Mar 062020
 

Why is the transformer core made of metal sheets?

You all know that the transformer core is made of metal sheets and someone will have wondered if it would not be easier to use a compact iron block.

If you are looking for an explanation for this purpose, you will not find it easily, so we will try to illustrate as simply as possible the reason why it is not possible to use an iron block instead of the sheets.

When a compact iron block is inserted into the core, when it is subjected to an alternating magnetic field, inside it run parasitic currents (Eddy currents also named Foucault’s currents) that overheat it. The same situation would occur if we would wrap a large loop in the transformer core and short the two ends together.

Why is the transformer core made of metal sheets

By SvjoOwn work, CC BY-SA 3.0, Link

Transformer core made of metal sheets

To neutralize these parasitic currents, the core must be cut into very thin sheets, insulating them from others, by means of a layer of varnish or oxide, so that even if one exceeds the other, the parasitic currents in a sheet could never be short-circuited with the currents of the adjacent sheets.

The standard sheets, which are currently on the market, have a thickness of 0.50 mm and only the “special” type has a thickness of 0.35 mm.

Obviously, those with a thickness of 0.35 mm have higher performance because the losses caused by dispersion and eddy currents are lower.

We should point out that parasitic currents and hysteresis losses increase considerably with increasing working frequency, then the sheet cores can only be used at low frequency up to a maximum of 20,000-30,000 Hz.

To work at frequencies between 10,000 – 100,000 Hz, ferrite cores composed of microscopic iron granite are joined together by special glues that isolate them from each other. The lighter the core the less iron powder is in your body, so the higher your working frequency will be.

Very heavy ferrite cores are capable of working up to a maximum frequency of 2Mhz, the lighter ones up to 20/50 MHz and those that are lighter than the previous ones even at frequencies above 100MHz.

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