Електрически двигател

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Въртящо магнитно поле като сума от магнитните вектори на трифазни намотки.
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Въртящо магнитно поле като сума от магнитните вектори на трифазни намотки.

Един електрически двигател превръща електрическата енергия в кинетична енергия. Обратната задача, превръщането на кинетичната енергия в електрическа се извършва от генератор или динамо.

Съдържание

[редактиране] Действие

Повечето електрически мотори работят под действието на електромагнетизъм, но също съществуват двигатели работещи под влиянието на други сили, например електростатични сили и пиезоелектричен ефект. Основния принцип под който електромагнитните двигатели работят е механична сила на всеки токоносещ проводник поставен в магнитно поле. Силата е описана от закона на Лоренц и е перпендикулярна на проводника и магнитното поле. Повечето магнитни двигатели са въртеливи на съществуват и линейни. Във въртящият двигател въртящата част (обикновенно се намира вътре) се нарича ротор, а неподвижната част се нарича статор. Роторът се върти защото проводниците и магнитното поле са поставени така че се оформя ротационна сила около оста на ротора. Двигателят съдържа електромагнити поставени в рамка.

[редактиране] Постояннотокови електрически двигатели

Електрически двигатели различни рамери.
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Електрически двигатели различни рамери.

Един от първите електрически двигатели е изобретен от Майкъл Фарадей през 1821г. Съставен е от свобдно висящ проводник потопен в живак. Поставен е постоянен магнит в центъра на живака. Когато през проводника премине ток, той се върти около магнита, показвайки че тока създава въртяшо магнитно поле около проводника. Този двигател често се демонстрира в училищата, но вместо живак се използва солена вода. Това е най-простата форма на електрически двигател. Нарича се еднополярен двигател. По усъвършенстван вариант е колелото на Бароу.

Съвременният правотоков двигател е изобретен случайно през 1873, когато Женоб Грам свързва въртящо динамо към второ подобно устройство, движещо се като двигател.

Класическият правотоков двигател има въртяща се котва под формата на електромагнит.

The classic DC motor has a rotating armature in the form of an electromagnet. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the classical motor going in the proper direction. (See the diagrams below.)

Обикновен постояннотоков двигател. Когато има ток в намотката, се създава магнитно поле около котвата. Лявата част на котватасе избутва от левия магнит и се приблиближава до дясната част, създавайки въртене.
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Обикновен постояннотоков двигател. Когато има ток в намотката, се създава магнитно поле около котвата. Лявата част на котватасе избутва от левия магнит и се приблиближава до дясната част, създавайки въртене.
Котвата продължава да се върти.
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Котвата продължава да се върти.
Когато котвата се разположи хоризонтално, комутатора обръща посоката на тока през намотките, обръщайки и магнитното поле. След това процеса се повтаря.
Увеличаване
Когато котвата се разположи хоризонтално, комутатора обръща посоката на тока през намотките, обръщайки и магнитното поле. След това процеса се повтаря.



[редактиране] Теория

Ако ротора на постояннотоков двигател се завърти от външна сила, двигателят ще действа като генератор и ще създаде електродвижещо напрежение (ЕДН). Това напрежение също се генерира и при нормалната работа на двигателя. Въртенето на двигателя създава напрежение, наречено обратно ЕДН, защото е обратно на входното напрежение. Затова спада на напрежението през мотора се състои от спада на напрежението на това обратно ЕДН и паразитното напрежение, което е резултат на външното съпротивление на котвените намотки. Тока през двигателя е:

I = (VappliedVbackemf) / Rarmature

Механичната сила произведена от двигателя е:

P = I * Vbackemf

Когато двигателя се включи или бъде застопорен, обратното ЕДН е пропорционално на скорстта на двигателя. Тогава имаме нулево обратно ЕДН. По тази причина тока през котвата е много по-голям. Тази висока стойност на тока причинява силни електрически полета, който карат двигателя да заработи. Когато двигателя се върти, обратното ЕДН се увеличава, докато стане равно на входното напрежение минус паразитното напрежение. Затова има малък ток през мотора. Следните три уравнения могат да се използват за намиране на скоростта, тока и обратното ЕДН на натоварен двигател:

Load = Vbackemf * I

Vapplied = I * Rarmature + Vbackemf

Vbackemf = speed * Fluxarmature

[редактиране] Контрол на скоростта

Скоростта на въртене на един постояннотоков двигател е пропорционална на входното напрежение, а ротационната сила е пропорционална на тока. Контрола на скоростта може да се осъществи чрез променливо напрежение, съпротивления или електронно контролиране. Посоката на въртене може да се промени както чрез смяна на полюсите на входните клеми, така и чрез реверсиране на полето, но не и двете едновременно. Това се осъществява чрез специални контактори (директни контактори).

