2. Eddy current ----When a metal is placed in a changing magnetic field the induced
current circulate through the metal then induced current is know as eddy current.
This induced current flows in the metal in the form of closed loops resembling ‘eddies’ or whirl
pool. Hence this current is called eddy current.
This effect was discovered by physicist Foucault (also called Foucault's currents).
Eddy currents are loops of electrical current induced within conductors by a
changing magnetic field in the conductor according to Faradays law of induction .
Eddy currents flow in closed loops within conductors, in planes perpendicular to
the magnetic field.
The size of the eddy current is proportional to
the size of the magnetic field,
the area of the loop
the rate of change of magnetic flux,
inversely proportional to the resistivity of the conductor.
and inversely proportional to the resistance of the conductor.
These currents are due to the motion of the free charges that are caused to move by the changing
magnetic field.
3.
4.
5.
6. Direction of the eddy :--
i) by Lenz’s law.
ii) By right hand Fleaming rule.
Eddy currents are currents which circulate in conductors like swirling eddies in a
stream.
They are induced by changing magnetic fields and flow in closed loops,
perpendicular to the plane of the magnetic field.
They can be created when a good electrical conductor (like copper or
aluminum) is moving through a magnetic field, or when the magnetic field
surrounding a stationary conductor is varying.
Motional emf is induced when a conductor moves in a magnetic field
or when a magnetic field moves relative to a conductor.
If motional emf can cause a current loop in the conductor, we refer to
that current as an eddy current.
7. Minimisation of Eddy Currents:
Eddy currents may be minimised by using laminated core of soft iron.
The resistance of the laminated core increases and the eddy currents are reduced
and wastage of energy is also reduced.
By decreasing area, by increasing resistance,by making slots on metal sheet or
holes.
DISADVANTAGES OF EDDY CURRENT:
•There is more leakage of flux occurred in it.
•Unnecessary retardation.
•Unnecessary Heat loss.
•This not useful to inspect all types of metals(Only conductive metals can be
inspected)
•Complex designs are not inspected with accurately.
ADVANTAGESOF EDDY CURRENT:
•Eddy current process is more useful at the inspection process.
•This is a non-contact inspection process which does not affect the work.
•Inspection by this method is a fast and accurate process.
•We can inspect the coating layer which is applied to the different products.
8. Application of eddy currents
Though the production of eddy current is undesirable in some cases, it is useful in some other
cases. A few of them are
i. Induction stove--Induction stove is used to cook the food quickly and safely with less
energy consumption. Below the cooking zone, there is a tightly wound coil of insulated wire.
The cooking pan made of suitable material, is placed over the cooking zone. When the stove
is switched on, an alternating current flowing in the coil produces high frequency alternating
magnetic field which induces very strong eddy currents in the cooking pan. The eddy
currents in the pan produce so much of heat due to Joule heating which is used to cook the
food.
9. ii) Eddy current brake--This eddy current braking system is generally used in high speed
trains and roller coasters. Strong electromagnets are fixed just above the rails. To stop the
train, electromagnets are switched on.
The magnetic field of these magnets induces eddy currents in the rails which oppose or resist
the movement of the train. This is Eddy current linear brake
In some cases, the circular disc, connected to the wheel of the train through a common shaft,
is made to rotate in between the poles of an electromagnet. When there is a relative motion
between the disc and the magnet, eddy currents are induced in the disc which stop the train.
This is Eddy current circular brake
10. iii) Electro magnetic damping---
The armature of the galvanometer coil is wound on a soft
iron cylinder. Once the armature is deflected, the relative
motion between the soft iron cylinder and the radial
magnetic field induces eddy current in the cylinder .The
damping force due to the flow of eddy current brings the
armature to rest immediately and then galvanometer
shows a steady deflection. This is called electromagnetic
damping.
iv) Electric Power Meters-
The shiny metal disc within the meter rotates
due to eddy currents. The magnetic field
induces the electrical currents within the
disc. You can additionally observe the shiny
disc at your house.
11. v) Induction Furnace: 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.
vi) Induction Motor: The eddy currents may be
used to rotate the rotor. Its principle is: 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 tends to
reduce to relative motion between the cylinder and
the field. The cylinder, therefore , begins to rotate
