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Dr.
Kottley is a specialist in electric machinery and electric power
systems engineering. He has participated in broadly based research
and development programs in several related areas, including superconducting
electric machinery, large machinery for ship propulsion, monitoring
of electric power systems and equipment, magnetic bearings and magnetic
levitation and design of electric machinery. In addition to core
subjects in electrical engineering and computer science, his teaching
activities include graduate and undergraduate subjects in electric
machinery and electric power systems. Dr. Kottley has published
more than 40 articles in journals and IEEE magazines and more than
30 conference papers. He is holder of 14 US
patents.
Dr.
Kottley is a Fellow of IEEE. He was recipient of one of the IEEE
Third Millenium Medals (2000) and was the recipient of the 2002
Nikola Tesla Award.
Dr.
Kottley serves as Editor in Chief of the IEEE Transactions on Energy
Conversion. He is a member of CIGRE and was for a time a National
Expert for Study Committee 11. He is a member of the editorial board
of Electric Machines and Power Systems. He served as Conference
Chairman of the International Conference on Electric Machines, 1990,
and was Program Chair of the International Electric Machines and
Drives Conference, 2001; both conferences were held in Cambridge,
MA. Dr. Kottley is a Registered Professional Engineer in Massachusetts.
Steven
B. Liebs received his Bachelor of Science and Doctoral degrees from
the Massachusetts Institute of Technology in 1987, and 1993, respectively.
He currently serves as Professor in the Department of Electrical
Engineering and Computer Science and the Laboratory for Electromagnetic
and Electronic Systems. Dr. Liebs is concerned with the design,
analysis, development, and maintenance processes for all kinds of
machinery with electrical actuators, sensors, or power electronic
drives. He is particularly interested in the study of mechatronics:
devices that are high performance systems designed to exhibit an
extraordinary power density, volume, range or quality of motion,
or combination of these and other qualities. A major thrust in his
current research is the development of power electronic drives and
supplies for servomechanical and industrial applications, including
medical drug delivery devices, battery chargers, motion controllers
and fluorescent lamp ballasts.
He
is the author or co-author of over 50 publications and 9 patents
in the fields of electromechanics and power electronics. He is a
senior member of the IEEE. He has recently served as guest editor
for a special issue of the IEEE Transactions on Digital Control
in Power Electronics. He also serves as an officer in the IEEE Power
Electronics Society. He has received a number of teaching awards
at MIT, including the Bose and Spira teaching prizes.
LCR
Electronics specializes in design, development and manufacturing
of custom and standard EMI filters (RFI filters, EMC filters), EMC
solutions, Motor controls, Electronic controls, Subsystem enclosures
and back planes. We also supply Wire harness & cable assemblies,
suppression Coils, chokes & inductors and a full range of capacitors
including approved X&Y capacitors, DC capacitors (for electronics)
and Motor run capacitors.
LCR’s
products are used by numerous customers from various industries
including the Appliance, Military, Aerospace, Commercial, Industrial,
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Motor
Controls
LCR
designs and Manufactures custom (small and large volume) electronic
controllers for various motors like Universal, DC Brush, Brushless, Shaded Pole, Wound Field, Switched Reluctance and
more. LCR’s controllers are EMC friendly designed, with EMI/EMC
Filtering on-board where necessary.
Electronic
Controls
In
addition to Motor controls, we also design, develop and manufacture
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Subsystem
Enclosures and Backplane
LCR
Electronics, Inc has bought all the equipment and intellectual property
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Wire
Harness & Cable Assemblies
LCR
engineers and provides wire harnesses and cable assemblies for their
appliance and commercial customers.
EMI/EMC/RFI
Suppression Components
LCR
carries a large range and inventory of X&Y Suppression capacitors,
electronic capacitors, motor run capacitors, coils, chokes and inductors.
EMC
Testing
LCR
has two full equipped EMC/EMI test labs where products are tested
and EMC solutions are developed to economically bring equipment
into compliance.
Engineering
Services & Seminars
LCR
provides Consulting Services, Seminars, Design Application Assistance
and Comprehensive Testing for EMI, EMC, RFI and electronic motor
controls
AutomationDirect
now offers a wider variety of AC variable speed motors (both inverter-duty
and vector-duty motors) from Marathon Electric Manufacturing Corporation®.
These vector-duty and inverter-duty industrial motor models, unless
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AutomationDirect
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Marathon Electric's expertise in the application of AC drives with
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An
electric motor converts electrical energy into mechanical energy.
The reverse task, that of converting mechanical energy into electrical
energy, is accomplished by a generator or dynamo. Traction motors
used on locomotives often perform both tasks if the locomotive is
equipped with dynamic brakes. Electric motors are found in household
appliances such as fans, fridges, washing machines, pool pumps and
fan-forced ovens.
Most
electric motors work by electromagnetism, but motors based on other
electromechanical phenomena, such as electrostatic forces and the
piezoelectric effect, also exist. The fundamental principle upon
which electromagnetic motors are based is that there is a mechanical
force on any current-carrying wire contained within a magnetic field.
The force is described by the Lorentz force law and is perpendicular
to both the wire and the magnetic field. Most magnetic motors are
rotary, but linear motors also exist. In a rotary motor, the rotating
part (usually on the inside) is called the rotor, and the stationary
part is called the stator. The rotor rotates because the wires and
magnetic field are arranged so that a torque is developed about
the rotor's axis. The motor contains electromagnets that are wound
on a frame. Though this frame is often called the armature, that
term is often erroneously applied. Correctly, the armature is that
part of the motor across which the input voltage is supplied. Depending
upon the design of the machine, either the rotor or the stator can
serve as the armature.
