Eddy currents
or Foucault currents are circular currents induced within the solid block of
conductor due to time-varying magnetic field around it. Eddy currents take time
to build up and can persists for long time (Actually, it is analogous to
turbulence in the water or swirls known as eddies). It is undesirable effect commonly
observed in the electric transformer, causes heating of the core. Overheating of
the transformer may lead to failure of the transformer.
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| Eddy Current in Unlaminated & Laminated Core |
According
to Faraday’s law of induction, the high frequency moving/ varying magnetic
field possess the ability to induce current in the conductor in its field. The coil
wound around the core of the transformer generates the electromagnetic field
which passes through core. The electrons in the core experience the Lorentz force.
Electrons start to move in direction such that the magnetic field produced due
to their motion will oppose the magnetic field which cause them to move. According
to Lenz’s law this induced current moves circular loop (swirls) within the solid
core and keep generating heat in the core. Sometimes transformer fails due to
excessive heat generated. That’s why the core of the transformer is made of
laminated plates stacked together instead of a solid metal core or poorly
conductive material (ferrite core). Core made of laminated plates stacked
together minimises the eddy currents.
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| Eddy Currents Formation |
Eddy
currents are not always undesirable. Eddy current used for electromagnetic braking
of locomotives. Since there is no contact with brake shoe and drums no
mechanical wear. Eddy currents are used for non- destructive testing of
materials. The phenomenon is fundamental of some of sensors (Proximity sensor).The
heating effect is used for electric heating of ores or metals to melt them in
foundry. Induction heating is innovative and clever way to cook the food.
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| Induction Cook-top Working Principle |
The induction
cooktop uses same phenomenon which causes transformer core to heat. Induction cooktop
has electric coil placed below, which produces magnetic field. The cookware to
be placed on top must be made up of conductive or ferromagnetic material and
with flat bottom so as to make more contact with magnetic field. The current in
the coil induces the small eddy currents of about one volt in the bottom of the
cookware causes to heat the bottom of pot. These currents come equipped with
their own magnetic fields result is a lot of swirling, churning, pushing and pulling
of the molecules within the material in cookware.
Other than commercial
and industrial use eddy currents has some attractive experiments to demonstrate
like levitation and repulsion of metallic objects. When magnet is allowed to
fall through copper tube, the speed of a magnet considerably reduces. This is
because eddy currents are produced in the copper tube due to motion of magnetic
field associated with magnet. The electromagnetic force produces by these eddy
currents in the copper tube opposes the magnet field of magnet, hence the
motion of magnet. Experiment explains the electromagnetic braking fundamentals.
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