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Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.

By Lenz's law, an eddy current creates a magnetic field that opposes the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material. Thus eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores or ferrite cores to minimize them.


When does Eddy current become significant?

Eddy current is not significant below 630 A. The IEC 61439-1 says that for more than 630 A Eddy current will be present and design has to ensure that additional thermal effects relating to the eddy currents and current displacement are considered in design (IEC 61439-1 : 2011, Sec. 10.10.2.2.3-b).


How is the effect of Eddy current managed in PrismaSeT I?

Induced Eddy current also depends on the design as explained below

• Fig 1 shows a single phase electrical conductor which is passing through a metallic plate. This will induce a magnetic field in the plate as shown in the figure and it will cause eddy current.
• Fig 2 shows the 3 phase electrical conductors which are passing through a common cut out in a metallic plate. In this case, there will not be any magnetic field induced in the metallic plate as vector summation of the magnetic fields induced by each phase will become zero.
• Fig 3 shows PrismaSeT International busbar architecture which is similar to the configuration of Fig 2. Hence, ideally there can not be any Eddy current induced in the supports.
• Fig 4 shows cutouts in PrismaSeT International form partitions when rated current is more than 630 A which is similar to Fig 2. Hence, ideally there can not be any Eddy current induced.

There are no defined limits by the standards for Eddy Current. But, by design there cannot be any high eddy current in PrismaSeT I. The entire PrismaSeT I offer is type tested for temperature rise as per IEC 61439-1. Hence, it is not required to be worried unless and until there is an excessive temperature rise.

Induced Magnetic Fields

Arrangement in PrismaSeT I

How does aluminum help to reduce Eddy current in PrismaSeT I for Gland Plate and Busbar Support instead of steel ?

Normally for steel, two types of magnetic losses occur, Eddy Current Loss and Hysteresis loss. As aluminum is a nonmagnetic material, Hysteresis loss does not occur and the induced eddy current is normally less compared to steel. Hence, when the rated current is more than 630 A and it is not possible to implement design as Fig. 2, aluminum sheet can be used in PrismaSeT I to minimize Eddy current effect (For example, cable gland plate).

Released for:Schneider Electric India