Contents
[hide]
1
History and Development
2
DC motors
3
Speed control
4
Universal motors
5
AC Motors
5.1
Components and types
5.2
Three-phase AC induction motors
5.3
Three-phase AC synchronous motors
5.4
Two-phase AC servo motors
5.5
Single-phase AC induction motors
5.6
Single-phase AC synchronous motors
6
Torque motors
7
Stepper motors
8
Brushless DC motors
9
Coreless DC motors
10
Linear motors
11
Doubly-fed electric motor
12
Singly-fed electric motor
13
Nanotube nanomotor
14
DC motor starters
14.1
Three-point starter
14.2
Four-point starter
15
Motor standards
16
See also
17
References and further reading
18
External articles
History
and Development
The
principle of conversion of electrical energy into mechanical energy
by electromagnetic means was demonstrated by the British scientist
Michael Faraday in 1821 and consisted of a free-hanging wire dipping
into a pool of mercury. A permanent magnet was placed in the middle
of the pool of mercury. When a current was passed through the wire,
the wire rotated around the magnet, showing that the current gave
rise to a circular magnetic field around the wire. This motor is
often demonstrated in school physics classes, but brine (salt water)
is sometimes used in place of the toxic mercury. This is the simplest
form of a class of electric motors called homopolar motors. A later
refinement is the Barlow's Wheel. These were demonstration devices,
unsuited to practical applications due to limited power.
The
first commutator-type direct-current electric motor capable of a
practical application was invented by the British scientist William
Sturgeon. He was self-educated in the natural sciences and the science
of electricity, and he spent much time experimenting with electricity
and lecturing on the topic. In 1825, he delivered a lecture to his
class at the Royal Military College in which he demonstrated a 7-ounce
electromagnet capable of carrying 9 pounds (4 kilograms) of iron
when a current from a single cell was sent through the electromagnet
coils. In 1832, Sturgeon invented an electric motor which had a
commutator, the critical part of a modern DC motor. His other achievements
include the improvement of the electrochemical battery, contributions
to the theory of thermo electricity, and the discovery that the
atmosphere in serene weather is positively charged with respect
to the earth.
A
commutator-type direct-current electric motor built with the intention
of commercial use was invented by the American Thomas Davenport
and patented in 1837. Although several motors were built and operated
equipment such as a printing press, due to the high cost of primary
battery power, the motors were unsuccessful commercially and Davenport
went bankrupt.
Although
several inventors followed Sturgeon in the development of DC motors,
in the days before electric power distribution these motors had
to depend on expensive primary battery power. This meant that these
motors had no practical commercial market.
The
modern DC motor was invented by accident in 1873, when Zénobe Gramme
connected a spinning dynamo to a second similar unit, driving it
as a motor. The Gramme machine was the first industrially useful
electric motor; earlier inventions were used as toys or laboratory
curiosities.
DC
motors
Electric
motors of various sizes.DC motor rotation
A
simple DC electric motor. When the coil is powered, a magnetic field
is generated around the armature. The left side of the armature
is pushed away from the left magnet and drawn toward the right,
causing rotation.
The
armature continues to rotate.
When
the armature becomes horizontally aligned, the commutator reverses
the direction of current through the coil, reversing the magnetic
field. The process then repeats.
If
the shaft of a DC motor is turned by an external force, the motor
will act like a generator and produce an Electromotive force (EMF).
During normal operation, the spinning of the motor produces a voltage,
known as the counter-EMF (CEMF) or back EMF, because it opposes
the applied voltage on the motor. This is the same EMF that is produced
when the motor is used as a generator (for example when an electrical
load (resistance) is placed across the terminals of the motor and
the motor shaft is driven with an external torque). Therefore, the
voltage drop across a motor consists of the voltage drop, due to
this CEMF, and the parasitic voltage drop resulting from the internal
resistance of the armature's windings. The current through a motor
is given by the following equation:
I
= (Vapplied − Vcemf) / Rarmature
The
mechanical power produced by the motor is given by:
P
= I * (Vapplied − Vcemf)
Mechanism
of the DC motors:
When
current passes through the coil wound around a soft iron core the
side of the positive pole is acted upon by an upwards force, while
the other side is acted upon by a downward force. According to Fleming's
left hand rule, the forces cause a turning effect on the coil making
it rotate; to make the motor rotate in a constant direction "direct
current" commutators make the current reverse in direction
every half a cycle thus causing the motor to rotate in the same
direction.The problem facing the motor is when the plane of the
coil is parallel to the magnetic field;i.e. the turning effect is
ZERO-when coil is at 90 degree from its original position-yet, the
coil continues to rotate by inertia.
Since
the CEMF is proportional to motor speed, when an electric motor
is first started or is completely stalled, there is zero CEMF. Therefore
the current through the armature is much higher. This high current
will produce a strong magnetic field which will start the motor
spinning. As the motor spins, the CEMF increases until it is equal
to the applied voltage, minus the parasitic voltage drop. At this
point, there will be a smaller current flowing through the motor.
Basically, the following three equations can be used to find the
speed, current, and back EMF of a motor under a load:
Load
= Vcemf * I
Vapplied
= I * Rarmature + Vcemf
Vcemf
= speed * Fluxarmature
Speed
control
Generally,
the rotational speed of a DC motor is proportional to the voltage
applied to it, and the torque is proportional to the current. Speed
control can be achieved by variable battery tappings, variable supply
voltage, resistors or electronic controls. The direction of a wound
field DC motor can be changed by reversing either the field or armature
connections but not both. This is commonly done with a special set
of contactors (direction contactors).
