INTRODUCTIONMagnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. The electromagnetic force is used to counteract the effects of the gravitational force.

Earnshaw's theorem proves that using only static ferromagnetism it is impossible to stably levitate against gravity as required for stable equilibrium. Earnshaw's theorem can be viewed as a consequence of the Maxwell equations, whichdo not allow the magnitude of a magnetic field in a free space to possess a maximum. But servomechanisms, the use of diamagnetic materials or superconductor permit this to occur. For a particle to be in a stable equilibrium, small perturbations ("pushes") on the particle in any direction should not break the equilibrium; the particle should "fall back" to its previous position. This means that the force field lines around the particle's equilibrium position should all point inwards, towards that position. If all of the surrounding field lines point towards the equilibrium point, then the divergence of the field at that point must be negative (i.e. that point acts as a sink). However, Gauss's Law says that the divergence of any possible electric force field is zero in free space. Diamagnets (which respond to magnetic fields with mild repulsion) are known to flout the theorem, as their negative susceptibility results in the requirement of a minimum rather than a maximum in the field’s magnitude. Stable levitation has been demonstrated for diamagnetic objects such as superconducting pellets and live creatures. Strong diamagnetism of superconductors allows the situation to be reversed, so that a magnet can be levitated above a superconductor.

We set out to lift a magnet by applying a magnetic field and then stabilizing the intrinsically unstable equilibrium with repulsive forces from a nearby diamagnetic material. Diamagnetic levitation can be used to levitate very light pieces of pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is predominantly diamagnetic, this technique has been used to levitate water droplets and even live animals, such as a grasshopper and a frog. However, the magnetic fields required for this are very high, typically in the range of 16 teslas, and therefore create significant problems if ferromagnetic materials are nearby.

MAGLEV METHODS

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There are several methods to obtain magnetic levitation. The following are a few general methods.

Mechanical constraint (Pseudo-levitation)

With a small amount of mechanical constraint for stability, pseudo-levitation is relatively straightforwardly achieved.

If two magnets are mechanically constrained along a single vertical axis, for example, and arranged to repel each other strongly, this will act to levitate one of the magnets above the other.

Another geometry is where the magnets are attracted, but constrained from touching by a tensile member, such as a string or cable.

Another example is the Zippe-type centrifuge where a cylinder is suspended under an attractive magnet, and stabilised by a needle bearing from below.

Direct diamagnetic levitation

A live frog levitates inside a 32 mm diameter vertical bore of a Bitter solenoid in a magnetic field of about 16 teslas at the High Field Magnet Laboratory of the Radboud University in Nijmegen the Netherlands.

A substance that is diamagnetic repels a magnetic field. All materials have diamagnetic properties, but the effect is very weak, and is usually overcome by the object's paramagnetic or ferromagnetic properties, which act in the opposite manner. Any material in which the diamagnetic component is strongest will be repelled by a magnet, though this force is not usually very large.

Earnshaw's theorem does not apply to diamagnets. These behave in the opposite manner to normal magnets owing to their relative permeability of μr < 1 (i.e. negative magnetic susceptibility).

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Diamagnetic levitation can be used to levitate very light pieces of pyrolytic graphite or bismuth above a moderately strong permanent magnet. As water is predominantly diamagnetic, this technique has been used to levitate water droplets and even live animals, such as a grasshopper and a frog. However, the magnetic fields required for this are very high, typically in the range of 16 teslas, and therefore create significant problems if ferromagnetic materials are nearby.

The minimum criterion for diamagnetic levitation is , where:

χ is the magnetic susceptibility ρ is the density of the material g is the local gravitational acceleration (-9.8 m/s2 on Earth) μ0 is the permeability of free space B is the magnetic field

is the rate of change of the magnetic field along the vertical axis

Assuming ideal conditions along the z-direction of solenoid magnet:

Water levitates at

Graphite levitates at

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Superconductors

Superconductors may be considered perfect diamagnets (μr = 0), completely expelling magnetic fields due to the Meissner effect. The levitation of the magnet is stabilized due to flux pinning within the superconductor. This principle is exploited by EDS (electrodynamic suspension) magnetic levitation trains, superconducting bearings, flywheels, etc.

