Editing Electric motor (section)

==Advanced types==
===Rotary===
====Ironless or coreless rotor motor====
[[File:Miniature_Coreless_DC_Motor.jpg|thumb|A miniature coreless motor]]
The coreless or ironless DC motor is a specialized permanent magnet DC motor.<ref name="Weiβmantel (2008)2" /> Optimized for rapid [[acceleration]], the rotor is constructed without an iron core. The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only wire and bonding material. The rotor can fit inside the stator magnets; a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux. A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the motor housing, and provides a return path for the flux.

Because the rotor is much lower mass than a conventional rotor, it can accelerate much more rapidly, often achieving a mechanical [[time constant]] under one millisecond. This is especially true if the windings use aluminum rather than (heavier) copper. The rotor has no metal mass to act as a heat sink; even small motors must be cooled. Overheating can be an issue for these designs.

The [[vibrating alert]] of cellular phones can be generated by cylindrical permanent-magnet motors, or disc-shaped types that have a thin multipolar disc field magnet, and an intentionally unbalanced molded-plastic rotor structure with two bonded coreless coils. Metal brushes and a flat commutator switch power to the rotor coils.

Related limited-travel actuators have no core and a bonded coil placed between the poles of high-flux thin permanent magnets. These are the fast head positioners for rigid-disk ("hard disk") drives. Although the contemporary design differs considerably from that of loudspeakers, it is still loosely (and incorrectly) referred to as a "voice coil" structure, because some earlier rigid-disk-drive heads moved in straight lines, and had a drive structure much like that of a loudspeaker.

====Pancake or axial rotor motor====
{{Main|Axial flux motor}}
The printed armature or pancake motor has windings shaped as a disc running between arrays of high-flux magnets. The magnets are arranged in a circle facing the rotor spaced to form an axial air gap.<ref name="Krishnan (1987)2">{{cite journal|last=Krishnan|first=R.|date=March 1987|title=Selection Criteria for Servo Motor Drives|journal=IEEE Transactions on Industry Applications|volume=IA-23|issue=2|pages=270–75|doi=10.1109/TIA.1987.4504902|s2cid=14777000}}</ref> This design is commonly known as the pancake motor because of its flat profile.

The armature (originally formed on a printed circuit board) is made from punched copper sheets that are laminated together using advanced composites to form a thin, rigid disc. The armature does not have a separate ring commutator. The brushes move directly on the armature surface making the whole design compact.

An alternative design is to use wound copper wire laid flat with a central conventional commutator, in a flower and petal shape. The windings are typically stabilized with electrical epoxy potting systems. These are filled epoxies that have moderate, mixed viscosity and a long gel time. They are highlighted by low shrinkage and low exotherm, and are typically UL 1446 recognized as a potting compound insulated with {{convert|180|C|F}}, Class H rating.

The unique advantage of ironless DC motors is the absence of [[Cogging torque|cogging]] (torque variations caused by changing attraction between the iron and the magnets). Parasitic eddy currents cannot form in the rotor as it is totally ironless, although iron rotors are laminated. This can greatly improve efficiency, but variable-speed controllers must use a higher switching rate (>40&nbsp;kHz) or DC because of decreased [[electromagnetic induction]].

These motors were invented to drive the capstan(s) of magnetic tape drives, where minimal time to reach operating speed and minimal stopping distance were critical. Pancake motors are widely used in high-performance servo-controlled systems, robotic systems, industrial automation and medical devices. Due to the variety of constructions now available, the technology is used in applications from high temperature military to low cost pump and basic servos.

Another approach (Magnax) is to use a single stator sandwiched between two rotors. One such design has produced peak power of 15&nbsp;kW/kg, sustained power around 7.5&nbsp;kW/kg. This yokeless axial flux motor offers a shorter flux path, keeping the magnets further from the axis. The design allows zero winding overhang; 100 percent of the windings are active. This is enhanced with the use of rectangular-crosssection copper wire. The motors can be stacked to work in parallel. Instabilities are minimized by ensuring that the two rotor discs put equal and opposing forces onto the stator disc. The rotors are connected directly to one another via a shaft ring, cancelling out the magnetic forces.<ref name=":02">{{Cite web|last=Blain|first=Loz|date=May 30, 2018|title=Magnax prepares to manufacture radically high-powered, compact axial flux electric motor|url=https://newatlas.com/magnax-axial-flux-electric-motor/54821|access-date=2018-06-18|website=newatlas.com|language=en}}</ref>

==== Servomotor ====
{{Main|Servomotor}}
A [[servomotor]] is a motor that is used within a position-control or speed-control feedback system. Servomotors are used in applications such as machine tools, pen plotters, and other process systems. Motors intended for use in a servomechanism must have predictable characteristics for speed, torque, and power. The speed/torque curve is important and is high ratio for a servomotor. Dynamic response characteristics such as winding inductance and rotor inertia are important; these factors limit performance. Large, powerful, but slow-responding servo loops may use conventional AC or DC motors and drive systems with position or speed feedback. As dynamic response requirements increase, more specialized motor designs such as coreless motors are used. AC motors' superior power density and acceleration characteristics tends to favor permanent magnet synchronous, BLDC, induction, and SRM drive approaches.<ref name="Krishnan (1987)2" />

