Traditional distribution transformers use cold-rolled grain-oriented (CRGO) silicon steel for their core. However, in recent years, manufacturers have preferred using amorphous iron materials. The amorphous core material is typically an amorphous iron alloy. It is mainly an iron alloy, with a small quantity of boron and silicon. Unlike CRGO steel, the amorphous material is non-crystalline in nature. Therefore, it lacks the long-range atomic order found in crystalline materials.
In a specialized manufacturing process, the molten alloy goes through an extremely rapid cooling process. The rate og cooling is extremely high, often more than a million degrees centigrade per second. This rapid cooling prevents the atoms from arranging themselves into a regular crystal lattice. Hence, it results in a disordered atomic structure.
Process of Production of Amorphous Metal
Firstly, the manufacturers mix the iron with boron, silicon, and some other elements. The main alloy elements are 80% iron, 15% boron, and 5% silicon. Then they melt these raw elements in an induction furnace at a temperature 1200°C to 1500°C. This ensures the homogeneous mixture of the raw elements. They inject this molten mixture through a nozzle onto a rotating copper roller.
At the same time, they apply a spray of chilled water or any other coolant. As the molten mixed metal hits the cold surface, it solidifies almost instantly at a cooling rate of approximately 1 million degrees centigrade per second. This rapid solidification makes a very thin ribbon on the roller. In this way, the manufacturer achieves the thickness of the sheets on ribbons around 0.02 to 0.03 mm. Thus, it is much thinner than the standard thicknesses (0.23 to 0.27 mm) of CRGO lamination.
Advantages of Amorphous Core
Since amorphous material is nearly non-crystalline, it experiences significantly lower hysteresis loss. In CRGO steel, the crystal grains alternately change their orientation along the alternating direction of the magnetic field. This repeated reorientation of grains contributes to the hysteresis loss. However, in amorphous iron material, due to the lack of crystal grains, there is no such orientation and reorientation process. Hence, hysteresis loss is minimum. This makes amorphous materials more energy efficient for transformer cores.
The non-crystalline feature of an amorphous core also reduces magnetostriction. Magnetostriction is a phenomenon in which, due to the orientation and reorientation of grains, the dimensions of each lamination slightly change during each half-cycle of the magnetizing current. This mechanical deformation generates an irritating humming sound in transformers with CRGO cores. This humming sound is almost absent or significantly reduced in transformers using amorphous core materials. This is because of the lack of grains in amorphous materials.
Since the thickness of the layers or laminations is much smaller, the overall electrical resistance of an amorphous core is significantly higher than that of an equivalent CRGO core. This higher resistance results in lower eddy current losses in transformers.
Disadvantages of Amorphous Core
The main disadvantage of an amorphous core is its low magnetic flux density. The maximum flux density of an amorphous core is about 1.5 T, compared to 1.9 T to 2.03 T for a CRGO core. As a result, an amorphous core must be larger in size than an equivalent CRGO core to handle the same power level.
Additionally, the manufacturing process of amorphous cores is more complex. The making process of the core is more expensive than that of equivalent CRGO cores.
Since the amorphous core is non-crystalline, it is also brittle. This makes handling the core and its laminations during the construction process more challenging than with CRGO cores. Special care must also be taken during winding assembly on the core.
