LINEAR INDUCTION MOTOR

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LINEAR INDUCTION MOTORS

• Y. LALITHA

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Overview

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

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Electric Motor Basic Principles

• Interaction between magnetic field and current
  carrying wire produces a force
• Opposite of a generator

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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

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2 pole brushed DC motor commutation
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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

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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

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Brushless DC Motors

• Essential difference - commutation is performed
  electronically with controller rather than
  mechanically with brushes

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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

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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
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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

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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.

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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

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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.

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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.

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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
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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."
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• 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.

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• 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.

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Photo NASA tests a prototype Maglev railroad,
2001
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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.
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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
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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

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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

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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
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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

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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)

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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!)

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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
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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
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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
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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

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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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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.

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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.
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• 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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• 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.

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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.
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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.
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• 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.

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• 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. 

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• 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.

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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 secondhere,
vs  is velocity of the linear travelling
field, t is the pole pitch.
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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.

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• 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.

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• 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

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• 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. 

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Classification of linear induction motor
application areas
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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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