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Thursday, 1 August 2019

Hysteresis Loop of Magnetic Material

Materials in which magnetisation can be induced are called magnetic material. In design practice these materials are tailored to optimise a specific device function performed under certain operating conditions like frequency, power level, temperature range etc. For example, in transformers and rotating machines, these materials have to serve as a medium for conversion or transformation of energies from one form to another. In the field of electronics the magnetic materials have to respond to varied frequency conditions. And so selection of proper magnetic material with proper composition of alloys to suit the requirements is of prime importance. A detailed understanding of types and behaviour of the magnetic material is essential before t13$ choice is decided for the usage of material for the magnetic function, may it be in electrical or electronics.

Residual Magnetism : 

Residual magnetism is defined as the magnetic flux density which still remains in a magnetic material even when the magnetising force is completely removed. It is also called 'Remanance', and is measured in webers / metre2.

Retentivity :

The power of retaining magnetism even after the inducing magnet is removed is called magnetic retentivity of the material. Steel possess this property while soft iron does not possess. It is the ratio of the residual flux density (Br) to the maximum flux density (Bm).

Retentivity = Br/Bm

In the sample saturation loop shown in Figure, the retentivity of the magnetic material can be obtained by actually taking the maximum value of Br and Bm.


In Figure, coercity is given by

Coercity = Hc/Hm

Hysteresis Loop :

When B is increased gradually from O, causes a saturation in the flux density i.e., any further increase in H, does not cause any increase in B, this is called saturation. The saturation value of flux density depends on the chemical composition and temperature. When the external field is reduced gradually, the B-H curve does not trace AO, and when the field is reduced to zero, there is still some residual magnetism 'OC'. Now if the external magnetic field is reversed to 'OD', the residual magnetism comes to zero. 'OD' is called coercive force, further increase of 'H' in this direction increases 'B' in the reverse direction upto the saturation point 'E'. The residual magnetism in the reverse direction of OF to neutralise this 'H' must be increased in the positive direction upto 'OC'.

Thus is evident that 'B' always lags behind 'H'. This property of lagging of 'B' behind 'H' is called Hysteresis. The cycle of operation to create the curve is ACDEFCA. which gives one hysteresis loop. Different materials have different shapes of hysteresis loops as shown in Figure.
Hysteresis Loop of different Magnetic Materials

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