In trains where the weight of the large electromagnet is a major design issue (a very strong magnetic field is required to levitate a massive train) superconductors are sometimes proposed for use for the electromagnet, since they can produce a stronger magnetic field for the same weight.

Diamagnetically-stabilized levitation

A permanent magnet can be stably suspended by various configurations of strong permanent magnets and strong diamagnets. When using superconducting magnets, the levitation of a permanent magnet can even be stabilized by the small diamagnetism of water in human fingers.

Rotational stabilization

A magnet can be levitated against gravity when gyroscopically stabilized by spinning it in a toroidal field created by a base ring of magnet(s). However, it will only remain stable until the rate of precession slows below a critical threshold — the region of stability is quite narrow both spatially and in the required rate of precession. The first discovery of this phenomenon was by Roy Harrigan, a Vermont inventor who patented a levitation device in 1983 based upon it. Several devices using rotational stabilization (such as the popular Levitron toy) have been developed citing this patent. Non-commercial devices have been created for university research laboratories, generally using magnets too powerful for safe public interaction.

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Servomechanisms

The attraction from a fixed strength magnet decreases with increased distance, and increases at closer distances. This is termed 'unstable'. For a stable system, the opposite is needed, variations from a stable position should push it back to the target position.

Stable magnetic levitation can be achieved by measuring the position and speed of the object being levitated, and using a feedback loop which continuously adjusts one or more electromagnets to correct the object's motion, thus forming a servomechanism.

Many systems use magnetic attraction pulling upwards against gravity for these kinds of systems as this gives some inherent lateral stability, but some use a combination of magnetic attraction and magnetic repulsion to push upwards.

This is termed Electromagnetic suspension (EMS). For a very simple example, some tabletop levitation demonstrations use this principle, and the object cuts a beam of light to measure the position of the object. The electromagnet is above the object being levitated; the electromagnet is turned off whenever the object gets too close, and turned back on when it falls further away. Such a simple system is not very robust; far more effective control systems exist, but this illustrates the basic idea. A practical demonstration of such system can be seen here. Of course in the real situation the problem becomes much more complex while the requirements of a MAGLEV suspension are difficult to achieve, i.e the electromagnetic suspension has to support very large mass (for axample 1T) wihtin a small air gap (in the region of mm). Also, the EMS system has to accomodate the rail irregulatrities while follow the track gradients. Nevertheless, all these requirements can be achieved using advance control strategies. A practical demonstration of a 25kg Electro-magnetic suspension setup is shown here. The Electromagnets are suspending 5mm below the track (rail). The control can be done using classical strategies as shown here or modern control strategies as shown here.

EMS magnetic levitation trains are based on this kind of levitation: The train wraps around the track, and is pulled upwards from below. The servo controls keep it safely at a constant distance from the track.

Induced currents/Eddy currents

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This is sometimes called ElectroDynamic Suspension (EDS).

Relative motion between conductors and magnets

If one moves a base made of a very good electrical conductor such as copper, aluminium or silver close to a magnet, an (eddy) current will be induced in the conductor that will oppose the changes in the field and create an opposite field that will repel the magnet (Lenz's law). At a sufficiently high rate of movement, a suspended magnet will levitate on the metal, or vice versa with suspended metal. Litz wire made of wire thinner than the skin depth for the frequencies seen by the metal works much more efficiently than solid conductors.

An especially technologically-interesting case of this comes when one uses a Halbach array instead of a single pole permanent magnet, as this almost doubles the field strength, which in turn almost doubles the strength of the eddy currents. The net effect is to more than triple the lift force. Using two opposed Halbach arrays increases the field even further. [3]

Halbach arrays are also well-suited to magnetic levitation and stabilisation of gyroscopes and electric motor and generator spindles.

Oscillating electromagnetic fields

A conductor can be levitated above an electromagnet (or vice versa) with an alternating current flowing through it. This causes any regular conductor to behave like a diamagnet, due to the eddy currents generated in the conductor. Since the eddy currents create their own fields which oppose the magnetic field, the conductive object is repelled from the electromagnet.

This effect requires non-ferromagnetic but highly conductive materials like aluminium or copper, as the ferromagnetic ones are also strongly attracted to the electromagnet (although at high frequencies the field can still be expelled) and tend to have a higher resistivity giving lower eddy currents. Again, litz wire gives the best results.

