AC Motor
AC Motor
AC Motor
INTRODUCTION
An AC motor is an electric motor that is driven by an alternating current. It
consists of two basic parts, an outside stationary stator having coils supplied
with alternating 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 types of AC motors, depending on the type of rotor used. The
first is the synchronous motor, which rotates exactly at the supply frequency
or a submultiple of the supply frequency. The magnetic field on the rotor is
either generated by current delivered through slip rings or by a permanent
magnet.
The second type is the induction motor, which turns slightly slower than the
supply frequency. The magnetic field on the rotor of this motor is created by
an induced current.
History
In 1882, Serbian inventor Nikola Tesla identified the rotating magnetic
induction field principle[citation needed] and pioneered the use of this rotating and
inducting electromagnetic field force to generate torque in rotating
machines. He exploited this principle in the design of a poly-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.
Three phase AC induction motors rated 746 W (1.000 hp) and 25 W (left),
with smaller motors from CD player, toy and CD/DVD drive reader head
traverse. (9 V battery shown, at bottom center, for size comparison.)
Disassembled 250 W 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.
There are two types of rotors used in induction motors: squirrel cage rotors
and wound rotors.
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 refers to the rotating exercise cage for pet animals. The motor
takes its name from the shape of its rotor "windings"- 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.
This is why, for example, a squirrel cage blower motor may cause the lights
in a home to dim as it starts, but doesn't dim the lights on startup when its
fan belt (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.
Slip
The motor would not start with the terminals open; connecting the common
to one other made the motor run one way, and connecting common to the
other made it run the other way. These motors were used in industrial and
scientific devices.
Applying AC to the coil created a field that progressed in the gap between
the poles. The plane of the stator core was approximately tangential to an
imaginary circle on the disc, so the traveling magnetic field dragged the disc
and made it rotate.
The stator was mounted on a pivot so it could be positioned for the desired
speed and then clamped in position. Keeping in mind that the effective speed
of the traveling magnetic field in the gap was constant, placing the poles
nearer to the center of the disc made it run relatively faster, and toward the
edge, slower.
It's possible that these motors are still in use in some older installations.
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.
Wound rotors
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.
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, YΔ) starting, where the motor coils are
initially connected in star 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.
Ns = 120F / p
where
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
where
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.
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 synchronously with the
rotating magnetic field produced by the polyphase electrical supply.
Synchronous motors are occasionally used as traction motors; the TGV may
be the best-known example of such use.
One use for this type of motor is its use in a power factor correction scheme.
They are referred to as synchronous condensers. This exploits a feature of
the machine where it consumes power at a leading power factor when its
rotor is over excited. It thus appears to the supply to be a capacitor, and
could thus be used to correct the lagging power factor that is usually
presented to the electric supply by inductive loads. The excitation is adjusted
until a near unity power factor is obtained (often automatically). Machines
used for this purpose are easily identified as they have no shaft extensions.
Synchronous motors are valued in any case because their power factor is
much better than that of induction motors, making them preferred for very
high power applications.
Repulsion motor
These motors are relatively costly, and are used where exact speed
(assuming an exact-frequency AC source) as well as rotation with a very
small amount of fast variations in speed (called 'flutter" in audio recordings)
is essential. Applications included tape recorder capstan drives (the motor
shaft could be the capstan). Their distinguishing feature is their rotor, which
is a smooth cylinder of a magnetic alloy that stays magnetized, but can be
demagnetized fairly easily as well as re-magnetized with poles in a new
location. Hysteresis refers to how the magnetic flux in the metal lags behind
the external magnetizing force; for instance, to demagnetize such a material,
one could apply a magnetizing field of opposite polarity to that which
originally magnetized the material.
Such motors have an external rotor with a cup-shaped housing and a radially
magnetized permanent magnet connected in the cup-shaped housing. An
interior stator is positioned in the cup-shaped housing. The interior stator has
a laminated core having grooves. Windings are provided within the grooves.
The windings have first end turns proximal to a bottom of the cup-shaped
housing and second end turns positioned distal to the bottom. The first and
second end turns electrically connect the windings to one another. The
permanent magnet has an end face rom the bottom of the cup-shaped
housing. At least one galvano-magnetic rotor position sensor is arranged
opposite the end face of the permanent magnet so as to be located within a
magnetic leakage of the permanent magnet and within a magnetic leakage of
the interior stator. The at least one rotor position sensor is designed to
control current within at least a portion of the windings. A magnetic leakage
flux concentrator is arranged at the interior stator at the second end turns at a
side of the second end turns facing away from the laminated core and
positioned at least within an angular area of the interior stator in which the at
least one rotor position sensor is located.
