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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 0,416,194 ,
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
A typical AC motor consists of two parts:
1. An outside stationary stator having coils
supplied with AC current to produce a rotating
magnetic field, and;
2. An inside rotor attached to the output shaft
that is given a torque by the rotating field.
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.
Slip ring
The slip ring or wound rotor motor is an
induction machine where the rotor comprises a
set of coils that are terminated in slip rings
to which external impedances can be connected.
The stator is the same as is used with a
standard squirrel cage motor.
By changing the impedance connected to the rotor
circuit, the speed/current and speed/torque
curves can be altered.
The slip ring motor is used primarily to start a
high inertia load or a load that requires a very
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 current from zero
speed to full speed. A secondary use of the slip
ring motor is to provide a means of speed
control. Because the torque curve of the motor
is effectively modified by the resistance
connected to the rotor circuit, the speed of the
motor can be altered. Increasing the value of
resistance on the rotor circuit will move the
speed of maximum torque down. If the resistance
connected to the rotor is increased beyond the
point where the maximum torque occurs at zero
speed, the torque will be further reduced.
When used with a load that has a torque curve
that increases with speed, the motor will
operate at the speed where the torque developed
by the motor is equal to the load torque.
Reducing the load will cause the motor to speed
up, and increasing the load will cause the motor
to slow down until the load and motor torque are
equal. Operated in this manner, the slip losses
are dissipated in the secondary resistors and
can be very significant. The speed regulation is
also very poor.
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).
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, and in many
roller-coasters where the rapid motion of the
motorless railcar is controlled by the rail.
Doubly-fed
electric motor
Doubly-fed electric motors have two independent
multiphase windings that actively participate in
the energy conversion process with at least one
of the winding sets electronically controlled
for variable speed operation. Two is the most
active multiphase winding sets possible without
duplicating singly-fed or doubly-fed categories
in the same package. As a result, doubly-fed
electric motors are 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.
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
with controlling speed near synchronous speed
limit applications.[3]
Singly-fed
electric motor
Singly-fed electric machines incorporate a
single multiphase winding set that is connected
to a power supply. Singly-fed electric machines
may be either induction or synchronous. The
active winding set can be electronically
controlled. Induction machines develop starting
torque at zero speed and can operate as
standalone machines. Synchronous machines must
have auxiliary means for startup, such as a
starting induction squirrel-cage winding or an
electronic controller. Singly-fed electric
machines have an effective constant torque speed
range up to synchronous speed 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 the this page, are
examples of singly-fed motors. By far,
singly-fed motors are the predominantly
installed type of motors.
Nanotube
nanomotor
Researchers at University of California,
Berkeley, recently developed rotational bearings
based upon multiwall carbon nanotubes. By
attaching a gold plate (with dimensions of the
order of 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. |