The effect can be used for stunts such as levitating a telephone book by concealing an aluminium plate within it.

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Stabilized permanent magnet suspension

In this method a repulsive magnet arrangement is used to provide lift and then any one or combination of the above stabilisation systems are used laterally. The vertical component of the lift magnets is stable in this arrangement, whereas the horizontal component is unstable, but, (depending on the geometry) rather smaller, and hence somewhat easier to stabilise.

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Application in MEGLAV VEHICLE

The main application of meglav is in meglav vehicle so while discussing magnetic levitation it is a must to discuss the technology used in meglav vehicle. The term "maglev" refers not only to the vehicles, but to the railway system as well, specifically designed for magnetic levitation and propulsion. All operational implementations of maglev technology have had minimal overlap with wheeled train technology and have not been compatible with conventional rail tracks. Because they cannot share existing infrastructure, these maglev systems must be designed as complete transportation systems.

Basically the there are three main forces involved in working of a meglav vehicle. All the forces work for one goal to stably levitate a considerable mass while making it move from one place to another.

LEVITATION. PROPULSION. LATERAL GUIDING

LEVITATION

The levitating force is the upward thrust which lifts the vehicle in the air. It counteracts the gravitational force and make the body float in air.There are 3 types of levitating systems.

For electromagnetic suspension (EMS), electromagnets in the train repel it away from a magnetically conductive (usually steel) track.

electrodynamic suspension (EDS) uses electromagnets on both track and train to push the train away from the rail.

stabilized permanent magnet suspension (SPM) uses opposing arrays of permanent magnets to levitate the train above the rail.

Another experimental technology, which was designed, proven mathematically, peer reviewed, and patented, but is yet to be built, is the magnetodynamic suspension (MDS), which uses the attractive magnetic force of a permanent magnet array near a steel track to lift the train and hold it in place.

ELECTROMAGNETIC SUSPENSION (EMS)

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The attraction from a fixed strength magnet decreases with increased distance, and increases at closer distances. This is termed 'unstable'. For a stable system, the opposite is needed; variations from a stable position should push it back to the target position.

Stable magnetic levitation can be achieved by measuring the position and speed of the object being levitated, and using a feedback loop which continuously adjusts one or more electromagnets to correct the object's motion, thus forming a servomechanism.

Many systems use magnetic attraction pulling upwards against gravity for these kinds of systems as this gives some inherent lateral stability, but some use a combination of magnetic attraction and magnetic repulsion to push upwards.

This is termed Electromagnetic suspension (EMS). For a very simple example, some tabletop levitation demonstrations use this principle, and the object cuts a beam of light to measure the position of the object. The electromagnet is above the object being levitated; the electromagnet is turned off whenever the object gets too close, and turned back on when it falls further away. Such a simple system is not very robust; far more effective control systems exist, but this illustrates the basic idea. Of course in the real situation the problem becomes much more complex while the requirements of a MAGLEV suspension are difficult to achieve, i.e the electromagnetic suspension has to support very large mass (for example 1T) wihtin a small air gap (in the region of mm). Also, the EMS system has to accomodate the rail irregulatrities while follow the track gradients. Nevertheless, all these requirements can be achieved using advance control strategies. EMS magnetic levitation trains are based on this kind of levitation: The train wraps around the track, and is pulled upwards from below. The servo controls keep it safely at a constant distance from the track.

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Electrodynamic suspension

In electrodynamic suspension (EDS), both the rail and the train exert a magnetic field, and the train is levitated by the repulsive force between these magnetic fields. The magnetic field in the train is produced by either electromagnets (as in JR-Maglev) or by an array of permanent magnets (as in Inductrack). The repulsive force in the track is created by an induced magnetic field in wires or other conducting strips in the track.

At slow speeds, the current induced in these coils and the resultant magnetic flux is not large enough to support the weight of the train. For this reason the train must have wheels or some other form of landing gear to support the train until it reaches a speed that can sustain levitation.

Onboard magnets and large margin between rail and train enable highest recorded train speeds (581 km/h).This system is inherently stable. Magnetic shielding for suppression of strong magnetic fields and wheels for travel at low speed are required. It can’t produce the propulsion force. So, LIM system is required.

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Fig. 9   The guideway of the electrodynamic suspension system is installed with guidance-levitation coils.

