Soft Iron Amorphous C Shape Core for High Performance Transformers

Amorphous C Shape Cores in Transformer Duty

An amorphous C shape core is wound from thin amorphous alloy ribbon, cut into two halves and re-clamped, which turns a material with excellent magnetic properties into an easily assembled component. In transformer duty the shape is used for distribution transformers, instrument transformers and reactor applications where the winding must be produced and tested before the magnetic circuit is closed. The magnetic circuit itself has no crystal structure, so domain walls move under very low driving force, coercivity is low and hysteresis loss is small compared with laminated silicon steel.

The trade-off is a lower saturation flux density than grain-oriented silicon steel, about 1.56 T against about 2.03 T, together with a lower stacking factor because the ribbon is only about 25 µm thick. Designers therefore work at a lower flux density, which reduces the cross section penalty but does not remove it, and they gain a large reduction in no-load loss and in the noise that comes from magnetostriction.

Core Loss and Efficiency in Distribution Duty

Because no-load loss is present whenever the transformer is energised, a reduction in core loss is a continuous saving over the whole life of the unit, and it is worth more in a network with high utilisation hours than the price difference between the two core technologies. The calculation is a present-value comparison of annual no-load energy cost against the capital cost of the core, and it should use the local tariff and the actual load profile rather than an assumed average.

Materials and Standards

Soft magnetic metallic materials, including iron-based amorphous alloys, are covered in IEC 60404-8-6, and semi-processed amorphous core alloys are specified in ASTM A900/A900M, with the Chinese strip series in GB/T 19345.1. Measurement conditions must be attached to every loss figure, because core loss depends on flux density, frequency, waveform and temperature. In practice the useful guarantee from a core supplier names the specific loss at a stated flux density and frequency, the stacking factor, the window dimensions and the AL value where the core is gapped.

Compared with ferrite, amorphous ribbon offers far higher saturation flux density and a much higher energy density for 50 Hz and medium frequency duty. Compared with laminated silicon steel it offers lower loss and lower magnetostriction. Neither comparison is complete without a check on the fill factor of the winding window, because the lower stacking factor of amorphous ribbon reduces the copper area available in a core of the same outline.

Assembly of Cut Cores

Amorphous ribbon is brittle after the annealing that develops its magnetic properties, so it is wound to the finished shape, annealed, impregnated, cut and then re-clamped with a controlled force. The cut faces are ground flat so that the two halves close with minimal extra reluctance. Any bending after annealing degrades the loss locally and permanently, which makes handling instructions and fixture design part of the technical specification rather than a workshop detail. The finished core is normally potted or varnished and mounted in a frame that also carries the clamping force, and the frame must not form a closed conductive loop around the core.

Instrument Transformer and Sensor Applications

In current and voltage instrument transformers, and in precision current sensors, the ratio error and phase displacement depend on the magnetising current at very low flux density. An amorphous core helps here because its permeability stays high at low excitation and its hysteresis loop is narrow, so the measuring error is small and stable over the working range. The same property supports residual current devices and differential protection cores, where a small unbalanced current has to be detected reliably. In all of these duties the core must be protected from dc magnetisation, which shifts the operating point along the hysteresis loop and can drive the core towards saturation.

Frequently Asked Questions

Q: What is the difference between an amorphous C core and a gapped ferrite core?
A: The amorphous core carries a much higher saturation flux density, about 1.56 T against 0.40 T to 0.50 T for power ferrite, so it suits lower frequency and higher power duty, while ferrite suits very high frequency duty because of its far higher resistivity.

Q: Why is the working flux density lower in an amorphous core?
A: Because saturation occurs near 1.56 T, so designs normally run at about 1.30 T to 1.40 T to keep magnetising current and loss in a controlled range.

Q: Can an amorphous C core be re-cut or modified on site?
A: No; the core is annealed to its final properties and any cutting exposes unannealed material and creates a new loss region, so dimensional changes must be made at the winding stage.

Q: How is the core held together in service?
A: By impregnation or varnishing of the ribbon layers plus a clamping frame or band that applies a defined pressure to the two halves without forming a closed conductive path.

Q: Which standards apply to amorphous core material?
A: IEC 60404-8-6 for soft magnetic metallic materials and ASTM A900/A900M for semi-processed amorphous core alloys, with strip dimensions and grade designations following the national series such as GB/T 19345.1 in China.

Q: Why does an amorphous core reduce transformer noise?
A: The amorphous structure has much lower magnetostriction than grain-oriented steel, so the dimensional change under alternating flux is smaller and the radiated sound power at the same flux density is lower.