Applications of eddy current

Application Of Eddy Current

1. Eddy currents

An eddy current is a  current set up in a conductor in response to a changing magnetic field. They flow in closed loops in a plane perpendicular to the magnetic field.  By Lenz law, the current swirls in such a way as to create a magnetic field opposing the change; for this to occur in a conductor, electrons swirl in a plane perpendicular to the magnetic field.

Because of the tendency of eddy currents to oppose, eddy currents cause a loss of energy. Eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which isn’t generally useful.

In The Brakes of Trains

During braking, the brakes expose the metal wheels to a magnetic field which generates eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels spin, the stronger is the effect, meaning that as the train slows the braking force is reduces, producing a smooth stopping motion.
In an eddy current brake the magnetic field may be created by a permanent magnet or an electromagnet, so the braking force can be turned on and off or varied by varying the electric current in the electromagnet windings. Another advantage is that since the brake does not work by friction, there are no brake shoe surfaces to wear out, necessitating replacement, as with friction brakes. A disadvantage is that since the braking force is proportional to the relative velocity of the brake, the brake has no holding force when the moving object is stationary, as is provided by static friction in a friction brake, so in vehicles it must be supplemented by a friction brake.
Eddy current brakes are used to slow high-speed trains and roller coasters, as a complement for friction brakes in semi-trailer trucks to help prevent brake wear and overheating, to stop powered tools quickly when power is turned off, and in electric meters used by electric utilities.

Electromagnetic damping

Used to design deadbeat galvanometers. Usually, the needle oscillates a little about its equilibrium position before it comes to rest. This causes a delay in taking the reading so to avoid this delay, the coil is wound over a non-magnetic metallic frame. As the coil is deflected, eddy currents set up in the metallic frame and thus, the needle comes to rest almost instantly.
Thus, the motion of the “coil is damped”. Certain galvanometers have a fixed core made up of nonmagnetic metallic material. When the coil oscillates, the eddy currents that generate in the core oppose the motion and bring the coil to rest.

The damping effect of Eddy Currents is used in some moving coil galvanometers to make them dead-beat. The coil of such galvanometres is wound on a light metal frame. When the coil and the frame rotate in the field of the permanent magnet, the eddy currents set up in the frame oppose the motion so that the coil returns to zero quickly.

The oscillations of moving coil ballistic Galvanometer are stopped by short circuiting the coil, this being due to the induced current in the coil itself.

A similar damping device is used in some instruments such as the balances, ammeters and voltmeters. A copper plate is attached to the moving system of the instruments such that it moves between the poles of a permanent magnet when the system is oscillating.

Electric Power Meters

The shiny metal disc in the electric power meter rotates due to eddy currents. The magnetic field induces the electric currents in the disc. You can also observe the shiny disc at your house.
On a single-phase AC supply, the electromechanical induction meter operates through electromagnetic induction by counting the revolutions of a non-magnetic, but electrically conductive, metal disc which is made to rotate at a speed proportional to the power passing through the meter. The number of revolutions is thus proportional to the energy usage. The voltage coil consumes a small and relatively constant amount of power, typically around 2 watts which is not registered on the meter. The current coil similarly consumes a small amount of power in proportion to the square of the current flowing through it, typically up to a couple of watts at full load, which is registered on the meter.
The disc is acted upon by two sets of induction coils, which form, in effect, a two phase linear induction motor. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees, due to the coil's inductive nature, and calibrated using a lag coil.^[16] This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current, voltage and phase angle (power factor) between them. A permanent magnet acts as an eddy current brake, exerting an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power or rate of energy usage. The disc drives a register mechanism which counts revolutions, much like the odometer in a car, in order to render a measurement of the total energy used.

Induction Furnace

In a rapidly changing magnetic fields, due to a large emf produced, large eddy currents are set up. Eddy currents produce temperature. Thus a large temperature is created. So a coil is wound over a constituent metal which is placed in a field of the highly oscillating magnetic field produced by high frequency. The temperature produced is enough to melt the metal. This is used to extract metals from ores. Induction furnace can be used to prepare alloys, by melting the metals at a very high temperature.

Speedometers

To know the speed of any vehicle, these currents are used. A speedometer consists of a magnet which keeps rotating according to the speed of our vehicle. Eddy currents are been produced in the drum. As the drum turns in the direction of the rotating magnet, the pointer attached to the drum  indicates the speed of the vehicle.
 the speedometer cable rotates, it turns the magnet at the same speed. The spinning magnet creates a fluctuating magnetic field inside the speed cup and, by the laws of electromagnetism, that means electric currents flow inside the cup as well. In effect, the speed cup turns into a kind of electricity generator. But, unlike in a proper generator (the kind that makes electricity for your home in a power plant), the currents in the speed cup have nowhere to go: there's nothing to carry their power away. So the currents just swim about uselessly in swirling eddies—we call them eddy currents for that very reason. Since they're electric currents, and they're moving in an electrical conductor inside a magnetic field, another law of electromagnetism says they will create motion.

Induction cooktops

An induction cooktop has a coil wound on ferromagnetic material under it. A high frequency alternating current in this coil produces a rapidly changing magnetic field. Placing a magnetic-based leads to a change in magnetic flux, because the magnetic field B varies. From Faraday's Law, E = Δ ϕ Δ t , {\displaystyle {\mathcal {E}}={\frac {\Delta \phi }{\Delta t}},} an emf (electromagnetic field) is induced. From Lenz's Law, eddy currents flow in the pan. The resistance in the pan to the oscillating AC current results in heat being produced directly in the base of the pan. Unlike gas cooking, the ceramic cooktop is not heated directly. This makes induction cookers much more energy efficient than traditional cookers.
The direction of the eddy current is determined by Lenz's Law. In addition to efficiency, other benefits include safety (because the cooktop is not heated directly) and speed of cooking.