Residual Magnetic Loss Testing for Soft Magnetic Materials: Principles, Methods, and Standards

A residual magnetic loss test quantifies the portion of total core loss in soft magnetic materials—such as ferrites, amorphous alloys, and soft magnetic composites—that cannot be attributed to hysteresis or eddy-current effects. Accurate determination of this loss component is essential for high-frequency transformer and inductor design, thermal management, and material qualification against IEC, ASTM, and GB standards. This article explains the test principles, sample requirements, measurement procedures, and typical applications of residual magnetic loss evaluation in accredited third-party laboratories.

Technical Overview

When a soft magnetic material is magnetized by an alternating field, the total power dissipated per unit volume, known as the total core loss, is conventionally decomposed into three components: hysteresis loss, eddy-current loss, and residual loss. The residual magnetic loss is defined as the difference between the measured total loss and the sum of the quasi-static hysteresis loss and the classical eddy-current loss. It is sometimes referred to as excess loss or anomalous loss in the technical literature.

The residual component originates from dynamic magnetization mechanisms that are not captured by the classical loss model, including magnetic aftereffects, damping of reversible and irreversible domain wall motion, spin-lattice relaxation, and ferromagnetic resonance effects. In high-resistivity materials such as manganese-zinc and nickel-zinc ferrites, eddy-current loss is strongly suppressed, and residual loss becomes the dominant dissipation mechanism at frequencies above roughly tens of kilohertz. In laminated and thin-gauge conductive materials, the residual contribution is described by statistical loss models associated with discrete domain-wall jumps.

Because the residual loss cannot be measured directly, the test is performed as a loss separation exercise. The total loss is measured over a range of frequencies and flux densities, the hysteresis and eddy-current components are characterized independently, and the residual loss is extracted by subtraction or by fitting to an established physical loss model. The results allow engineers to predict temperature rise, optimize operating frequency, and select materials for power conversion applications.

Testing Samples

Soft Ferrite Cores

Toroidal, E-shaped, RM, and PQ cores made of MnZn or NiZn ferrites are the most common specimens. Closed magnetic circuits with uniform cross sections are preferred so that the magnetizing field distribution is well defined. Test windings are typically applied by the laboratory according to the core geometry and the target frequency range.

Soft Magnetic Powder Composites

Iron powder, sendust, high-flux, and iron-silicon compressed ring cores exhibit distributed air gaps and high electrical resistivity, making residual loss a significant fraction of their total loss. Ring specimens with consistent pressing and annealing history are required for reproducible results.

Amorphous and Nanocrystalline Ribbons

Wound toroidal cores produced from rapidly quenched ribbons are tested after stress-relief annealing. Thin ribbon thickness and elevated resistivity reduce eddy-current loss, so careful separation of the residual component is particularly important for accurate high-frequency characterization.

Laminated Electrical Steel

Epstein strip sets, single-sheet specimens, and ring-stamped laminations of oriented or non-oriented silicon steel can also be evaluated. For these materials, the residual loss is often analyzed using statistical excess-loss models in combination with total loss measurements at power and medium frequencies.

Testing Items

• Total core loss density as a function of frequency, peak flux density, and temperature
• Loss per cycle, obtained by dividing total loss by frequency, for separation analysis
• Quasi-static hysteresis loss determined from low-frequency loop area measurements
• Classical eddy-current loss calculated from resistivity, thickness, and excitation parameters
• Residual loss magnitude and its share of total loss under specified conditions
• Frequency, flux density, and temperature dependence of the residual loss component
• Complex permeability and loss tangent at low excitation levels for correlation purposes
• Fitted Steinmetz or statistical loss model coefficients for simulation and design use

Testing Methods

Total Core Loss Measurement

The primary method uses a ring specimen with primary and secondary windings. A programmable power amplifier excites the primary winding while digital feedback maintains a sinusoidal flux density waveform in the core. An alternating-current B-H analyzer or digital wattmeter method records the induced voltage and magnetizing current, reconstructs the dynamic hysteresis loop, and computes the total loss from the loop area multiplied by the frequency. Measurements are typically performed at several spot frequencies covering the intended operating range.