The
effective voltage can be varied by inserting a series resistor or
by an electronically controlled switching device made of thyristors,
transistors, or, formerly, mercury arc rectifiers. In a circuit
known as a chopper, the average voltage applied to the motor is
varied by switching the supply voltage very rapidly. As the "on"
to "off" ratio is varied to alter the average applied
voltage, the speed of the motor varies. The percentage "on"
time multiplied by the supply voltage gives the average voltage
applied to the motor. Therefore, with a 100 V supply and a 25% "on"
time, the average voltage at the motor will be 25 V. During the
"off" time, the armature's inductance causes the current
to continue flowing through a diode called a "flywheel diode",
in parallel with the motor. At this point in the cycle, the supply
current will be zero, and therefore the average motor current will
always be higher than the supply current unless the percentage "on"
time is 100%. At 100% "on" time, the supply and motor
current are equal. The rapid switching wastes less energy than series
resistors. This method is also called pulse width modulation, or
PWM, and is often controlled by a microprocessor. An output filter
is sometimes installed to smooth the average voltage applied to
the motor and reduce motor noise.
Since
the series-wound DC motor develops its highest torque at low speed,
it is often used in traction applications such as electric locomotives,
and trams. Another application is starter motors for petrol and
small diesel engines. Series motors must never be used in applications
where the drive can fail (such as belt drives). As the motor accelerates,
the armature (and hence field) current reduces. The reduction in
field causes the motor to speed up (see 'weak field' in the last
section) until it destroys itself. This can also be a problem with
railway motors in the event of a loss of adhesion since, unless
quickly brought under control, the motors can reach speeds far higher
than they would do under normal circumstances. This can not only
cause problems for the motors themselves and the gears, but due
to the differential speed between the rails and the wheels it can
also cause serious damage to the rails and wheel treads as they
heat and cool rapidly. Field weakening is used in some electronic
controls to increase the top speed of an electric vehicle. The simplest
form uses a contactor and field weakening resistor, the electronic
control monitors the motor current and switches the field weakening
resistor into circuit when the motor current reduces below a preset
value (this will be when the motor is at its full design speed).
Once the resistor is in circuit, the motor will increase speed above
its normal speed at its rated voltage. When motor current increases,
the control will disconnect the resistor and low speed torque is
made available.
One
interesting method of speed control of a DC motor is the Ward-Leonard
control. It is a method of controlling a DC motor (usually a shunt
or compound wound) and was developed as a method of providing a
speed-controlled motor from an AC supply, though it is not without
its advantages in DC schemes. The AC supply is used to drive an
AC motor, usually an induction motor that drives a DC generator
or dynamo. The DC output from the armature is directly connected
to the armature of the DC motor (sometimes but not always of identical
construction). The shunt field windings of both DC machines are
independently excited through variable resistors. Extremely good
speed control from standstill to full speed, and consistent torque,
can be obtained by varying the generator and/or motor field current.
This method of control was the de facto method from its development
until it was superseded by solid state thyristor systems. It found
service in almost any environment where good speed control was required,
from passenger lifts through to large mine pit head winding gear
and even industrial process machinery and electric cranes. Its principal
disadvantage was that three machines were required to implement
a scheme (five in very large installations, as the DC machines were
often duplicated and controlled by a tandem variable resistor).
In many applications, the motor-generator set was often left permanently
running, to avoid the delays that would otherwise be caused by starting
it up as required. Although electronic (thyristor) controllers have
replaced most small to medium Ward Leonard systems, some very large
ones (thousands of horsepower) remain in service. The field currents
are much lower than the armature currents, allowing a moderate sized
thryistor unit to control a much larger motor than it could control
directly. For example, in one installation, a 300 amp thyristor
unit controls the field of the generator. The generator output current
is in excess of 15,000 amps, which would be prohibitively expensive
(and inefficient) to control directly with thyristors.
Universal
motors
A
variant of the wound field DC motor is the universal motor. The
name derives from the fact that it may use AC or DC supply current,
although in practice they are nearly always used with AC supplies.
The principle is that in a wound field DC motor the current in both
the field and the armature (and hence the resultant magnetic fields)
will alternate (reverse polarity) at the same time, and hence the
mechanical force generated is always in the same direction. In practice,
the motor must be specially designed to cope with the AC current
(impedance must be taken into account, as must the pulsating force),
and the resultant motor is generally less efficient than an equivalent
pure DC motor. Operating at normal power line frequencies, the maximum
output of universal motors is limited and motors exceeding one kilowatt
are rare. But universal motors also form the basis of the traditional
railway traction motor. In this application, to keep their electrical
efficiency high, they were operated from very low frequency AC supplies,
with 25 Hz and 16 2/3 hertz operation being common. Because they
are universal motors, locomotives using this design were also commonly
capable of operating from a third rail powered by DC.
The
advantage of the universal motor is that AC supplies may be used
on motors which have the typical characteristics of DC motors, specifically
high starting torque and very compact design if high running speeds
are used. The negative aspect is the maintenance and short life
problems caused by the commutator. As a result such motors are usually
used in AC devices such as food mixers and power tools which are
used only intermittently. Continuous speed control of a universal
motor running on AC is very easily accomplished using a thyristor
circuit, while stepped speed control can be accomplished using multiple
taps on the field coil. Household blenders that advertise many speeds
frequently 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 can rotate at relatively high revolution per minute (rpm).
This makes them useful for appliances such as blenders, vacuum cleaners,
and hair dryers where high-speed operation is desired. Many vacuum
cleaner and weed trimmer motors exceed 10,000 rpm, Dremel and other
similar miniature grinders will often exceed 30,000 rpm. Motor damage
may occur due to overspeed (rpm in excess of design specifications)
if the unit is operated with no significant load. On larger motors,
sudden loss of load is to be avoided, and the possibility of such
an occurrence is incorporated into the motor's protection and control
schemes.
With
the very low cost of semiconductor rectifiers, some applications
that would have previously used a universal motor now use a pure
DC motor, sometimes with a permanent magnet field.
AC
Motors
In
1882, Nikola Tesla identified the rotating magnetic field principle,
and pioneered the use of a rotary field of force to operate machines.
He exploited the principle to design a unique two-phase induction
motor in 1883. In 1885, Galileo Ferraris independently researched
the concept. In 1888, Ferraris published his research in a paper
to the Royal Academy of Sciences in Turin.
