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LINEAR INDUCTION MOTOR

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Linear motors are electric induction motors that produce motion in a straight line rather than rotational motion. In a traditional electric motor, the rotor (rotating part) spins inside the stator(static part); in a linear motor, the stator is unwrapped and laid out flat and the "rotor" moves past it in a straight line. Linear motors often use superconducting magnets, which are cooled to low temperatures to reduce power consumption – PowerPoint PPT presentation

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Title: LINEAR INDUCTION MOTOR


1
LINEAR INDUCTION MOTORS
  • Y. LALITHA

2
Overview
  • DC Motors (Brushed and Brushless)
  • Brief Introduction to AC Motors
  • Linear Motors
  • Linear Induction Motors

2
Y.Lalitha
3
Electric Motor Basic Principles
  • Interaction between magnetic field and current
    carrying wire produces a force
  • Opposite of a generator

3
Y.Lalitha
4
Conventional (Brushed) DC Motors
  • Permanent magnets for outer stator
  • Rotating coils for inner rotor 
  • Commutation performed with metal contact brushes
    and contacts designed to reverse the polarity of
    the rotor as it reaches horizontal

4
Y.Lalitha
5
2 pole brushed DC motor commutation
5
Y.Lalitha
6
Conventional (Brushed) DC Motors
  • Common Applications
  • Small/cheap devices such as toys, electric tooth
    brushes, small drills
  • Lab 3
  • Pros
  • Cheap, simple
  • Easy to control - speed is governed by the
    voltage and torque by the current through the
    armature
  • Cons
  • Mechanical brushes - electrical noise, arcing,
    sparking, friction, wear, inefficient, shorting

6
Y.Lalitha
7
DC Motor considerations
  • Back EMF - every motor is also a generator
  • More current more torque more voltage more
    speed
  • Load, torque, speed characteristics
  • Shunt-wound, series-wound (aka universal motor),
    compound DC motors

7
Y.Lalitha
8
Brushless DC Motors
  • Essential difference - commutation is performed
    electronically with controller rather than
    mechanically with brushes

8
Y.Lalitha
9
Brushless DC Motor Commutation
  • Commutation is performed electronically using a
    controller (e.g. HCS12 or logic circuit)
  • Similarity with stepper motor, but with less
    poles
  • Needs rotor positional closed loop feedback hall
    effect sensors, back EMF, photo transistors

9
Y.Lalitha
10
BLDC (3-Pole) Motor Connections
  • Has 3 leads instead of 2 like brushed DC
  • Delta (greater speed) and Wye (greater torque)
    stator windings 

Delta               Wye
10
Y.Lalitha
11
Brushless DC Motors
  • Applications
  • CPU cooling fans
  • CD/DVD Players
  • Electric automobiles
  • Pros (compared to brushed DC)
  • Higher efficiency
  • Longer lifespan, low maintenance
  • Clean, fast, no sparking/issues with brushed
    contacts
  • Cons
  • Higher cost
  • More complex circuitry and requires a controller

11
Y.Lalitha
12
AC Motors
  • Two main types of AC motor, Synchronous and
    Induction.
  • Synchronous motors supply power to both the rotor
    and the stator, where induction motors only
    supply power to the stator coils, and rely on
    induction to generate torque.

12
Y.Lalitha
13
AC Induction Motors (3 Phase)
  • Use poly-phase (usually 3) AC current to create a
    rotating magnetic field on the stator
  • This induces a magnetic field on the rotor, which
    tries to follow stator - slipping required to
    produce torque
  • Workhorses of the industry - high powered
    applications

13
Y.Lalitha
14
AC induction Motors
  • Induction motors only supply current to the
    stator, and rely on a second induced current in
    the rotor coils.
  • This requires a relative speed between the
    rotating magnetic field and the rotor. If the
    rotor somehow matches or exceeds the magnetic
    field speed, there is condition called slip.
  • Slip is required to produce torque, if there is
    no slip, there is no difference between the
    induced pole and the powered pole, and therefore
    no torque on the shaft.

14
Y.Lalitha
15
Synchronous AC Motors
  • Current is applied to both the Rotor and the
    Stator.
  • This allows for precise control (stepper motors),
    but requires mechanical brushes or slip rings to
    supply DC current to the rotor.
  • There is no slip since the rotor does not rely on
    induction to produce torque.

