The following is a brief guide to some of the commonly used testing methods & standards for magnet and magnetic assemblies.
Flux Density Check
This test uses a gaussmeter to measure the flux density of a magnet at a determined distance. Typically the measurement is made at the magnet surface, or at the distance at which the flux will be used in the magnetic circuit. This test verifies that the magnet material will perform as predicted when the measurement matches the calculated values.
Helmholtz Coil Check for Br, Hc, BHmax and Orientation Angle
This test uses a Helmholtz coil to verify magnetic values. A Helmholtz coil is a set of coils placed at a determined distance with a known number of turns. When a permanent magnet of known volume is placed at the center of both coils, the magnetic flux of the magnet produces a current in the coils which can be related to a measurement of flux (Maxwells) based on the displacement and number of turns. By taking the displacement caused by the magnet, the magnet volume, the permeance coefficient and the recoil permeability of the magnet, we can determine values such as Br, Hc, BHmax and orientation angle for a given magnet.
Helmholtz coils can be used to determine the relationship between the mechanical zero axis and the magnetic zero axis. This is accomplished by placing the magnet in the coil and measuring the flux output in three different axes. The perfectly aligned magnet will have a maximum displacement in the orientation direction, and a null displacement in either of the other two directions. As a displacement is measured in the other two directions, these can be used to calculate the angle variation by vector summation using C2= SQRT(A2+B2).
B-H Curve Tracing
A permeameter can be used to trace the entire or a portion of the hysteresis loop of a material. A sample of the material to be tested is magnetized to saturation and then demagnetized while the flux output is measured and plotted in a closed circuit condition. The resulting curve is the B-H curve for the material.
A Search-Coil is a coil of known turns that is placed around a magnet. As the magnet is moved into and out of the coil, the lines of magnetic flux produced by the magnet produce a current in the coil, which are integrated in the flux meter to provide a reading of the total flux emanating from the magnet. Search coils are designed specifically for certain magnet geometries. They can be used to verify the saturation of magnets after the magnetizing process. Typical applications are motor and generator rotor assemblies.
Flux Maps, analogous to geographical contour maps, can be produced of a magnet array by scanning the array with a device equipped with a 3-axis Hall Effect probe, and plotting the resulting data as a flux contour map. The data can be presented in many other ways, but the contour plot is a useful way of visualizing the effect of the magnet array.
A pull test is used to determine the holding strength of a magnet assembly. This strength is directly related to the square of the flux density and the magnetic gap between the assembly and the surface to be adhered to. Although there are various methods that can be used to verify this value, Magnet Sales has developed a simple test stand that allows the magnet assembly to be pulled away from a 3 inch steel plate in a vertical motion. It is critical to the accuracy of the test to have the assembly pulled away evenly from the plate and not lifted from one end first.