Measurement of magnetic properties

Complete parameters measurement

The main magnetic characteristics of permanent magnets are called intrinzic (ie internal). This means, that these characteristcics do not show themselves outwardly, such as temperature or weight, but must be measured in a particular system. Changes to this system due to the measured magnet subsequently correspond to its magnetic parameters. In a similar way f.i. electrical resistance is measured. Measurement of magnetic properties, however, requires disproportionately complicated apparatus. To measure the full characterization is used so called hysterezigraph. In principle we talk about electromagnetic circuit with an air gap, into which the measured magnet is inserted. The control computer continuously changes the current by electromagnet coil and also measures the intensity of the magnetic field H and the magnetic induction in the measured magnet B.


From the recorded course of B vs. H the hysteresis curve is obtained, from this curve are subtracted landmarks - coercivity, remanence, and maximum product - see chapter Terms in magnetism.
Hysteresis curve may be measured at different temperatures . You can obtain information on the thermal behavior of the magnet.
For more information see standard IEC 404-5 (1982) magnetic material Part 5: Test measure for hard magnet (permanent magnet) material properties.

Measurement of the relative magnitudes

Nevertheless, we can find out relevant data about the quality of the magnet by simpler methods. We talk about measurement of the surface magnetic field and magnetic flux by Webermeter. In the first case, we mostly talk about measurements with Hall probes, in the second case, we mean a measuring coil passed through by magnet itself.
An important condition for the accuracy of such measurements is comparison with the ethanol of the same type and size of the magnet, which has a known background characteristics (measured by hysterezigraph).

Measurement of dimensions

Measurement of dimensions
Dimensions of permanent magnets is measured by conventional lenghts gauges: a caliper or micrometer. Currently there are many instruments with digital display. It is necessary to verify that they are not sensitive to the presence of the magnetic field.
In case of more complex shapes in some cases it is necesary to meassure their dimensions and geometry (parallelism, concentricity, etc.) by special three-dimensional measuring devices with computer analysis.

Measurement of holding forces

Holding force of permanent magnets to ferromagnetic materials is very often used, especially to iron and its alloys.
Very often the question about holding force of the magnets emerges. It should be emphasized that this force depends on several factors. The first is the actual shape and material of the magnet, in addition to it, also material which is attracted by the magnet, its thickness and surface finish. Holding force changes very significantly with distance from the pad - see chart.

The graph of the holding force of the magnet distance

If all conditions are defined, the holding force can be measured using a tensile load cell - see picture.

Schema load cell

By default, this power is measured and reported as a parameter to the magnetic holders. It is a magnet with a metal shell, which encloses the magnetic circuit.
The holding force of magnets themselves can be measured on request request, taking into account the fact, that different conditions may vary this parameter.

Temperature tests

Various kinds of magnets have different heat resistance. Exceeding the temperature limits leads to a gradual loss of magnetization. This transition consists of reversible and irreversible components and do not have a sharp boundary - see the expressions on magnetism - Temperature and time characteristics. The thermal resistance shows the basic measurement of hysteresis loops depending on the temperature - see Measurement of magnetic properties. For easy monitoring of thermal resistance is used the surface magnetic induction and magnetic flux measurement. The magnet is subjected to a temperature cycle to ... tk ... to, where to is room temperature and tested tk is the critical temperature. Exposure time tk is governed by the size of the magnet, at least 20 min. Temperature values before and after cycle are compared. The maximum allowable drop is 5%.
Temperature behavior of magnets is affected by the presence of magnetic field. At the favorable orientation with the vector of internal magnetization effects of temperature to some extent diminish, the opposite orientation on a contrary causes increases. This plays an important role in testing kits such as magnets in electric motors. In these cases, the heat resistance test must be conducted on the assembly similar to actual use.

Corrosion tests

The various types of magnets significantly differ in their resistance to corrosion - see picture.

Sequence of corrosion resistance

To improve the corrosion resistance especially of FeNdB magnets different finishes are used.
For testing the corrosion resistance there are two basic tests: salt spray (Salt Spray Test - ISO 9227:2006) and vapor pressure test (Pressure Cooker Test, Autoclave Test - IEC68-2-66). Tests are carried out in a closed corrosive chamber at the following conditions:

Salt spray test

Temperature: 35 oC (±2 oC)
Concentration: 5%±0.1% NaCl
pH: 6,6 – 7,2
Exposure time: 24 - 72 hours depending on the type of finish (see finish)
Evaluation criterion: Sample with no visible signs of corrosion

Test in a pressure steam

Temperature: 125°C (±5°C)
Relative humidity: 100 %
Pressure: 270 kPa (absolute)
Exposure time: 7 days
Evaluation criterion: Loss of sample up to 5 mg/cm2.


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