15
Y.Lalitha
16
Linear Motors
Linear motors are electric induction motors that
produce motion in a straight line rather than
rotational motion. In a traditional electric
motor, the rotor (rotating part) spins inside
the stator(static part) in a linear motor, the
stator is unwrapped and laid out flat and the
"rotor" moves past it in a straight line. Linear
motors often use superconducting magnets, which
are cooled to low temperatures to reduce power
consumption
17
The basic principle behind the linear motor was
discovered in 1895, but practical devices were
not developed until 1947. During the 1950s,
British electrical engineer Eric
Laithwaite started to consider whether linear
motors could be used in electric weaving
machines. Laithwaite's research at Imperial
College, London attracted international
recognition in the 1960s following a speech to
the Royal Institution entitled "Electrical
Machines of the Future."
18
  • In the 1960s, Eric Laithwaite's research into
    linear motors led to renewed interest in the idea
    of a magnetically levitated or "maglev" train.
  • Around this time MIT scientist Henry Kolm
    proposed a "magnaplane" running on rails that
    could carry 20,000 people at 200 mph (320 kph).
  • This prompted a US research program and led to a
    working prototype that was tested in Colorado in
    1967.
  • However, the US program ran into political
    difficulties and was shelved in 1975.
  • The early 1990s brought an ambitious proposal to
    link Las Vegas, Los Angeles, San Diego, and San
    Fransisco with a maglev railroad, but that
    project has since run into more political
    problems.

19
  • By contrast, maglev has been enthusiastically
    developed by
  • Germany and Japan.
  • German engineers first produced a working
    prototype in 1971 and developed the Transrapid
    system a year later.
  • With considerable support from the German
    government, this has been progressively refined
    into a viable train that has been tested at
    speeds of up to 271 mph (433 kph).
  • Strictly speaking, the Transrapid uses magnetic
    attraction rather than the magnetic repulsion
    normally associated with maglev the copper
    magnets are fixed to a "skirt" that runs
    underneath, and is attracted up toward, the steel
    track.

20
Photo NASA tests a prototype Maglev railroad,
2001
21
In a traditional DC electric motor,a central core
of tightly wrapped magnetic material (known as
the rotor) spins at high speed between the fixed
poles of a magnet (known as the stator) when an
electric current is applied. In an AC induction
motor, electromagnets positioned around the edge
of the motor are used to generate a rotating
magnetic field in the central space between
them. This "induces" (produces) electric
currents in a rotor, causing it to spin. In
an electric car, DC or AC motors like these are
used to drive gears and wheelsand convert
rotational motion into motion in a straight line.
22
Basics of Linear Motors 1,4
  • Analogous to Unrolled DC Motor
  • Force (F) is generated when the current (I)
    (along vector L) and the flux density (B)
    interact
  • F LI x B

I
23
Benefits of Linear Motors
  • High Maximum Speed
  • Limited primarily by bus voltage, control
    electronics
  • High Precision
  • Accuracy, resolution, repeatability limited by
    feedback device, budget
  • Zero backlash No mechanical transmission
    components.
  • Fast Response
  • Response rate can be over 100 times that of a
    mechanical transmission ? faster accelerations,
    settling time (more throughput)
  • Stiffness
  • No mechanical linkage, stiffness depends mostly
    on gain current
  • Durable
  • Modern linear motors have few/no contacting parts
    ? no wear

24
Downsides of Linear Motors
  • Cost
  • Low production volume (relative to demand)
  • High price of magnets
  • Linear encoders (feedback) are much more
    expensive than rotary encoders, cost increases
    with length
  • Higher Bandwidth Drives and Controls
  • Lower force per package size
  • Heating issues
  • Forcer is usually attached to load ? I2R losses
    are directly coupled to load
  • No (minimal) Friction
  • No automatic brake

25
Components of Linear Motors
  • Forcer (Motor Coil)
  • Windings (coils) provide current (I)
  • Windings are encapsulated within core material
  • Mounting Plate on top
  • Usually contains sensors (hall effect and
    thermal)
  • Magnet Rail
  • Iron Plate / Base Plate
  • Rare Earth Magnets of alternating polarity
    provide flux (B)
  • Single or double rail

F lI x B
26
Types of Linear Motors
  • Iron Core
  • Coils wound around teeth of laminations on
    forcer
  • Ironless Core
  • Dual back iron separated by spacer
  • Coils held together with epoxy
  • Slotless
  • Coil and back iron held together with epoxy

27
Linear Motor Types Iron Core
  • Distinguishing Feature
  • Copper windings around forcer laminations over a
    single magnet rail
  • Advantages
  • Highest force available per unit volume
  • Efficient Cooling
  • Lower cost
  • Disadvantages
  • High attractive force between forcer magnet
    track
  • Cogging iron forcer affects thrust force as it
    passes over each magnet (aka velocity ripple)

28
Linear Motor Types Ironless
Top View
  • Distinguishing Feature
  • Forcer constructed of wound coils held together
    with epoxy and running between two rails (North
    and South)
  • Also known as Aircore or U-channel motors
  • Advantages
  • No attractive forces in forcer
  • No Cogging
  • Low weight forcer - No iron means higher
    accel/decel rates
  • Disadvantages
  • Low force per package size
  • Lower Stiffness limited max load without
    improved structure
  • Poor heat dissipation
  • Higher cost (2x Magnets!)

