Transcript Stator

电气技术专业英语
朱一纶 主编
中国电力出版社
Unit 8 Electric motor
课件制作: 吴岱曦
Index
• Text
– 1. Induction Motor
– 2. DC motor
• Reading materials
– 1. Stepping motor
– 2. Servo controllers
– 3. Linear motor
• Exercises
– 1.
– 2.
– 3.
– 4.
Put the Phrases into English
Put the Phrases into Chinese
Sentence Translation
Translation
Text
• An electric motor (Fig 8.1) uses electrical
energy to produce mechanical energy. The
reverse process, which using mechanical
energy to produce electrical energy, is
accomplished by a generator or dynamo.
• Electric motors are found in household
appliances such as fans, refrigerators,
washing machines, pool pumps, floor
vacuum , and fan-forced ovens, and many
industrial applications are dependent upon
motors (or machines)too, which range
from the size of one's thumb to the size of
a railroad locomotive.
• All loads moved by electric motors are
really moved by magnetism. The purpose
of every component in a motor is to help
harness, control, and use magnetic force.
To move a load fast does not require more
magnets, you just move the magnets fast.
• To move a heavier load or to decrease
acceleration time (accelerate faster),
however, more magnets (more torque) are
needed. This is the basis for all motor
applications.
Fig 8.1 Various Motors
1. Induction Motor
• The induction motor is a three phase AC
motor and is the most widely used
machine. Its characteristic features are:
– Simple and rugged construction
– Low cost and minimum maintenance
– High reliability and sufficiently high efficiency
• An induction motor has basically two
parts :
– An outside stationary stator having coils
supplied with AC current to produce a rotating
magnetic field;
– An inside rotor attached to the output shaft
that is given a torque by the rotating field.
Stator (Fig 8.2)
• The stator is made up of a number of
stampings with slots to carry three phase
windings. It is wound for a definite
number of poles. The windings are
geometrically spaced 120 degrees apart.
Fig 8.2 Stator
• Stator laminations are stacked together
forming a hollow cylinder. Coils of
insulated wire are inserted into slots of the
stator core.
• Each grouping of coils, together with the
steel core it surrounds, form an
electromagnet. Electromagnetism is the
principle behind motor operation. The
stator windings are connected directly to
the power source.
Rotor (Fig 8.3)
• Two types of rotors are used in Induction
motors - squirrel-cage rotor and wound
rotor.
Fig 8.3 rotor of an induction motor
• (a) lamination - left
• (b) The construction of the squirrel cage rotor - right
• The construction of the squirrel cage rotor
is reminiscent of rotating exercise wheels
found in cages of pet squirrel. The rotor
consists of a stack of steel laminations
with evenly spaced conductor bars around
the circumference.
• The laminations are stacked together to
form a rotor core. Aluminum is die cast in
the slots of the rotor core to form a series
of conductors around the perimeter of the
rotor. Current flow through the conductors
forms the electromagnet.
• The conductor bars are mechanically and
electrically connected with end rings. The
rotor core mounts on a steel shaft to form
a rotor assembly.
• The enclosure (Fig 8.4) consists of a frame
and two end brackets (or bearing
housings). The stator is mounted inside
the frame. The rotor fits inside the stator
with a slight air gap separating it from the
stator.
• There is no direct physical connection
between the rotor and the stator. The
enclosure also protects the electrical and
operating parts of the motor from harmful
effects of the environment in which the
motor operates.
• Bearings, mounted on the shaft, support
the rotor and allow it to turn. A fan, also
mounted on the shaft, is used on the
motor for cooling.
Fig 8.4 Induction Motor
• The wound rotor motor or slip ring 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（Fig 8.5）.
Fig 8.5 torque curve modified by R2
• 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.
2. DC motor
• By far the most common DC motor types
are the brushed and brushless types,
which use internal and external
commutation respectively to create an
oscillating AC current from the DC source so they are not purely DC machines in a
strict sense.
• The classic DC motor design generates an
in a wound rotor with a split ring
commutator (Fig 8.3), and either a wound
or permanent magnet stator, a rotor
consists of a coil wound around a rotor
which is then powered by any type of DC
voltage source.
Fig 8.6 DC 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.
• Some of the problems of the brushless DC
motor are eliminated in the brushless
design. In this motor, the mechanical
"rotating switch" or commutator /
brushgear assembly is replaced by an
external electronic switch synchronized to
the rotor's position.
• Brushless motors are typically 85-90%
efficient, whereas DC motors with brush
gear are typically 75-80% efficient.
• Brushless DC motors are commonly used
where precise speed control is necessary,
as in computer disk drives or in video
cassette recorders, the spindles within CD
drives, and mechanisms within office
products such as fans, laser printers and
photocopiers.
• 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.
This is the End of the Text
Reading materials
1. Stepping motor
• Stepping motors can be viewed as electric
motors without commutators. Typically, all
windings in the motor are part of the
stator, and the rotor is either a permanent
magnet or, in the case of variable
reluctance motors, a toothed block of
some magnetically soft material.
• For example, a variable reluctance
stepping motor has three windings,
typically connected as shown in the
schematic diagram in Figure 8.7, with one
terminal common to all windings. In use,
the common wire typically goes to the
positive supply and the windings are
energized in sequence.
