1K107 Nanocrystalline Toroidal Core: Permeability, Saturation and Coating Options

What a 1K107 Nanocrystalline Toroidal Core Is

A 1K107 toroidal core is a wound ring core made from a nanocrystalline soft magnetic alloy ribbon. The alloy is produced by rapid solidification of a molten metal onto a rotating chill surface, which yields a thin, continuous ribbon in an amorphous state; a subsequent controlled heat treatment under a magnetic field crystallises it into a structure of nanometre-scale grains dispersed in a residual amorphous matrix. The resulting material combines high permeability with low core loss, which is why it is used in high-accuracy current transformers, common-mode chokes and small high-frequency power transformers.

The practical benefit is that a toroidal core of a given outer diameter can achieve a much higher inductance and a much lower error over a wide frequency band than a ferrite core of the same size, or can achieve the same performance in a smaller volume.

Material Composition and Magnetic Properties

The alloy is iron based, with iron accounting for roughly 82 % of the composition and the balance made up of silicon, boron, niobium, copper and small additions of carbon, molybdenum and nickel. The ribbon is supplied to the core manufacturer in the amorphous state and crystallised after winding, so the finished core has a grain structure distinct from both the as-cast ribbon and a conventional crystalline alloy.

A saturation induction of 1.25 T together with a wide usable temperature range means that current transformers, common-mode chokes and magnetic amplifiers built on this material are less sensitive to current imbalance and lose less of their performance as the temperature rises - an important property where the core sits in a sealed enclosure next to a power stage.

Ribbon Winding, Coating and Mechanical Protection

Because the annealed ribbon is brittle, the finished toroid is protected by a coating or a wrapping. Epoxy resin coating gives a hard, dimensionally stable surface; alternative wraps such as paper tape cushion the winding against the sharp edge of the ribbon. The coating serves a second purpose: it provides the inter-turn insulation that prevents a short circuit between successive layers of the metallic ribbon, and it raises the withstand voltage between the core and the wire wound onto it. Cores are available in a range of standard dimensions and with alternative coating thicknesses, and custom sizes are wound to order.

Applications and Frequency Range

The two properties that decide where a nanocrystalline core is used are permeability and loss behaviour against frequency. In metering current transformers the high permeability keeps the excitation current low, which is what allows accuracy classes such as 0.2, 0.5 or 1 to be held over the required burden range in accordance with the accuracy class definitions of IEC 61869-2. In common-mode chokes the high permeability delivers a large common-mode impedance in a small package, and the low loss allows the choke to operate from about 10 kHz up to the very high frequency region. In power transformers the useful band is roughly 10 kHz to 100 kHz, which includes the switching frequency range of many converters.

Typical applications therefore include revenue metering current transformers, precision instrument transformers, common-mode chokes in power supplies and inverters, magnetic amplifiers and pulse transformers, and small high-frequency transformers for switched-mode conversion.

Measurement, Specification and Quality Control

Magnetic characteristics of soft magnetic materials are measured by the methods of the IEC 60404 series, with the individual material specifications for magnetically soft metallic materials covering the alloy classes themselves. Rated values such as amplitude permeability, core loss and saturation induction are quoted at stated frequency, flux density and temperature, so a comparison between cores is only meaningful when the measurement conditions match. Effective magnetic parameters of a finished toroid - effective path length, effective cross-sectional area and effective volume - are derived from the core dimensions by the calculation methods of IEC 60205, which is the basis on which inductance and loss figures for a wound component are predicted.

Because the saturating flux density of nanocrystalline material is materially lower than that of grain-oriented silicon steel, a designer must ensure the core does not saturate under the worst-case DC bias or current unbalance, and must check that the heat treatment and coating processes have not degraded the permeability. Cores are supplied with a rated inductance factor and loss figure per unit mass, measured on a sample basis.

Frequently Asked Questions

Q: What does the 1K107 designation refer to?
A: It is a material grade code for an iron-based nanocrystalline soft magnetic alloy. The grade fixes the alloy composition and the heat-treatment route, which together determine the permeability, the saturation induction and the loss behaviour of the finished core.

Q: Why is a nanocrystalline core better than ferrite in a common-mode choke?
A: Nanocrystalline ribbon reaches a higher saturation induction and a much higher permeability than most ferrite grades, so a smaller core can provide the same common-mode impedance and can tolerate a larger differential current without saturating.

Q: What is the saturation induction of this material?
A: Approximately 1.25 T, compared with about 0.4 T for typical manganese-zinc ferrite. The higher value allows a smaller cross-section for the same flux, which is why the material is attractive in compact transformer and choke designs.

Q: How is the mechanical fragility of the ribbon handled?
A: The wound core is coated with epoxy resin or wrapped with paper tape. The coating protects the brittle ribbon from handling damage and provides the inter-laminar insulation that prevents adjacent turns of the ribbon from short-circuiting.

Q: Which standard governs the measurement of the magnetic properties?
A: The IEC 60404 series covers magnetic materials and their measurement methods, and the effective parameters of a finished core - path length, cross-sectional area and volume - are calculated according to IEC 60205.

Q: What accuracy classes can be achieved in a metering current transformer?
A: Classes 0.1, 0.2, 0.5, 1 and 3 are defined for measuring current transformers in IEC 61869-2. A high-permeability nanocrystalline core is normally chosen where the tighter classes have to be met across a wide burden and current range.