Electric motor

An electric motor is a machine that converts electrical energy into mechanical energy. Most
electric motors operate through the interaction between the motor's magnetic field and electric
current in a wire winding to generate force in the form of torque applied on the motor's shaft. An
electric generator is mechanically identical to an electric motor, but operates in reverse, converting
mechanical energy into electrical energy.

An industrial electric motor

Electric motors can be powered by direct current (DC) sources, such as from batteries or rectifiers,
or by alternating current (AC) sources, such as a power grid, inverters or electrical generators.

Electric motors may be classified by considerations such as power source type, construction,
application and type of motion output. They can be brushed or brushless, single-phase, two-
phase, or three-phase, axial or radial flux, and may be air-cooled or liquid-cooled.

Standardized motors provide power for industrial use. The largest are used for ship propulsion,
pipeline compression and pumped-storage applications, with output exceeding 100 megawatts.

Applications include industrial fans, blowers and pumps, machine tools, household appliances,
power tools, vehicles, and disk drives. Small motors may be found in electric watches. In certain
applications, such as in regenerative braking with traction motors, electric motors can be used in
reverse as generators to recover energy that might otherwise be lost as heat and friction.

Electric motors produce linear or rotary force (torque) intended to propel some external
mechanism. This makes them a type of actuator. They are generally designed for continuous
rotation, or for linear movement over a significant distance compared to its size. Solenoids also
convert electrical power to mechanical motion, but over only a limited distance.
Components

Rotor (left) and stator (right)

An electric motor has two mechanical parts: the rotor, which moves, and the stator, which does not.
Electrically, the motor consists of two parts, the field magnets and the armature, one of which is
attached to the rotor and the other to the stator. Together they form a magnetic circuit.[1] The
magnets create a magnetic field that passes through the armature. These can be electromagnets
or permanent magnets. The field magnet is usually on the stator and the armature on the rotor, but
these may be reversed.

Salient-pole rotor

Rotor
The rotor is the moving part that delivers the mechanical power. The rotor typically holds
conductors that carry currents, on which the magnetic field of the stator exerts force to turn the
shaft.[2]

Stator
The stator surrounds the rotor, and usually holds field magnets, which are either electromagnets
(wire windings around a ferromagnetic iron core) or permanent magnets. These create a magnetic
field that passes through the rotor armature, exerting force on the rotor windings. The stator core is
made up of many thin metal sheets that are insulated from each other, called laminations. These
laminations are made of electrical steel, which has a specified magnetic permeability, hysteresis,
and saturation. Laminations reduce losses that would result from induced circulating eddy currents
that would flow if a solid core were used. Mains powered AC motors typically immobilize the wires
within the windings by impregnating them with varnish in a vacuum. This prevents the wires in the
winding from vibrating against each other which would abrade the wire insulation and cause
premature failures. Resin-packed motors, used in deep well submersible pumps, washing
machines, and air conditioners, encapsulate the stator in plastic resin to prevent corrosion and/or
reduce conducted noise.[3]

Gap
An air gap between the stator and rotor allows it to turn. The width of the gap has a significant
effect on the motor's electrical characteristics. It is generally made as small as possible, as a large
gap weakens performance. Conversely, gaps that are too small may create friction in addition to
noise.

Armature
The armature consists of wire windings on a ferromagnetic core. Electric current passing through
the wire causes the magnetic field to exert a force (Lorentz force) on it, turning the rotor. Windings
are coiled wires, wrapped around a laminated, soft, iron, ferromagnetic core so as to form magnetic
poles when energized with current.

Electric machines come in salient- and nonsalient-pole configurations. In a salient-pole motor the
rotor and stator ferromagnetic cores have projections called poles that face each other. Wire is
wound around each pole below the pole face, which become north or south poles when current
flows through the wire. In a nonsalient-pole (distributed field or round-rotor) motor, the
ferromagnetic core is a smooth cylinder, with the windings distributed evenly in slots around the
circumference. Supplying alternating current in the windings creates poles in the core that rotate
continuously.[4] A shaded-pole motor has a winding around part of the pole that delays the phase
of the magnetic field for that pole.

Commutator
 Commutator in a universal motor
from a vacuum cleaner. Parts: (A)
commutator, (B) brush

A commutator is a rotary electrical switch that supplies current to the rotor. It periodically reverses
the flow of current in the rotor windings as the shaft rotates. It consists of a cylinder composed of
multiple metal contact segments on the armature. Two or more electrical contacts called brushes
made of a soft conductive material like carbon press against the commutator. The brushes make
sliding contact with successive commutator segments as the rotator turns, supplying current to the
rotor. The windings on the rotor are connected to the commutator segments. The commutator
reverses the current direction in the rotor windings with each half turn (180°), so the torque applied
to the rotor is always in the same direction.[5] Without this reversal, the direction of torque on each
rotor winding would reverse with each half turn, stopping the rotor. Commutated motors have been
mostly replaced by brushless motors, permanent magnet motors, and induction motors.

Shaft
The motor shaft extends outside of the motor, where it satisfies the load. Because the forces of the
load are exerted beyond the outermost bearing, the load is said to be overhung.[6]

Bearings
The rotor is supported by bearings, which allow the rotor to turn on its axis by transferring the force
of axial and radial loads from the shaft to the motor housing.[6]

History

Early motors
 Faraday's electromagnetic
experiment, 1821, the first
demonstration of the conversion
of electrical energy into motion[7]

Before modern electromagnetic motors, experimental motors that worked by electrostatic force
were investigated. The first electric motors were simple electrostatic devices described in
experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the
1740s.[8][9] The theoretical principle behind them, Coulomb's law, was discovered but not
published, by Henry Cavendish in 1771. This law was discovered independently by Charles-
Augustin de Coulomb in 1785, who published it so that it is now known by his name.[10] Due to the
difficulty of generating the high voltages they required, electrostatic motors were never used for
practical purposes.

