S= (Ns Nr) /Ns

where

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 a

nameplate rating of 1725 RPM at full load, while its calculated speed is

1800 RPM.

The speed in this type of motor has traditionally been altered by having

additional sets of coils or poles in the motor that can be switched on and off

to change the speed of magnetic field rotation. However, developments in

power electronics mean that the frequency of the power supply can also now

be varied to provide a smoother control of the motor speed.

Three-phase AC synchronous motors

If 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 a

synchronous 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 transistorized

variable-frequency drives. This greatly eases the problem of starting the

massive rotor of a large synchronous motor. They may also be started as

induction motors using a squirrel-cage winding that shares the common

rotor: once the motor reaches synchronous speed, no current is induced in

the 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 may

be the best-known example of such use.

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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 of

the machine where it consumes power at a leadingpower factorwhen its

rotor is over excited. It thus appears to the supply to be a capacitor, and

could thus be used to correct the lagging power factor that is usually

presented to the electric supply by inductive loads. The excitation is adjusted

until a near unity power factor is obtained (often automatically). Machines

used for this purpose are easily identified as they have no shaft extensions.

Synchronous motors are valued in any case because theirpower factoris

much better than that of induction motors, making them preferred for very

high power applications.

Some of the largest AC motors arepumped-storage hydroelectricity

generators that are operated as synchronous motors to pump water to a

reservoir at a higher elevation for later use to generate electricity using thesame machinery. Six 350-megawatt generators are installed in the Bath

County Pumped Storage Station in Virginia, USA. When pumping, each unit

can produce 563,400 horsepower (420 megawatts).[5]

Repulsion motor

Repulsion motors are wound-rotor single-phase AC motors that are similar

to 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 repulsion

instead of attraction as in other motors. Several types of repulsion motors

have been manufactured, but the repulsion-start induction-run (RS-IR)

motor has been used most frequently. The RS-IR motor has a centrifugal

switch that shorts all segments of the commutator so that the motor operates

as an induction motor once it has been accelerated to full speed. Some of

these motors also lift the brushes out of contact with the commutator once

the commutator is shorted. RS-IR motors have been used to provide high

starting torque per ampere under conditions of cold operating temperatures

and poor source voltage regulation. Few repulsion motors of any type are

sold as of 2005.

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Other types of rotors

Single-phase AC synchronous motors

Small 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 so

they do not slip backward against the mains frequency. Instead, they rotate

synchronously with the mains frequency. Because of their highly accurate

speed, such motors are usually used to power mechanical clocks, audio

turntables, and tape drives; formerly they were also much used in accurate

timing instruments such as strip-chart recorders or telescope drive

mechanisms. The shaded-pole synchronous motoris one version.

If a conventional squirrel-cage rotor has flats ground on it to create salient

poles and increase reluctance, it will start conventionally, but will run

synchronously, although it can provide only a modest torque at synchronous

speed.

Because inertia makes it difficult to instantly accelerate the rotor from

stopped to synchronous speed, these motors normally require some sort of

special feature to get started. Some include a squirrel-cage structure to bring

the rotor close to synchronous speed. Various other designs use a smallinduction motor (which may share the same field coils and rotor as the

synchronous motor) or a very light rotor with a one-way mechanism (to

ensure that the rotor starts in the "forward" direction). In the latter instance,

applying AC power creates chaotic (or seemingly chaotic) jumping

movement back and forth; such a motor will always start, but lacking the

anti-reversal mechanism, the direction it runs is unpredictable. The

Hammond 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 motors

These motors are relatively costly, and are used where exact speed

(assuming an exact-frequency AC source) as well as rotation with a very

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small amount of fast variations in speed (called 'flutter" in audio recordings)

is essential. Applications included tape recorder capstan drives (the motor

shaft could be the capstan). Their distinguishing feature is their rotor, which

is a smooth cylinder of a magnetic alloy that stays magnetized, but can be

demagnetized fairly easily as well as re-magnetized with poles in a new

location. Hysteresis refers to how the magnetic flux in the metal lags behind

the external magnetizing force; for instance, to demagnetize such a material,

one could apply a magnetizing field of opposite polarity to that which

originally magnetized the material.

These motors have a stator like those of capacitor-run squirrel-cage

induction motors. On startup, when slip decreases sufficiently, the rotor

becomes magnetized by the stator's field, and the poles stay in place. The

motor then runs at synchronous speed as if the rotor were a permanent

magnet. 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 motors

Such motors have an external rotor with a cup-shaped housing and a radially

magnetized permanent magnet connected in the cup-shaped housing. An

interior 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-shaped

housing and second end turns positioned distal to the bottom. The first and

second end turns electrically connect the windings to one another. The

permanent magnet has an end face rom the bottom of the cup-shaped

housing. At least one galvano-magnetic rotor position sensor is arranged

opposite the end face of the permanent magnet so as to be located within a

magnetic leakage of the permanent magnet and within a magnetic leakage of

the interior stator. The at least one rotor position sensor is designed to

control current within at least a portion of the windings. A magnetic leakage

flux concentrator is arranged at the interior stator at the second end turns at a

side of the second end turns facing away from the laminated core and

positioned at least within an angular area of the interior stator in which the at

least one rotor position sensor is located.

