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Eddy Current Losses

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Transformers are very competent devices. The losses in transformer dictate the design of the transformer. Power transformers in particular are designed so that losses are minimized. The losses can be broken down into several categories. Core Losses In p

Transformers are very competent devices. The losses in transformer dictate the design of the transformer. Power transformers in particular are designed so that losses are minimized. The losses can be broken down into several categories.

Core Losses

In power transformers the largest loss occurs in the core. Core losses can be divided into two separate parts as follows:

Eddy Current Losses

The cores of power transformers are generally made of soft iron or steel. Because iron and steel are good conductors; a current can be induced into the core when the core is subjected to a moving magnetic field. Thus, unless special precautions are taken, large circulating currents will be induced into the core of the transformer. These currents are called as eddy currents.

When alternating current flows through the winding, a changing magnetic field is established in the core.

As this field expands and contracts it induces a voltage into the core. The induced EMF causes eddy current to flow. The eddy currents produce a power loss which is proportional to the current squared.

If the eddy current could be reduced, the power loss would also be reduced. Eddy currents can be reduced by changing the construction of the core. If the cross sectional area of the core is large it has very little resistance and large eddy currents can flow. It can be reduced by using many thin sheets of metal for the core rather than using a solid block of metal. A large eddy current can flow through the low resistance solid core. A core having the same magnetic characteristics can be formed of several thin sheets.

The sheets are coated with an insulating varnish so that no current can flow from one sheet to the other. Thus any eddy current produced are restricted to a single sheet a metal. Because the cross sectional area of a sheet is quite small, the resistance of each individual sheet is relatively high which keeps the magnitude of the eddy currents Thus the power loss is much low in a core made of thin sheets.

The thin sheets which make up the core are called laminations. Frequently, an iron core is constructed by interleaving "E" and "I" laminations.

Hysteresis another kind of Core loss is discuss in details in my article Hysteresis Vs. Copper Loss Vs. Induction Loss

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Posted on May 1, 2011

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Nikhil Agarwal

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