Amorphous Rolled Transformer Core: Material, Processing and Core Data

What an Amorphous Rolled Transformer Core Is

An amorphous rolled transformer core is wound from thin ribbon of amorphous alloy, a metallic glass that is cast directly from the melt instead of being rolled from a crystalline ingot. The ribbon is wound into a closed magnetic circuit and annealed, producing a core whose principal duty is to hold the no-load loss of a distribution transformer at a very low level. The alloy combines excellent magnetic properties with high strength, high hardness and high electrical resistivity, and it behaves as a superior core material for energy-efficient transformers.

Because the ribbon is thin and the core is wound rather than stacked, the finished body is compact, mechanically stiff and stable in service, and it can be supplied as a bare wound core or inside a protective enclosure.

Material Science: Why the Disordered Atomic Structure Matters

For several thousand years the metals and alloys used in engineering have been crystalline materials, in which the atomic three-dimensional arrangement is ordered and forms a periodic lattice. When molten alloy is cooled at an extremely high rate, the atoms cannot rearrange into that lattice in time and remain frozen in the disordered arrangement of the liquid at room temperature. The result is a solid with no long-range ordered, periodic structure, which is why these alloys are called amorphous, or metallic glasses. The birth of amorphous alloys is regarded as a revolution in metallurgical materials science.

The absence of a periodic lattice removes the grain boundaries and the magnetocrystalline anisotropy that generate hysteresis loss in conventional sheet steel, so the advantages of amorphous alloy as a transformer core material are pronounced:

High saturation magnetic flux density, so the core can be made small for a given power rating.

Low coercive force, so a small exciting current reverses the flux every cycle.

Low loss, equivalent to one third to one fifth of the loss of silicon steel sheet.

Low exciting current, which cuts the reactive burden placed on the supply.

Good temperature stability, so the magnetic data remain usable across the service temperature range.

Processing Technology and Its Constraints

Amorphous ribbon does not behave like conventional lamination stock, and four production steps dominate the final quality of the core.

These four constraints explain why amorphous cores are specified by the core maker together with the transformer designer: a dimension that is convenient for punching a lamination may be impossible for a wound amorphous body, and any stress introduced after annealing will raise loss again.

Structural Features for Three-Phase Transformers

The three-phase five-column amorphous core is designed for Dyn11 connection-group transformers, and its core size is arranged so that it retains the physical envelope of amorphous cores already used in the market, which means existing assembly fixtures, equipment and capacity can be reused without redesign. An advanced distributed stack structure is used in the core connector design, and dimension E is at least 7 % smaller than that of commonly used amorphous cores, which effectively reduces the overall size of the transformer. The excitation power of the core is low, and this significantly reduces the noise of the transformer in operation, which matters in residential and urban distribution installations where audible noise is a planning constraint.

Because the loss of the magnetic circuit is already small, the remaining design freedom lies in the winding and in the cooling arrangement, and a compact core leaves more room for both. Non-standard core sizes are wound to drawing when a customer envelope differs from the standard series.

Finished Core Dimension and Electrical Data

The table below gives representative finished core sizes together with effective cross-section, mean effective path length, core weight and the total flux at 25 degrees Celsius. Effective cross-section and path length are the two values that drive the winding design, and the flux figure sets the usable working point of the core.

The series extends to an outer diameter of 40 mm, a height of 15 mm and a core weight of 70 g, and both the small cores used in metering and control transformers and the larger bodies used in distribution transformers come from the same wound-ribbon process. When a design needs a taller window for more copper, the core can be wound to a non-standard inner diameter, outer diameter and height combination.

Frequently Asked Questions

Q: Why does an amorphous core lose less energy than a silicon steel core?
Its atoms are frozen in a disordered arrangement instead of a periodic lattice, so grain boundaries and magnetocrystalline anisotropy do not contribute hysteresis loss, and the very thin ribbon also limits eddy-current loss; the total loss is about one third to one fifth of silicon steel sheet.

Q: Which processing step is the most critical?
Annealing is the core technology of the whole process, because the excellent low-loss characteristics are obtained only after a correctly controlled anneal, and any mechanical stress introduced afterwards can degrade them again.

Q: What are the main production difficulties?
The alloy is hard and difficult to cut with conventional tools, the very thin and not perfectly flat ribbon gives a low fill factor, and the material is sensitive to mechanical stress, so tightening measures must be controlled.

Q: Which connection group suits a three-phase five-column amorphous core?
The three-phase five-column amorphous core is designed for Dyn11 connection-group transformers and keeps the physical envelope of the amorphous cores already used in the market.

Q: What does the distributed stack structure change?
It is applied in the core connector design and makes dimension E at least 7 % smaller than commonly used amorphous cores, which reduces the overall footprint of the transformer.

Q: Does the amorphous core reduce transformer noise?
Yes, because the core needs a low excitation power, and the reduced magnetostrictive excitation lowers the audible noise emitted by the transformer during operation.