Stabilized Permanent Magnet suspension

SPM maglev systems differ from EDS maglev in that they use opposing sets of rare earth magnets (typically neodymium alloys in a Halbach array) in the track and vehicle to create permanent, passive levitation; i.e., no power is required to maintain permanent levitation. With no current required for levitation, the system has much less electromagnetic drag, thus requiring much less power to move a given cargo at a given speed.

Because of Earnshaw's theorem, SPM maglev systems require a mechanism to create lateral stability (i.e., controlling the side-to-side movement of the vehicle). One way to provide this stability is to use a set of coils along the bottom of the magnet array on the vehicle being levitated, which centers the vehicle over the rails by means of small amounts of current. Because the voice coils are not needed to provide lift and there is almost no drag, this system uses less power than other maglev systems: when the vehicle is centered over the rails, it uses no power. As the vehicle navigates a curve, the controller moves the vehicle to a ‘balance point’ inside the curve so that the (magnetic) centripetal pull of the magnetic rails in the ground offset the vehicle’s (kinetic) centrifugal momentum. This balance point varies based on the vehicle’s weight, which the controller automatically accounts for, resulting in zero steady state power consumption.

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INDUCTRACK SYSTEM:

The inductrack guide way would contain two rows of tightly packed levitation coils, which would act as the rails. Each of these “rails” would be lined by two Halbach arrays carried underneath the maglev vehicle: one positioned directly above the “rail” and one along the inner side of the “rail”. The Halbach arrays above the coils would provide levitation while the Halbach arrays on the sides would provide lateral guidance that keeps the train in a fixed position on the track.

The track is actually an array of electrically-shorted circuits containing insulated wire. In one design, these circuits are aligned like rungs in a ladder. As the train moves, a magnetic field repels the magnets, causing the train to levitate.

There are two inductrack designs. Inductrack I and II. Inductrack I is designed for high speeds, while inductrack II is suited for slow speeds. Inductrack trains could levitate higher with greater stability. As long as it’s moving a few miles per hour, an inductrack train will levitate nearly an inch above the track. A greater gap above the track means that the train would not require complex sensing systems to maintain stability. Permanent magnets had not been used before because scientists thought that they would not create enough levitating force. The inductrack design bypasses this problem by arranging the magnets in a Halbach array. The magnets are configured so that the intensity of the magnetic field concentrates above the array instead of below it which generates higher magnetic field.

The inductrack II design incorporates two Halbach arrays to generate a stronger magnetic field at lower speeds. Dr. Richard post at the Livermore National Laboratory in

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California came up with this concept in response to safety and cost concerns. The prototype tests caught the attention of NASA, which awarded a contract to Dr.post and his team to explore the possibility of using the inductrack system to launch satellites into orbit.

PROPULSION

This is a horizontal force which causes the movement of train. An EDS system can provide both levitation and propulsion using an onboard linear motor. EMS systems can only levitate the train using the magnets onboard, not propel it forward. As such, vehicles need some other technology for propulsion. A linear motor (propulsion coils) mounted in the track is one solution. Over long distances where the cost of propulsion coils could be prohibitive, a propeller or jet engine could be used.

It requires 3 parameters.

• Large electric power supply• Metal coil lining, a guide way or track.• Large magnet attached under the vehicle.

PRINCIPLES OF LINEAR MOTOR

Its principle is similar to induction motor having linear stator and flat rotor. The 3-phase supply applied to the stator produces a constant speed magnetic wave, which further produces a repulsive force.

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Maglev vehicles are propelled primarily by one of the following three options:

1.A linear synchronous motor (LSM) in which coils in the guideway are excited by a three phase winding to produce a traveling wave at the speed desired; Trans Rapid in Germany employs such a system.2. A Linear Induction Motor (LIM) in which an electromagnet underneath the vehicle induces current in an aluminum sheet on the guideway.3. A reluctance motor is employed in which active coils on the vehicle are pulsed at the proper time to realize thrust.

LATERAL GUIDING:

Guidance or steering refers to the sideward forces that are required to make the vehicle follow the guideway. The necessary forces are supplied in an exactly analogous fashion to the suspension forces, either attractive or repulsive. The same magnets on board the vehicle, which supply lift, can be used concurrently for guidance or separate guidance magnets can be used.