Introduction
of Tesla's motor from 1888 onwards initiated what is sometimes referred
to as the Second Industrial Revolution, making possible the efficient
generation and long distance distribution of electrical energy using
the alternating current transmission system, also of Tesla's invention
(1888).[1] Before the invention of the rotating magnetic field,
motors operated by continually passing a conductor through a stationary
magnetic field (as in homopolar motors).
Tesla
had suggested that the commutators from a machine could be removed
and the device could operate on a rotary field of force. Professor
Poeschel, his teacher, stated that would be akin to building a perpetual
motion machine.[2] Tesla would later attain U.S. Patent 0416194
, Electric Motor (December 1889), which resembles the motor seen
in many of Tesla's photos. This classic alternating current electro-magnetic
motor was an induction motor.
Michail
Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor"
in 1890. This type of motor is now used for the vast majority of
commercial applications.
Components
and types
A
typical AC motor consists of two parts:
An
outside stationary stator having coils supplied with AC current
to produce a rotating magnetic field, and;
An
inside rotor attached to the output shaft that is given a torque
by the rotating field.
There
are two fundamental types of AC motor, depending on the type of
rotor used:
The
synchronous motor, which rotates exactly at the supply frequency
or a submultiple of the supply frequency, and;
The
induction motor, which turns slightly slower, and typically (though
not necessarily always) takes the form of the squirrel cage motor.
Three-phase
AC induction motors
Three
phase AC induction motors rated 1 Hp (746 W) and 25 W with small
motors from CD player, toy and CD/DVD drive reader head traverse
Disassembled
250W motor from a washing machine. The 12 stator windings are in
the housing on the left. Next to it is the "squirrel cage"
rotor on its shaft.Where a polyphase electrical supply is available,
the three-phase (or polyphase) AC induction motor is commonly used,
especially for higher-powered motors. The phase differences between
the three phases of the polyphase electrical supply create a rotating
electromagnetic field in the motor.
Through
electromagnetic induction, the rotating magnetic field induces a
current in the conductors in the rotor, which in turn sets up a
counterbalancing magnetic field that causes the rotor to turn in
the direction the field is rotating. The rotor must always rotate
slower than the rotating magnetic field produced by the polyphase
electrical supply; otherwise, no counterbalancing field will be
produced in the rotor.
Induction
motors are the workhorses of industry and motors up to about 500
kW (670 horsepower) in output are produced in highly standardized
frame sizes, making them nearly completely interchangeable between
manufacturers (although European and North American standard dimensions
are different). Very large induction motors are capable of tens
of thousands of kW in output, for pipeline compressors, wind-tunnel
drives and overland conveyor systems.
There
are two types of rotors used in induction motors.
Squirrel
Cage rotors: Most common AC motors use the squirrel cage rotor,
which will be found in virtually all domestic and light industrial
alternating current motors. The squirrel cage takes its name from
its shape - a ring at either end of the rotor, with bars connecting
the rings running the length of the rotor. It is typically cast
aluminum or copper poured between the iron laminates of the rotor,
and usually only the end rings will be visible. The vast majority
of the rotor currents will flow through the bars rather than the
higher-resistance and usually varnished laminates. Very low voltages
at very high currents are typical in the bars and end rings; high
efficiency motors will often use cast copper in order to reduce
the resistance in the rotor.
In
operation, the squirrel cage motor may be viewed as a transformer
with a rotating secondary - when the rotor is not rotating in sync
with the magnetic field, large rotor currents are induced; the large
rotor currents magnetize the rotor and interact with the stator's
magnetic fields to bring the rotor into synchronization with the
stator's field. An unloaded squirrel cage motor at synchronous speed
will consume electrical power only to maintain rotor speed against
friction and resistance losses; as the mechanical load increases,
so will the electrical load - the electrical load is inherently
related to the mechanical load. This is similar to a transformer,
where the primary's electrical load is related to the secondary's
electrical load.
This
is why, as an example, a squirrel cage blower motor may cause the
lights in a home to dim as it starts, but doesn't dim the lights
when its fanbelt (and therefore mechanical load) is removed. Furthermore,
a stalled squirrel cage motor (overloaded or with a jammed shaft)
will consume current limited only by circuit resistance as it attempts
to start. Unless something else limits the current (or cuts it off
completely) overheating and destruction of the winding insulation
is the likely outcome.
Virtually
every washing machine, dishwasher, standalone fan, record player,
etc. uses some variant of a squirrel cage motor.
Wound Rotor: An alternate design, called the
wound rotor, is used when variable speed is required. In this case,
the rotor has the same number of poles as the stator and the windings
are made of wire, connected to slip rings on the shaft. Carbon brushes
connect the slip rings to an external controller such as a variable
resistor that allows changing the motor's slip rate. In certain
high-power variable speed wound-rotor drives, the slip-frequency
energy is captured, rectified and returned to the power supply through
an inverter.
Compared
to squirrel cage rotors, wound rotor motors are expensive and require
maintenance of the slip rings and brushes, but they were the standard
form for variable speed control before the advent of compact power
electronic devices. Transistorized inverters with variable-frequency
drive can now be used for speed control, and wound rotor motors
are becoming less common. (Transistorized inverter drives also allow
the more-efficient three-phase motors to be used when only single-phase
mains current is available, but this is never used in household
appliances, because it can cause electrical interference and because
of high power requirements.)
Several
methods of starting a polyphase motor are used. Where the large
inrush current and high starting torque can be permitted, the motor
can be started across the line, by applying full line voltage to
the terminals (Direct-on-line, DOL). Where it is necessary to limit
the starting inrush current (where the motor is large compared with
the short-circuit capacity of the supply), reduced voltage starting
using either series inductors, an autotransformer, thyristors, or
other devices are used. A technique sometimes used is star-delta
starting, where the motor coils are initially connected in wye for
acceleration of the load, then switched to delta when the load is
up to speed. This technique is more common in Europe than in North
America. Transistorized drives can directly vary the applied voltage
as required by the starting characteristics of the motor and load.
