Pole Spacing Effects on Magnetization
Magnet pole spacing affects the uniformity and magnitude of the flux density in the pipe. The effects of pole spacing are closely tied to those of velocity. Geometric constraints for passing bends define the maximum length of the magnetizing assembly, which sets the maximum backing bar length or pole spacing.
The need for a uniform magnetic field through the pipe wall defines the minimum pole spacing. Nonuniformity makes analysis of inspection signals difficult. The level of nonuniformity in the applied field increases for high magnetization levels and for short pole spacings. The level of nonuniformity is strongly influenced by the velocity of the tool.
Long pole spacings are generally desirable because they produce a uniform field through the pipe wall. A long pole spacing can decrease the applied field, though, because long pole spacings increase the magnetic reluctance. Reluctance is a measure of the magnetic resistance of a system. Reluctance is a function of cross-sectional area, pole spacing, and material permeability. The magnetic reluctance increases as the length between poles increases. Therefore, stronger magnets are required to achieve saturation as the pole spacing increases.