29
Linear Motor Types Slotless
Side View
  • Distinguishing Feature
  • Mix of ironless and iron core coils with back
    iron contained within aluminum housing over a
    single magnet rail
  • Advantages over ironless
  • Lower cost (1x magnets)
  • Better heat dissipation
  • Structurally stronger forcer
  • More force per package size
  • Advantages over iron core
  • Lighter weight and lower inertia forcer
  • Lower attractive forces
  • Less cogging

Front View
30
Linear Motor Types Slotless
Side View
  • Disadvantages
  • Some attractive force and cogging
  • Less efficient than iron core and ironless - more
    heat to do the same job

Front View
31
Linear Motor Type Comparison 2
Linear Brushless DC Motor Type Linear Brushless DC Motor Type Linear Brushless DC Motor Type
Feature Iron Core Ironless Slotless
Attraction Force Most None Moderate
Cost Medium High Lowest
Force Cogging Highest None Medium
Power Density Highest Medium Medium
Forcer Weight Heaviest Lightest Moderate
32
Components of a Complete Linear Motor System 3
  • Motor components
  • Base/Bearings
  • Servo controller/feedback elements
  • Typical sensors include Hall Effect (for
    position) and thermal sensors
  • Cable management

33
Applications
  • Small Linear Motors
  • Packaging and Material Handling
  • Automated Assembly
  • Reciprocating compressors and alternators
  • Large Linear Induction Machines (3 phase)
  • Transportation
  • Materials handling
  • Extrusion presses

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35
Linear Induction Motor
  • Linear Induction motor abbreviated as LIM.
  • Basically a special purpose motor that is in use
    to achieve rectilinear motion rather than
    rotational motion as in the case of conventional
    motors.
  • This is quite an engineering marvel, to convert a
    general motor for a special purpose with more or
    less similar working principle, thus enhancing
    its versatility of operation.

36
Linear induction motors invented by Charles
Wheatstone in 1840 m., since this time linear
induction motors are investigated, produced and
improved and nowadays are used in mechatronic
systems whose examples are High-speed transport
and catapult, Industry transport systems,
Batching systems, Vertical transport systems,
Semiconductors and electronics industry,
Explosion localizing systems, Industry robots
and machine-tools, Protection and control
systems of power energetic, Medical
instruments, Computer engineering.
37
  • Advantages
  • Direct electromagnetic force (no
  • mechanical elements, no limitations for speed).
  • Economical and cheap maintenance.
  • Easy expansion for any linear motion of system
    topology.
  • Exact positioning in closed loop systems.
  • Possibility to provide inductor and
  • windings separate cooling.
  • The power factor developed by naturally cooling
    LIM is 1 N / cm2 . Almost 2 N / cm2 can be
    obtained with an air cooling and from 2,5 - 3 N /
    cm2 with liquids 3.
  • All electro-mechanical controlled systems used
    for an induction motors can be adopted for a LIM
    without any bigger changes.
  • Disadvantages
  • Power factor and efficiency are less than of
    rotary motors because of a ratio of large air gap
    between inductors and pole pitch(g / t) gt1/ 250 .
  • The longitudinal end effect reduces power factor
    and efficiency. This can be noticed only with
    fast speed and small pole number motors.
  • Influence of the longitudinal end effect can be
    reduced with special motor design methods.
  • Extra vibrations with distortions can be noticed
    because of uncompensated normal force .

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39
  • In recent years, attempts to develop new means of
    high-speed, efficient transportation have led to
    considerable world-wide interest in high-speed
    trains.
  • This in turn has generated interests in the
    linear induction motor which is considered to be
    one of the most suitable propulsion systems for
    super-high-speed trains.
  • Research and experiments on linear induction
    motors are being actively pursued in a number of
    countries, among them Japan.
  • Unfortunately, many researchers, in their desire
    to achieve immediate practical results, have
    concentrated on experiments with large-scale
    testing equipment and large-size test trains,
    leaving the theoretical aspects of the linear
    induction motor neglected so that few useful
    results have been produced.

40
In spite of extensive experimental efforts, there
has been no reported test result on a linear
induction motor with proven feasibility for
high-speed trains higher than, say, 200km/h.
This situation is partly due to the fact that up
to now no sound theoretical basis for linear
induction motor has been established so that many
researchers have based their ideas on theories
and experiences from the rotary induction motor.
41
Construction of a Linear Induction Motor
Construction wise a LIM is similar to three phase
induction motor. . If the stator of the poly
phase  induction motor shown in the figure is cut
along the section aob and laid on a flat surface,
then it forms the primary of the LIM housing the
field system, and consequently the rotor forms
the secondary consisting of flat aluminum
conductors with ferromagnetic core for effective
flux linkage.
42
  • There are several ways and types of construction
    of a Linear Motor or Linear Induction Motor.
  • The simplest form of construction of a Linear
    Motor is as simple as a three phase induction
    motor.
  • It has three phase winding housed in slots in a
    field system.
  • It is simply the primary winding on a stator in
    case of an induction motor.
  • This is obtained if we cut the stator of an
    induction motor from middle. 
  • In case of a moving object like in a train the
    primary winding is mounted on the body of
    vehicle.