Ефективното напрежение може да се променя чрез поставяне на сериини съпротивления или електронно-контролирано изключващо устройство направено от тиристори, транзистори или по-рано от Живачни токоизправители. В електрическа верига позната като прекъсвач, сумарното напрежение подавано към двигателя се променя много бързо. Докато честотата на "включено" до "изключено" състояние променя входното напрежение, скоростта на двигателя се променя. Процента на времето във включено състояние умножен по входното напрежение дава входното напрежение. Следователно със 100 V напрежение и 25% "включено" време, входното напрежение ще е 25 V. През времето когато е в изключено състояние, индукцията на котвата създава ток, който продължава да преминава през диод, наречен "диод маховик", в паралел със захранването. В тази точка на цикъла, захранващия ток ще е нула и затова сумарния ток на двигателя винаги ще е по-голям от захранващия, освен ако включеното състояние е 100%. При 100%-но включено ссътояние, захранващия и двигателния ток са еднакви. Бързото превключване губи по-малко енергия от използването на серийни резистори. Един изходен филтър се свързва понякога за да умекоти входното напрежение и да намали шума. Този метод се нарича широка пулсираща модулация или ШПМ и често се контролира от микропроцесор.

Откакто простояннотоковите мотори имат голям въртящ момент при ниски скорости, те често се използват за силова употреба при локомотивите и трамваите. Друго приложение е пускови двигатели за петролови и малки дизелови двигатели. Серийните мотори никога не трябва да се използват на места където силата може да спадне рязко(при късане на ремък). Докато мотора ускорява, котвение (от там на полето) ток намалява. Това намаляване в полето кара мотора да увеличи скоростта си, докато се самоунищожи. Това може да е проблем със железопътните мотори в случаи на отлепяне от релсите, освен осъществяване на контрол двигателят може да развие скорост далеч над номиналната. Това не само може да осъществи проблеми за самите двигатели и трансисиите, но заради триенето, релсите и колелата могат да бъдат сериозно повредени. Въртящото поле се използва в някои електрически управления за да се увеличи максималната скорост на електрическо превозно средство. Най-простото утройство използва контактор и резистор, електронни контроли показват тока на мотора и превключват резистора в схемата, когато тока падне под дадена стойност (това става когато двигателят достигне максимално предвидената си скорост на въртене). Когато се включи съпротивлението, моторът ще повиши скоростта си над номиналната при номинално напрежение. Когато токът на двигателя се повиши, управлението ще изключи резистора и въртящото поле ще намалее.

[редактиране] Универсални двигатели

Вариант на постояннотоковият двигател е универсалният двигател. Името произлиза от факта, че той може да се захрани от постоянен или променлив ток, но в практиката обикновенно се захранват от променлив ток. Принципът се състои в това, че в постояннотоковия двигател в полето и в котвата (от там сумарният магнитен поток) ще се сменят по едно и също време (обратна поляризация) и механичната сила, която се генерира е винаги в една и съща посока. В практиката, двигателят трябва да е специално проектиран, така че да може да се захранва от променлив ток (съпротивлението трябва да бъде предвидено, както и силата на пулсация). Резултантният двигател е по-малко ефективен от еквивалентия чист постояннотоков двигател. Работейки при номинална постоянна работа, изходната мощност е ограничена и рядко надминава един киловат. Но универсалните двигатели са в основата на традиционната железница. В това си приложение, за да запазят високата си ефективност, често те работят при нискочестотно променливотоково захранване с 25 и 16 2/3 Hz.

