An EMS system can provide both levitation and propulsion using an onboard linear motor.The polarity of the stators (Figure 1) at the track will quickly change its polarity continuously to move the Maglev train. Stators at the sides are excited sequentially. The electromagnets onboard 'chase' the current forward along the track. The continuous magnetic field moving forward. Speed controlled by frequency of alternating current.
Electrodynamic Suspension Systems (EDS)
EDS 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.
The process of propulsion for EDS is same as EMS except that the stators will stop for a moment after polarity change. Referred as "pull- then neutral- then push” system. Coils or aluminum sheet at the sides. The direction of current of the particular segment is reversed. Polarity of that segment changes which causes repulsion.
Propulsion coils on the guideway are used to exert a force on the magnets in the train and make the train move forward. The propulsion coils that exert a force on the train are effectively a linear motor. The linear motor in the guideway functions just like a conventional electric motor with its stator cut open and stretched out over the length of the guideway. Instead of a magnetic rotary field, the current in the windings generates a magnetic field of travelling waves, which pulls the vehicle without contact. By changing the intensity and frequency of the driving current, speed and thrust can be continuously adjusted. When the motor is operated as a generator, the direction of the energy flow is reverted and used for contactless braking.
An alternating current flowing through the coils generates a continuously varying magnetic field that moves forward along the track. The frequency of the alternating current is synchronized to match the speed of the train. The offset between the field exerted by magnets on the train and the applied field create a force moving the train forward.
A linear motor or linear induction motor is essentially a multi-phase alternating current (AC) electric motor that has had its stator "unrolled" so that instead of producing a torque (rotation) it produces a linear force along its length. The most common mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (F = qv × B).
Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications
The force is produced by a moving linear magnetic field acting on conductors in the field. Any conductor, be it a loop, a coil or simply a piece of plate metal, that is placed in this field will have eddy currents induced in it thus creating an opposing magnetic field. The two opposing fields will repel each other, thus forcing the conductor away from the stator and carrying it along in the direction of the moving magnetic field.Free body diagram of a U-channel linear motor. The view is perpendicular to the channel axis. The two coils at center are mechanically connected, and are energized in ‘quadrature’ (with a phase difference of 90° (π/2 radians)). If the bottom coil (as shown) leads in phase, then the motor will move downward (in the drawing), and vice versa.
The Maglev Track
The magnetized coil running along the track, called a guideway, repels the large magnets on the train's undercarriage, allowing the train to levitate between 0.39 and 3.93 inches (1 to 10 cm) above the guideway. Once the train is levitated, power is supplied to the coils within the guideway walls to create a unique system of magnetic fields that pull and push the train along the guideway. The electric current supplied to the coils in the guideway walls is constantly alternating to change the polarity of the magnetized coils. This change in polarity causes the magnetic field in front of the train to pull the vehicle forward, while the magnetic field behind the train adds more forward thrust.Maglev trains float on a cushion of air, eliminating friction. This lack of friction and the trains' aerodynamic designs allow these trains to reach unprecedented ground transportation speeds of more than 310 mph (500 kph), or twice as fast as Amtrak's fastest commuter train. In comparison, a Boeing-777 commercial airplane used for long-range flights can reach a top speed of about 562 mph (905 kph). Developers say that maglev trains will eventually link cities that are up to 1,000 miles (1,609 km) apart. At 310 mph, you could travel from Paris to Rome in just over two hours.
Germany and Japan are both developing maglev train technology, and both are currently testing prototypes of their trains. (The German company "Transrapid International" also has a train in commercial use -- more about that in the next section.) Although based on similar concepts, the German and Japanese trains have distinct differences. In Germany, engineers have developed an electromagnetic suspension (EMS) system, called Transrapid. In this system, the bottom of the train wraps around a steel guideway. Electromagnets attached to the train's undercarriage are directed up toward the guideway, which levitates the train about 1/3 of an inch (1 cm) above the guideway and keeps the train levitated even when it's not moving. Other guidance magnets embedded in the train's body keep it stable during travel. Germany has demonstrated that the Transrapid maglev train can reach 300 mph with people onboard.
The propulsion system of the superspeed maglev system is installed in the guideway in the form of a linear motor. This "guideway motor" has two big advantages: firstly, the vehicle is much lighter, and secondly, the driving power is flexible. This means that in up-grade or acceleration sections more power is installed in the guideway than in sections where the route runs at grade.
Additionally, a lot of energy is saved because only that section of the linear motor on which the vehicle moves is powered. Moreover, this prevents two vehicles from being in the same section at the same time and absolutely leaves no chance of trains meeting on the same track.
The double-track guideway consists of beams made of concrete or steel and is installed at grade or elevated, so the guideway can be adapted to any terrain.
