Traditionally, cold-rolled grain-oriented (CRGO) silicon steel has been widely used for the cores of distribution transformers and station service transformers due to its excellent magnetic properties and relatively low core losses. However, in recent years, a modern trend has emerged in the choice of core materials, especially for distribution transformers and, to some extent, station service transformers. This trend involves the use of amorphous iron materials. The amorphous core material is typically an amorphous iron alloy, composed mainly of iron, with small amounts of boron and silicon. Unlike CRGO steel, the amorphous material is non-crystalline in nature. That means it lacks the long-range atomic order found in crystalline materials. This non-crystalline structure is achieved through a specialized manufacturing process, where the molten alloy is rapidly cooled at an extremely high rate, often more than a million degrees Celsius per second. This rapid cooling prevents the atoms from arranging themselves into a regular crystal lattice, resulting in a disordered atomic structure.
Let us now discuss the detailed manufacturing process of this amorphous core material.
Process of Production of Amorphous Metal
First iron is mixed with boron, silicon and some other elements. The main alloy elements are 80% iron, 15% boron and 5% silicon. These raw elements are melted in an induction furnace at a temperature 1200°C to 1500°C. This ensures the homogeneous mixture of the raw elements. This molten mixture is then ejected to through a nozzle to a rotating roller made of copper. At the same time the roller is cooled rapidly with cold water or any other coolant. The speed of the spin of rotor is about 20-30 meters per second or higher. As the molten mixed metal hits the cold surface, it solidifies almost instantly at a cooling rate approximately 1 million degrees centigrade per second. This rapid solidification makes very thin ribbon on the roller. The thickness can be achieved around 0.02 to 0.03 mm which is much thinner than 0.23 to 0.27 mm which is the standard thickness of CRGO lamination.
Advantages of Amorphous Core
We have seen two major properties of amorphous core materials. First, they are non-crystalline in nature. Second, the thickness of each lamination in an amorphous core is significantly less than that of a CRGO (Cold Rolled Grain Oriented) steel lamination.
Since amorphous material is nearly non-crystalline, it experiences significantly lower hysteresis loss when used as a transformer core material. In the case of CRGO steel, during each cycle of magnetizing current the crystal grains have to reorient themselves along the direction of the magnetic field. This repeated reorientation of grains within the crystalline structure contributes the hysteresis loss. However, in amorphous iron material due to 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 that use amorphous core materials.
Therefore, both hysteresis and eddy current losses are much smaller in amorphous core there by it increases the transformer efficiency.
| Features | CRGO | Amorphous |
|---|---|---|
| Atomic Structure | Crystalline, Orderly, Repeat | Non-Crystalline, Random Arrangement |
| Saturation Flux Density | 2.03 T | 1.56 T |
| Sheet Thickness | 0.23–0.3 mm | 0.025 mm |
| Electrical Resistivity | 45 Ω-m | 130 Ω-m |
| Available Form | Sheet, Roll | Thin Sheet, Ribbon |
| Lamination Factor | 96% | 86% |
| Core Losses | 100% | 60–75% |
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, making them more expensive than 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.