Editing Electric motor (section)

===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.