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ECM motors are increasingly being found in forced-air furnaces and HVAC

systems to save on electricity costs as modern HVAC systems are running

their fans for longer periods of time (duty cycle).[6] The cost effectiveness of

using ECM motors in HVAC systems is questionable, given that the repair

(replacement) costs are likely to equal or exceed the savings realized by

using such a motor.[citation needed]

Watthour-meter motors

These are essentially two-phase induction motors with permanent magnets

that retard rotor speed, so their speed is quite accurately proportional to

wattage 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 coils

with very few turns of heavy-gauge wire, hence quite-low inductance. These

coils are in series with the load.

The core structure, seen face-on, is akin to a cartoon mouth with one tooth

above and two below. Surrounding the poles ("teeth") is the common flux

return path. The upper pole (high-inductance winding) is centered, and the

lower ones equidistant. Because the lower coils are wound in opposition, the

three poles cooperate to create a "sidewise" traveling flux. The disc is

between 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 motors

Representative are low-torque synchronous motors with a multi-pole hollow cylindricalmagnet (internal poles) surrounding the stator structure. An aluminum cup supports the

magnet. The stator has one coil, coaxial with the shaft. At each end of the coil are a pair

of 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 coil's flux

directly, 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 four

poles, unshaded alternating with shaded, that creates a circumferential traveling field to

which the rotor's magnetic poles rapidly synchronize. Some stepping motors have asimilar structu

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Motor controller

A motor controller is a device or group of devices that serves to govern in some

predetermined manner the performance of an electric motor.[1] A motor controller might

include a manual or automatic means for starting and stopping the motor, selectingforward or reverse rotation, selecting and regulating the speed, regulating or limiting the

torque, and protecting against overloads and faults.[2]

Scope of motor controller applications

The scope of motor control technology must be very wide to accommodate the widevariety of motor applications in various fields.

Domestic applications

Electric motors are used domestically inpersonal care products, small and large

appliances, 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 have

provisions for controlling the speed of the motor. Built-in circuit breakers protect some

appliance motors, but most are unprotected except that the household fuse or circuit

breaker 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 as

electric pencil sharpeners. Motor controllers for these types of equipment are built into

the equipment. Some quite sophisticated motor controllers are used to control the motors

in computer disc drives. Medical equipment may include very sophisticated motorcontrollers.

Commercial applications

Commercial buildings have larger heating ventilation and air conditioning (HVAC)

equipment than that found in individual residences. In addition, motors are used for

elevators, escalators and other applications. In commercial applications, the motor controlfunctions are sometimes built into the motor-driven equipment and sometimes installed

separately.

Industrial applications

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Many industrial applications are dependent upon motors (or machines), which range from

the size of one's thumb to the size of a railroad locomotive. The motor controllers can be

built into the driven equipment, installed separately, installed in an enclosure along withother machine control equipment or installed in motor control centers. Motor control

centers are multi-compartment steel enclosures designed to enclose many motor

controllers. 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 other

so they can work the motors together as a team.

Types of motor controllers

Motor controllers can be manually, remotely or automatically operated. They may includeonly the means for starting and stopping the motor or they may include other

functions.[4][2][5]

An electric motor controller can be classified by the type of motor it is to drive such as

permanent 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 starters

A small motor can be started by simply plugging it into an electrical receptacle or by

using a switch or circuit breaker. A larger motor requires a specialized switching unit

called a motor starter or motorcontactor. When energized, a direct on line (DOL) starterimmediately connects the motor terminals directly to the power supply. A motor soft

starter connects the motor to the power supply through a voltage reduction device andincreases the applied voltage gradually or in steps.[4][2][5]

Adjustable-speed drives

An adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnected

combination of equipment that provides a means of driving and adjusting the operatingspeed of a mechanical load. An electrical adjustable-speed drive consists of an electric

motor and a speed controller or power converter plus auxiliary devices and equipment. In

common usage, the term drive is often applied to just the controller.[4][5]

Speed controls for AC induction motors

Recent developments in drive electronics have allowed efficient and

convenient speed control of these motors, where this has not traditionally

been the case. The newest advancements allow for torque generation down

to zero speed. This allows the polyphase AC induction motor to compete in

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areas where DC motors have long dominated, and Variable frequency

drives

Phase vector drives

Phase vector drives (or simply vector drives) are an improvement overvariablefrequency drives(VFDs) in that they separate the calculations of magnetizing current and

torque generating current. These quantities are represented by phase vectors, and are

combined 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 DSPdevice.

Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position

and 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 drives

Direct torque control has better torque control dynamics than the PI-current controller

based 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 to

the faster control it has better capabilities to damp mechanical resonances and thus extend

the life of the mechanical system.

Servo controllers

Main article: Servo drive

Main article: Servomechanism

Servo controllers is a wide category of motor control. Common features are:

precise closed loop position control

fast acceleration rates precise speed control

Servo motors may be made from several motor types, the most common being

brushed DC motor

brushless DC motors

AC servo motors

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Servo controllers use position feedback to close the control loop. This is commonly

implemented with encoders, resolvers, and Hall effect sensors to directly measure the

rotor's position. Others measure the backEMF in the undriven coils to infer the rotorposition, and therefore are often called "sensorless" controllers.

A servo may be controlled usingpulse-width modulation (PWM). How long the pulseremains high (typically between 1 and 2 milliseconds) determines where the motor will

try to position itself. Another control method is pulse and direction.

Stepper motor controllers

A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase

motor. 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 with

steppers 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 a

separate higher voltage drive circuit which is responsible for commutation and current

limiting.

Relevant circuits to motor control

H-bridge

Main article: H-bridge

DC 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 as

two NPN and two PNP transistors. One NPN and one PNP transistor are activated at a

time. 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 backEMF it creates.

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