The invention of the electrochemical battery by Alessandro Volta in 1799[11] made the production of
persistent electric currents possible. Hans Christian Ørsted discovered in 1820 that an electric
current creates a magnetic field, which can exert a force on a magnet. It only took a few weeks for
André-Marie Ampère to develop the first formulation of the electromagnetic interaction and present
the Ampère's force law, that described the production of mechanical force by the interaction of an
electric current and a magnetic field.[12]

Michael Faraday gave the first demonstration of the effect with a rotary motion on 3 September
1821 in the basement of the Royal Institution.[13] A free-hanging wire was dipped into a pool of
mercury, on which a permanent magnet (PM) was placed. When a current was passed through the
wire, the wire rotated around the magnet, showing that the current gave rise to a close circular
magnetic field around the wire.[14] Faraday published the results of his discovery in the Quarterly
Journal of Science, and sent copies of his paper along with pocket-sized models of his device to
colleagues around the world so they could also witness the phenomenon of electromagnetic
rotations.[13] This motor is often demonstrated in physics experiments, substituting brine for (toxic)
mercury. Barlow's wheel was an early refinement to this Faraday demonstration, although these
and similar homopolar motors remained unsuited to practical application until late in the century.
 Jedlik's "electromagnetic self-
rotor", 1827 (Museum of Applied
Arts, Budapest). The historic
motor still works perfectly
today.[15]

An electric motor presented to

Kelvin by James Joule in 1842,
Hunterian Museum, Glasgow

In 1827, Hungarian physicist Ányos Jedlik started experimenting with electromagnetic coils. After
Jedlik solved the technical problems of continuous rotation with the invention of the commutator,
he called his early devices "electromagnetic self-rotors". Although they were used only for
teaching, in 1828 Jedlik demonstrated the first device to contain the three main components of
practical DC motors: the stator, rotor and commutator. The device employed no permanent
magnets, as the magnetic fields of both the stationary and revolving components were produced
solely by the currents flowing through their windings.[16][17][18][19][20][21][22]

DC motors
The first commutator DC electric motor capable of turning machinery was invented by English
scientist William Sturgeon in 1832.[23] Following Sturgeon's work, a commutator-type direct-
current electric motor was built by American inventors Thomas Davenport and Emily Davenport,[24]
which he patented in 1837. The motors ran at up to 600 revolutions per minute, and powered
machine tools and a printing press.[25] Due to the high cost of primary battery power, the motors
were commercially unsuccessful and bankrupted the Davenports. Several inventors followed
Sturgeon in the development of DC motors, but all encountered the same battery cost issues. As
no electricity distribution system was available at the time, no practical commercial market
emerged for these motors.[26]

After many other more or less successful attempts with relatively weak rotating and reciprocating
apparatus Prussian/Russian Moritz von Jacobi created the first real rotating electric motor in May
1834. It developed remarkable mechanical output power. His motor set a world record, which
Jacobi improved four years later in September 1838.[27] His second motor was powerful enough to
drive a boat with 14 people across a wide river. It was also in 1839/40 that other developers
managed to build motors with similar and then higher performance.

In 1827–1828, Jedlik built a device using similar principles to those used in his electromagnetic
self-rotors that was capable of useful work.[28][29][30][31][32][33][16][22] He built a model electric
vehicle that same year.[34]

A major turning point came in 1864, when Antonio Pacinotti first described the ring armature
(although initially conceived in a DC generator, i.e. a dynamo).[12] This featured symmetrically
grouped coils closed upon themselves and connected to the bars of a commutator, the brushes of
which delivered practically non-fluctuating current.[35][36] The first commercially successful DC
motors followed the developments by Zénobe Gramme who, in 1871, reinvented Pacinotti's design
and adopted some solutions by Werner Siemens.

A benefit to DC machines came from the discovery of the reversibility of the electric machine,
which was announced by Siemens in 1867 and observed by Pacinotti in 1869.[12] Gramme
accidentally demonstrated it on the occasion of the 1873 Vienna World's Fair, when he connected
two such DC devices up to 2 km from each other, using one of them as a generator and the other
as motor.[37]

The drum rotor was introduced by Friedrich von Hefner-Alteneck of Siemens & Halske to replace
Pacinotti's ring armature in 1872, thus improving the machine efficiency.[12] The laminated rotor
was introduced by Siemens & Halske the following year, achieving reduced iron losses and
increased induced voltages. In 1880, Jonas Wenström provided the rotor with slots for housing the
winding, further increasing the efficiency.

In 1886, Frank Julian Sprague invented the first practical DC motor, a non-sparking device that
maintained relatively constant speed under variable loads. Other Sprague electric inventions about
this time greatly improved grid electric distribution (prior work done while employed by Thomas
Edison), allowed power from electric motors to be returned to the electric grid, provided for electric
distribution to trolleys via overhead wires and the trolley pole, and provided control systems for
electric operations. This allowed Sprague to use electric motors to invent the first electric trolley
system in 1887–88 in Richmond, Virginia, the electric elevator and control system in 1892, and the
electric subway with independently powered centrally-controlled cars. The latter were first installed
in 1892 in Chicago by the South Side Elevated Railroad, where it became popularly known as the
"L". Sprague's motor and related inventions led to an explosion of interest and use in electric
motors for industry. The development of electric motors of acceptable efficiency was delayed for
several decades by failure to recognize the extreme importance of an air gap between the rotor and
stator. Efficient designs have a comparatively small air gap.[38][a] The St. Louis motor, long used in
classrooms to illustrate motor principles, is inefficient for the same reason, as well as appearing
nothing like a modern motor.[40]

