.INTRODUCTION
Magnetic levitation is the latest in transportation technology and has been the interest of many countries around the world. The idea has been around since 1904 when Robert Goddard, an American Rocket scientist, created a theory that trains could be lifted off the tracks by the use of electromagnetic rails. Many assumptions and ideas were brought about throughout the following years, but it was not until the 1970’s that Japan and Germany
The motion of the Maglev train is based purely on magnetism and magnetic fields. This magnetic field is produced by using high-powered electromagnets. By using magnetic fields, the Maglev train can be levitated above its track, or guideway, and propelled forward. Wheels, contact with the track, and moving parts are eliminated on the Maglev train, allowing the Maglev train to essentially move on air without friction.
BASIC PRINCIPLE OF MAGLEV TRAIN
Maglev can be used for both low and high speed transportation. The low speed Maglev is used for short distance travel. Birmingham , England
PROPULSION SYSTEM
Electrodynamic Propulsion is the basis of the movement in a Maglev system. The basic principle that electromagnetic propulsion follows is that “opposite poles attract each other and like poles repel each other”. This meaning that the north pole of a magnet will repel the north pole of a magnet while it attracts the south pole of a magnet. Likewise, the south pole of a magnet will attract the north pole and repel the south pole of a magnet. It is important to realize these three major components of this propulsion system. They are:
· A large electrical power source Metal coils that line the entire guideway Guidance magnets used for alignment.
The Maglev system does not run by using a conventional engine or fossil fuels. The interaction between the electromagnets and guideway is the actual motor of the Maglev system. To understand how Maglev works without a motor, we will first introduce the basics of a traditional motor. A motor normally has two main parts, a stator and a rotor. The outer part of the motor is stationary and is called the stator. The stator contains the primary windings of the motor. The polarity in the stator is able to rapidly change from north and south. The inner part of the motor is known as the rotor, which rotates because of the outer stator. The secondary windings are located within the rotor. A current is applied to the secondary wingings of the rotor from a voltage in the stator that is caused by a magnetic force in the primary windings. As a result, the rotor is able to rotate.
Now that we have an understanding of how motors work, we can describe how Maglev uses a variation on the basic ideas of a motor. Although not an actual motor, the Maglev’s propulsion system uses an electric synchronous motor or a linear synchronous motor. The Maglev system works in the same general way the compact motor does, except it is linear, “meaning it is stretched as far as the track goes”. The stators of the Maglev system are usually in the guiderails, whereas the rotors are located within the electromagnetic system on the train. The sections of track that contain the stators are known as stator packs. This linear motor is essential to any Maglev system. The picture below gives an idea of where the stator pack and motor windings are located.
· Parts of the Electromagnetic System
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The guideway for Maglev systems is made up of magnetized coils, for both levitation and propulsion, and the stator packs. “An alternating current is then produced, from the large power source, and passes through the guideway, creating an electromagnetic field which travels down the rails”. As defined by the Encarta Online dictionary, an alternating current is “a current that reverses direction.” The strength of this current can be made much greater than the normal strength of a magnet by increasing the number of winds in the coils. The current in the guideway must be alternating so the polarity in the magnetized coils can change. The alternating current allows a pull from the magnetic field in front of the train, and a push from the magnetic field behind the train. This push and pull motion work together allowing the train to reach maximum velocities well over 300 miles per hour.
PROPULSION SYSTEM IN EDS
This propulsion is unique in that the current is able to be turned on and off quickly. Therefore, at one instance there can be a positive charge running through a section of the track, and within a second it could have a neutral charge. This is the basic principle behind slowing the vehicle down and breaking it. The current through the guiderails is reversed causing the train to slow, and eventually to competely stop. Additionally, by reversing the current, the train would go in the reverse direction. This propulsion system gives the train enough power to accelerate and decelerate fairly quickly, allowing the train to easily climb steep hills.
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fig PROPULSION SYSTEM IN EDS |
The levitation, guidance, and propulsion of the electromagnetic suspension system must work together in order for the Maglev train to move. All of the magnetic forces are computer controlled to provide a safe and hazard free ride. The propulsion system works hand in hand with the suspension system on the Maglev system.
MAGNETIC LEVITATION SYSTEM
Magnetic levitation means “to rise and float in air”. The Maglev system is made possible by the use of electromagnets and magnetic fields. The basic principle behind Maglev is that if you put two magnets together in a certain way there will be a strong magnetic attraction and the two magnets will clamp together. This is called "attraction". If one of those magnets is flipped over then there will be a strong magnetic repulsion and the magnets will push each other apart. This is called "repulsion". Now imagine a long line of magnets alternatively placed along a track. And a line of alternatively placed magnets on the bottom of the train. If these magnets are properly controlled the trains will lift of the ground by the magnetic repulsion or magnetic attraction. On the basis of this principle, Magnetic Levitation is broken into two main types of suspension or levitation,
1. Electromagnetic Suspension.
2. Electrodynamic Suspension.
A third type of levitation, known an Inductrack, is also being developed in the United States
.ELECTROMAGNETIC SUSPENSION SYSTEM(EMS )
Electromagnetic Suspension or EMS is the first of the two main types of suspension used with Maglev. This suspension uses conventional electromagnets located on structures attached to the underside of the train; these structures then wrap around a T-shaped guiderail. This guiderail is ferromagnetic, meaning it is made up of such metals as iron, nickel, and cobalt, and has very high magnetic permeability. The magnets on the train are then attracted towards this ferromagnetic guiderail when a “current runs through the guiderail and the electromagnets of the train are turned on”. This attraction lifts the car allowing it to levitate and move with a frictionless ride. “Vehicle levitation is analyzed via on board computer control units that sample and adjust the magnetic force of a series of onboard electromagnets as they are attracted to the guideway”.
