Introduction to Magnetic Levitation

Introduction to Magnetic Levitation

The magnetic levitation is based on the creation of magnetic forces which exist because of various things. It can either be caused by a permanent magnet made up of solid material with induced north and south pole or the other way through which a magnetic field established is through an electric field changing linearly with time. Yet another way to create a magnetic field is through the use of direct current.

Two fundamental principles are involved for studying the concept of magnetic levitation. The first principle is in the form of a law which is suggested by Michael Faraday commonly known as Faraday's laws of electromagnetic induction.

Faraday's law states that if there is a change in the magnetic field on a coil of wire, a change in voltage is observed. It can also be said that with change in voltage, a change in magnetic field occurs. This is due to induced current in the coil as a result of that change in voltage. The strength of the magnetic field is proportional to current flowing through coil. It current is higher, stronger magnetic field is produced with greater magnetic forces.

The direction of magnetic forces is given using Lenz's law which states that the emf induced in an electric circuit always acts in such a way that the current driven by it in the circuit will oppose the change in the magnetic flux producing the emf. This means that if a current is induced in the coil then the        magnetic field produced will be prependicular to the direction of current. Due to prediction of direction of magnetic field, it can be maximized by setting up a suitable set up and magnetic levitation finds number of applications such as in transportation and other industrial applications.

Maglev is a combination of superconducting magnets and linear motor technology which in turn realizes super high speed transport system. It is safe, reliable, with less impact on environment and requires minimum maintenance. In Maglev system, a vehicle rims levitated from the guide way with use of electromagnetic forces between superconducting magnets and coils on the ground. Principle of Maglev can be understood from the Fig. 9.12.1.

The coils are installed on the side walls of the guideway. The superconducting magnets are attached to the vehicle itself. Due to high speed of vehicle, there is induced current in the coils during the instant when it passes the coils. The coils act as electromangets for small duration. Due to the interaction between the coils on the guide way and the magnets on the vehicle allows the vehicle to stay levitated above a track for a few centimeters. One side of vehicle experiences a magnetic force tending to push it from the bottom while at the same time, there is pulling force from the top part of coil which pulls the other side of the vechicle away from the coil. This forms basis for levitation principle for the track. It is also required to take proper care so that the vehicle does not slide from side to side. Let us try to understand this levitation principle in detail with various forces involved and their interactions.

For simplicity in understanding, let us consider a permanent magnet which is moving at a speed of v m/s across a conducting ladder as shown in the Fig. 9.12.2.

This moving magnet tends to drag the conducting ladder along with it because of application of horizontal tractive force, F = BIl where B is flux density in Wb/m²,

I is current flowing through the conductor and l is the active length of the conductor under the influence of magnetic field. The plane of motion of magnet and the plane of conducting ladder are perpendicular to each other due to which maximum force is exerted and its direction can be found using Fleming's left hand rule. 

In addition to this horizontal tractive force, a vertical force also exists between the moving magnet and conducting ladder which pushes the magnet away from the ladder in the upward direction. Let I be the current flowing through conductor Q. The front view of above figure is shown in the Fig. 9.12.3. Let us initially assume that the magnet is moving at a low speed.

The magnetic flux ϕ at the centre of the magnet is maximum so the emf induced in conductor Q is maximum. If the conductor is assumed to have low inductance then the induced current will also reach its maximum value approximately at the same instant when voltage reaches its maximum value. This current flows through conductors P and R with magnitude I/2 . The currents in conductors P, Q and R will produce their own magnetic fields, which can be found by using Right hand rule. These magnetic fields produced by the conductors will interact with the magnetic field of the magnet so that vertical force is exerted on the magnet. The front half of magnet is pushed upwards while the rear half of magnet is pulled downwards. With respect to centre of magnet, this vertical forces of attraction and repulsion, being equal and opposite, cancels each other due to symmetry and only horizontal tractive force is present. 

Now let us consider that the magnet is moving at a very high speed. This condition is represented in the Fig. 9.12.4 with the front view as shown earlier.

Due to inductance associated with the conductor, current in conductor Q reaches its maximum value a fraction of time, At, after voltage reaches its maximum value. This time interval At depends on L/R time constant of conductor circuit. This delay is very small at low speed such that voltage and current reach their maximum value virtually at the same time and place. ∆t large speeds, this time dealy At is sufficient enough to produce a large shift in space between the points where the voltage and current achieve their maximum values.

By the time current in Q reaches its maximum value, the centre of magnet is already ahead of conductor Q by a distance given as vAt. The currents in conductors P, Q and R are established as explained earlier and their interaction with the field of magnet exerts a vertical force in such a way that the front end of magnet is pushed downwards while the rear end is pulled upwards. This is basic principle of magnetic levitation which means floating in air.

This principle is used in ultra high speed trains running at speeds in the range of 300 km/hr and which float in the air about 100 mm to 300 mm above the track. These trains do not need traditional steel rail and will not require any wheels.

A powerful superconducting magnet is mounted at the bottom of the train which induces current in the rail. This produces a vertical force called levitation force which keeps the train pushed up in the air above the track.

Linear induction motor is employed to propel the train. Linear induction motor consists of a flat stator which produces a flux that moves in a straight line from its one end to the other at a linear synchronous speed. This speed is given by,

v _s = 2 w f

where v _s = Linear synchronous speed

W = Width of one pole pitch in m

f = Supply frequency in Hz.

The above speed is independent of number of poles. The rotor consists of a plate made up of aluminium, or copper or iron. The flux moves linearly and drags the rotor plate along with it in the same direction. Practically the stator moves while the rotor plate is kept stationary in the applications.

It is employed in high speed trains which uses principle of magnetic levitation as explained earlier. The rotor consists of a thick aluminium plate which is fixed to the ground and extends over the entire length of the track. The stator which is linear is bolted in the Fig. 9.12. 5.

The slip in case of linear induction motor is given as,

Slip, s = v _s – v / v _s where v _s = 2 wf

And v = Actual speed in m/s

The force or thrust exerted is given by,

F = p_m / v _s

where P_in is active power supplied to the rotor. The flow of active power remains same as that in case of normal induction motor.

Review Question

1. Write a short note on magnetic levitation.