SOLUTION

Silicon steels are used for electrical transformer cores and cores of other electrical devices for the following reasons:-

1. Low hysteresis loss.
2. High permeability.
3. Virtually eliminated aging.
4. High resistance.

The function of a transformer’s core is to provide a low-reluctance/high permeability magnetic circuit by which magnetic flux, created by the magnetomotive force in the primary winding, can efficiently couple with the secondary winding.

Laminations of transformer cores are manufactured from a silicon-iron alloy (iron with around 3% silicon content), which is more generally termed ‘silicon steel, ‘electrical steel’, ‘transformer steel’, or by some trade name such as ‘Stalloy’. This ferromagnetic alloy is specifically designed to have all of the characteristics described.

⇒ Low-reluctance/high permeability magnetic circuit, in order to maximize the flux density within the core.

⇒ Low-remanence/low-coercivity/low-area hysteresis loop: To minimize the energy loss per magnetization/demagnetization cycle; this energy loss being termed its ‘hysteresis loss’

⇒ High resistivity conducting path: To minimize any ‘eddy currents’ (circulating currents) resulting from undesirable voltages induced into the core; eddy currents are further minimized by manufacturing the core from laminations.

The silicon content of the iron acts to both reduce the core’s hysteresis losses and to increase its resistivity by a factor of around 4.5, which acts to reduces the magnitude of any circulating currents (eddy currents) that result from voltages induced into the core.

There are two general categories of silicon-steel, termed ‘grain orientated steel‘ and ‘non-orientated steel‘:

⇒ Grain-orientated steel has a silicon content of around 3% and is manufactured in such a way that its magnetic properties are optimized along the direction of its grain, enabling its flux density to be increased by as much as 20% in that direction.

⇒ Non-orientated steel, with a silicon content of 2-3.5%, has a randomly orientated grain with similar magnetic properties in all directions, but the resulting flux density can be significantly lower than for grain-orientated steel.

From the point of view of the varying flux, the core of a transformer is simply another winding and, therefore, it will induce a voltage into it. If the core was manufactured from a solid piece of silicon steel, then it would behave like a heavy, short-circuited, single winding, and a large current would circulate around it.

In eddy current, the magnetic field that induces a voltage in the secondary of a transformer also induces a voltage in the core. This causes circulating (or eddy currents) in the core. Making the laminations as thin as possible reduces this loss. The laminations are coated with a thin insulating material to prevent current from flowing between them.