It requires the following arrangements:

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Guideway levitating coil Moving magnet

Also some systems use Null Flux systems (also called Null Current systems). These use a coil which is wound so that it enters two opposing, alternating fields. When the vehicle is in the straight ahead position, no current flows, but if it moves off-line this creates a changing flux that generates a field that pushes it back into line.

STABILITY:

Earnshaw's theorem shows that any combination of static magnets cannot be in a stable equilibrium. However, the various levitation systems achieve stable levitation by violating the assumptions of Earnshaw's theorem. Earnshaw's theorem assumes that the magnets are static and unchanging in field strength and that permeability is constant everywhere. EMS systems rely on active electronic stabilization. Such systems constantly measure the bearing distance and adjust the electromagnet current accordingly. All EDS systems are moving systems (no EDS system can levitate the train unless it is in motion).

Because Maglev vehicles essentially fly, stabilisation of pitch, roll and yaw is required by magnetic technology. In addition translations, surge (forward and backward motions), sway (sideways motion) or heave (up and down motions) can be problematic with some technologies.

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Pros and cons of different technologies

Each implementation of the magnetic levitation principle for train-type travel involves advantages and disadvantages. Time will tell us which principle, and whose implementation, wins out commercially.

Technology Pros Cons

EMS (Electromagnetic suspension)

Magnetic fields inside and outside the vehicle are less than EDS; proven, commercially available technology that can attain very high speeds (500 km/h); no wheels or secondary propulsion system needed

The separation between the vehicle and the guideway must be constantly monitored and corrected by computer systems to avoid collision due to the unstable nature of electromagnetic attraction; due to the system's inherent instability and the required constant corrections by outside systems, vibration issues may occur.

EDS (Electrodynamic)

Onboard magnets and large margin between rail and train enable highest recorded train speeds (581 km/h) and heavy load capacity; has recently demonstrated (December 2005) successful operations using high temperature superconductors in its onboard magnets, cooled with inexpensive liquid nitrogen

Strong magnetic fields onboard the train would make the train inaccessible to passengers with pacemakers or magnetic data storage media such as hard drives and credit cards, necessitating the use of magnetic shielding; limitations on guideway inductivity limit the maximum speed of the vehicle; vehicle must be wheeled for travel at low speeds.

Inductrack Failsafe Suspension - no power Requires either wheels or track

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System (Permanent Magnet EDS)

required to activate magnets; Magnetic field is localized below the car; can generate enough force at low speeds (around 5 km/h) to levitate maglev train; in case of power failure cars slow down on their own safely; Halbach arrays of permanent magnets may prove more cost-effective than electromagnets

segments that move for when the vehicle is stopped. New technology that is still under development (as of 2008) and as yet has no commercial version or full scale system prototype.

Neither Inductrack nor the Superconducting EDS are able to levitate vehicles at a standstill, although Inductrack provides levitation down to a much lower speed. Wheels are required for these systems. EMS systems are wheel-less.

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complexities faced in magnetic levitation

Most of the levitation techniques have various complexities.

Many of the active suspension techniques have a fairly narrow region of stability.

Magnetic fields have no built-in damping. This can permit vibration modes to exist

that can cause the item to leave the stable region. Eddy currents can be stabilizing if

a suitably shaped conductor is present in the field, and other mechanical or

electronic damping techniques have been used in some cases.

Power and current requirements can be reasonably large to generate sufficiently

strong magnetic fields using electromagnets to lift significant mass.

Superconductors require very low temperatures to operate, often helium cooling is

employed.

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Comparison

Compared to conventional trains

Major comparative differences between the two technologies lie in backward-compatibility, rolling resistance, weight, noise, design constraints, and control systems.

Backwards Compatibility Maglev trains currently in operation are not compatible with conventional track, and therefore require all new infrastructure for their entire route. By contrast conventional high speed trains such as the TGV are able to run at reduced speeds on existing rail infrastructure, thus reducing expenditure where new infrastructure would be particularly expensive (such as the final approaches to city terminals), or on extensions where traffic does not justify new infrastructure.

Efficiency Due to the lack of physical contact between the track and the vehicle, maglev trains experience no rolling resistance, leaving only air resistance and electromagnetic drag, potentially improving power efficiency.[13]

Weight The weight of the large electromagnets in many EMS and EDS designs is a major design issue. A very strong magnetic field is required to levitate a massive train. For this reason one research path is using superconductors to improve the efficiency of the electromagnets, and the energy cost of maintaining the field.

