Fundamental Electrical and Electronic Principles, Third Edition

Magnetic Fields and Circuits 123However, as the coil magnetisation current is slowly increased, sothe molecular magnets start to rotate towards a particular orientation.This results in a certain degree of polarisation of the material, asshown in Fig. 4.12 . As the coil current continues to be increased, sothe molecular magnets continue to become more aligned. Eventually,the coil current will be sufficient to produce complete alignment. Thismeans that the flux will have reached its maximum possible value.Further increase of the current will produce no further increase offlux. The material is then said to have reached magnetic saturation, asillustrated in Fig. 4.13 .un-magnetisedpartially magnetisedFig. 4.11Fig. 4.12saturationFig. 4.13Typical magnetisation curves for air and a magnetic material are shownin Fig. 4.14. Note that the flux density produced for a given value of His very much greater in the magnetic material. The slope of the graph isB / H μ 0 μ r , and this slope varies. Since μ 0 is a constant, then the valueof μ r for the magnetic material must vary as the slope of the graphvaries.B(T)magnetic materialair0H (At/m)Fig. 4.14