43
  • The rotor is made by aluminium or copper plates
    in parallel.
  • In order to complete the flux path a
    ferromagnetic material is placed with the plates.
  • As the primary is on vehicle or object and
    secondary is in form of plates so they will have
    unequal length.
  • For larger distance primary is kept small and for
    very small and limited distance secondary is kept
    small.
  • Normally two sided primary winding is used.
  • In this configuration the two field system, one
    on either side of secondary are used. 

44
  • The essential difference between the linear
    induction motor and the rotary induction motor is
    the open linear air gap, which has both an entry
    end and an exit end.
  • The end effect, which is caused by the
    open-endedness of the air gap, produces
    considerable distortion in magnetic field
    distribution and peculiar phenomena which are not
    observed in the rotary induction motor, but which
    considerably influence the characteristics of the
    linear induction motor.
  • The first reports on the end effect of the linear
    induction motor were made many years ago in
    connection with the arch motor, an induction
    motor in which one part of the stator core was
    removed.

45
Working of a Linear Induction Motor
When the primary is excited by a balanced three
phase supply, a rotating electromagnetic flux is
induced in primary. The synchronous speed of the
field is given by the equation ns2 fs/p Here,
fs  is supply frequency in Hz, p is the
number of poles, ns is the synchronous speed of
the rotation of magnetic field in
revolutions per second. The developed field will
results in a linear travelling field, the
velocity of which is given by the
equation, vs2 t fs  meter per second here,
vs  is velocity of the linear travelling
field, t is the pole pitch.
46
For a slip of s, the speed of conducting slave in
a linear motor is given by vr(1-s)vs
  • Linear Induction Motor is similar in construction
    to a circular motor that has been opened out
    flat.
  • The magnetic field now sweeps across the flat
    motor face instead of rotating.

47
  • The stator generally consists of a multi phase
    winding in a laminated iron core.
  • When energized from an AC supply a travelling
    wave magnetic field is produced.
  • Travel direction can be reversed by swapping two
    phases.
  • The reaction plate is the equivalent of the
    rotor.
  • For single sided applications this is usually a
    conductive sheet of aluminium or copper backed
    by steel, and for double sided applications only
    a conductive sheet is used.
  • Currents induced in the reaction plate by the
    stator travelling field create a secondary
    magnetic field. It is the reaction between these
    two fields which produces the linear thrust.

48
  • Application of Linear Induction Motor or LIM
  • Although these motors are not frequently used.
  • There are only a few instances where the linear
    motor is used or is utilized in a proper way.
  • It seems that these motors are technically,
    feasible but due to economical point of view
    these motors are not frequently used. However the
    possible applications of a Linear Induction Motor
    are listed below Application for Stationary
    Field System
  • Automatic sliding doors in an electrical train
  • Metallic belt conveyer
  • Mechanical handling equipment, such as propulsion
    of a
  • train of tubs along a certain route
  • Shuttle-propelling application

49
  • Applications for the moving field system
  • High and medium speed applications have been
    tried with linear motor propulsion of vehicles
    with air cushion or magnetic suspension.
  • High speed application as a travelling crane
    motor where the field system is suspended from
    loist. 

50
Classification of linear induction motor
application areas
51
References
  • 1 S. Cetinkunt, Mechatronics, John Wiley
    Sons, Inc., Hoboken 2007.
  • 2 J. Barrett, T. Harned, J. Monnich, Linear
    Motor Basics, Parker Hannifin Corporation,
    http//www.parkermotion.com/whitepages/linearmotor
    article.pdf
  • 3 Trilogy Linear Motor Linear Motor
    Positioners, Parker Hannifin Corporation, 2008,
    http//www.parkermotion.com/pdfs/Trilogy_Catalog.p
    df
  • 4 Rockwell Automation, http//www.rockwellautoma
    tion.com/anorad/ products/linearmotors/questions.h
    tml
  • 5 J. Marsh, Motor Parameters Application Note,
    Parker-Trilogy Linear Motors, 2003.
    http//www.parkermotion.com/whitepages/ Linear_Mot
    or_Parameter_Application_Note.pdf
  • 6 Greg Paula, Linear motors take center stage,
    The American Society of Mechanical Engineers,
    1998.

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