Editing Electric motor (section)

==Self-commutated motor==
===Brushed DC motor===
{{main|DC motor}}
Most DC motors are small permanent magnet (PM) types. They contain a [[Brush (electric)|brushed]] internal mechanical commutation to reverse motor windings' current in synchronism with rotation.<ref name="Weiβmantel (2008)2">{{cite book|last=Weiβmantel|first=H|title=§2.1 Motors with Commutator in Chapter 2 – Motors with Continuous Rotation|author2=Oesingmann, P.|author3=Möckel, A.|pages=13–160}} in {{harvnb|Stölting|Kallenbach|Amrhein|2008|p=5}}</ref>

====Electrically excited DC motor====
{{Main|Brushed DC electric motor}}
[[File:Electric_motor_cycle_2.png|right|thumb|Workings of a brushed electric motor with a two-pole rotor and PM stator. ("N" and "S" designate polarities on the inside faces of the magnets; the outside faces have opposite polarities.)]]
A commutated DC motor has a set of rotating windings wound on an [[Armature (electrical)|armature]] mounted on a rotating shaft. The shaft also carries the commutator. Thus, every brushed DC motor has AC flowing through its windings. Current flows through one or more pairs of brushes that touch the commutator; the brushes connect an external source of electric power to the rotating armature.

The rotating armature consists of one or more wire coils wound around a laminated, [[Soft magnetic material|magnetically "soft"]] [[Ferromagnetism|ferromagnetic]] core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet (an [[electromagnet]]). The magnetic field produced interacts with a stationary magnetic field produced by either PMs or another winding (a field coil), as part of the motor frame. The force between the two magnetic fields rotates the shaft. The commutator switches power to the coils as the rotor turns, keeping the poles from ever fully aligning with the magnetic poles of the stator field, so that the rotor keeps turning as long as power is applied.

Many of the limitations of the classic commutator DC motor are due to the need for brushes to maintain contact with the commutator, creating friction. The brushes create sparks while crossing the insulating gaps between commutator sections. Depending on the commutator design, the brushes may create [[short circuits]] between adjacent sections—and hence coil ends. Furthermore, the rotor coils' [[inductance]] causes the voltage across each to rise when its circuit opens, increasing the sparking. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their [[resistivity]], limits the motor's output. Crossing the gaps also generates [[electrical noise]]; sparking generates [[Radio frequency interference|RFI]]. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance or replacement. The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the rotor.

While most commutators are cylindrical, some are flat, segmented discs mounted on an insulator.

Large brushes create a large contact area, which maximizes motor output, while small brushes have low mass to maximize the speed at which the motor can run without excessive sparking. (Small brushes are desirable for their lower cost.) Stiffer brush springs can be used to make brushes of a given mass work at a higher speed, despite greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a trade-off between output power, speed, and efficiency/wear.

DC machines are defined as follows:<ref name="Dorf (1997)2">{{cite book |last=Liu|first=Chen-Ching|display-authors=etal|year=1997|editor-last=Dorf|editor-first=Richard C. |section=§66.1 Generators |url=https://books.google.com/books?id=qP7HvuakLgEC&pg=PA1456 |edition=3rd|publisher=CRC Press|page=1456|isbn=0-8493-8574-1 |title=The Electrical Engineering Handbook}}</ref>

* Armature circuit – A winding that carries the load, either stationary or rotating.
* Field circuit – A set of windings that produces a magnetic field.
* Commutation: A mechanical technique in which rectification can be achieved, or from which DC can be derived.

[[File:Serie_Shunt_Coumpound.svg|thumb|A: shunt B: series C: compound f = field coil]]
The five types of brushed DC motor are:

* Shunt-wound 
* Series-wound 
* Compound (two configurations):
** Cumulative compound
** Differentially compounded
* Permanent magnet (not shown)
* Separately excited (not shown).

==== Permanent magnet ====
{{main|Permanent-magnet electric motor}}
A permanent magnet (PM) motor does not have a field winding on the stator frame, relying instead on PMs to provide the magnetic field. Compensating windings in series with the armature may be used on large motors to improve commutation under load. This field is fixed and cannot be adjusted for speed control. PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.

To minimize overall weight and size, miniature PM motors may use high energy magnets made with [[neodymium]]; most are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed [[Electric motor#Singly fed electric motor|singly-fed]] synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles (to ensure starting, regardless of rotor position) and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.

===Electronic commutator (EC)===
==== Brushless DC ====
{{Main|Brushless DC electric motor}}
Some of the problems of the brushed DC motor are eliminated in the BLDC design. In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. BLDC motors are typically 85%+ efficient, reaching up to 96.5%,<ref name="Nozawa (2009)2">{{cite web|last=Nozawa|first=Tetsuo |date=2009|title=Tokai University Unveils 100W DC Motor with 96% Efficiency|url=http://techon.nikkeibp.co.jp/english/NEWS_EN/20090403/168295/|url-status=live|archive-url=https://web.archive.org/web/20110101131311/http://techon.nikkeibp.co.jp/english/NEWS_EN/20090403/168295/|archive-date=2011-01-01|publisher=Tech-On – Nikkei Electronics}}</ref> while brushed DC motors are typically 75–80% efficient.