A servo system differs from some stepper motor applications in that position feedback is continuous while the motor is running. A stepper system inherently operates open-loop—relying on the motor not to "miss steps" for short term accuracy—with any feedback such as a "home" switch or position encoder external to the motor system.<ref name="Patrick (1997)2">{{cite conference|last=Patrick|first=Dale R.|author2=Fardo, Stephen W.|year=1997|title=Chapter 11|edition=2nd|publisher=Fairmont Press, Inc.|isbn=978-0-88173-239-9|book-title=Rotating Electrical Machines and Power Systems}}</ref>

====Stepper motor====
{{Main|Stepper motor}}
[[File:Stepper_motor.svg|thumb|A stepper motor with a soft iron rotor, with active windings shown. In 'A' the active windings tend to hold the rotor in position. In 'B' a different set of windings are carrying a current, which generates torque and rotation.]]
Stepper motors are typically used to provide precise rotations. An internal rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of electronically switched external magnets. A stepper motor may also be thought of as a cross between a DC electric motor and a rotary 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, the stepper motor may not rotate continuously; instead, it moves in steps—starting and then stopping—advancing 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, and it may change direction, stop, speed up or slow down at any time.

Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to "cog" to a limited number of positions. [[Stepper motor#Microstepping|Microstepping]] drivers can proportionally control the power to the field windings, allowing the rotors to position between cog points and rotate smoothly. Computer-controlled stepper motors are one of the most versatile positioning systems, particularly as part of a digital [[Servomechanism|servo-controlled]] system.

Stepper motors can be rotated to a specific angle in discrete steps with ease, and hence stepper motors are used for read/write head positioning in early [[Hard disk drive|disk drives]], where the precision and speed they offered could correctly position the read/write head. As drive density increased, precision and speed limitations made them obsolete for hard drives—the precision limitation made them unusable, and the speed limitation made them uncompetitive—thus newer hard disk drives use voice coil-based head actuator systems. (The term "voice coil" in this connection is historic; it refers to the structure in a cone-type [[loudspeaker]].)

Stepper motors are often used in computer printers, optical scanners, and digital photocopiers to move the active element, the print head carriage ([[inkjet printers]]), and the [[platen]] or feed rollers.

So-called quartz analog wristwatches contain the smallest commonplace stepping motors; they have one coil, draw little power, and have a permanent magnet rotor. The same kind of motor drives battery-powered quartz clocks. Some of these watches, such as [[chronographs]], contain more than one stepper motor.

Closely related in design to three-phase AC synchronous motors, stepper motors and SRMs are classified as variable reluctance motor type.{{Sfn|Bose|2006|pp=569–70, 891}}

===Linear===
{{Main|Linear motor}}
A linear motor is essentially any electric motor that has been "unrolled" so that, instead of producing [[torque]] (rotation), it produces a straight-line force along its length.

Linear motors are most commonly [[Linear induction motor|induction motors]] or stepper motors. Linear motors are commonly found in roller-coasters where the rapid motion of the motorless railcar is controlled by the rail. They are also used in [[maglev train]]s, where the train "flies" over the ground. On a smaller scale, the 1978 era HP 7225A pen plotter used two linear stepper motors to move the pen along the X and Y axes.<ref>{{cite journal|last=Fenoglio|first=John A.|author2=Chin, Bessie W.C.|author3=Cobb, Terry R.|date=February 1979|title=A High-Quality Digital X-Y Plotter Designed for Reliability, Flexibility and Low Cost|url=http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1979-02.pdf|url-status=live|journal=Hewlett-Packard Journal|archive-url=https://web.archive.org/web/20120327132234/http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1979-02.pdf|archive-date=27 March 2012|access-date=9 February 2012}}</ref>

=== Non-magnetic ===

==== Electrostatic ====
{{Main|Electrostatic motor}}
An electrostatic motor is based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high-voltage power supply, although small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon. Electrostatic motors find frequent use in micro-electro-mechanical systems ([[MEMS]]) where their drive voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. The molecular machinery that runs living cells is often based on linear and rotary electrostatic motors.{{Citation needed|date=March 2013}}

==== Piezoelectric ====
{{Main|Piezoelectric motor}}
A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a [[Piezoelectricity|piezoelectric material]] when an [[electric field]] is applied. Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or [[Ultrasonic motor|ultrasonic]] vibrations to produce linear or rotary motion.<ref>{{Cite book |last=Horn |first=Alexander |url={{google books |plainurl=y |id=IdCg2dDZQr8C|page=38}} |title=Ultra-fast Material Metrology |publisher=John Wiley & Sons |year=2009 |isbn=978-3-527-62793-6 |language=en}}</ref> In one mechanism, the elongation in a single plane is used to make a series of stretches and position holds, similar to the way a caterpillar moves.<ref>{{cite book |last1=Shafik |first1=Amro |title=Nanopositioning Technologies |last2=Ben Mrad |first2=Ridha |date=2016 |publisher=Springer International Publishing |isbn=978-3-319-23853-1 |page=39 |chapter=Piezoelectric Motor Technology: A Review |doi=10.1007/978-3-319-23853-1_2}}</ref>

==== Electric propulsion ====
{{Main|Electrically powered spacecraft propulsion}}
An electrically powered spacecraft propulsion system uses electric motor technology to propel spacecraft in outer space. Most systems are based on electrically accelerating propellant to high speed, while some systems are based on [[electrodynamic tether]]s principles of propulsion to the [[magnetosphere]].<ref>{{Cite web |title=Launch Assist Tethers |url=http://www.tethers.com/LaunchAssist.html |archive-url=https://web.archive.org/web/20171116050844/http://www.tethers.com/LaunchAssist.html |archive-date=2017-11-16 |access-date=2017-09-15 |website=www.tethers.com}}</ref>