The small distance of about 10mm needs to be constantly monitored in order to avoid contact between the train’s rails and the guiderail. This distance is also monitored by computers, which will automatically adjust the strength of the magnetic force to bring this distance back to around 10mm, if needed. This small elevation distance and the constant need for monitoring the Electromagnetic Suspension System is one of its major downfalls.
CR0SS SECTION OF ELECTROMAGNETIC SUSPENSION SYSTEM
The train also needs a way to stay centered above the guideway. To do this, guidance coils and sensors are placed on each side of the train’s structures to keep it centered at all points during its ride, including turns. Again, the gap should be around 10mm, so computers are used to control the current running through the guidance magnets and keep the gap steady. In addition to guidance, these magnets also allow the train to tilt, pitch, and roll during turns. To keep all distances regulated during the ride, the magnets work together with sensors to keep the train centered. However, the guidance magnets and levitation magnets work independently.
There are several advantages to this system. First, the train interlocks with the guiderail making it impossible to derail. Noise is extremely limited with this system because there is no contact between the train and its track. In addition, there aren’t many moving parts, which reduces the noise and maintenance of the system. With fewer parts, there is less wear and tear on the system. The Maglev train is also able to travel on “steep gradients and tight curves”. Figure [4] shows the metal beams which attach to the underside of the train. An example of Electromagnetic Suspension is shown in Figure [5] below. Before a Maglev system can be made, a choice must be made between using this type of suspension or Electrodynamic Suspension.
.ELECTRODYNAMIC SUSPENSION SYSTEM
The second of the two main types of suspension systems in use is the Electrodynamic Suspension (EDS). EDS uses superconducting magnets (SCM) located on the bottom of the train to levitate it off of the track. By using super cooled superconducting magnets, the electrical resistance in superconductors allows current to flow better and creates a greater magnetic field. The downside to using an EDS system is that it requires the SCMs to be at very cold temperatures, usually around 5 K (-268ºC) to get the best results and the least resistance in the coils. The Japanese Maglev, which is based on an EDS system, uses a cooling system of liquid nitrogen and helium.
To understand what’s really going on here, let’s start from the inside out. The first major difference between EDS and EMS is the type of track. Whereas with EMS the bottom of the train hooks around the edges of the track, an EDS train literally floats on air, as shown in the figure.
THE ELECTRODYNAMIC SUSPENSION SYSTEM
The outside guides act like the cushions used to prevent gutter balls in bowling only an EDS train has a magnetic safety net to keep the train centered, unlike your traditional bowling ally. If the train is knocked in the horizontal direction, the field on the side it shifts to becomes greater and the field on the opposite side weakens due to this increase in distance. Therefore, in order to restore equal magnetic forces from each side, the train is pushed back into the center of the guideway and the strength of the magnetic fields reduces to their normal strength. This is one reason why EDS is a much more stable suspension system. A second reason why the Electrodynamic Suspension system is more stable is that it is able to carry a much heavier weight load without having its levitation greatly affected. As the gap between the train and vehicle decreases, forces between the SCMs located on the train and the magnets on the track repel each other and increase as the train gets heavier. For example, if weight is added to the train, it is going to want to get closer to the track; however it cannot do so because repulsion forces grow stronger as the poles on the train sink closer to the similar poles on the guideway. The repulsive forces between the magnets and coils lift the train, on average, about 4 to 6 inches above the track, which virtually eliminates any safety issues regarding the train losing levitation and hitting its guideway. This brings us to the next thing we encounter as we move out from the center of the guideway. Levitation coils repel the SCMs underneath the train, providing the restoring forces to keep the train aligned.
Propulsion coils are located next. The propulsion system of the Electrodynamic Suspension system is quite similar to Electromagnetic propulsion, but does vary slightly. To propel the train, the guideway has coils running along the top and bottom of the SCMs. Induced current within these coils creates alternating magnetic fields that attract or repel the SCMs, sending the train in the forward or reverse direction. Because the trains are moving by magnetic waves that push and pull it forward, it’s virtually impossible for trains to collide since they are in essence “riding the same magnetic waves”.
No engine or other power source is required to keep the train moving except the initial speed that is required to begin levitation. Therefore wheels are required to keep the train moving until about 100 km/hr (65 mph) where it can then begin to levitate.
Finally, the guideway has rails that encompass the outside of the train. Within these rails are the propulsion coils and levitation coils needed to keep the train moving and levitating above the bottom of the track. Because the train has its own safety net of magnetic force to keep it centered, the rails simply provide a place for other coils to be located and used. This railway provides no other means of support for the train since the bulk of the train is floating above the entire track.
EDS suspension has several positive and negative aspects to it. To begin, initial costs are high and most countries do not have the money or feel the need to spend it on this kind of transportation. Once up and running however, an EDS Maglev runs only on electricity so there is no need for other fuels. This reduction in fuel will prove to be very important to the sustainability of Maglev. One huge disadvantage of the EDS system is the great cost and inconvenience of having to keep the super cooled superconductive magnets at 5K. Another drawback is that in the event of a power failure, a Maglev train using EDS would slam onto the track at great speeds. This is a second reason for the wheels that are primarily used to get the train moving quickly enough for levitation. The wheels would need to have a shock system designed to compensate for the weight of the car and its passengers as the train falls to the track. In Japan United States EMS and EDS suspension systems are the two main systems in use, but there is a possibility for a third to soon join the pack.
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