Electric motors revolutionized industry. Industrial processes were no longer limited by power
transmission using line shafts, belts, compressed air or hydraulic pressure. Instead, every machine
could be equipped with its own power source, providing easy control at the point of use, and
improving power transmission efficiency. Electric motors applied in agriculture eliminated human
and animal muscle power from such tasks as handling grain or pumping water. Household uses
(sauch as washing machines, dishwashers, fans, air conditioners and refrigerators) of electric
motors reduced heavy labor in the home and made higher standards of convenience, comfort and
safety possible. Today, electric motors consume more than half of the electric energy produced in
the US.[41]

AC motors
In 1824, French physicist François Arago formulated the existence of rotating magnetic fields,
termed Arago's rotations, which, by manually turning switches on and off, Walter Baily
demonstrated in 1879 as in effect the first primitive induction motor.[42][43][44][45] In the 1880s
many inventors were trying to develop workable AC motors[46] because AC's advantages in long-
distance high-voltage transmission were offset by the inability to operate motors on AC.

The first alternating-current commutatorless induction motor was invented by Galileo Ferraris in
1885. Ferraris was able to improve his first design by producing more advanced setups in 1886.[47]
In 1888, the Royal Academy of Science of Turin published Ferraris's research detailing the
foundations of motor operation, while concluding at that time that "the apparatus based on that
principle could not be of any commercial importance as motor."[45][48][49]

Possible industrial development was envisioned by Nikola Tesla, who invented independently his
induction motor in 1887 and obtained a patent in May 1888. In the same year, Tesla presented his
paper A New System of Alternate Current Motors and Transformers to the AIEE that described
three patented two-phase four-stator-pole motor types: one with a four-pole rotor forming a non-
self-starting reluctance motor, another with a wound rotor forming a self-starting induction motor,
and the third a true synchronous motor with separately excited DC supply to rotor winding. One of
the patents Tesla filed in 1887, however, also described a shorted-winding-rotor induction motor.
George Westinghouse, who had already acquired rights from Ferraris (US$1,000), promptly bought
Tesla's patents (US$60,000 plus US$2.50 per sold hp, paid until 1897),[47] employed Tesla to
develop his motors, and assigned C.F. Scott to help Tesla; however, Tesla left for other pursuits in
1889.[50][51][52][53] The constant speed AC induction motor was found not to be suitable for street
cars,[46] but Westinghouse engineers successfully adapted it to power a mining operation in
Telluride, Colorado in 1891.[54][55][56] Westinghouse achieved its first practical induction motor in
1892 and developed a line of polyphase 60 hertz induction motors in 1893, but these early
Westinghouse motors were two-phase motors with wound rotors. B.G. Lamme later developed a
rotating bar winding rotor.[50]

Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the

three-phase induction motor in 1889, of both types cage-rotor and wound rotor with a starting
rheostat, and the three-limb transformer in 1890. After an agreement between AEG and
Maschinenfabrik Oerlikon, Doliwo-Dobrowolski and Charles Eugene Lancelot Brown developed
larger models, namely a 20-hp squirrel cage and a 100-hp wound rotor with a starting rheostat.
These were the first three-phase asynchronous motors suitable for practical operation.[47] Since
1889, similar developments of three-phase machinery were started Wenström. At the 1891
Frankfurt International Electrotechnical Exhibition, the first long distance three-phase system was
successfully presented. It was rated 15 kV and extended over 175 km from the Lauffen waterfall on
the Neckar river. The Lauffen power station included a 240 kW 86 V 40 Hz alternator and a step-up
transformer while at the exhibition a step-down transformer fed a 100-hp three-phase induction
motor that powered an artificial waterfall, representing the transfer of the original power source.[47]
The three-phase induction is now used for the vast majority of commercial applications.[57][58]
Mikhail Dolivo-Dobrovolsky claimed that Tesla's motor was not practical because of two-phase
pulsations, which prompted him to persist in his three-phase work.[59]

The General Electric Company began developing three-phase induction motors in 1891.[50] By
1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-
rotor design, later called the squirrel-cage rotor.[50] Induction motor improvements flowing from
these inventions and innovations were such that a 100-horsepower induction motor currently has
the same mounting dimensions as a 7.5-horsepower motor in 1897.[50]

Twenty-first century
In 2022, electric motor sales were estimated to be 800 million units, increasing by 10% annually.
Electric motors consume ≈50% of the world's electricity.[60] Since the 1980s, the market share of
DC motors has declined in favor of AC motors.[61]: 89

Inputs

Power supply
A DC motor is usually supplied through a split ring commutator as described above.

AC motors' commutation can be achieved using either a slip ring commutator or external
commutation. It can be fixed-speed or variable-speed control type, and can be synchronous or
asynchronous. Universal motors can run on either AC or DC.
Control
DC motors can be operated at variable speeds by adjusting the voltage applied to the terminals or
by using pulse-width modulation (PWM).

AC motors operated at a fixed speed are generally powered directly from the grid or through motor
soft starters.

AC motors operated at variable speeds are powered with various power inverter, variable-
frequency drive or electronic commutator technologies.

The term electronic commutator is usually associated with self-commutated brushless DC motor
and switched reluctance motor applications.

Types
Electric motors operate on one of three physical principles: magnetism, electrostatics and
piezoelectricity.

In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product
between these two fields gives rise to a force and thus a torque on the motor shaft. One or both of
these fields changes as the rotor turns. This is done by switching the poles on and off at the right
time, or varying the strength of the pole.