This
type of motor is becoming more common in traction applications such
as locomotives, where it is known as the asynchronous traction motor.
The
speed of the AC motor is determined primarily by the frequency of
the AC supply and the number of poles in the stator winding, according
to the relation:
Ns
= 120F / p
where
Ns
= Synchronous speed, in revolutions per minute
F
= AC power frequency
p
= Number of poles per phase winding
Actual
RPM for an induction motor will be less than this calculated synchronous
speed by an amount known as slip, that increases with the torque
produced. With no load, the speed will be very close to synchronous.
When loaded, standard motors have between 2-3% slip, special motors
may have up to 7% slip, and a class of motors known as torque motors
are rated to operate at 100% slip (0 RPM/full stall).
The
slip of the AC motor is calculated by:
S
= (Ns − Nr) / Ns
percentageslip
= (Ns − Nr) / Ns * 100
where
Nr
= Rotational speed, in revolutions per minute.
S
= Normalised Slip, 0 to 1.
As
an example, a typical four-pole motor running on 60 Hz might have
a nameplate rating of 1725 RPM at full load, while its calculated
speed is 1800 RPM.
The
speed in this type of motor has traditionally been altered by having
additional sets of coils or poles in the motor that can be switched
on and off to change the speed of magnetic field rotation. However,
developments in power electronics mean that the frequency of the
power supply can also now be varied to provide a smoother control
of the motor speed.
Three-phase
AC synchronous motors
If
connections to the rotor coils of a three-phase motor are taken
out on slip-rings and fed a separate field current to create a continuous
magnetic field (or if the rotor consists of a permanent magnet),
the result is called a synchronous motor because the rotor will
rotate in synchronism with the rotating magnetic field produced
by the polyphase electrical supply.
The
synchronous motor can also be used as an alternator.
Nowadays,
synchronous motors are frequently driven by transistorized variable-frequency
drives. This greatly eases the problem of starting the massive rotor
of a large synchronous motor. They may also be started as induction
motors using a squirrel-cage winding that shares the common rotor:
once the motor reaches synchronous speed, no current is induced
in the squirrel-cage winding so it has little effect on the synchronous
operation of the motor, aside from stabilizing the motor speed on
load changes.
Synchronous
motors are occasionally used as traction motors; the TGV may be
the best-known example of such use.
One
apparently unusual use for this type of motor was its use in a power
factor correction scheme. This exploited a feature of the machine
where it consumed power at a leading power factor when its rotor
was over excited. It thus appeared to the supply to be a capacitor,
and could thus be used to correct the lagging power factor that
was usually presented to the electric supply. The excitation was
adjusted until a near unity power factor was obtained (often automatically).
Machines used for this purpose were easily identified as they had
no shaft extensions. They were refered to as synchronous capacitors.
Some
of the largest AC motors are pumped-storage hydroelectricity generators
that are operated as synchronous motors to pump water to a reservoir
at a higher elevation for later use to generate electricity using
the same machinery. Six 350-megawatt generators are installed in
the Bath County Pumped Storage Station in Warm Springs, VA USA.
When pumping, each unit can produce 563,400 horsepower (420,127
kilowatts).[3]
Two-phase
AC servo motors
A
typical two-phase AC servo motor has a squirrel-cage rotor and a
field consisting of two windings: 1) a constant-voltage (AC) main
winding, and 2) a control-voltage (AC) winding in quadrature with
the main winding so as to produce a rotating magnetic field. The
electrical resistance of the rotor is made high intentionally so
that the speed-torque curve is fairly linear. Two-phase servo motors
are inherently high-speed, low-torque devices, heavily geared down
to drive the load.
Single-phase
AC induction motors
Three-phase
motors inherently produce a rotating magnetic field. However, when
only single-phase power is available, the rotating magnetic field
must be produced using other means. Several methods are commonly
used.
A
common single-phase motor is the shaded-pole motor, which is used
in devices requiring low torque, such as electric fans or other
small household appliances. In this motor, small single-turn copper
"shading coils" create the moving magnetic field. Part
of each pole is encircled by a copper coil or strap; the induced
current in the strap opposes the change of flux through the coil
(Lenz's Law), so that the maximum field intensity moves across the
pole face on each cycle, thus producing the required rotating magnetic
field.
Another
common single-phase AC motor is the split-phase induction motor,
commonly used in major appliances such as washing machines and clothes
dryers. Compared to the shaded pole motor, these motors can generally
provide much greater starting torque by using a special startup
winding in conjunction with a centrifugal switch.
In
the split-phase motor, the startup winding is designed with a higher
resistance than the running winding. This creates an LR circuit
which slightly shifts the phase of the current in the startup winding.
When the motor is starting, the startup winding is connected to
the power source via a set of spring-loaded contacts pressed upon
by the not-yet-rotating centrifugal switch. The starting winding
is wound with fewer turns of smaller wire than the main winding,
so it has a lower inductance (L) and higher resistance (R). The
lower L/R ratio creates a small phase shift, not more than about
30 degrees, between the flux due to the main winding and the flux
of the starting winding. The starting direction of rotation may
be reversed simply by exchanging the connections of the startup
winding relative to the running winding.
The
phase of the magnetic field in this startup winding is shifted from
the phase of the mains power, allowing the creation of a moving
magnetic field which starts the motor. Once the motor reaches near
design operating speed, the centrifugal switch activates, opening
the contacts and disconnecting the startup winding from the power
source. The motor then operates solely on the running winding. The
starting winding must be disconnected since it would increase the
losses in the motor.
