Linear Motors

LINEAR INDUCTION MOTORSY. LALITHA

2Overview DC Motors (Brushed and Brushless) Brief Introduction to AC Motors Linear Motors Linear Induction MotorsY.Lalitha3

Electric Motor Basic PrinciplesInteraction between magnetic field and current carrying wire produces a forceOpposite of a generatorY.Lalitha3left: current carrying wireF=BILpair of force produces torque - spins the rotorright: electromagnet with metal core wrapped by wire coilscoil creates N and S poles - becomes attracted to S and N poles on stator, respectively

the idea, is then how to create a dynamically changing magnetic flux to keep the rotor spinning constantly

faraday's law concerning generators:generated emf = rate of change of magnetic flux4Conventional (Brushed) DC MotorsPermanent magnets for outer statorRotating coils for inner rotorCommutation performed with metal contact brushes and contacts designed to reverse the polarity of the rotor as it reaches horizontalY.Lalitha

42 pole DC electric motorDirect Current

a better picture of rotation/commutation next slide

52 pole brushed DC motor commutationY.Lalitha5important to note that with this simple 2 pole motor, when rotor rotates 90 degrees from this picture, there will be 0 torque.

Unable to start from rest at that 90deg position

in practice, a real DC motor use more than 2 poles to eliminate - zero torque zone, and shorting of battery6Conventional (Brushed) DC MotorsCommon Applications:Small/cheap devices such as toys, electric tooth brushes, small drillsLab 3Pros:Cheap, simpleEasy to control - speed is governed by the voltage and torque by the current through the armatureCons:Mechanical brushes - electrical noise, arcing, sparking, friction, wear, inefficient, shortingY.Lalitha6mechanical brushes could be metallic or carbon7

DC Motor considerationsBack EMF - every motor is also a generatorMore current = more torque; more voltage = more speedLoad, torque, speed characteristics

Shunt-wound, series-wound (aka universal motor), compound DC motors

Y.Lalitha7under no load conditions, motor will rotate at a speed such that the back emf equals the applied voltage plus voltage drop across armaturegenerally highest torque at zero speed, zero torque at max speedincrease current to increase torqueincrease voltage to increase speedshunt wound, series wound DC motors: Here, the stator is an electromagnet instead of permanent magnet.shunt has stator and armature connected in parallel. series has stators and armature connected in series.Has different loading characteristicsseries wound DC is also known as universal motor and can run on both AC and DC because both stator and rotor polarity can be switched 8

Brushless DC MotorsEssential difference - commutation is performed electronically with controller rather than mechanically with brushesY.Lalitha

8Brushed DC motor- 'conventional'/'inrunner' configuration:flipped inside out - stator is now coil, rotor is permanent magnet that spins on the insidetypically less torque, but high RPM

'outrunner' configuration - rotor spins on the outside around stator.typically high torque but lower RPMEnergize the stator electromagnet coils sequentially (very much like a stepper motor) to make the rotor rotate9

Brushless DC Motor CommutationCommutation is performed electronically using a controller (e.g. HCS12 or logic circuit)Similarity with stepper motor, but with less # polesNeeds rotor positional closed loop feedback: hall effect sensors, back EMF, photo transistors

Y.Lalitha9How to know when to energize coils?cannot do this in open loop like stepper due to smaller number of poles on stator; needs feedback2 ways to sense rotor position: -hall effect sensor (detects magnetic fields)-sensorless (back emf on the un-energized coils)-photo transistors (encoders, lab3 slot and detector)10

Delta Wye

BLDC (3-Pole) Motor ConnectionsHas 3 leads instead of 2 like brushed DCDelta (greater speed) and Wye (greater torque) stator windingsY.Lalitha10left diagram (delta):sequentially energize each of the 3 leads to make rotor turn

if more poles/windings on stator, typically still arranged into 3 groups - hence still 3 leadswye - greater torque at low speedsdelta - greater speeddelta, wye in AC transformers - neutral wires -phase to neutral voltages available for wye.only phase to phase voltage available for delta

11

Brushless DC MotorsApplicationsCPU cooling fansCD/DVD PlayersElectric automobilesPros (compared to brushed DC)Higher efficiencyLonger lifespan, low maintenanceClean, fast, no sparking/issues with brushed contactsConsHigher costMore complex circuitry and requires a controllerY.LalithaAC 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. 12Y.Lalitha13

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

Y.Lalitha13There are also single phase - require external starterAC current through the stator windings creates a time varying magnetic field.This induces an emf across the conductive rotor (often a 'squirrel cage'This makes the rotor a magnet, which then interacts with the magnetic field of the stator.The goal is to make a rotating magnetic field with the stator.Induction motors require (slip)workhorse of industry - rugged construction; no brushes to wear out - reliable, low maintenanceAC induction MotorsInduction 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. 14Y.LalithaSynchronous 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. 15Y.LalithaLinear Motors

Linear motors are electricinduction motors that produce motion in a straight line rather than rotational motion. In a traditionalelectric motor, therotor(rotating part) spins inside thestator(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 usesuperconducting magnets, which are cooled to low temperatures to reduce power consumption

The basic principle behind the linear motor was discovered in 1895, but practical devices were not developed until 1947.