Предимството на универсалният двигател е, че могат да се захранват от променлив ток и да имат характеристиките на постояннотоков двигтел, по-специално голям начален въртящ момент и малки размери при високи скорости на въртене. Отрицателен аспект е поддръжката и малкият им живот заради комутатора. Затова такова двигатели се използват в променливотокови устройства като домакински миксери, мощни инструменти които се използват с прекъсване. Продължителният контрол на скоростта на универсалният двигател с променливотоково захранване много лесно се осъществява с тиристорна схема, докато стъпковият контрол може да се осъществи чрез много намотки. Домакинските роботи, които се рекламират заради многото си скорости комбинират намотките с няколко разклонения и диод, който може да се свърже последователно с двигателя.

[редактиране] Променливотокови двигатели

През 1882, Никола Тесла открива принципите на въртящото магнитно поле и въвежда използването на въртящият момент за работа с машини. Той използва този принцип, за да създаде един уникален дву-фазов индукционен двигател през 1883г. През 1885г., Галилео Ферарис независмо открива тази концепция. През 1888г., Ферарис публикува неговото откритие във вестникът на Кралската Акаемия на науките в Торино.


Introduction of Tesla's motor from 1888 onwards initiated what is known as the Second Industrial Revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888).[1] Before the invention of the rotating magnetic field, motors operated by continually passing a conductor through a stationary magnetic field (as in homopolar motors).

Tesla had suggested that the commutators from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine.[2] Tesla would later attain Щатски патент 0416194, 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.

Stator energy Rotor energy Total energy supplied Power developed
10 90 100 900
50 50 100 2500

In the induction motor, the field and armature were ideally of equal field strengths and the field and armature cores were of equal sizes. The total energy supplied to operate the device equaled the sum of the energy expended in the armature and field coils.[3] The power developed in operation of the device equaled the product of the energy expended in the armature and field coils.[4]

Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. A successful commercial polyphase system of generation and long-distance transmission was designed by Almerian Decker at Mill Creek No. 1[5] in Redlands California.[6]

[редактиране] Структурни елементи и видове

Обикновенния променливеотоков двигател се състои от две части:

  1. Външен неподвижен статор с намотки, захранени с променлив ток, които създават въртящо магнитно поле и;
  2. Вътрешен ротор закрепен към вала.

Има два основни типа променливотокови двигатели, според вида на ротора:

  • Синхронен двигател, който се върти точно със скоростта на захранваща честота или по-бавно и;
  • Индукционен двигател, който се завърта много по-бавно и обикновенно (не винаги) ротора е кафезен

[редактиране] Three-phase AC induction motors

Three phase AC induction motors rated 1 Hp (746 W) and 25 W with small motors from CD player, toy and CD/DVD drive reader head traverse
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Three phase AC induction motors rated 1 Hp (746 W) and 25 W with small motors from CD player, toy and CD/DVD drive reader head traverse

Where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is 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 rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no counterbalancing field will be produced in the rotor.

Induction motors are the workhorses of industry and motors up to about 500 kW (670 horsepower) 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 synchronous motors are capable of tens of thousands of kW in output, for pipeline compressors and wind-tunnel drives.

There are two types of rotors used in induction motors.

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 takes its name from its shape - 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 transformer with a rotating 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 into synchronization with the stator's field. An unloaded squirrel cage motor at synchronous 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, as an example, a squirrel cage blower motor may cause the lights in a home to dim as it starts, but doesn't dim the lights when its fanbelt (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.

Virtually every washing machine, dishwasher, standalone fan, record player, etc. uses some variant of a squirrel cage motor.

Wound Rotor: 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 stator and 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.

Compared to 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. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available, but this is never used in household appliances, because it can cause electrical interference and because of high power requirements.)

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 starting, where the motor coils are initially connected in wye 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 the relation:

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 as slip, 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).

The slip of the AC motor is calculated by:

S = (NsNr) / 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.

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.

[редактиране] 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 in synchronism with the rotating 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 operation of 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.

[редактиране] 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, and 2) a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. 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.

[редактиране] Single-phase AC induction motors

Three-phase motors inherently 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.

A common single-phase motor is the shaded-pole motor, which is used in devices requiring low 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 (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle, thus producing the required rotating magnetic field.

Another common single-phase AC motor is the split-phase induction motor, 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 not-yet-rotating 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 lower L/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 the starting 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.

In a capacitor start motor, a starting capacitor is inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.

