Eddy Current Loss

An electric current passed through a conductor creates a magnetic field; this field varies in proportion to the current if alternating. If the ac current carrying conductor is wound around an electrically conductive core – for example in a transformer – the varying magnetic field will, according to Faraday’s laws of induction, induce currents in the core. These will flow in closed loops at right angles to the magnetic field, and are known as eddy currents.

Eddy currents are also created if a magnet moves in relation to a conductor, for example within a generator. In any case, the value of a current within a given loop relates to the magnetic field strength, the loop area and the rate of change of flux.

If an eddy current of magnitude I flows through a core path of resistance r, it will dissipate energy in the form of heat according to the power equation power = I2R. Since this represents energy being expended for no useful purpose, it is considered as an eddy current loss, sometimes called iron loss.

Eddy current magnitude can be reduced by designing the core as a set of thin sheets or laminations, in parallel to the magnetic field. Each sheet must be insulated from its neighbours with a thin coating of varnish or oxide film. By laminating the core, the area of each section and hence the induced EMF is reduced.

Eddy currents can also be reduced by using a magnetic material such as silicon steel, with a higher resistivity.