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24 x 7 Basics The Professional Engineers

ELECTROMAGNETIC INDUCTION



EDDY CURRENTS


Assume you are going in a train and the motorman applies the brakes all of unexpected. Do you know how the brakes are been applied so as to stop the train? The response to this is eddy currents. Give us a chance to contemplate what eddy currents are and its different uses.

WHAT ARE EDDY CURRENTS?


Eddy currents are the currents induced in the body of a conductor when the amount of magnetic flux linked with it changes.

Eddy currents are the currents induced in a metallic plate when it is kept in a time varying magnetic field. Magnetic flux linked with the plate changes and so the induced current is set up. Eddy currents are sometimes so strong, that metallic plate becomes red hot.

An eddy current is a present set up in a conductor in light of a changing magnetic field. They flow in closed loops in a plane opposite to the magnetic field. By Lenz law, the present swirls so as to make a magnetic field contradicting the change; for this to happen in a conductor, electrons swirl in a plane opposite to the magnetic field.

As a result of the inclination of eddy currents to restrict, eddy currents cause a loss of energy. Eddy currents transform more useful forms of energy, for example, kinetic energy, into heat, which isn't commonly helpful.

Applications: Induction furnace (heating effect of eddy current)


In induction furnace, the metal to be heated is placed in a rapidly varying magnetic field produced by high frequency alternating current. Strong eddy currents are set up in the metal produce so much heat that the metal melts. This process is used in extracting a metal from its ore. The arrangement of heating the metal by means of strong induced currents is called the induction furnace.

Induction Motor:


The eddy currents may be used to rotate the rotor. When a metallic cylinder (or rotor) is placed in a rotating magnetic field, eddy currents are produced in it. According to Lenz’s law, these currents tend to reduce to relative motion between the cylinder and the field. The cylinder, therefore, begins to rotate in the direction of the field. This is the principle of induction motion.

APPLICATIONS OF EDDY CURRENTS


* Induction furnace (heating effect of eddy current)
* Dead-beat galvanometer (electromagnetic damping)
* Electric brakes
* Diathermy (Strong 50 MHz A.C. passed through human body, also called deep heat treatment)
* Induction motor
* Car-speedometer
* Energy meter

SOME PRACTICAL APPLICATIONS


During braking, the brakes uncover the metal wheels to a magnetic field which creates eddy currents in the wheels. The magnetic cooperation between the applied field and the eddy currents acts to back the wheels off. The quicker the wheels turn, the more grounded is the impact, implying that as the train eases back the braking power is decreases, delivering a smooth stopping movement.

Used to plan deadbeat galvanometers. Normally, the needle oscillates a little about its equilibrium position before it stops. This causes a delay in taking the perusing so to maintain a strategic distance from this postponement, the coil is twisted over a non-magnetic metallic edge. As the coil is redirected, eddy currents set up in the metallic edge and along these lines, the needle stops in a flash. In this manner, the movement of the "coil is damped". Certain galvanometers have a fixed core made of nonmagnetic metallic material. At the point when the coil oscillates, the eddy currents that create in the center restrict the motion and carry the coil to rest.

The sparkling metal plate in the electric power meter turns because of eddy currents. The magnetic field prompts the electric currents in the disc. You can likewise watch the sparkling disc at your home.

In a quickly changing magnetic field, because of a huge EMF created, enormous eddy currents are set up. Eddy currents produce temperature. Consequently an enormous temperature is made. So a curl is wound over a constituent metal which is set in a field of the highly oscillating magnetic field created by high frequency. The temperature created is sufficient to dissolve the metal. This is utilized to concentrate metals from minerals. Induction heater can be utilized to get ready combinations, by dissolving the metals at a high temperature.

To know the speed of any vehicle, these currents are utilized. A speedometer consists of a magnet which continues pivoting as per the speed of our vehicle. Eddy currents are been created in the drum. As the drum turns toward the rotating magnet, the pointer connected to the drum shows the speed of the vehicle.

SPECIAL POINTS


1. These currents are produced in metal bodies
(a)They are placed in a time varying magnetic field.
(b)They move in a magnetic field such that their flux through them changes or they cut away magnetic field lines.

2. These currents are produced only in closed path within the entire volume of metal body or surface area of metal body. Therefore their measurement is impossible.

3. These “circulatory” currents are always produced in planes perpendicular to magnetic lines of force.

4. The resistance of bulk conductor is usually low, eddy currents often have large magnitudes and heat up the conductor. That’s why these are sometimes undesirable.

5. By ‘Laminations’, slotting process, the resistance path for circulation of eddies increases, resulting in to weakening them and also reducing losses caused by them. Slots and laminations intercept the conducting paths and decrease the magnitude of eddy currents. That’s why a laminated metal core is always preferred to be used in an appliance like dynamo, transformer, choke will etc.