Another variation is the Permanent Split-Capacitor (PSC) motor (also known as a capacitor start and run motor). This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch and the second winding is permanently connected to the power source. PSC motors are frequently used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps 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.

Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a repulsion motor, the armature brushes are shorted together rather than connected in series with the field. 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. 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 2006.

[редактиране] Single-phase AC synchronous motors

Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). 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 motor is one version.

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. Various designs use a small induction 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).

[редактиране] Torque motors

A torque motor is a specialized form of induction motor which is capable of operating indefinitely at stall (with the rotor blocked from turning) without damage. In this mode, the motor will apply a steady torque to the load (hence the name). A common application of a torque motor would be the supply- and take-up reel motors in a tape drive. In this application, driven from a low voltage, the characteristics 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 a higher torque), the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches.

[редактиране] Stepper motors

Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a large iron core with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the motor may not rotate continuously; instead, it "steps" from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forwards or backwards.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position "between" the "cog" points and thereby rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.

Stepper motors can be rotated to a specific angle with ease, and hence stepper motors are used in computer disk drives, where the high precision they offer is necessary for the correct functioning of, for example, a hard disk drive or CD drive.

[редактиране] Permanent magnet motor

A permanent magnet motor is the same as the conventional dc machine except the fact that the field winding is replaced by permanent magnets. By doing this, the machine would act like a constant excitation dc machine (separately excited dc machine).

These motors usually have a small rating, ranging up to a few horsepower. They are used in small appliances, battery operated vehicles, for medical purposes, in other medical equipment such as x-ray machines. These motors are also used in toys, and in automobiles as auxiliary motors for the purposes of seat adjustment, power windows, sunroof, mirror adjustment, blower motors, engine cooling fans and the like.

[редактиране] Brushless DC motors

Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. This limits the maximum speed of the machine. The current density per unit area of the brushes limits the output of the motor. The imperfect electric contact also 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.

These problems are eliminated in the brushless motor. In this motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switch synchronised to the motor's position. Brushless motors are typically 85-90% efficient, whereas DC motors with brushgear are typically 75-80% efficient.

Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors. In effect, they act as three-phase synchronous motors containing their own variable frequency drive electronics. A specialized class of brushless DC motor controllers utilize EMF feedback through the main phase connections instead of Hall effect sensors to determine position and velocity. These motors are used extensively in electric radio-controlled vehicles.

Brushless DC motors are commonly used where precise speed control is necessary, computer disk drives or in video cassette recorders the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products such as fans, laser printers and photocopiers. They have several advantages over conventional motors:

  • Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to much-improved life of the fan's bearings.
  • Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers.
  • The same Hall effect devices that provide the commutation can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan okay" signal.
  • The motor can be easily synchronized to an internal or external clock, leading to precise speed control.
  • Brushed motors cannot be used in the vacuum of space because they will weld themselves into an immovable position.

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

[редактиране] Coreless DC motors

Nothing in the design of any of the motors described above requires that the iron (steel) portions of the rotor actually rotate; torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless DC motor, a specialized form of a brush DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the 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 being impregnated with epoxy resins.

Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air.

These motors were commonly used to drive the capstan(s) of magnetic tape drives and are still widely used in high-performance servo-controlled systems.

[редактиране] 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 by setting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground.

[редактиране] Nano motor

Картинка:Http://en.wikipedia.org/wiki/Image:Nanomotor.gif
Nanomotor constructed at UC Berkeley. The motor is about 500nm across: 300 times smaller than the diameter of a human hair

Researchers at University of California, Berkeley, have developed rotational bearings based upon multiwall carbon nanotubes. By attaching a gold plate (with dimensions of order 100nm) to the outer shell of a suspended multiwall carbon nanotube (like nested carbon cylinders), they are able to electrostatically rotate the outer shell relative to the inner core. These bearings are very robust; Devices have been oscillated thousands of times with no indication of wear. The work was done in situ in an SEM. These nanoelectromechanical systems (NEMS) are the next step in miniaturization that may find their way into commercial aspects in the future.

Notice: The thin vertical string seen in the middle, is the nanotube to which the rotor is attached. When the outer tube is sheared, the rotor is able to spin freely on the nanotube bearing.

The process and technology can be seen in this render.