During the 1950s, British electrical engineerEric Laithwaitestarted 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."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.By contrast, maglev has been enthusiastically developed byGermany 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.

Photo: NASA tests a prototype Maglev railroad, 2001

In a traditional DCelectric 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 ACinduction 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 anelectric car, DC or AC motors like these are used to drivegearsandwheelsand convert rotational motion into motion in a straight line.Hannes DaeppBasics of Linear Motors [1],[4]

I

Analogous to Unrolled DC MotorForce (F) is generated when the current (I) (along vector L) and the flux density (B) interactF = LI x B

Hannes DaeppBenefits of Linear Motors High Maximum SpeedLimited primarily by bus voltage, control electronicsHigh PrecisionAccuracy, resolution, repeatability limited by feedback device, budgetZero backlash: No mechanical transmission components.Fast ResponseResponse rate can be over 100 times that of a mechanical transmission faster accelerations, settling time (more throughput)StiffnessNo mechanical linkage, stiffness depends mostly on gain & currentDurableModern linear motors have few/no contacting parts no wear23Typical max speeds: 3-5 m/s with 1 micron resolution, 5+ m/s (>200 ips) with less resolutionBudget is main restraint on controller bandwidth

Higher stiffness (spring rate), though limited by motor peak force, available current, and feedbakc resolutionHannes DaeppDownsides of Linear MotorsCostLow production volume (relative to demand)High price of magnetsLinear encoders (feedback) are much more expensive than rotary encoders, cost increases with lengthHigher Bandwidth Drives and ControlsLower force per package sizeHeating issuesForcer is usually attached to load I2R losses are directly coupled to loadNo (minimal) FrictionNo automatic brake24Linear encoders are usually around $500 for 100 mm travel encoder, cost increases with length. Rotary encoders are relatively inexpensive tend to be under 100 dollarsNo mechanical reduction between motor and load, thus servo response (bandwidth) must be faster. Includes higher encoder bandwidth and servo update ratesLinear motors are not compact force generators when compared to rotary motor with transmission offering mechanical advantage. Example 3/8 diam. Ball screw produces 100 lb of thrust, while 15 lb of linear thrust typically requires 2 x 1.5 cross section.Heat management techniques such as air and water cooling options (both common, popular) have to be appliedSuppose its traveling at 3 m/s and loses power. Without resistance, it will quickly reach end of end of system, mechanical stops.

Hannes DaeppComponents of Linear Motors

Forcer (Motor Coil)Windings (coils) provide current (I)Windings are encapsulated within core materialMounting Plate on topUsually contains sensors (hall effect and thermal)Magnet RailIron Plate / Base PlateRare Earth Magnets of alternating polarity provide flux (B)Single or double railF = lI x B

Hannes DaeppTypes 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

26Iron core: base plate with magnets, basically a brushless DC motor laid out. Magnetic back iron keeps it down by maintaining magnetic attraction to place

Ironless: NO back iron.

Slotless: Just one rail, often uses non-ferrous housing to support coil assembly (so that its not limited purely to epoxy, but isnt magnetic)Hannes Daepp

Iron PlateRare earth magnetsLaminated forcer assembly and mounting plateCoil wound AroundForcer laminationHall effectand thermalsensorsLinear Motor Types: Iron Core Distinguishing FeatureCopper windings around forcer laminations over a single magnet railAdvantages:Highest force available per unit volume Efficient CoolingLower cost Disadvantages:High attractive force between forcer & magnet trackCogging: iron forcer affects thrust force as it passes over each magnet (aka velocity ripple)27-- Highest force per unit volume is because laminations concentrate flux field-- iron forcer also aids in heat dissipation. Cooling tubes can be routed through laminations to improve thermal managment-- only 1 row of magnets lower costDisadvantages:-- Since the forcer consists of iron, it is attracted to the permanentmagnets. Bearings are used to support the force. Can be up to 10 x thrust force, meaning that choice of bearings is critical.-- Cogging: Since the forcer is made of iron and it passes over magnets, there is avariation in the thrust force as it passes each magnet. This is referred to as cogging andaffects low speed smoothness (velocity ripple) [8]

27Hannes DaeppDistinguishing FeatureForcer constructed of wound coils held together with epoxy and running between two rails (North and South)Also known as Aircore or U-channel motorsAdvantages:No attractive forces in forcer No CoggingLow weight forcer - No iron means higher accel/decel rates

Top View

ForcerMountingPlateRare EarthMagnetsHorseshoeShapedbackironWinding, heldby epoxyHall Effect and ThermalSensors in coilFront ViewLinear Motor Types: Ironless Disadvantages:Low force per package sizeLower Stiffness; limited max load without improved structurePoor heat dissipationHigher cost (2x Magnets!)

