# Current flowing within a coil = eddy currents?

Discussion in 'General Electronics Chat' started by Silhorn, Apr 11, 2013.

1. ### Silhorn Thread Starter New Member

Apr 9, 2013
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0
Hello,

I am trying to understand what eddy currents are.

Let's write up a scenario:
I have a coil of wire not connected to a load (open circuit). It is being exposed to a changing magnetic field. This changing magnetic field induces a voltage into the coil but current does not flow because their is no load.

I now connect the coil to a load. Because of the impedance of the load a current will begin to flow.

From the googling I have done so far, it seems that the current flowing in the coil is actually an eddy current?

If this is the case then if I were to add a core into this coil, an eddy current will flow in this core but how could it since this core is not part of a closed circuit so the current has not where to flow?

2. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Eddy current is current that flows in the core, and not in the coil itself. The core is a closed circuit. Current can flow around the surface in circles. Magnetic fields cause current to flow in a circular pattern, inducing circular flow within the core.

screen1988 and #12 like this.
3. ### MrChips Moderator

Oct 2, 2009
12,449
3,363
Google eddy current.

In a coil of wire the flow of the induced current is restricted by the dimension and shape of the wire. This is not called eddy current.

In the core, the induced current becomes eddy current because the direction of the current flow deviates from a straight line resulting in a circular current.

"The swirling current current set up in the conductor is due to electrons experiencing a Lorentz force that is perpendicular to their motion".

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4. ### GopherT AAC Fanatic!

Nov 23, 2012
6,061
3,825
Yes, think about Eddie current as current that lost as heat to the core instead of inductive resistance from the coil. One method to reduce eddy current losses is to break to magnetic "circuit" in the core. Toroidal cores are often used with a small gap cut into the ring. Also, cores made of iron powder are used with each particle of iron being coated with a very thin layer of insulating material to avoid a majority of current flow. Ferrite is another option for materials - magnetically active but electrically non-conductive.

5. ### crutschow Expert

Mar 14, 2008
13,028
3,238
An eddy current looks like a virtual secondary winding with some high resistance load. As such there is a current flowing that dissipates parasitic power independent of any other currents in the real windings.

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6. ### Silhorn Thread Starter New Member

Apr 9, 2013
18
0
Ahh ok,

So when the emf comes into contact with the core producing an emf which produces the eddy currents. This will mean the magnitude of the eddy current is dependent upon the emf value and core resistance, since you cannot have current without voltage.

The eddy currents will not only heat the core but it will produce another emf which opposes the original emf. This weakened original emf will then need to draw more current to sustain itself and that is actually how the power loss is acquired?

So pretty much the same thing as how an induction cook top works...

7. ### GopherT AAC Fanatic!

Nov 23, 2012
6,061
3,825
Yes, but the core of transformers and inductors break the circuit, as mentioned above, buy not using a solid chunk of iron as the core.

Transformer cores are generally stacks of this plates of silicon-steel that are insulated with micron-thick MgOxide. The thinner the plates the better. You can tell one aspect of a transformer's quality by the thickness (thinness) of the plates. Thicker plates allow more eddy-current heating and run hotter.

Other cores use non-conductive yet magnetic ferrite ceramics and insulated iron powders (iron powder dipped in acid to make non-conductive salts and oxides on the surface).

In summary, preventing electrical current flow in the core made of ferromagnetic material (generally iron, iron/nickel alloy or ferrite).

8. ### Brownout Well-Known Member

Jan 10, 2012
2,375
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No, magnetic fields cause eddy currents. Not EMF.

9. ### Silhorn Thread Starter New Member

Apr 9, 2013
18
0
Doesn't a changing magnetic field induce an emf?

10. ### Brownout Well-Known Member

Jan 10, 2012
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998
Under certain conditions.

11. ### GopherT AAC Fanatic!

Nov 23, 2012
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Only if current can flow in the core material.