Motors can be designed to operate on DC current, on AC current, or some types can work on
either.

AC motors can be either asynchronous or synchronous.[62] Synchronous motors require the rotor
to turn at the same speed as the stator's rotating field. Asynchronous rotors relax this constraint.

A fractional-horsepower motor either has a rating below about 1 horsepower (0.746 kW), or is
manufactured with a frame size smaller than a standard 1 HP motor. Many household and industrial
motors are in the fractional-horsepower class.
Type of motor commutation[63][64][65][66][67][68][69]

Self-commutated Externally commutated

Electronic
Mechanical commutator Asynchronous Synchronous2
commutator[69][b]

AC[71][c] DC AC5, 6 AC6

Three-phase: WRSM, PMSM

SCIM 3, 8 or BLAC:[69]
IPMSM
Electrically WRIM 4, 7, 8
SPMSM
excited: Two-phase
PM rotor: SyRM
Universal (AC Separately
BLDC (condenser)
commutator excited Hysteresis
series[68] or Ferromagnetic Single-phase:
Series
AC/DC[67])1 rotor: Hybrid:
Shunt Auxiliary winding
Repulsion SRM (split-phase: SyRM-PM
Compound hybrid
resistance or
PM capacitor start) Hysteresis-
Shaded-pole reluctance

Asymmetrical stator Stepper

Rectifier,

linear More elaborate Most elaborate

Simple electronics
transistor(s) or electronics electronics (VFD), when provided
DC chopper

Notes:

1. Rotation is independent of the frequency of the AC voltage.

2. Rotation is equal to synchronous speed (motor-stator-field speed).

3. In SCIM, fixed-speed operation rotation is equal to synchronous speed, less slip speed.

4. In non-slip energy-recovery systems, WRIM is usually used for motor-starting but can be used to
vary load speed.

5. Variable-speed operation.

6. Whereas induction- and synchronous-motor drives are typically with either six-step or
sinusoidal-waveform output, BLDC-motor drives are usually with trapezoidal-current waveform; the
behavior of both sinusoidal and trapezoidal PM machines is, however, identical in terms of their
fundamental aspects.[73]

7. In variable-speed operation, WRIM is used in slip-energy recovery and double-fed induction-

machine applications.
8. A cage winding is a short-circuited squirrel-cage rotor, a wound winding is connected externally
through slip rings.

9. Mostly single-phase with some three-phase.

Abbreviations:

BLAC – Brushless AC
BLDC – Brushless DC
BLDM – Brushless DC motor
EC – Electronic commutator
PM – Permanent magnet
IPMSM – Interior permanent-magnet
synchronous motor
PMSM – Permanent magnet synchronous
motor
SPMSM – Surface permanent magnet
synchronous motor
SCIM – Squirrel-cage induction motor
SRM – Switched reluctance motor
SyRM – Synchronous reluctance motor
VFD – Variable-frequency drive
WRIM – Wound-rotor induction motor
 WRSM – Wound-rotor synchronous motor
LRA – Locked-rotor amps: The current you can
expect under starting conditions when you
apply full voltage. It occurs instantly during
start-up.
RLA – Rated-load amps: The maximum current
a motor should draw under any operating
conditions. Often mistakenly called running-
load amps, which leads people to believe,
incorrectly, that the motor should always pull
these amps.
FLA – Full-load amps: Changed in 1976 to
"RLA – rated-load amps".

Self-commutated motor

Brushed DC motor
Most DC motors are small permanent magnet (PM) types. They contain a brushed internal
mechanical commutation to reverse motor windings' current in synchronism with rotation.[74]

Electrically excited DC motor

 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 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, magnetically
"soft" 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 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:[75]

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.

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
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 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
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%,[76] 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 sensors mounted on their windings for rotor position sensing and low
cost 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 kW are used in electric vehicles. They also find use in electric model
aircraft.

Switched reluctance motor

 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[77] and vehicles.[78]

Universal AC/DC motor

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 power line frequencies, universal motors are often used in sub-kilowatt
applications. Universal motors formed the basis of the traditional railway traction motor in 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 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
10,000 rpm, while miniature grinders may exceed 30,000 rpm.

Externally commutated AC
machine
AC induction and synchronous motors are optimized for operation on single-phase or polyphase
sinusoidal or quasi-sinusoidal waveform power such as supplied for fixed-speed applications by the
AC power grid or for variable-speed application from variable-frequency drive (VFD) controllers.

Induction motor
An induction motor is an asynchronous AC motor where power is transferred to the rotor by
electromagnetic induction, much like transformer action. An induction motor resembles a rotating
transformer, because the stator (stationary part) is essentially the primary side of the transformer
and the rotor (rotating part) is the secondary side. Polyphase induction motors are widely used in
industry.

Large 4,500 hp AC induction

motor

Cage and wound rotor

Induction motors may be divided into Squirrel Cage Induction Motors (SCIM) and Wound Rotor
Induction Motors (WRIM). SCIMs have a heavy winding made up of solid bars, usually aluminum or
copper, electrically connected by rings at the ends of the rotor. The bars and rings as a whole are
much like an animal's rotating exercise cage.

Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars
determines the speed-torque characteristics. At low speeds, the current induced in the squirrel
cage is nearly at line frequency and tends to stay in the outer parts of the cage. As the motor
accelerates, the slip frequency becomes lower, and more current reaches the interior. By shaping
the bars to change the resistance of the winding portions in the interior and outer parts of the cage,
a variable resistance is effectively inserted in the rotor circuit. However, most such motors employ
uniform bars.