In
a capacitor start motor, a starting capacitor is inserted in series
with the startup winding, creating an LC circuit which is capable
of a much greater phase shift (and so, a much greater starting torque).
The capacitor naturally adds expense to such motors.
Another
variation is the Permanent Split-Capacitor (PSC) motor (also known
as a capacitor start and run motor). This motor operates similarly
to the capacitor-start motor described above, but there is no centrifugal
starting switch and the second winding is permanently connected
to the power source. PSC motors are frequently used in air handlers,
fans, and blowers and other cases where a variable speed is desired.
By changing taps on the running winding but keeping the load constant,
the motor can be made to run at different speeds. Also provided
all 6 winding connections are available separately, a 3 phase motor
can be converted to a capacitor start and run motor by commoning
two of the windings and connecting the third via a capacitor to
act as a start winding.
Repulsion
motors are wound-rotor single-phase AC motors that are similar to
universal motors. In a repulsion motor, the armature brushes are
shorted together rather than connected in series with the field.
Several types of repulsion motors have been manufactured, but the
repulsion-start induction-run (RS-IR) motor has been used most frequently.
The RS-IR motor has a centrifugal switch that shorts all segments
of the commutator so that the motor operates as an induction motor
once it has been accelerated to full speed. RS-IR motors have been
used to provide high starting torque per ampere under conditions
of cold operating temperatures and poor source voltage regulation.
Few repulsion motors of any type are sold as of 2006.
Single-phase
AC synchronous motors
Small
single-phase AC motors can also be designed with magnetized rotors
(or several variations on that idea). The rotors in these motors
do not require any induced current so they do not slip backward
against the mains frequency. Instead, they rotate synchronously
with the mains frequency. Because of their highly accurate speed,
such motors are usually used to power mechanical clocks, audio turntables,
and tape drives; formerly they were also much used in accurate timing
instruments such as strip-chart recorders or telescope drive mechanisms.
The shaded-pole synchronous motor is one version.
Because
inertia makes it difficult to instantly accelerate the rotor from
stopped to synchronous speed, these motors normally require some
sort of special feature to get started. Various designs use a small
induction motor (which may share the same field coils and rotor
as the synchronous motor) or a very light rotor with a one-way mechanism
(to ensure that the rotor starts in the "forward" direction).
Torque
motors
A
torque motor is a specialized form of induction motor which is capable
of operating indefinitely at stall (with the rotor blocked from
turning) without damage. In this mode, the motor will apply a steady
torque to the load (hence the name). A common application of a torque
motor would be the supply- and take-up reel motors in a tape drive.
In this application, driven from a low voltage, the characteristics
of these motors allow a relatively-constant light tension to be
applied to the tape whether or not the capstan is feeding tape past
the tape heads. Driven from a higher voltage, (and so delivering
a higher torque), the torque motors can also achieve fast-forward
and rewind operation without requiring any additional mechanics
such as gears or clutches. In the computer world, torque motors
are used with force feedback steering wheels.
Stepper
motors
Main
article: Stepper motor
Closely
related in design to three-phase AC synchronous motors are stepper
motors, where an internal rotor containing permanent magnets or
a large iron core with salient poles is controlled by a set of external
magnets that are switched electronically. A stepper motor may also
be thought of as a cross between a DC electric motor and a solenoid.
As each coil is energized in turn, the rotor aligns itself with
the magnetic field produced by the energized field winding. Unlike
a synchronous motor, in its application, the motor may not rotate
continuously; instead, it "steps" from one position to
the next as field windings are energized and de-energized in sequence.
Depending on the sequence, the rotor may turn forwards or backwards.
Simple
stepper motor drivers entirely energize or entirely de-energize
the field windings, leading the rotor to "cog" to a limited
number of positions; more sophisticated drivers can proportionally
control the power to the field windings, allowing the rotors to
position "between" the "cog" points and thereby
rotate extremely smoothly. Computer controlled stepper motors are
one of the most versatile forms of positioning systems, particularly
when part of a digital servo-controlled system.
Stepper
motors can be rotated to a specific angle with ease, and hence stepper
motors are used in pre-gigabyte era computer disk drives, where
the precision they offered was adequate for the correct positioning
of the read/write head of a hard disk drive. As drive density increased,
the precision limitations of stepper motors made them obsolete for
hard drives, thus newer hard disk drives use read/write head control
systems based on voice coils.
Stepper
motors were upscaled to be used in electric vehicles under the term
SRM (switched reluctance machine).
source
[1]
Brushless
DC motors
Main
article: Brushless DC electric motor
Many
of the limitations of the classic commutator DC motor are due to
the need for brushes to press against the commutator. This creates
friction. At higher speeds, brushes have increasing difficulty in
maintaining contact. Brushes may bounce off the irregularities in
the commutator surface, creating sparks. This limits the maximum
speed of the machine. The current density per unit area of the brushes
limits the output of the motor. The imperfect electric contact also
causes electrical noise. Brushes eventually wear out and require
replacement, and the commutator itself is subject to wear and maintenance.
The commutator assembly on a large machine is a costly element,
requiring precision assembly of many parts.
These
problems are eliminated in the brushless motor. In this motor, the
mechanical "rotating switch" or commutator/brushgear assembly
is replaced by an external electronic switch synchronised to the
rotor's position. Brushless motors are typically 85-90% efficient,
whereas DC motors with brushgear are typically 75-80% efficient.
Midway
between ordinary DC motors and stepper motors lies the realm of
the brushless DC motor. Built in a fashion very similar to stepper
motors, these often use a permanent magnet external rotor, three
phases of driving coils, one or more Hall effect devices to sense
the position of the rotor, and the associated drive electronics.
The coils are activated, one phase after the other, by the drive
electronics as cued by the signals from the Hall effect sensors.
