Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 2B

Physics Past Exam • 2017/2018 • Pack 1 • Question 2B

02:38

Video Transcript

Explain why the soft iron core in a transformer is made up of insulated laminas, sheets.

Now, if we sketch out a basic transformer, we see it consists of three parts. There is the core of the transformer and then on either side the primary coil and the secondary coil. When alternating current runs through the primary coil, this creates a changing magnetic field through the windings of this coil.

The purpose of the core is to transmit this changing magnetic field through the windings of the secondary coil. When it does this, EMF is induced in the secondary coil and current begins to flow. Now, we’ve said that this alternating current that runs through the primary coil sets up a changing magnetic field to the windings of this coil. And that’s true.

Unfortunately, from the perspective of the efficiency of the transformer, it also has another effect which is the setup of potential differences across different parts of the core itself. When this happens, current starts to flow not in the wires as we want them to but through the core material itself in small loops, called eddy currents.

These eddy currents are responsible for energy loss due to heat and an overall loss in efficiency for the transformer. Let’s say we were to change our perspective so we’re looking at the transformer core from the side. If the core was manufactured from one single chunk of iron, then that would mean that the eddy currents that form in the core are fairly unrestricted in where they can go. They can be as small as in one small part of the core or they can run all through out of it in.

And as we said, it’s these eddy currents which create efficiency losses for our transformer. In order to combat these losses, a new design for the core was developed. In this new construction, the core instead of being made of one piece of material is made of many electrically insulated laminas or sheets of iron. We can imagine each lamina almost as a slice of the core with many slices again electrically insulated stacked one on top of another.

Here is the impact of this redesigned core. When eddy currents do form in one of the isolated laminas within the core, they don’t have very far to go. They’re forced to stay within that particular sheet and can’t jump and spread throughout the rest of the core. This has the overall effect of minimizing energy lost due to unintended current flow. In our transformer then, more of the energy from the primary coil is able to be carried over to the secondary coil; that is, the transformer efficiency goes up.

Here is how we can explain the reason for this redesigned core. We can say that the transformer core is made up of insulated laminas in order to decrease the energy dissipated due to eddy currents. That’s the effect of constructing the core this way.

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