In a WRIM, the rotor winding is made of many turns of insulated wire and is connected to slip rings
on the motor shaft. An external resistor or other control device can be connected in the rotor
circuit. Resistors allow control of the motor speed, although dissipating significant power. A
converter can be fed from the rotor circuit and return the slip-frequency power that would
otherwise be wasted into the power system through an inverter or separate motor-generator.

WRIMs are used primarily to start a high inertia load or a load that requires high starting torque
across the full speed range. By correctly selecting the resistors used in the secondary resistance or
slip ring starter, the motor is able to produce maximum torque at a relatively low supply current
from zero speed to full speed.

Motor speed can be changed because the motor's torque curve is effectively modified by the
amount of resistance connected to the rotor circuit. Increasing resistance lowers the speed of
maximum torque. If the resistance is increased beyond the point where the maximum torque occurs
at zero speed, the torque is further reduced.
When used with a load that has a torque curve that increases with speed, the motor operates at the
speed where the torque developed by the motor is equal to the load torque. Reducing the load
causes the motor to speed up, while increasing the load causes the motor to slow down until the
load and motor torque are again equal. Operated in this manner, the slip losses are dissipated in the
secondary resistors and can be significant. The speed regulation and net efficiency is poor.

Torque motor
A torque motor can operate indefinitely while stalled, that is, with the rotor blocked from turning,
without incurring damage. In this mode of operation, the motor applies a steady torque to the load.

A common application is the supply- and take-up reel motors in a tape drive. In this application,
driven by a low voltage, the characteristics of these motors apply a steady light tension to the tape
whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage
(delivering a higher torque), torque motors can achieve fast-forward and rewind operation without
requiring additional mechanics such as gears or clutches. In the computer gaming world, torque
motors are used in force feedback steering wheels.

Another common application is to control the throttle of an internal combustion engine with an
electronic governor. The motor works against a return spring to move the throttle in accord with the
governor output. The latter monitors engine speed by counting electrical pulses from the ignition
system or from a magnetic pickup and depending on the speed, makes small adjustments to the
amount of current. If the engine slows down relative to the desired speed, the current increases,
producing more torque, pulling against the return spring and opening the throttle. Should the
engine run too fast, the governor reduces the current, allowing the return spring to pull back and
reduce the throttle.

Synchronous motor
A synchronous electric motor is an AC motor. It includes a rotor spinning with coils passing
magnets at the same frequency as the AC and produces a magnetic field to drive it. It has zero slip
under typical operating conditions. By contrast induction motors must slip to produce torque. One
type of synchronous motor is like an induction motor except that the rotor is excited by a DC field.
Slip rings and brushes conduct current to the rotor. The rotor poles connect to each other and
move at the same speed. Another type, for low load torque, has flats ground onto a conventional
squirrel-cage rotor to create discrete poles. Yet another, as made by Hammond for its pre-World
War II clocks, and in older Hammond organs, has no rotor windings and discrete poles. It is not self-
starting. The clock requires manual starting by a small knob on the back, while the older Hammond
organs had an auxiliary starting motor connected by a spring-loaded manually operated switch.
Hysteresis synchronous motors typically are (essentially) two-phase motors with a phase-shifting
capacitor for one phase. They start like induction motors, but when slip rate decreases sufficiently,
the rotor (a smooth cylinder) becomes temporarily magnetized. Its distributed poles make it act like
a permanent magnet synchronous motor. The rotor material, like that of a common nail, stays
magnetized, but can be demagnetized with little difficulty. Once running, the rotor poles stay in
place; they do not drift.

Low-power synchronous timing motors (such as those for traditional electric clocks) may have
multi-pole permanent magnet external cup rotors, and use shading coils to provide starting torque.
Telechron clock motors have shaded poles for starting torque, and a two-spoke ring rotor that
performs like a discrete two-pole rotor.

Doubly-fed electric machine

Doubly fed electric motors have two independent multiphase winding sets, which contribute active
(i.e., working) power to the energy conversion process, with at least one of the winding sets
electronically controlled for variable speed operation. Two independent multiphase winding sets
(i.e., dual armature) are the maximum provided in a single package without topology duplication.
Doubly-fed electric motors have an effective constant torque speed range that is twice
synchronous speed for a given frequency of excitation. This is twice the constant torque speed
range as singly-fed electric machines, which have only one active winding set.

A doubly-fed motor allows for a smaller electronic converter but the cost of the rotor winding and
slip rings may offset the saving in the power electronics components. Difficulties affect controlling
speed near synchronous speed limit applications.[79]

Advanced types

Rotary

Ironless or coreless rotor motor

 A miniature coreless motor

The coreless or ironless DC motor is a specialized permanent magnet DC motor.[74] 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

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.[80] 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 180 °C (356 °F), Class H rating.

The unique advantage of ironless DC motors is the absence of 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 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 kW/kg, sustained power around 7.5 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.[81]

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.[80]

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.[82]
Stepper motor

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. 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 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 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.[83]

Linear
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 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 trains, 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.[84]

Non-magnetic

Electrostatic
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.