Noise. Because the major source of noise of a maglev train comes from displaced air, maglev trains produce less noise than a conventional train at equivalent speeds. However, the psychoacoustic profile of the maglev may reduce this benefit: A study concluded that maglev noise should be rated like road traffic while conventional trains have a 5-10 dB "bonus" as they are found less annoying at the same loudness level. [14][15]

Design Comparisons Braking and overhead wire wear have caused problems for the Fastech 360 railed Shinkansen. Maglev would eliminate these issues. Magnet reliability at higher temperatures is a countervailing comparative disadvantage (see suspension types), but new alloys and manufacturing techniques have resulted in magnets that maintain their levitational force at higher temperatures.

As with many technologies, advances in linear motor design have addressed the limitations noted in early maglev systems. As linear motors must fit within or straddle their track over the full length of the train, track design for some EDS and EMS maglev systems is challenging for anything other than point-to-point services. Curves must be gentle, while

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switches are very long and need care to avoid breaks in current. An SPM maglev system, in which the vehicle permanently levitated over the tracks, can instantaneously switch tracks using electronic controls, with no moving parts in the track. A prototype SPM maglev train has also navigated curves with radius equal to the length of the train itself, which indciates that a full-scale train should be able to navigate curves with the same or narrower radius as a conventional train.

Control Systems EMS Maglev needs very fast-responding control systems to maintain a stable height above the track; this needs careful design in the event of a failure in order to avoid crashing into the track during a power fluctuation. Other maglev systems do not necessarily have this problem. For example, SPM maglev systems have a stable levitation gap of several centimeters.

Compared to aircraft

For many systems, it is possible to define a lift-to-drag ratio. For maglev systems these ratios can exceed that of aircraft (for example Inductrack can approach 200:1 at high speed, far higher than any aircraft). This can make maglev more efficient per kilometre. However, at high cruising speeds, aerodynamic drag is much larger than lift-induced drag. Jet transport aircraft take advantage of low air density at high altitudes to significantly reduce drag during cruise, hence despite their lift-to-drag ratio disadvantage, they can travel more efficiently at high speeds than maglev trains that operate at sea level (this has been proposed to be fixed by the vactrain concept). Aircraft are also more flexible and can service more destinations with provision of suitable airport facilities.

Unlike airplanes, maglev trains are powered by electricity and thus need not carry fuel. Aircraft fuel is a significant danger during takeoff and landing accidents. Also, electric trains emit little carbon dioxide emissions, especially when powered by nuclear or renewable sources.

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Recent advancement

In the far future Maglev trains are hoped to be used to transport vast volumes of water to far regions at a greater speed eliminating droughts. Far more, space is an open door to maglev trains to propel humans and cargo into space at a lower cost. But most important is the New York-London tunnel, which runs under the Atlantic’s water, to form the last stage of the intercontinental highway. Scientists hope future technologies can get the train to operate at

a 6000km/h, since theoretically the speed limit is limitless. But still it’s a long way to go. Transrapid International is developing an electromagnetic suspension system (EMS). They have already demonstrated that it can reach 500Km/h with people on board. This speed can get a passenger from Paris to Rome in 2 hours. The Swiss are considering a new 700km system. The developers of these trains will most likely be connecting major cities up to 1600km away from each other, linking the busiest routes and exploiting their niche by being the fastest mode of accessible transport. The costs of producing the guideway at the moment still remain quite high at $10 million to $30million per mile.

If these technologies have the potential to reach 6000km/hr then why so far only 517km/hr have been materialized? Well it is due to the fact that the speed of the vehicle is limited by the air drag and the electromagnetic drag. Now electromagnetic drag has been overcome by the use of Halbach array of magnets. And as for the air drag scientist are working over the vacuum tubes for maglev vehicle but it has its own disadvantage as any defect in the body of the vehicle would eventually put the life of people travelling. So a great work is still to be done to overcome the air drag so as to improve the efficiency and cost efficiency.

Another area that still requires development is the development of the high temperature superconductors. As of now the working of the superconductor needs less temperature which is obtained by liquid nitrogen.

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