3. Special steels
page 17
Logo
Interactive graphic of soft iron core
Picture 3.15 The magnetic particles in a soft iron nail will line up with the magnetic field when the current is switched on. This makes the field stronger and is why we use iron - or electrical steels - as the core of electromagnets.

Electrical steels
Electrical steels are used to make the iron cores of motors, transformers and generators. These cores concentrate the magnetic field and make the electromagnet stronger. The steel must be easy to magnetise and to demagnetise.
Properties of electrical steels
When a steel core is magnetised and then demagnetised, it absorbs energy (and heats up). Electrical steels are designed to reduce this power loss.

Electrical steels are also designed to have a high permeability. This means that the electrical current needed to produce the magnetism should be as low as possible.

Structures
There are two types of structure that electrical steels can have:
  • grain oriented
  • non oriented.
Grain oriented electrical steels
Grain oriented electrical steel has a symmetrical structure in which cubic crystals are all arranged in the direction in which the steel plate has been rolled. This is called the GOSS orientation. It is achieved by a series of rolling and annealing treatments.

Steel of this type has much better power and permeability properties in the direction of rolling than it has at 90 ° to this direction.

The low power loss is achieved by adding up to 3% silicon to the molten steel. This has the disadvantage, however, that it reduces the magnetic saturation of the steel. The magnetic saturation is measured in Tesla (T).

Non-oriented electrical steel
Non-oriented electrical steels have similar magnetic properties in all directions. The cubic crystals that make up the structure are randomly oriented.

The steel is annealed at 800 °C in a wet hydrogen atmosphere to remove as much carbon as possible. This is because carbon causes an increase in power loss over time which is known as magnetic aging.

Photo of industrial transformer
Picture 3.15 A large industrial transformer. The coils inside are wound on special cores made from electrical steel.
Uses of electrical steels
Grain oriented steels are used in transformers, transducers and magnetic amplifier cores.

Non-oriented steels are used in rotating machines such as motors, generators and alternators.

Question 3-6.
a) What are two desirable properties of electrical steels?

b) Which type of structure results from rolling and annealing treatments?


Summary                   Close
  • electrical steels are used to make the cores of electromagnets
  • they have a high permeability so that it is easy to produce a magnetic field in them