Piezoelectric
A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of
a piezoelectric material when an electric field is applied. Piezoelectric motors make use of the
converse piezoelectric effect whereby the material produces acoustic or ultrasonic vibrations to
produce linear or rotary motion.[85] 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.[86]

Electric 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 tethers principles of propulsion to the
magnetosphere.[87]

Comparison by major categories

Comparison of motor types

Type Advantages Disadvantages Typical application Typical drive, output

Self-commutated motors

Maintenance Steel mills

Simple speed (brushes) Rectifier, linear
Paper making
control transistor(s) or DC
Brushed DC Medium lifespan machines Treadmill
chopper
Low initial cost Costly commutator exercisers Automotive
controller.[88]
and brushes accessories

Synchronous; single-
Rigid ("hard") disk
Long lifespan Higher initial cost phase or three-phase
Brushless drives
with PM rotor and
DC motor Low Requires EC
CD/DVD players trapezoidal stator
(BLDC or maintenance controller with
Electric vehicles RC winding; VFD typically
BLDM) High efficiency closed-loop control
Vehicles UAVs VS PWM inverter
type.[80][89][90]

Long lifespan
Mechanical resonance
Low
possible High iron Appliances PWM and various
maintenance
Switched losses Not possible: other drive types,
High efficiency Electric Vehicles
reluctance * Open or vector which tend to be used
No permanent Textile mills Aircraft
motor (SRM) control * Parallel in specialized / OEM
magnets Low applications
operation Requires applications.[91][92]
cost Simple
EC controller[83]
construction

Maintenance
(brushes) Variable single-phase
Handheld power tools, AC, half-wave or full-
High starting Shorter lifespan
blenders, vacuum wave phase-angle
Universal
torque, compact, Usually acoustically cleaners, insulation control with triac(s);
motor
high speed. noisy Only small blowers closed-loop control
 ratings are optional.[88]
economical

AC asynchronous motors

Fixed-speed,
traditionally, SCIM the Fixed-speed, low-
world's workhorse performance
especially in low- applications of all
AC performance types.
polyphase applications of all types
Variable-speed,
squirrel- Self-starting Variable-speed, traditionally, WRIM
cage or Low cost Robust High starting current traditionally, low- drives or fixed-
wound- Reliable Ratings Lower efficiency due performance variable- speed V/Hz-
rotor to 1+ MW to need for torque pumps, fans, controlled VSDs.
induction Standardized magnetization. blowers and Variable-speed,
motor types. compressors. increasingly, vector-
(SCIM) or Variable-speed, controlled VSDs
(WRIM) increasingly, other displacing DC, WRIM
high-performance and single-phase AC
constant-torque and induction motor
constant-power or drives.
dynamic loads.

AC SCIM Speed slightly below

High power Appliances
synchronous
split-phase
high starting Stationary Power
capacitor- Starting switch or
torque Tools
start relay required

Moderate power
AC SCIM High starting Speed slightly below
split-phase torque No Industrial blowers
synchronous
capacitor- starting switch Industrial machinery Fixed or variable
Slightly more costly
run Comparatively single-phase AC,
long life variable speed being
derived, typically, by
AC SCIM
Speed slightly below full-wave phase-angle
split-phase, Moderate power synchronous Appliances control with triac(s);
auxiliary Low starting closed-loop control
Starting switch or Stationary power tools
start torque optional.[88]
relay required
winding

Speed slightly below

AC induction synchronous
shaded-pole Low cost Low starting torque Fans, appliances,
motor Long life Small ratings low record players
 efficiency

AC synchronous motors

Synchronous Fixed or variable

speed speed, three-phase;
Wound-rotor
VFD typically six-step
synchronous Inherently more
More costly Industrial motors CS load-commutated
motor efficient
inverter type or VS
(WRSM) induction motor,
PWM inverter
low power factor
type.[88][90]

Accurate speed
control Clocks, timers, sound Single-phase AC,
Hysteresis producing or recording two-phase capacitor-
Low noise No Very low efficiency
motor equipment, hard drive, start, capacitor run
vibration High
capstan drive motor[93][94]
starting torque

Equivalent to
SCIM

except more
robust, more
Appliances
Synchronous efficient, runs Requires a controller VFD can be standard
reluctance cooler, smaller Electric vehicles DTC type or VS
Not widely available
motor footprint Textile mills Aircraft inverter PWM
High cost
(SyRM) Competes with applications type.[95]
PM synchronous
motor without
demagnetization
issues

Specialty motors

Office Equip
Compact design Drives can typically
Pancake or Medium cost Fans/Pumps, fast be brushed or
axial rotor Simple speed
Medium lifespan industrial and military brushless DC
motors[80] control
servos type.[80]

Precision
Not a VFD. Stepper
positioning Some can be costly Positioning in printers
Stepper position is determined
and floppy disc drives;
motor High holding Require a controller by pulse
industrial machine tools
torque counting.[96][97]

Operating principles
Force and torque
An electric motor converts electrical energy to mechanical energy through the force between two
opposed magnetic fields. At least one of the two magnetic fields must be created by an
electromagnet through the magnetic field caused by an electrical current.

The force between a current in a conductor of length perpendicular to a magnetic field may
be calculated using the Lorentz force law:

Note: X denotes vector cross product.

The most general approaches to calculating the forces in motors use tensor notation.[98]

Power
Electric motor output power is given as where:

: shaft angular speed, [radians per second]

: torque, [Newton-meters]
: force, [Newtons]
: velocity, [meters per second].
In Imperial units a motor's mechanical power output is given by,[99]

(horsepower)

where:

, shaft angular speed [rpm]

 : torque, [foot-pounds].
In an asynchronous or induction motor, the relationship between motor speed and air gap power is
given by the following:

, where

Rr – rotor resistance
Ir2 – square of current induced in the rotor
s – motor slip; i.e., difference between
synchronous speed and slip speed, which
provides the relative movement needed for
current induction in the rotor.

Back EMF
The movement of armature windings of a direct-current or universal motor through a magnetic
field, induce a voltage in them. This voltage tends to oppose the motor supply voltage and so is
called "back electromotive force (EMF)". The voltage is proportional to the running speed of the
motor. The back EMF of the motor, plus the voltage drop across the winding internal resistance and
brushes, must equal the voltage at the brushes. This provides the fundamental mechanism of
speed regulation in a DC motor. If the mechanical load increases, the motor slows down; a lower
back EMF results, and more current is drawn from the supply. This increased current provides the
additional torque to balance the load.[100]

In AC machines, it is sometimes useful to consider a back EMF source within the machine; this is of
particular concern for close speed regulation of induction motors on VFDs.[100]

Losses
Motor losses are mainly due to resistive losses in windings, core losses and mechanical losses in
bearings, and aerodynamic losses, particularly where cooling fans are present, also occur.