28-- No back iron in forcer, but is usually topped with an aluminum bar for mounting the load and for heat removal

Advantages:-- No attractive forces (no iron in forcer), so no additional forces on bearings. Motor is also easier to handle, install-- ironless forcer no cogging. Great for extreme velocity control. Usually used with air bearings due to their ultra-smooth characteristics

Disadvantages:-- since forcer is just coils with epoxy below plate, heat must leave the coil to aluminum plate via coil or through the air gap in magnet rail. High thermal resistance makes heat dissipation an issue.-- Weak structure relative to iron core, since forcer is made of coils and epoxy (as opposed to iron). Also limits max sizes and forces to which these motors can be manufactured without adding additional structural members-- Double rail, along with thermal and structural limitations, contributes to lower force per package size28Hannes DaeppDistinguishing FeatureMix of ironless and iron core: coils with back iron contained within aluminum housing over a single magnet railAdvantages over ironless:Lower cost (1x magnets)Better heat dissipationStructurally stronger forcerMore force per package sizeAdvantages over iron core:Lighter weight and lower inertia forcerLower attractive forcesLess cogging

Side ViewFront View

BackironMountingplateCoilassemblyThermalsensorRare EarthMagnets

IronplateLinear Motor Types: Slotless29Hybrid between iron core and ironless linear motor designs

v. Ironless-- less weight than ironless. Higher accelerations-- Housing provides considerably improved heat dissipation-- housing makes structure better than ironless; can handle larger loads-- force per package size between ironless and iron core. Better thermal management also means that it can handle higher currents than ironless and thus generate higher forces

v. Iron Core-- Light weight forcer (aluminum v. iron) means higher throughput in light load applications-- back iron causes 5-7 times less attractive force than with iron core-- larger magnetic gap between magnets and forcer backiron results in less cogging better velocity control29Hannes DaeppDisadvantagesSome attractive force and coggingLess efficient than iron core and ironless - more heat to do the same job

Side ViewFront View

BackironMountingplateCoilassemblyThermalsensorRare EarthMagnets

IronplateLinear Motor Types: Slotless 30Hybrid between iron core and ironless linear motor designs

30Hannes DaeppLinear Brushless DC Motor TypeFeatureIron CoreIronlessSlotlessAttraction ForceMostNoneModerateCostMediumHighLowestForce CoggingHighestNoneMediumPower DensityHighestMediumMediumForcer WeightHeaviestLightestModerateLinear Motor Type Comparison [2]3131Hannes DaeppComponents of a Complete Linear Motor System [3]Motor componentsBase/BearingsServo controller/feedback elementsTypical sensors include Hall Effect (for position) and thermal sensorsCable management

3232Y.LalithaApplications Small Linear MotorsPackaging and Material HandlingAutomated AssemblyReciprocating compressors and alternators Large Linear Induction Machines (3 phase)TransportationMaterials handlingExtrusion presses

33Packaging: Particular notice in semiconductor industry, where precision is critical and motions of under 1 micron are often desired

Most widely known use of linear motors is in transportationAutomotive indsutry has been quick to pick up on linear motors because it allows more flexibility can simply change fixtures for different cars instead of customizing assembly to one vehicle [6]33

Linear Induction MotorLinear Induction motorabbreviated 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. 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 powerenergetic, Medical instruments, Computer engineering.

Advantages:Direct electromagnetic force (nomechanical 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 andwindings 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 / ) >1/ 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 .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. 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.

Construction of a Linear Induction Motor

Construction wise a LIM is similar tothree phase induction motor.. If the stator of the poly phase induction motorshown 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.

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.

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.

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. Working of a Linear Induction MotorWhen 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 :ns=2 fs/p Here, fs is supply frequency in Hz,p is the number of poles,nsis 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,vs=2 t fs meter per secondhere, vs is velocity of the linear travelling field, t is the pole pitch.

For a slip of s, the speed of conducting slave in a linear motor is given byvr=(1-s)vsLinear 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.

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.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 SystemAutomatic sliding doors in an electrical trainMetallic belt conveyerMechanical handling equipment, such as propulsion of a train of tubs along a certain routeShuttle-propelling application

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.

Classification of linear induction motor application areas

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/linearmotorarticle.pdf[3] Trilogy Linear Motor & Linear Motor Positioners, Parker Hannifin Corporation, 2008, http://www.parkermotion.com/pdfs/Trilogy_Catalog.pdf[4] Rockwell Automation, http://www.rockwellautomation.com/anorad/products/linearmotors/questions.html[5] J. Marsh, Motor Parameters Application Note, Parker-Trilogy Linear Motors, 2003. http://www.parkermotion.com/whitepages/Linear_Motor_Parameter_Application_Note.pdf[6] Greg Paula, Linear motors take center stage, The American Society of Mechanical Engineers, 1998.