CRGO is the abbreviation for Cold Rolled Grain Oriented Steel, which is used in particular to laminate any heavy transformer as a core. CRGO steel has exceptionally high mechanical elasticity and magnetic properties in the rolling direction. These crystals are often arranged by cold rolling at room temperature to coordinate the direction of grain formation in a specific direction. Since the magnetic properties during cold rolling depend on individual micro-crystals and direction of growth orientation.
This can be further improved on the basis of different production processes and even heat treatment or annealing. While manufacturing CRGO Steel Cores the level of carbon, nitrogen, and oxygen must be kept at a very minimum level to avoid the formation of carbides and oxides.
As these formed compounds alloys must be kept below 1 micrometer in size as these compounds are responsible for increasing the hysteresis losses and decreasing the permeability of the material. Grain size in cold rolled grain oriented steel is always in larger and uniform sizes due to large grain size magnetic losses being reduced. In general, CRGO steel cores come in a normal range of 0.3 mm to 0.25 mm. When these materials are stacked or wound together to give the transformers and other electrical devices a strong magnetic core.
As a thumb rule, 3 percent of the silicon composition for CRGO steel is considered better as more silicon makes the sheet very brittle, which increases the electrical resistivity and decreases the losses due to eddy current.
However, there are two major factors that affect the CRGO steel material
When an external magnetic field is induced, CRGO steel grains will eventually orient in the direction of applied magnetizing force. Since the energy is required to change the steel’s grain orientation in the applied magnetic field direction, it will be lost as heat energy. A magnetic force is needed which ultimately depends on the generated heat.
A material’s magnetic permeability is often used to calculate a CRGO steel’s capacity to support magnetic field formation to that specific material. This can also be said to be the degree of magnetization of the particular material when applied to it by an external magnetic field. For every magnetic material’s magnetic permeability is not a single constant but varies depending on the magnetic field, temperature, environment, and other external and internal factors.
In order to improve the operating efficiency of transformer machines, 5 types of cores are being used. Among these, four popular cores are being discussed here.
As the name itself suggests, as the two main coils are positioned together between primary and secondary coils, the core is made of non-magnetic material, an AC (Alternating Current) flows through the main primary coil which creates a magnetic field where an electromotive force is induced into it. Since the amount of magnetic flux attached to the secondary coil is much smaller. That makes the air core of the transformer very inefficient.
An iron core has the ability to hold a large amount of magnetic flux compared to an air core. Whereas they possess a very high permeability of around 1400 as compared to the air core. This means the iron core has the ability to carry as much flux as 1400 of the air core. Iron core possesses higher eddy current loss and much better efficiency.
Hot rolled silicon steel core
Switching over further to a hot-rolled silicon steel core gave a substantial reduction in core losses to 2 watts/kg at a flux density of 1.5 Teslas. The core material will have more permeability when magnetized in a rolling direction and thus reduces the core losses.
Cold-rolled grain-oriented silicon steel core
They are cold-rolled steels, as the name itself suggests, which are commonly used for the most effective material at the core. At their peak magnetic flux density of 1.9 teslas, they had very high core losses of 0.7 to 0.9 watts per kg when cold-rolled in grain-oriented directions.
Why CRGO Core used in transformers?
C.R.G.O (Cold Rolled Grain Oriented) core is used in transformers to reduce eddy current losses and increase corrosion resistance. It also contributes to a reduction in coercive force resulting in a decrease in the loss of reverse magnetization in transformers.
But before we first let us know what silicon steel
Silicon steel is nothing more than soft magnetic steel, which has a very high magnetic level of saturation relative to other permanent magnetic material and is also used in large transformers, inductive motors, inverters and power generators. This material holds a very small amount of silicon in the total volume of about 3 percent, which is responsible for rising electrical resistivity. Where grain-oriented steel finds its application in static electrical devices whereas non-grain oriented in rotating electrical machines such as induction motors.
More technically, cold rolled grain-oriented and non-oriented steel silicon steel is a soft magnetic metal alloy commonly used as the core material for transformer production, as well as other large electrical and electronic devices. In heavy manufacturing industries, they are typically supplied with a low frequency of electrical voltage, because eddy currents are produced at these frequencies much less than in other ferrite or iron cores.
Alloying iron with silicon has seen an effective change in electrical resistivity about 3% as more addition of silicon would lead to material brittleness and mechanical difficulties with manufacturing the core. Which should usually be kept in a thickness range of 0.35mm to 0.65mm. To encounter loss due to resistivity, chemical vapor depositions have been developed to enrich the external surface lamination with silicon material. However, most standard cold-rolled grain-oriented steel is being used in a range of about 2 to 3 percent of the mass.
These are distinguished by a texture called GOSS when it comes to crystal orientation and structure. The irregular texture seen in electric steel is formed during annealing at very high temperatures due to irregular growth in grain structure.
Electrical Steel is often referred to as miracle steel in electrical engineering because these materials are specifically designed for efficient power conversion with enhanced electrical and magnetic properties.
Popular steel manufacturing industries have grown in technology and high-quality standards to continually expand and contribute to the electrical industries over the many years when some of the renowned steel manufacturers are Tata Steel(India), Thyssunkrup(Russia), Baosteel(China) and Nippon(Japan) etc.
Although a magnetic core manufacturing industry, Nicore India has always played a very important role in developing high-quality silicon steel cores that contribute to transformers and other heavy-duty industries.
Here is the youtube video explaining the difference between hot rolled and cold rolled steel.