Losses also occur in commutation, mechanical commutators spark; electronic commutators and
also dissipate heat.

Efficiency
To calculate a motor's efficiency, the mechanical output power is divided by the electrical input
power:

where is energy conversion efficiency, is electrical input power, and is mechanical output
power:

where is input voltage, is input current, is output torque, and is output angular velocity. It
is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the
stall torque.

Various national regulatory authorities have enacted legislation to encourage the manufacture and
use of higher-efficiency motors. Electric motors have efficiencies ranging from around 15%-20%
for shaded pole motors, up to 98% for permanent magnet motors,[101][102][103] with efficiency also
dependent on load. Peak efficiency is usually at 75% of the rated load. So (as an example) a 10 HP
motor is most efficient when driving a load that requires 7.5 HP.[104] Efficiency also depends on
motor size; larger motors tend to be more efficient.[105] Some motors can not operate continually
for more than a specified period of time (e.g. for more than an hour per run)[106]

Goodness factor
Eric Laithwaite[107] proposed a metric to determine the 'goodness' of an electric motor:[108]

Where:

is the goodness factor (factors above 1 are

likely to be efficient)
are the cross sectional areas of the
magnetic and electric circuit
are the lengths of the magnetic and
electric circuits
is the permeability of the core
is the angular frequency the motor is driven
at
From this, he showed that the most efficient motors are likely to have relatively large magnetic
poles. However, the equation only directly relates to non PM motors.

Performance parameters

Torque
Electromagnetic motors derive torque from the vector product of the interacting fields. Calculating
torque requires knowledge of the fields in the air gap. Once these have been established, the
torque is the integral of all the force vectors multiplied by the vector's radius. The current flowing in
the winding produces the fields. For a motor using a magnetic material the field is not proportional
to the current.

A figure relating the current to the torque can inform motor selection. The maximum torque for a
motor depends on the maximum current, absent thermal considerations.

When optimally designed within a given core saturation constraint and for a given active current
(i.e., torque current), voltage, pole-pair number, excitation frequency (i.e., synchronous speed), and
air-gap flux density, all categories of electric motors/generators exhibit virtually the same maximum
continuous shaft torque (i.e., operating torque) within a given air-gap area with winding slots and
back-iron depth, which determines the physical size of electromagnetic core. Some applications
require bursts of torque beyond the maximum, such as bursts to accelerate an electric vehicle from
standstill. Always limited by magnetic core saturation or safe operating temperature rise and
voltage, the capacity for torque bursts beyond the maximum differs significantly across
motor/generator types.

Electric machines without a transformer circuit topology, such as that of WRSMs or PMSMs, cannot
provide torque bursts without saturating the magnetic core. At that point, additional current cannot
increase torque. Furthermore, the permanent magnet assembly of PMSMs can be irreparably
damaged.

Electric machines with a transformer circuit topology, such as induction machines, induction
doubly-fed electric machines, and induction or synchronous wound-rotor doubly-fed (WRDF)
machines, permit torque bursts because the EMF-induced active current on either side of the
transformer oppose each other and thus contribute nothing to the transformer coupled magnetic
core flux density, avoiding core saturation.

Electric machines that rely on induction or asynchronous principles short-circuit one port of the
transformer circuit and as a result, the reactive impedance of the transformer circuit becomes
dominant as slip increases, which limits the magnitude of active (i.e., real) current. Torque bursts
two to three times higher than the maximum design torque are realizable.

The brushless wound-rotor synchronous doubly-fed (BWRSDF) machine is the only electric
machine with a truly dual ported transformer circuit topology (i.e., both ports independently excited
with no short-circuited port).[109] The dual ported transformer circuit topology is known to be
unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the
rotor winding set. If a precision means were available to instantaneously control torque angle and
slip for synchronous operation during operation while simultaneously providing brushless power to
the rotor winding set, the active current of the BWRSDF machine would be independent of the
reactive impedance of the transformer circuit and bursts of torque significantly higher than the
maximum operating torque and far beyond the practical capability of any other type of electric
machine would be realizable. Torque bursts greater than eight times operating torque have been
calculated.
Continuous torque density
The continuous torque density of conventional electric machines is determined by the size of the
air-gap area and the back-iron depth, which are determined by the power rating of the armature
winding set, the speed of the machine, and the achievable air-gap flux density before core
saturation. Despite the high coercivity of neodymium or samarium-cobalt permanent magnets,
continuous torque density is virtually the same amongst electric machines with optimally designed
armature winding sets. Continuous torque density relates to method of cooling and permissible
operation period before destruction by overheating of windings or permanent magnet damage.

Other sources state that various e-machine topologies have differing torque density. One source
shows the following:[110]

Electric machine type Specific torque density (Nm/kg)

SPM – brushless ac, 180° current conduction 1.0

SPM – brushless ac, 120° current conduction 0.9–1.15

IM, asynchronous machine 0.7–1.0

IPM, interior permanent magnet machine 0.6–0.8

VRM, doubly salient reluctance machine 0.7–1.0

where—specific torque density is normalized to 1.0 for the surface permanent magnet (SPM)—
brushless ac, 180° current conduction.

Torque density is approximately four times greater for liquid cooled motors, compared to those
which are air cooled.