ECM motors are increasingly being found in forced-air furnaces and HVAC
systems to save on electricity costs as modern HVAC systems are running
their fans for longer periods of time (duty cycle).[6] The cost effectiveness of
using ECM motors in HVAC systems is questionable, given that the repair
(replacement) costs are likely to equal or exceed the savings realized by
using such a motor.[citation needed]
Watthour-meter motors
The core structure, seen face-on, is akin to a cartoon mouth with one tooth
above and two below. Surrounding the poles ("teeth") is the common flux
return path. The upper pole (high-inductance winding) is centered, and the
lower ones equidistant. Because the lower coils are wound in opposition, the
three poles cooperate to create a "sidewise" traveling flux. The disc is
between the upper and lower poles, but with its shaft definitely in front of
the field, so the tangential flux movement makes it rotate.
Domestic applications
Electric motors are used domestically in personal care products, small and large
appliances, and residential heating and cooling equipment. In most domestic applications,
the motor controller functions are built into the product. In some cases, such as bathroom
ventilation fans, the motor is controlled by a switch on the wall. Some appliances have
provisions for controlling the speed of the motor. Built-in circuit breakers protect some
appliance motors, but most are unprotected except that the household fuse or circuit
breaker panel disconnects the motor if it fails.
Commercial applications
Commercial buildings have larger heating ventilation and air conditioning (HVAC)
equipment than that found in individual residences. In addition, motors are used for
elevators, escalators and other applications. In commercial applications, the motor control
functions are sometimes built into the motor-driven equipment and sometimes installed
separately.
Industrial applications
Many industrial applications are dependent upon motors (or machines), which range from
the size of one's thumb to the size of a railroad locomotive. The motor controllers can be
built into the driven equipment, installed separately, installed in an enclosure along with
other machine control equipment or installed in motor control centers. Motor control
centers are multi-compartment steel enclosures designed to enclose many motor
controllers. It is also common for more than one motor controller to operate a number of
motors in the same application. In this case the controllers communicate with each other
so they can work the motors together as a team.
An electric motor controller can be classified by the type of motor it is to drive such as
permanent magnet, servo, series, separately excited, and alternating current.
A motor controller is connected to a power source such as a battery pack or power supply,
and control circuitry in the form of analog or digital input signals.
Motor starters
Adjustable-speed drives
Phase vector drives (or simply vector drives) are an improvement over variable
frequency drives (VFDs) in that they separate the calculations of magnetizing current and
torque generating current. These quantities are represented by phase vectors, and are
combined to produce the driving phase vector which in turn is decomposed into the
driving components of the output stage. These calculations need a fast microprocessor,
typically a DSP device.
Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position
and phase currents. Rotor position can be obtained through an encoder, but is often
sensed by the reverse EMF generated on the motor leads.
In some configurations, a vector drive may be able to generate full rated motor torque at
zero speed.
Direct torque control has better torque control dynamics than the PI-current controller
based vector control. Thus it suits better to servo control applications. However, it has
some advantage over other control methods in other applications as well because due to
the faster control it has better capabilities to damp mechanical resonances and thus extend
the life of the mechanical system.
Servo controllers
Main article: Servo drive
Main article: Servomechanism
Servo motors may be made from several motor types, the most common being
• brushed DC motor
• brushless DC motors
• AC servo motors
Servo controllers use position feedback to close the control loop. This is commonly
implemented with encoders, resolvers, and Hall effect sensors to directly measure the
rotor's position. Others measure the back EMF in the undriven coils to infer the rotor
position, and therefore are often called "sensorless" controllers.
A servo may be controlled using pulse-width modulation (PWM). How long the pulse
remains high (typically between 1 and 2 milliseconds) determines where the motor will
try to position itself. Another control method is pulse and direction.
Modern stepper controllers drive the motor with much higher voltages than the motor
nameplate rated voltage, and limit current through chopping. The usual setup is to have a
positioning controller, known as an indexer, sending step and direction pulses to a
separate higher voltage drive circuit which is responsible for commutation and current
limiting.