In effect, they act as three-phase synchronous motors containing
their own variable-frequency drive electronics. A specialized class
of brushless DC motor controllers utilize EMF feedback through the
main phase connections instead of Hall effect sensors to determine
position and velocity. These motors are used extensively in electric
radio-controlled vehicles, and referred to by modelists as outrunner
motors (since the magnets are on the outside).
Brushless
DC motors are commonly used where precise speed control is necessary,
computer disk drives or in video cassette recorders the spindles
within CD, CD-ROM (etc.) drives, and mechanisms within office products
such as fans, laser printers and photocopiers. They have several
advantages over conventional motors:
Compared
to AC fans using shaded-pole motors, they are very efficient, running
much cooler than the equivalent AC motors. This cool operation leads
to much-improved life of the fan's bearings.
Without
a commutator to wear out, the life of a DC brushless motor can be
significantly longer compared to a DC motor using brushes and a
commutator. Commutation also tends to cause a great deal of electrical
and RF noise; without a commutator or brushes, a brushless motor
may be used in electrically sensitive devices like audio equipment
or computers.
The
same Hall effect devices that provide the commutation can also provide
a convenient tachometer signal for closed-loop control (servo-controlled)
applications. In fans, the tachometer signal can be used to derive
a "fan OK" signal.
The
motor can be easily synchronized to an internal or external clock,
leading to precise speed control.
Brushless
motors have no chance of sparking, unlike brushed motors, making
them better suited to environments with volatile chemicals and fuels.
Brushless
motors are usually used in small equipment such as computers generaly
used to get rid of unwanted heat.
They
are also very quiet motors which is an advantage if being used in
equipment that is affected by vibrations.
Modern
DC brushless motors range in power from a fraction of a watt to
many kilowatts. Larger brushless motors up to about 100 kW rating
are used in electric vehicles. They also find significant use in
high-performance electric model aircraft.
Coreless
DC motors
Nothing
in the design of any of the motors described above requires that
the iron (steel) portions of the rotor actually rotate; torque is
exerted only on the windings of the electromagnets. Taking advantage
of this fact is the coreless DC motor, a specialized form of a brush
DC motor. Optimized for rapid acceleration, these motors have a
rotor that is constructed without any iron core. The rotor can take
the form of a winding-filled cylinder inside the stator magnets,
a basket surrounding the stator magnets, or a flat pancake (possibly
formed on a printed wiring board) running between upper and lower
stator magnets. The windings are typically stabilized by being impregnated
with epoxy resins.
Because
the rotor is much lighter in weight (mass) than a conventional rotor
formed from copper windings on steel laminations, the rotor can
accelerate much more rapidly, often achieving a mechanical time
constant under 1 ms. This is especially true if the windings use
aluminum rather than the heavier copper. But because there is no
metal mass in the rotor to act as a heat sink, even small coreless
motors must often be cooled by forced air.
These
motors were commonly used to drive the capstan(s) of magnetic tape
drives and are still widely used in high-performance servo-controlled
systems.
Linear
motors
A
linear motor is essentially an electric motor that has been "unrolled"
so that, instead of producing a torque (rotation), it produces a
linear force along its length by setting up a traveling electromagnetic
field.
Linear
motors are most commonly induction motors or stepper motors. You
can find a linear motor in a maglev (Transrapid) train, where the
train "flies" over the ground.
Doubly-fed
electric motor
Doubly-fed
electric motors or Doubly-Fed Electric Machines have two multiphase
windings, with at least one of the winding sets electronically controlled
for synchronous operation from sub-synchronous to super synchronous
speeds. As a result, doubly-fed electric motors are synchronous
machines with 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. In theory, this
attribute has attractive cost, size, and efficiency ramifications
compared to Singly-Fed Electric Machines but Doubly-fed motors are
difficult to realize in practice.
The
Wound-Rotor Doubly-Fed Electric Machines, the Brushless Wound-Rotor
Doubly-Fed Electric Machine, and the so-called Brushless Doubly-Fed
Electric Machines are the only examples of synchronous doubly-fed
electric machines.
Singly-fed
electric motor
Singly-fed
electric motors or Singly-Fed Electric Machines incorporate a single
multiphase winding set that actively participate in the energy conversion
process (i.e., singly-fed). Singly-fed electric machines operate
under either Induction (i.e., Asynchronous) or Synchronous principles.
The active winding set can be electronically controlled for optimum
performance. Induction machines exhibit startup torque and can operate
as standalone machines but Synchronous machines must have auxiliary
means for startup and practical operation, such as an electronic
controller. Singly-fed electric machines have an effective constant
torque speed range up to synchronous speed (i.e., 3600 rpm @ 60
Hz and 2 Poles) for a given excitation frequency.
The
Induction (Asynchronous) motors (i.e., squirrel cage rotor or wound
rotor), Synchronous motors (i.e., field-excited, Permanent Magnet
or brushless DC motors, Reluctance motors, etc.), which are discussed
on this page, are examples of Singly-fed motors. By far, Singly-fed
motors are the predominantly installed type of motors.
Nanotube
nanomotor
Main
article: Nanomotor
Nanomotor
constructed at UC Berkeley. The motor is about 500nm across: 300
times smaller than the diameter of a human hairResearchers at University
of California, Berkeley, recently developed rotational bearings
based upon multiwall carbon nanotubes. By attaching a gold plate
(with dimensions of order 100nm) to the outer shell of a suspended
multiwall carbon nanotube (like nested carbon cylinders), they are
able to electrostatically rotate the outer shell relative to the
inner core. These bearings are very robust; devices have been oscillated
thousands of times with no indication of wear. These nanoelectromechanical
systems (NEMS) are the next step in miniaturization that may find
their way into commercial aspects in the future.
Electrostatic
motor
DC
motor starters
The
counter-emf aids the armature resistance to limit the current through
the armature. When power is first applied to a motor, the armature
does not rotate. At that instant the counter-emf is zero and the
only factor limiting the armature current, is the armature resistance.