Loss Separation and Extraction of Residual Loss

The separation procedure plots loss per cycle against frequency. The intercept at zero frequency gives the hysteresis loss per cycle, the frequency-squared term represents classical eddy-current loss, and the remainder is attributed to residual loss. For conductive laminations, the eddy-current term may be calculated analytically from material resistivity and sheet thickness, and the excess or residual contribution is modeled with a square-root-of-frequency dependence. For ferrites, empirical fitting of multi-term loss expressions over frequency and flux density is used to isolate the residual component. Cross-validation can be performed using calorimetric loss measurement at high frequencies where electrical methods lose accuracy.

Instrumentation and Test Conditions

Typical setups include wideband power amplifiers, digitizing B-H analyzers, impedance or LCR instruments for low-level characterization, and temperature-controlled chambers covering, for example, 25 degrees Celsius to 100 degrees Celsius or higher. Waveform control, winding resistance compensation, and air-flux correction are applied to minimize systematic error. Reportable conditions include frequency range, peak flux density, waveform factor, temperature, and specimen geometry.

Relevant Standards

Common reference frameworks include the IEC 60404 series for magnetic materials, such as IEC 60404-2 for the Epstein frame method, IEC 60404-3 for the single-sheet tester, and IEC 60404-6 for ring specimen measurement in the 20 hertz to 200 kilohertz range; the IEC 62044 series for measuring methods applicable to soft ferrites; ASTM standards such as ASTM A343 for wattmeter-based alternating-current magnetic measurements; and corresponding GB/T national standards. These standards define specimen preparation, winding procedures, measurement uncertainty, and reporting requirements.

Applications

• Qualification and grading of ferrite materials for switch-mode power supply transformers and output inductors
• Design verification of wireless power transfer coils and EV charging components operating at high frequency
• Thermal modeling and efficiency prediction for onboard chargers, DC-DC converters, and motor drive magnetics
• Material development and research into loss mechanisms in ferrites, amorphous alloys, nanocrystalline ribbons, and powder composites
• Incoming material inspection and quality control in core manufacturing and component assembly
• Generation of loss datasets and model coefficients for electromagnetic simulation and magnetics design software

FAQ

What is residual magnetic loss?

Residual magnetic loss is the part of total core loss that remains after subtracting the quasi-static hysteresis loss and the classical eddy-current loss. It arises from dynamic magnetization processes such as domain wall relaxation, magnetic aftereffects, and spin damping, and it becomes especially significant at high frequencies.

Why is residual loss important at high frequency?

In high-resistivity ferrites, eddy-current loss is very small, and above approximately ten to one hundred kilohertz the residual component can dominate the total loss. Ignoring it leads to underestimated heating, oversized or unreliable magnetics, and reduced converter efficiency.

How is residual loss separated from the other loss components?

The laboratory measures total loss at multiple frequencies, extracts the hysteresis term from the zero-frequency intercept of the loss-per-cycle curve, calculates or fits the eddy-current term, and determines the residual loss as the remainder. Regression against established loss models improves accuracy across the operating envelope.

What sample preparation is required?

Closed-circuit ring or core specimens with uniform cross section are preferred. The laboratory applies calibrated primary and secondary windings, records geometry and mass, and, where applicable, verifies that the specimen is in the intended annealed or stress-relieved state before testing.

Which standards govern the residual magnetic loss test?

The underlying total loss measurements are governed by the IEC 60404 series, the IEC 62044 series for soft ferrites, ASTM methods for alternating-current magnetic properties, and equivalent GB/T national standards. Loss separation itself is typically performed according to recognized physical models agreed between the laboratory and the client.