Eddy Currents:Detailed Overview, 5 Important Applications

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Here we will be studying about eddy currents and what is meant by electromagnetic damping. But changing magnetic flux also induces currents in bulk pieces of conductors, and their flow pattern resembles that of swirling eddies in the water.

François Arago, a mathematician and even the 25th prime minister of France, first observed the eddy currents in 1824. Later a physicist named Foucault discovered these currents, which are termed explicitly as eddy currents.

A Simple Demonstration of Eddy Current

The cause and effect of eddy currents can be understood by a simple experiment, as mentioned. A copper’s plate is swinging like pendulums.

Eddy Current
Eddy Current, Image By – Chetvorno, Eddy currents due to magnet, CC0 1.0

These produce hindrances in the swinging motion of the plate, and hence, the swinging motion is damped. In some time, the plate comes to rest in the magnetic field. 

This electromagnetic damping effect can be reduced by cutting down the available area for the flow of eddy currents. Hence, if we can introduce rectangular slots and holes in the plate, and because of the fact that the magnetic moments of the induced currents depend on the area enclosed by it, we can reduce the electromagnetic damping and the plate swings more freely.

POWER OF EDDY CURRENTS

The dissipation power of eddy currents can be expressed as:

image004 1

Where,

P refers to the power lost per unit mass.

Bp refers to the maximum magnetic fields.

d refers to the thickness.

f refers to the frequency.

k refers to a constant .

ρ refers to the resistivity.

D refers to the density.

Eddy current is decreased by using laminations in the metal core. Because of this, the magnitude is substantially reduced.

As the dissipation of energy in the form of heat is depended on squares of the magnitude of the eddy currents, the heat loss and subsequently, the energy loss is decreased. Energy losses can further be reduced by using thinner lamination with very low-carbon content iron or soft iron and wires with larger cross-sections.

eddy currents in a slab with and with-out laminations
eddy currents in a slab with and with-out lamination, Image Credit – Chetvorno, Laminated core eddy currents 2, CC0 1.0

Here’s a straightforward experiment where we can notice electromagnetic damping.

Two hollow thin cylindrical pipes of same geometrical orientations but one made up of aluminum and the other a PVC pipe is being clamped vertically. A cylindrical magnet having a diameter a little less than that of the cylinder’s diameters is dropped through both the pipes in such a way that they don’t get to touch the inner walls of the cylindrical pipes. The magnet dropped through the PVC pipe takes the same time to come out of the pipe as it would take if dropped from the same height without any pipe. The magnet in the aluminum pipe comparatively takes a longer time to come out of the pipe.

This is due to the eddy currents which are produced in the aluminum pipe that opposes the changing magnetic flux when the magnet moves through the aluminum pipe. As PVC is an insulator, no eddy currents are formed in it. This phenomenon where a retarding force due to the eddy currents restricts the motion of an object is known as electromagnetic damping.

APPLICATIONS OF EDDY CURRENTS

Although eddy currents are undesirable in some applications, there are many applications in which eddy currents are a necessity for their working. Some of them are magnetic braking in trains, electromagnetic damping, induction furnace, electrical power meters, levitation, identification of metals, Vibration and position sensing, structural testing, etc. Some of them have been explained in details as follows:

  • Magnetic braking in trains: As we know that the trains are quite heavy and can move at great speeds, hence, the braking system of trains should be very powerful and smooth. Eddy currents make this possible. Strong electromagnets can induces eddy currents in the rails. As there is no friction involved as there are no mechanical linkages; hence, the braking system becomes very smooth. But this application is used in some electrically powered trains only.
  • Induction furnace: They are used to melt iron, steel, copper, aluminum, and other precious metals for welding purposes, reshaping purposes, or for making alloys. In an induction furnace, the eddy current produces very high temperatures suitable enough to melt the metals.
  • Electromagnetic damping: Few measuring instruments like galvanometers make use of the effect of eddy currents in opposing the motion. They have a fixed core made up of a non-magnetic but metallic material in which the eddy currents are generated when the coil oscillates, which in turn opposes the motion of the coil and brings it to rest position quickly.
  • Repulsive effects and levitation: when a changing magnetic field is applied, it induces eddy currents that exhibit the behaviour of diamagnetic-like repulsion due to which a metal or any conductive material will experience a repulsion force.

For more about eddy current application, may read article on eddy current testing, eddy current sensor and eddy current brake.

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