The BLDC motor's characteristic trapezoidal [[counter-electromotive force]] (CEMF) waveform is derived partly from the stator windings being evenly distributed, and partly from the placement of the rotor's permanent magnets. Also known as electronically commutated DC or inside-out DC motors, the stator windings of trapezoidal BLDC motors can be single-phase, two-phase or three-phase and use [[Hall effect sensor]]s mounted on their windings for rotor position sensing and low cost [[Closed-loop controller|closed-loop commutator control]].

BLDC motors are commonly used where precise speed control is necessary, as in computer disk drives or 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:

* They are more efficient than AC fans using shaded-pole motors, running much cooler than the AC equivalents. This cool operation leads to much-improved life of the fan's bearings.
* Without a commutator, the life of a BLDC motor can be significantly longer compared to a brushed DC motor with a commutator. Commutation tends to cause electrical and RF noise; without a commutator or brushes, a BLDC motor may be used in electrically sensitive devices like audio equipment or computers.
* The same [[Hall effect]] sensors that provide the commutation can provide a convenient [[tachometer]] signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal as well as provide running speed feedback.
* The motor can be synchronized to an internal or external clock, providing precise speed control.
* BLDC motors do not spark, making them better suited to environments with volatile chemicals and fuels. Sparking also generates ozone, which can accumulate in poorly ventilated buildings.
* BLDC motors are usually used in small equipment such as computers and are generally used in fans to remove heat.
* They make little noise, which is an advantage in equipment that is affected by vibrations.

Modern BLDC motors range in power from a fraction of a watt to many kilowatts. Larger BLDC motors rated up to about 100&nbsp;kW are used in electric vehicles. They also find use in electric [[model aircraft]].

====Switched reluctance motor====
{{Main|Switched reluctance motor}}
[[File:Switched_reluctance_motor_6-4.svg|right|thumb|6/4 pole switched reluctance motor]]
The switched reluctance motor (SRM) has no brushes or permanent magnets, and the rotor has no electric currents. Torque comes from a slight misalignment of poles on the rotor with poles on the stator. The rotor aligns itself with the magnetic field of the stator, while the stator field windings are sequentially energized to rotate the stator field.

The magnetic flux created by the field windings follows the path of least magnetic sending the flux through rotor poles that are closest to the energized poles of the stator, thereby magnetizing those poles of the rotor and creating torque. As the rotor turns, different windings are energized, keeping the rotor turning.

SRMs are used in some appliances<ref name="Bush (2009)2">{{cite web|last=Bush|first=Steve |date=2009 |title=Dyson vacuums 104,000&nbsp;rpm brushless DC technology|url=http://www.electronicsweekly.com/Articles/13/08/2010/46377/dyson-vacuums-104000rpm-brushless-dc-technology.htm|archive-url=https://web.archive.org/web/20120411205153/http://www.electronicsweekly.com/Articles/13/08/2010/46377/dyson-vacuums-104000rpm-brushless-dc-technology.htm|archive-date=2012-04-11|publisher=Electronics Weekly Magazine}}</ref> and vehicles.<ref>{{Cite web|date=March 11, 2018|title=Tesla Model 3 Motor – Everything I've Been Able To Learn About It (Welcome To The Machine) |website=CleanTechnica|url=https://cleantechnica.com/2018/03/11/tesla-model-3-motor-in-depth/|access-date=2018-06-18 |language=en-US}}</ref>

===Universal AC/DC motor===
{{main|Universal motor}}
[[File:Universalmotor_3.JPG|thumb|Modern low-cost universal motor, from a vacuum cleaner. Field windings are dark copper-colored, toward the back, on both sides. The rotor's laminated core is gray metallic, with dark slots for winding the coils. The commutator (partly hidden) has become dark from use; it is toward the front. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing.]]
A commutated, electrically excited, series or parallel wound motor is referred to as a universal motor because it can be designed to operate on either AC or DC power. A universal motor can operate well on AC because the current in both the field and the armature coils (and hence the resultant magnetic fields) synchronously reverse polarity, and hence the resulting mechanical force occurs in a constant direction of rotation.

Operating at normal [[Utility frequency|power line frequencies]], universal motors are often used in sub-kilowatt applications. Universal motors formed the basis of the traditional railway traction motor in [[Railway electrification system#Low-frequency alternating current|electric railways]]. In this application, using AC power on a motor designed to run on DC would experience efficiency losses due to [[eddy current]] heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid (un-laminated) iron. They are now rarely used.

An advantage is that AC power may be used on motors that specifically have high starting torque and compact design if high running speeds are used. By contrast, maintenance is higher and lifetimes are shortened. Such motors are used in devices that are not heavily used, and have high starting-torque demands. Multiple taps on the field coil provide (imprecise) stepped speed control. Household blenders that advertise many speeds typically combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC). Universal motors also lend themselves to [[TRIAC#Application|electronic speed control]] and, as such, are a choice for devices such as domestic washing machines. The motor can agitate the drum (both forwards and in reverse) by switching the field winding with respect to the armature.

Whereas SCIMs cannot turn a shaft faster than allowed by the power line frequency, universal motors can run at much higher speeds. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable. They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed {{nowrap|10,000 rpm}}, while miniature grinders may exceed {{nowrap|30,000 rpm}}.