In a transformer the loss of energy due to eddy currents can be reduced by

In a transformer the loss of energy due to eddy currents can be reduced by

Right Answer is:

Using laminated sheets

SOLUTION

To reduce the eddy-current losses (as well as for ease of assembly), transformer cores are not solid but, instead, manufactured from laminated sheets — with each lamination lightly insulated from its neighbors. This is achieved by coating one side of each lamination, before assembly, with a thin layer of insulating material, such as varnish (smaller transformers), a dried flour/chalk solution (larger transformers), or by simply allowing a thin layer of corrosion to form on the surface of each lamination prior to assembly.

Using a solid iron core results in a large circulating current. Laminations have the effect of significantly reducing eddy-current losses. So, the core is made up of a stack of thin (0.5 mm) sheets.

The lines of magnetic flux can still run around the core within the plane of the laminations. The situation for the eddy currents is different. The surface of each sheet carries an insulating oxide layer formed during the heat treatment. This prevents current from circulating from one lamination across to its neighbors. Clearly, the current in each lamination will be less than the very large current we had with the solid core, but then there are more of these small currents.

Example

Let’s suppose the core of a transformer was solid, rather than laminated. And let’s suppose that the voltage induced into that core is, say, 10 V, and the resistance of the core is, say, 1Ω. The eddy-current loss, therefore, would be:

P = V2/R = 102/1 = 100 watt

Let’s suppose a transformer’s core is made up of just four laminations. The voltage induced into each of the four laminations would then be one-quarter (2.5) of the voltage induced into a solid core of equivalent size. At the same time, the cross-sectional area of each lamination will be just one-quarter that of the solid core, making its resistance four times greater. The eddy-current loss, per lamination, therefore, would be:

P = V2/R = 2.52/1 = 1.56 watt

So the total eddy current loss for four laminations will be

Total eddy current loss = 4 × 1.56 = 6.25 watt

In this example, the total eddy-current loss in a core comprising four laminations is one-sixteenth of the eddy-current loss for a solid core. Or, to put it another way, for a laminated core, the total eddy-current loss is inversely proportional to the square of the number of laminations.

So the eddy-current loss per lamination is so small, that the sum of those individual losses is far less than the losses that would occur in a solid core of the equivalent cross-sectional area.

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