French physicist Léon Foucault discovered eddy currents in September 1855. Eddy current are those which are produced or induced in the masses of metals, whenever these metals are moved in the magnetic field, or the magnetic field moves through the metals. The direction of the eddy currents is always in the direction opposite to the cause (motion) producing them.
An eddy current (also called Foucault currents) is a swirl (like a whirlpool) of current that is induced in a solid conducting mass. The eddy currents are usually of small intensity but may be enormous. They always incur power loss i.e. I^2 R loss, which causes the output of the machine to decrease.
When AC current flows in a conductor, the resistance offered to the conductor is somewhat greater than the resistance that would be offered to dc current by the same conductor. The reasons for the increase in resistance, are due to the fact that when an AC current flows in a conductor, it causes voltages to be set up inside of the conductor. The voltage set up in the conductor cause small independent currents, called eddy currents. The eddy currents flowing through the resistance of the conductor consume power and therefore represent a power loss, or an increase in resistance, in the circuit.
TRANSFORMER
The transformer core is a conductor. The changing magnetic flux in the core of a transformer induces a voltage into any conductors which surround it and also in the core.
The voltage induced in the core causes the current to circulate in the core. This current is called eddy current. The eddy current flowing through the resistance of the core produce heat.
The amount of heat due to eddy current is dependent on
(a) Eddy current and (b) induced voltage.
1. Each lamination of the transformer core is insulated within a layer of oxide.
2. The oxide has much higher resistance than the rest of the silicon steel lamination.
3. The eddy current would have to flow through the oxide layers in order to circulate through the core.
4. The high resistance of the oxide on each lamination effectively reduces the flow of eddy current.
5. Thus laminating the core reduces the eddy current and its associated heat loss.
FORMULA FOR EDDY CURRENT LOSS
It is difficult to determine the magnitude of eddy current and actual resistance values directly.
Eddy current losses are caused by induced electric currents, called eddies since they tend to flow in closed paths within
the magnetic material itself.
The eddy current loss is sinusoidally excited material, neglecting saturation can be expressed by the relationship.
Eddy current loss = Pe = Ke Bm^2 t^2 f^2 V watts
Ke – eddy current coefficient and its
value depends upon the
nature of the material
Bm^2 – maximum flux density in T
t^2 – thickness of lamination in m
f^2 – frequency of flux in Hz
V – volume of material in m^3.
The eddy-current loss per unit volume of
a magnetic core subjected to a time-varying flux is given by
Eddy current loss = Pe = [1.645/ρ] t^2
f^2 Bm^2 watts / m^3
ρ = resistivity of the material.
METHODS OF MINIMIZING THE EDDY CURRENT
Laminated core means the core is made of a number of thin sheets, called laminations. The sheets are painted (varnished) to provide insulation between them.
Eddy currents always tend to flow at right angles to the direction of the flux; if the resistance of the path is increased by laminating the cores etc., the power loss can be reduced because the eddy current loss varies as the square of the thickness of the laminations.
The lamination thickness varies from 0.5 to 5 mm in electrical machines and from 0.01 to 0.5 mm in devices used in electronics circuits operating at higher frequencies.
At radio frequencies, the eddy current loss is very high because the loss is proportional to the square of the frequency. Granulated or powdered-iron cores are used to make radio frequency transformers.
The main drawback of laminated core is that the total cross-sectional area of the magnetic material is reduced by the total thickness of the insulation.
This is generally taken into account by allowing about 10% reduction in the thickness of core when making the magnetic calculations.
STACKING FACTOR
It is defined as the ratio of the effective area to the overall area.
Ks = effective area / overall area
Ks = effective area / overall area
It is also defined as the ratio of the volume occupied by the magnetic material to the total volume of the core.
Ks = volume occupied by magnetic material / total volume of the core.
The stacking factor is important in calculating flux densities in magnetic parts. It is usually less than 1.0. It approaches 1.0 as the lamination thickness increases.
In powdered iron and ferrite magnetic parts, there is an equivalent staking factor that is approximately equal to the ratio of the volume of the magnetic particles to overall volume.
STACKING FACTOR FOR LAMINATED CORES
Laminated Thickness (mm) Staking Factor
0.0127 0.50
0.0254 0.75
0.0508 0.85
0.1 – 0.25 0.90
0.27 – 0.36 0.95
APPLICATIONS OF EDDY CURRENTS
1. Eddy current heating is used for heating metals; for examples melting, hardening and other heat-treatment processes.
2. Eddy current damping is used in permanent magnet moving coil instruments.
3. Eddy current braking is used in induction energy meters.
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