Test Methods and Procedures for Permanent Magnet Materials

Permanent magnet materials must undergo rigorous testing to ensure their performance, consistency, and suitability for industrial applications. Several standardized methods exist to evaluate flux density, total flux, and magnetic hysteresis characteristics. Below is an overview of the most widely used testing techniques.

Flux Density Test (Gaussmeter)

Flux density can be measured on the surface of a magnet or at a specific distance using a Gaussmeter equipped with either an axial or transverse probe.

• How it works: The probe contains a Hall Effect sensor that generates an output voltage proportional to the magnetic flux density.
• Accuracy considerations: Since measurements are highly position-sensitive, probes must be placed at the exact same location for each test. Brass fixtures are typically used to maintain consistent positioning.
• Calibration: A Zero Gauss chamber or a magnet sample with a known flux density is required for calibration.

Note: The Gaussmeter provides surface flux density in Gauss or Tesla, which differs from the intrinsic residual induction (Br) values found in material specifications.

Total Flux Test (Fluxmeter with Helmholtz Coil)

The total magnetic flux of a magnet can be measured using a Fluxmeter connected to a Helmholtz coil.

• Coil setup: A Helmholtz coil consists of two identical coils spaced apart by their radius. To achieve accurate measurements, the coil diameter should be at least three times larger than the largest magnet dimension.
• Procedure: The magnet is placed at the midpoint between the coils and then withdrawn, ensuring its flux lines cross perpendicularly through the coil. The induced voltage is captured by the Fluxmeter, providing data in Webers, Maxwells, or milliVolt-seconds (mVs).
• Modern advantage: Newer Fluxmeters allow users to input magnet volume to directly calculate Br and related values in Tesla.

Important operating steps:

• Fluxmeters must be de-drifted (a process that may take 10–30 minutes).
• The magnet orientation must remain perpendicular to the coil plane.
• The coil should be positioned away from external magnetic interference or metal surfaces.

Hysteresis Test (Permeameter)

The Hysteresis Test, also known as the B-H curve test, measures intrinsic and normal magnetic properties, typically at different temperatures.

• Equipment: A DC magnetizer and a Fluxmeter connected to a search coil wound around the sample.
• Sample preparation: The magnet material must be machined into precise shapes (commonly cubes) for accurate results.
• Test process: The magnet is cycled through:
  1. Origin to saturation
  2. Demagnetization
  3. Reverse saturation
  4. Return to original saturation

The recorded data generates a B-H curve (hysteresis curve), showing parameters such as:

• Br: Residual Induction
• Hcb: Coercive Force (normal)
• Hcj: Intrinsic Coercivity
• BHmax: Maximum Energy Product

This test provides critical performance data but is costly and time-consuming, making it impractical for high-volume testing of finished parts. Instead,magnet manufacturers typically supply one B-H curve per lot of magnets.

Conclusion

Testing permanent magnet materials is vital for ensuring product reliability in industries such as electronics, automotive, and renewable energy. While the Gaussmeter test provides localized flux density, the Helmholtz coil with Fluxmeter measures total flux output, and the Hysteresis test delivers deep insights into intrinsic material properties. Together, these methods provide a comprehensive evaluation of magnet performance, supporting quality control and application-specific design.