Electrical steel is kind of special steel which is tailored to exhibit certain specific magnetic properties such as small hysteresis area (small energy dissipation per cycle or low core loss) and high permeability. It is also called lamination steel, silicon (Si) steel, silicon electrical steel or transformer steel. The steel contains specific percentage of silicon in it which is responsible for its unique property. In mild steel there is much loss in electrical energy due to hysteresis and eddy current and hence use of mild steel is uneconomical when it is used in the electrical devices. The hysteresis loss is shown in Fig.1. The hysteresis loss is proportional to the area of the respective loops shown in the figure.
Fig 1 Comparison of hysteresis loss in electrical steel (left) and mild steel (right)
Electrical steel is an iron alloy of iron which may have from zero to 6.5 % silicon but usually has silicon content up to 3.2 % (higher concentrations usually provoke brittleness during cold rolling). Manganese and aluminum can be added up to 0.5 %. Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss. However due to the silicon the grain structure hardens and embrittles the steel, which adversely affects the workability of the steel, especially during rolling. When alloying, the concentration levels of carbon, sulphur, oxygen and nitrogen should be kept low since these elements indicate the presence of carbides, sulphides, oxides and nitrides in the steel. These compounds, even in particles sizes as small as one micrometer in diameter, increase hysteresis losses and decrease magnetic permeability. The presence of carbon has a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For this reason, the carbon level is kept to 0.005 % or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen. Typical properties of electrical steel are given below.
- Melting point – Around 1500 deg C
- Specific gravity – 7.65
- Resistivity – Around 47.2 x 10?? Ohm Meter
The magnetic properties of electrical steel are dependent on heat treatment since the increase of average grain size decreases the hysteresis loss. Hysteresis loss is determined by a standard test, and for common grades of electrical steel may range from about 2 to 10 watts per kilogram at 60 Hz and around 1.5 tesla of magnetic field strength. Semi processed electrical steels are supplied in a state so that after punching of the final shape, a final heat treatment develops the desired 150 micrometer grain size. The fully processed steels are usually supplied with insulating coating, full heat treatment, and defined magnetic properties, for applications where the punching operation does not significantly degrade the properties of steel. Excessive bending, incorrect heat treatment, or even rough handling of core steel can adversely affect its magnetic properties and may also increase noise due to magnetostriction. Magnetic properties of electrical steels are tested using the internationally standardised Epstein frame method. Manufacturing process flow for electrical steel is given in Fig 2
Fig 2 Manufacturing process flow for electrical steel
Types of electrical steels
- Non Oriented Electrical Steel – This steel has uniform magnetic characteristics in rolling and in other directions and is widely used in the iron core materials or rotary machines from large transformers to small electric precision motors. This steel is made without special processing to control grain orientation. It usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, i.e., it is isotropic. Cold rolled non grain oriented electrical steel is normally abbreviated as CRNGO. CRNGO is less expensive than CRGO. It is used when cost is more important than efficiency and for applications where the direction of magnetic flux is not constant for example electric motors and generators with moving parts etc. It can also be used when there is insufficient space to orient components to take advantage of the directional properties of grain oriented electrical steel. Micro structure of CRNGO is at Fig 3.
Fig 3 Microstructure of CRNGO
- Grain Oriented Electrical Steel – This type of steel has superior magnetic characteristics in the direction of rolling. This steel is used in the manufacturing of large, medium and small-sized transformers, distribution transformers and reactors. This type of electrical steel normally has a silicon level of 3 %. It is processed in such a way that the optimum properties are developed in the rolling direction, due to a tight control of the grain orientation relative to the sheet. During cold rolling the grains get elongated in one direction and get narrowed in the perpendicular direction. Hence due to the shape of the grain it becomes easier for the domains to change polarity back and forth along the elongated direction. The enlargement of grain takes place when the steel is heated at temperatures around critical temperature followed by slow cooling. Larger grains help reducing the losses due to hysteresis. The grain size in case of CRGO is 2 to 5 and in case of HRGO it is 5-20. The magnetic flux density is increased by 30% in the coil rolling direction, although its magnetic saturation is decreased by 5%. It is used for the cores of power and distribution transformers. Cold rolled grain oriented electrical steel is normally abbreviated as CRGO. CRGO is usually supplied in coil form and it has to be cut into “laminations”. These laminations are then used to form a transformer core which is an integral part of a transformer. Grain oriented steel is used in large power and distribution transformers and also in certain audio output transformers. Micro structure of CRGO is given in Fig 4
Fig 4 Microstructure of CRGO
Electrical steel is usually coated to increase electrical resistance between laminations which reduces eddy currents, to provide resistance to corrosion and rust and to act as a lubricant during die cutting. There are various coatings, both organic and inorganic. These coatings are used depending on the application of the electric steel. The type of coating selected depends on the heat treatment of the laminations, whether the finished lamination will be immersed in oil, and the working temperature of the finished apparatus. Very early practice was to insulate each lamination with a layer of paper or a varnish coating, but this reduced the stacking factor of the core and limited the maximum temperature of the core. ASTM A973-03 classifies different types of coating for electrical steels.