Maglev Trains And The Technology Behind Them (magnetism)

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Maglev Trains And The Technology Behind Them

Introduction

Magnets

Magnetism is a phenomenon that occurs when a moving charge exerts a force on other moving charges. The magnetic force caused by these moving charges sets up a field which in turn exerts a force on other moving charges. This magnetic field is found to be perpendicular to the velocity of the current. The force of the field decays with distance from the charge. Most magnets we come across are weak permanent magnets, such as fridge magnets and door catches. A permanent magnet is a material that is naturally magnetic, they set up magnetic fields by electrons circling an atom setting up magnetic fields.  They are based on oxides of barium and iron. They have low field strength and would not be strong enough for use in Maglev trains. Lately, developments in magnet research have found rare earth-permanent magnets that have a much stronger magnetic field. These new magnets have become an important part of our everyday life being used in many everyday applications such as computers, CD players and mobile phones. It is these high performance magnets that are used in Maglev trains. The rare earth elements are scandium 21, yttrium 39 and lanthanide’s 57-71.

The principle of a Maglev (Magnetic Levitation) train is that it floats on a magnetic field and is propelled by a linear induction motor. They follow guidance tracks with magnets and have been seen to have great potential in the transport world.

An example of a maglev train from

Simple Theory

A maglev train will float about 10mm above the guideway on a magnetic field. It is propelled not by an onboard engine, but by the guideway itself by changing the magnetic field of the electromagnets along the length of the guideway. Once the train is pulled into the next section the magnetism switches so that the train is pulled on again.

The repulsive force between magnets with like poles facing explains how permanent magnets can be used to provide a levitating and thrust force to the train.

Magnet Strength

We know that magnets have a north and south pole and if we bring them together they will repel each other. We can see this effect in the picture below with the area between the two magnets having the iron filings being repelled away from the area in between the two magnets.

We can use this theory to determine the strength of repulsion. We use the following formulas.

The magnetic flux density is another word for the strength of the magnetic field. Lines of magnetic force of flux are used to describe the shape of a magnetic field near a magnet or solenoid (a long coil through which current is flowing, establishing a magnetic field). The density of the lines of force increase in proportion to the field strength. It can be calculated by using the equation

B=O/A

where B is the flux density O is the flux and A is the cross sectional area of the material.

Magnet flux in o is produced by current-turns NI. The formula for this is given to be

O =   NI

where     = the permeance of the magnetic circuit. Permeance is a quantity which says how much flux there is in the material for a given number of current turns round the material. The permeance is bigger for larger cross sections and smaller for long sections of material.  Also to find the strength of the Flux density in a straight conductor when can use the Equation

F=BIl


In this equation F = the force in Newtons  on a straight conductor of length l which carries current I. By using these equations we can come up with an equation that tells us the attractive force between two magnets from which we would be able to calculate the strengths of the distance apart they are from each other.

In this formula F is the force, M1 and M2 are the flux strengths of the two magnets, d is the distance between the magnets and u is the relative permeability of the medium between the magnets. By using some figures from a Physics World magazine is possible to see why magnetic levitation is possible. By using them we can find the size of the magnet that is needed to levitate two people at 20mm.  First to find the force we multiply the weight of two people at 70N each by the gravitational constant 9.8 and then multiply this figure by a safety factor of two we get 2744N. And is using the figures for a possible levitation magnet Nd Fe B we can find that the magnet flux strength M=1.2 x 10 Wb. Given this we find that the circular pole face to give this figure is 94mm. This is not very big and proves that levitation is possible with the right conditions.  

But according to a theorem by Earnshaw  it is not possible to achieve static levitation using any combination of fixed magnets and electric charges. Static levitation means stable suspension of an object against gravity. It was found that there are several exceptions to this theory. They are Quantum effects, as Earnshaw’s theorem only takes classic physical laws into account, this is not used in Maglev trains. Feedback is another exception to the theory and one which is used in Maglev trains, if you can detect the position of an object then use it in a control system in which you can vary the strength of electromagnets which are acting on the object it is possible to keep it levitated. The system must be programmed to weaken the strength of the magnet whenever the object approaches it and strengthen as it moves away. This is an exception as Earnshaw’s theorem only takes into account that the magnets are fixed. This is the type of Maglev that was used in Birmingham airport. The other exception is diamagnetism. A diamagnetic material is one that expels a magnetic field. It is possible to levitate superconductors and other diamagnetic materials which magnetise in the opposite sense to a magnetic field in which they are placed. This system can only be used with the strongest magnetic fields that technology has produced. This system has been used to levitate water droplets and even frogs. A superconductor is a material that below a certain temperature has zero resistance. Resistance is undesirable because it cause the loss of energy. In theory once set in motion the electricity will flow forever in a closed loop. It is referred to as a “Macroscopic Quantum Phenomenon”.

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Different Types of Maglev Technology

There are essentially two types of Maglev trains depending on the different ways they levitate. One is the electromagnetic suspension or EMS uses electromagnets on the train which are attracted to an iron rail. The vehicle magnets wrap around the iron guideway, and the attractive force in the +z direction lifts the train. The electrodynamic suspension, or EDS is levitated by forces caused by induced circulating currents in a passive conductive guideway. In either case, the levitating magnets are mounted to a number of “bogies” which in most Maglev systems are connected to ...

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