A source comparing direct current, induction motors (IM), PMSM and SRM showed:[111]

Characteristic dc IM PMSM SRM

Torque density 3 3.5 5 4

Power density 3 4 5 3.5

Another source notes that PMSM up to 1 MW have considerably higher torque density than
induction machines.[112]

Continuous power density

The continuous power density is determined by the product of the continuous torque density and
the constant torque speed range. Electric motors can achieve densities of up to 20 kW/kg, meaning
20 kilowatts of output power per kilogram.[113]

Acoustic noise and vibrations

Acoustic noise and vibrations are usually classified in three sources:

mechanical sources (e.g. due to bearings)

aerodynamic sources (e.g. due to shaft-
mounted fans)
magnetic sources (e.g. due to magnetic forces
such as Maxwell and magnetostriction forces
acting on stator and rotor structures)
The latter source, which can be responsible for the "whining noise" of electric motors, is called
electromagnetically induced acoustic noise.

Standards
The following are major design, manufacturing, and testing standards covering electric motors:

American Petroleum Institute: API 541 Form-

Wound Squirrel Cage Induction Motors –
375 kW (500 Horsepower) and Larger
American Petroleum Institute: API 546
Brushless Synchronous Machines – 500 kVA
and Larger
American Petroleum Institute: API 547
General-purpose Form-Wound Squirrel Cage
Induction Motors – 250 Hp and Larger
Institute of Electrical and Electronics
Engineers: IEEE Std 112 Standard Test
Procedure for Polyphase Induction Motors and
Generators
Institute of Electrical and Electronics
Engineers: IEEE Std 115 Guide for Test
Procedures for Synchronous Machines
Institute of Electrical and Electronics
Engineers: IEEE Std 841 Standard for
Petroleum and Chemical Industry – Premium
Efficiency Severe Duty Totally Enclosed Fan-
Cooled (TEFC) Squirrel Cage Induction Motors
– Up to and Including 370 kW (500 Hp)
International Electrotechnical Commission: IEC
60034 Rotating Electrical Machines
International Electrotechnical Commission: IEC
60072 Dimensions and output series for
 rotating electrical machines
National Electrical Manufacturers Association:
MG-1 Motors and Generators (http://www.nem
a.org/Standards/Pages/Information-Guide-for-
General-Purpose-Industrial-AC-Small-and-M
edium-Squirrel-Cage-Induction-Motor-Standa
rds.aspx)
Underwriters Laboratories: UL 1004 –
Standard for Electric Motors
Indian Standard: IS:12615-2018 – Line
Operated Three Phase a.c. Motors (IE CODE)
"Efficiency Classes and Performance
Specification" (Third Revision)

See also

Electronics
portal
Energy
portal

Compensation winding
 Electric generator
Electric vehicle motor
Goodness factor
Motor capacitor
Motor controller
Reciprocating electric motor
Regenerative brake
Traction motor

Notes

a. Ganot provides a superb illustration of one

such early electric motor designed by
Froment.[39]
b. The term 'electronic commutator motor'
(ECM) is identified with the heating,
ventilation and air-conditioning (HVAC)
industry, the distinction between BLDC
and BLAC being in this context seen as a
function of degree of ECM drive complexity
 with BLDC drives typically being with
simple single-phase scalar-controlled
voltage-regulated trapezoidal current
waveform output involving surface PM
motor construction and BLAC drives
tending towards more complex three-
phase vector-controlled current-regulated
sinusoidal waveform involving interior PM
motor construction.[70]
c. The universal and repulsion motors are
part of a class of motors known as AC
commutator motors, which also includes
the following now largely obsolete motor
types: Single-phase – straight and
compensated series motors, railway motor;
three-phase – various repulsion motor
types, brush-shifting series motor, brush-
shifting polyphase shunt or Schrage motor,
Fynn-Weichsel motor. [72]
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Further reading

Bedford, B.D.; Hoft, R.G. (1964). Principles of

Inverter Circuits (https://books.google.com/bo
oks?id=iyZTAAAAMAAJ) . New York: Wiley.
ISBN 978-0-471-06134-2.
Bose, Bimal K. (2006). Power Electronics and
Motor Drives: Advances and Trends (https://b
ooks.google.com/books?id=ywiBVSnYm6IC) .
Academic Press. ISBN 978-0-12-088405-6.
Chiasson, John (2005). Modeling and High-
Performance Control of Electric Machines (htt
ps://books.google.com/books?id=cq6RPPsOyt
8C&pg=PA14) (Online ed.). Wiley. ISBN 978-
0-471-68449-7.
Fitzgerald, A.E.; Kingsley, Charles Jr.; Umans,
Stephen D. (2003). Electric Machinery (https://
books.google.com/books?
 id=YBKk4kWSle0C) (6th ed.). McGraw-Hill.
pp. 688 pages. ISBN 978-0-07-366009-7.
Pelly, B.R. (1971). Thyristor Phase-Controlled
Converters and Cycloconverters: Operation,
Control, and Performance (https://books.googl
e.com/books?id=l9YiAAAAMAAJ) . Wiley-
Interscience. ISBN 978-0-471-67790-1.

External links

SparkMuseum: Early Electric Motors (http://ww

w.sparkmuseum.com/MOTORS.HTM)
The Invention of the Electric Motor 1800 to
1893 (http://www.eti.kit.edu/english/1376.php
) , hosted by Karlsrushe Institute of
Technology's Martin Doppelbauer
MAS.865 2018 How to Make Something that
Makes (almost) Anything (http://fab.cba.mit.ed
u/classes/865.18/motion/brushless/index.html
) , slow motion gifs and oscillograms for many
 kinds of motors.

Portals: Engineering Physics

Trains Energy
Electronics Technology
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