CRGO core Laminations Of Electrical Transformer

Cold Rolled Grain Oriented (CRGO) silicon steels are used for laminations of the Power Transformers. Cold Rolled Grain Oriented (CRGO) sheets will have superior magnetic properties in the direction of rolling. The crystals are aligned in the direction by cold rolling followed by heat treatment process. Magnetic properties of the CRGO steel Sheets are dependent on the magnetic properties of the individual crystals of the material and the direction of orientation of the crystal. The properties of the CRGO silicon steels are improved by composition, manufacturing process, heat treatment, laser irradiation etc.

Thickness of the CRGO sheets will be of the order of 0.33mm to 0.25mm. These CRGO Steel Laminations are stacked together to form a magnetic core for the Transformer.The Commercially available CRGO steel sheets will have 3% of Silicon. Higher the Silicon content increases the resistivity and reduces the eddy current losses. But Silicon Content above 3.5% makes the CRGO silicon steel sheets brittle. Silicon content of 3% to 3.25% is used in commercial grades of CRGO steel.

The following features or properties influence for selecting the CRGO steel sheet as magnetic core circuits for transformers.

• Maximum magnetic induction to obtain high induction amplitude in an alternating field
• Core loss will be independent of the load of the transformer. By using CRGO steel sheets core loss is low result in reduction of the constant losses.
• Low apparent power input (Low hysteresis loss) results in low no load current
• High grade surface insulation
• Good mechanical processing properties
• Low magnetostriction results in low noise level

In order to appreciate the CRGO Steel material first we have to understand the core losses in the Transformer.

Core losses of a transformer consist of Hysteresis and Eddy Current losses. About 80% of the no load losses are due to properties of the material and remaining 20% of the core losses are due to design and manufacturing process.

In an AC Transformer voltage and current waveforms vary in cycles, means when the direction of current flow for one half cycle is in one direction and for other half life the direction reverses. So the direction magnitude of Field applied to the core of the Transformer also alters for every half cycle. Consider the core is magnetized by the magnetic field (H) called magnetizing force. The magnetic flux density (B) of the core of Transformer will increase and saturates at the knee point and then starts decreasing as magnetizing force (H) decreases. An instant (D in fig.) will arise where the magnetizing force (H) will become zero i.e, no magnetic field force is applied to the material (This arise due to the current wave during passing from one half cycle to other half cycle it touches zero point) but still magnetic flux density is not zero in the material but have some value. This is called Magnetic Retentivity. The value of H at zero B is called Corrective Force( C in fig.). Coercive Force required making the magnetic flux (B) zero by applying the magnetic field in opposite direction. When the flux applied in the opposite direction to the material in opposite direction in another cycle the same phenomenon takes place. This complete loop is called hysteresis loop. Hysteresis loop is significant because the area under the hysteresis loop gives the total Hysteresis Loss of the Magnetic Material.

So from hysteresis loop of BH curve we understand that lower the Magnetic Retentivity in the magnetic material lower is the loss of magnetic field applied to make the magnetic flux zero. CRGO steel have the properties of soft magnetic materials which have smaller hysteresis loop or BH curve. So Hysteresis loss is much smaller in CRGO core compared to ordinary steel material.

In simple when a magnetic field is applied all the grains of the magnetic material will orient in the direction of magnetizing force. In another cycle this grains will orient in opposite direction in the direction of magnetizing force. The energy required to change the orientation of the magnetic grains in the direction of the magnetic field is lost in the form of heat. This loss is called hysteresis loss.

When an alternating magnetic flux is applied to the iron core small emf will be induced due to change in flux linkage. This induced emf will cause small circulating current s called eddy currents. Eddy current flowing through the material causes I²R losses in the material.

In order to reduce the Eddy Current loss:

Use of Material having high electrical resistivity:

By using superior grain orientation CRGO steel will have higher electrical resistivity. Thus eddy current loss can be reduced.

Lamination Cores are used:

By using thin laminations the core thickness is reduced so eddy current losses are reduced. So CRGO steel laminations are used as a core material.

Insulation between laminations:

Oxide coating is provided for CRGO laminations to reduce eddy current losses.

CRGO Steel Laminations are stacked together to form a magnetic core for the Transformer.

Cold rolled silicon steel sheets have superior grain orientation. By cold rolling it is made that all crystals are aligned in the direction of rolling. In Electric Steel the percentage of silicon content will be 0 to 6%. Increasing the silicon in the steel will results in narrowing of Hysteresis loop (Area of the hysteresis loop reduces) thus Hysteresis loss reduces and resistivity of the material increases by adding the silicon content in the steel reducing the Eddy Current Loss.