• The rotor in this motor has 4 teeth and
the stator has 6 poles, with each winding
wrapped around two opposite poles. With
winding number 1 energized, the rotor
teeth marked X are attracted to this
winding's poles.
• If the current through winding 1 is turned
off and winding 2 is turned on, the rotor
will rotate 30 degrees clockwise so that
the poles marked Y line up with the poles
marked 2. To rotate this motor
continuously, we just apply power to the 3
windings in sequence.
Fig8.7 variable reluctance motor
• All of the commutation must be handled
externally by the motor controller, and
typically, the motors and controllers are
designed so that the motor may be held in
any fixed position as well as being rotated
one way or the other.
• Stepping motors come in a wide range of
angular resolution. The coarsest motors
typically turn 90 degrees per step, while
high resolution permanent magnet motors
are commonly able to handle 1.8 or even
0.72 degrees per step.
• With an appropriate controller, most
permanent magnet motors can be run in
half-steps, and some controllers can
handle smaller fractional steps.
• Stepping motors can be used in simple
open-loop control systems; these are
generally adequate for systems that
operate at low accelerations with static
loads, but closed loop control may be
essential for high accelerations,
particularly if they involve variable loads.
2. Servo controllers
• Servo controller is a wide category of
motor control. Common features are:
– precise closed loop position control
– fast acceleration rates
– precise speed control
• 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 electromotive
force (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.
• For some applications, there is a choice
between using servomotors and stepping
motors. Both types of motors offer similar
opportunities for precise positioning, but
they differ in a number of ways.
Servomotors require analog feedback
control systems of some type.
• Typically, this involves a potentiometer to
provide feedback about the rotor position,
and some mix of circuitry to drive a
current through the motor inversely
proportional to the difference between the
desired position and the current position.
3. Linear motor
• 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 rollercoasters[1] where the rapid motion of the
motorless railcar is controlled by the rail.
• Many designs have been put forward for
linear motors, falling into two major
categories, low-acceleration and highacceleration linear motors. Lowacceleration linear motors are suitable for
maglev trains and other ground-based
transportation applications.
• The Shanghai Maglev Train connects the
rapid transit network 30.5 km to the
Shanghai Pudong International Airport
(Fig 8.8). High-acceleration linear motors
are normally quite short, and are designed
to accelerate an object up to a very high
speed and then release the object, like
roller coasters.
Fig 8.8 The Shanghai Maglev Train
• They are usually used for studies of
hypervelocity collisions, as weapons, or as
mass drivers for spacecraft propulsion.
• The high-acceleration motors are usually
of the linear induction design (LIM) with
an active three-phase winding on one side
of the air-gap and a passive conductor
plate on the other side.
• The low-acceleration, high speed and high
power motors are usually of the linear
synchronous design (LSM), with an active
winding on one side of the air-gap and an
array of alternate-pole magnets on the
other side. These magnets can be
permanent magnets or energized magnets.
This is the End of
the Reading materials
Exercises
• Ready to go?
1. Put the Phrases into English
•
•
•
•
•
1)
2)
3)
4)
5)
异步电动机
三相交流电动机
定子绕组
旋转（磁）场
鼠笼式转子
Show the Answer
•
•
•
•
•
6)
7)
8)
9)
10)
绕线式电动机
最大转矩
在严格意义上
直流电机
速度控制
Show the Answer
2. Put the Phrases into Chinese
• 1）to produce mechanical energy
• 2）floor vacuum
• 3）decrease acceleration time
• 4）produce a rotating magnetic field
• 5）the output shaft
Show the Answer
• 6）geometrically spaced 120 degrees
apart
• 7）speed/torque curve
• 8）a means of speed control
• 9）oscillating current
• 10）wear out and require replacement
Show the Answer
3. Sentence Translation
• 1）Electric motors are found in household
appliances such as fans, refrigerators,
washing machines.
• 1）很多家用电器里都有电动机，例如电风
扇，冰箱，洗衣机。
• 2) To move a load fast does not require
more magnets, you just move the
magnets fast.
• 2）要让负载转的更快并不需要增加磁场强
度，你只要增加磁场的转速就可以了。
• 3) Two types of rotors are used in
induction motors - squirrel-cage rotor and
wound rotor.
• 3）异步电动机的转子分成两种 – 鼠笼式转
子和绕线式转子。
• 4) By changing the impedance connected
to the rotor circuit, the speed/current and
speed/torque curves can be altered.
• 4）改变转子电路的阻抗，可以改变速度/电
流曲线和速度/转矩曲线。
• 5) 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.
• 5）绕线式电动机最常用的场合是拖动惯性
比较大的负载或者需要高起动转矩的负载
起动。
• 6) The imperfect electric contact also
causes electrical noise.
• 6）不完善的电接触还会产生电噪声。
• 7) At higher speeds, brushes have
increasing difficulty in maintaining contact.
• 7）在更高的速度下，电刷更难保持接触状
态。
• 8) In brushless DC motor, the mechanical
"rotating switch" or commutator assembly
is replaced by an external electronic
switch synchronized to the rotor's position.
• 8）在无刷直流电动机中，机械“旋转开关”
或换向器被和转子位置同步的外置电子开
关所取代。
4．Translation
• 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.
Show the Answer
This is the End of the
Exercises
Good work everyone !!!
(习题答案仅供参考)