The high attractive force, proportional to the magnetic flux density squared, between a moving magnet and its surrounding structure creates an instability in that structure. This instability, usually ignored in low flux density devices, limits the design in many high flux machines. In disk file spindle motors, a design with neodymium magnets can be 50% smaller and lighter that an equivalent output ferrite magnet design which has 1/3 the flux density. However, the higher attractive force of the neodymium is an excellent driver of the motor structure. This problem would be most severe when any harmonic of the driving frequencies match a resonant frequency of the motor or surrounding structures. Vibration levels have been shown to be affected by shaping of the magnetic field through shaping of the magnets or magnetic structure.

Normal distribution of a magnetic flux density. The diamagnetic wedge area is bypassed by the magnetic flux. Non-classic magnetic saturated flux density. The diamagnetic wedge area is crossed by the magnetic flux. The residual moisture in the resin is boiled by the flux energy (via eddy currents) and the winding disintegrate.This wedge part as indicated in figure 1 was composed of a two-component material (resin and fibre), which contained approximately 10% residual moisture. In figure 1b the magnetic flux left the soft iron, crossed the wedge and heated up the mixture of resin, fibre and water. First the water boiled, than the resin melted and consequently the remaining soft fibre frame alone was unable to retain the massive copper windings of the rotor. Because the rotor was spinning at 3000 rpm, the generator was destroyed with the consequent risk to the operators. The reason for the damage was the poor design of the generator, which allowed magnetic flux densities about 2,2 Tesla to occur, where the saturation limit in soft iron is 2,16 [1]. The magnetic flux density phenomenon is also theme of the experiment described in this paper. Here, a travelling magnetic field at a constant speed affects a soft iron washer.