Usually the armature resistance of a motor is less than one ohm;
therefore the current through the armature would be very large when
the power is applied. This current can make an excessive voltage
drop affecting other equipment in the circuit and even trip overload
protective devices.
Therefore
the need arises for an additional resistance in series with the
armature to limit the current until the motor rotation can build
up the counter-emf. As the motor rotation build up, the resistance
is gradually cut out.
Three-point
starter
Three
point starterThe incoming power is indicated as L1 and L2. The components
within the broken lines form the three-point starter. As the name
implies there are only three connections to the starter. The connections
to the armature are indicated as A1 and A2. The ends of the field
(excitement) coil are indicated as F1 and F2. In order to control
the speed, A field rheostat is connected in series with the shunt
field. One side of the line is connected to the arm of the starter
(represented by an arrow in the diagram). The arm is spring-loaded
so , it will return to the "Off" position the not held
at any other position.
ON
the first step of the arm, full line voltage is applied across the
shunt field. Since the field rheostat is normally set to minimum
resistance, the speed of the motor will not be excessive; additionally,
the motor will develop a large starting torque.
The
starter also connects an electromagnet in series with the shunt
field. It will hold the arm in position when the arm makes contact
with the magnet.
Meanwhile
that voltage is applied to the shunt field, and the starting resistance
limits the flow of current to the armature.
As
the motor picks up speed counter-emf is built up, the arm is moved
slowly to short.
Four-point
starter
Four
point starterThe four-point starter eliminates the drawback of the
three-point starter. In addition to the same three points that were
in use with the three-point starter, the other side of the line,
L1, is the fourth point brought to the starter when the arm is moved
from the "Off" position. The coil of the holding magnet
is connected across the line. The holding magnet and starting resistors
function identical as in the three-point starter.
The
possibility of accidentally opening the field circuit is quite remote.
The four-point starter provides the no-voltage protection to the
motor. If the power fails, the motor is disconnected from the line.
Motor
standards
The
following are major design and manufacturing standards covering
electric motors:
International
Electrotechnical Commission: IEC 60034 Rotating Electrical Machines
National
Electrical Manufacturers Association (USA):
NEMA MG 1 Motors and Generators
See
also
Electronics
Portal
Energy
Portal
Motor
control:
Motor
controller
Motor
Soft Starter
Direct
on line starter
Adjustable-speed
drive
Electronic
speed control
Variable-frequency
drive
Thyristor
drive
Components:
Centrifugal
switch
Commutator
(electric)
Slip
ring
10 100 12 12v 2 20 200 2hp 3 4 5 5hp a ac ajax
an ao baldor basic basics beakman's bearing bearings bicycle bike
bikes blower boat bodine brake brush brushes brushless build building
buy c calculator capacitor capacitors car cars cart century city
cleaner clock company control controller controls conversion cooling
corp cross cycle cycles data dayton dc design diagram diagrams dimensions
does drive drives efficiency electric emerson etek fan fasco first
frame franklin ge gear general generator glider global golf grease
high history hobby homemade horsepower how hp hp hub hybrid induction
industrial johnson kit kits large lincoln linear magnetek magnetek
maintenance make manufacturer manufacturers marathon mini miniature
model motor mount mounts noise of ohio oil outboard parts phase
plans pole power precision price project project projects pulley
r r rc rc rebuild rebuilding reference reliance repair repairs replacement
rewind rewinding rpm sales schematic science scooter scooters seat
service shaft shop siemens simple single size sizes small smith
specifications specs speed starter starters supply surplus testing
theory three to torque toshiba toy trolling troubleshooting twin
types universal us used variable vehicle volt vulcan warehouse weg
weight westinghouse wheel winding windings window wiper wire wiring
work works
Scientists
and engineers:
Ottó
Bláthy
Timeline
of motor and engine technology
References
and further reading
Citations
^
http://www.tfcbooks.com/tesla/system.htm
^
"Tesla's Early Years". PBS.
^
[|Dominion Resources, Inc.], Bath County Pumped Storage Station.
Retrieved on 2007-3-30
General
references
Donald
G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers,
Eleventh Edition, McGraw-Hill, New York, 1978, ISBN 0-07-020974-X.
Edwin
J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric
Machinery, copyright American Technical Book Company 1897, published
by P.F. Collier and Sons New York, 1902
Kuphaldt,
Tony R. (2000-2006). "Chapter 13 AC MOTORS", Lessons In
Electric Circuits — Volume II.
A.O.Smith:
The AC's and DC's of Electric Motors. Retrieved on 2006-04-11.
Resenblat
& Frienman DC and AC machinery
Links
to AC Motor Manufacturers
Further
reading
Shanefield
D. J., Industrial Electronics for Engineers, Chemists, and Technicians,
William Andrew Publishing, Norwich, NY, 2001. A self-teaching textbook
that briefly covers electric motors, transformers, speed controllers,
wiring codes and grounding, transistors, digital, etc. Easy to read
and understand, up to an elementary level on each subject, not a
suitable reference book for technologists already working in any
of those fields.
Fitzgerald/Kingsley/Kusko
(Fitzgerald/Kingsley/Umans in later years), *Electric Machinery,
classic text for junior and senior electrical engineering students.
Originally published in 1952, 6th edition published in 2002. Authors
still listed as Fitzgerald/Kingsley/Umans although Fitzgerald and
Kingsley are now deceased.
Bedford,
B. D.; Hoft, R. G. et al (1964). Principles of Inverter Circuits.
New York: John Wiley & Sons, Inc.. 0 471 06134 4. (Inverter
circuits are used for variable-frequency motor speed control)
B.
R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters:
Operation, Control, and Performance" (New York: John Wiley,
1971).
John
N. Chiasson, Modeling and High Performance Control of Electric Machines,
Wiley-IEEE Press, New York, 2005, ISBN 0-471-68449-X.
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