National Physical Laboratory

Magnetic Materials

The measurement of the properties of magnetic materials can be made on a wide range of material types and geometries and for a large number of technologically important parameters. Standard measurement methods are continuously modified so that real world measurement conditions can be produced in the laboratory. An example is the application of stress to soft magnetic materials to reproduce the conditions experienced in high speed rotating machines. This generates material data that when used in the modelling of an electrical machine results in fewer model and prototype iterations and faster entry to market.

Magnetically soft materials

DC Measurements

The normal magnetisation curve, hysteresis loop, remanence and coercivity of bars, rods, rings and strips of ferromagnetic material can be determined in accordance with the methods of IEC 60404 Part 4. The principles of these methods have been modified to allow the measurement of the DC properties under operational levels of stress.

DC properties for ring cores can be determined up to 150 °C.

Figure showing how stress alters the DC hysteresis loss of various magnetic materials
Figure showing how stress alters the DC hysteresis loss of various magnetic materials. It can be seen that the material exhibiting the worse properties (largest loss value) with no stress applied has superior properties when the stress increases beyond around 250 MPa.


AC Measurements

The specific total loss, the specific apparent power and AC permeability of sheets, strips and ring cores of electrical steels and magnetic alloys can be measured in accordance with IEC 60404 Parts 2, 3 and 6.

Because the waveforms applied to magnetic materials in actual machines and devices can be very different from sinusoidal conditions, a digital approach is used to generate these waveforms. For waveforms like PWM, the number of harmonics can be large and the frequency of these harmonics will be several tens of kHz. By measuring the energy loss (the specific total loss) for these conditions, the performance of a device/machine using this material can be better predicted and subsequently optimised. The material selected for a particular application can also be better chosen by knowing the properties for the actual waveform conditions experienced during use.

AC properties for ring cores can be determined up to 150 °C.

Magnetically hard materials

The demagnetisation curve and from this the remanence, intrinsic coercivity, maximum energy product can be measured for a range of magnetically hard materials, including rare earth magnets, in accordance with IEC 60404 Part 5.

Figure showing the effect of reversal speed on the measured intrinsic coercivity of a NdFeB magnet
Figure showing the effect of reversal speed on the measured intrinsic coercivity of a NdFeB magnet. When using pulsed magnetic fields to determine DC properties, this behaviour is important and must be known to establish reliable results.


To determine the temperature dependence of the demagnetisation curve, to make measurements on thin magnets (<5 mm) and on non-standard shapes, a new measurement method is being established that will allow the full loop properties to be determined at the operating temperature in a few tens of milliseconds. Dynamical effects such as magnetic viscosity and eddy currents and magnetostatic corrections due to self-demagnetisation are corrected using the long standing expertise of the group in these measurements.

Modelling of magnetic systems

NPL use the finite element modelling software of Vector Fields to model magnetic systems.

The material data that is required by the software is measured using one of the NPL measurement methods or by modifying one of these to allow the application of the operational conditions.

Shielding of the magnetic field
To establish the shielding of the magnetic field from MRI machines that could be achieved with a single sheet, a finite element model was constructed, solved and analysed. From the figure above, the shielding factor was determined and compared with experiment. This helped a solution to shielding the increasing magnetic field used for MRI to be found.


Using the measurement methods introduced here and the Vector Fields FEM software it is possible to offer a complete solution to your magnetic system problems.

Reference materials

Low relative magnetic permeability (feebly magnetic) measurements and reference materials

Portable relative magnetic permeability measurement equipment used in the non-destructive testing of feebly magnetic materials exhibits a non-linear behaviour and must be regularly calibrated.

Reference materials with relative magnetic permeability in the range 1.002 to 1.6 are available.

Low relative magnetic permeability reference materials in the range 1.004 to 1.1 are made at NPL
Low relative magnetic permeability reference materials in the range 1.004 to 1.1 are made at NPL. Fixed values of 1.27 and 1.63 are also available. The relative magnetic permeability can be determined at magnetic fields in the range of 5 kA/m to 100 kA/m. The measurement method is UKAS accredited.


Bars, rods and strips of material can be tested in accordance with BS 5884 at field strengths up to 100 kA/m. The relative magnetic permeability of finished components can be measured using a permeability indicator. Volume susceptibility of less than 0.00001 can also be measured.

AC conductivity standard sets

Portable eddy current measurement equipment used in the non-destructive testing of materials must be calibrated at their operating frequency (typically 60 kHz).

NPL boxed sets of AC conductivity standards are available for purchase. They comprise up to eight square blocks of material, each 80 mm x 80 mm x 10 mm, having nominal conductivities of approximately: 59, 36, 29, 22, 19, 14, 9.6 and 2.2 MS/m (where 58.0 MS/m = 100.0% on the International Annealed Copper Scale, IACS).

Set of NPL AC conductivity standards covering the conductivity range 2.2 to 59 MS/m
Set of NPL AC conductivity standards covering the conductivity range 2.2 to 59 MS/m. The conductivity can be determined at frequencies in the range of 10 to 100 kHz. The measurement method is UKAS accredited.


People

Mike Hall

Last Updated: 13 Mar 2012
Created: 12 Mar 2012