The following properties are important when a magnetic material is used as a core.
Permeability is given by the flux density B and the magnetizing force H by the following relation
If the permeability of the material is high sufficient magnetic flux density can be produced with the lower value of the magnetizing force.pemeability of the material is not constant. It varies with H.Initially with the increase in H, its value increases. It becomes maximum at a certain value of H.Therefore there is no increase in the flux density with the increase in H and the material is said to be in saturation.
As shown in below figure after C point there is a small increase in B with more increase in H.This means that the material is approaching saturation. After the saturation zone, the material is fully saturated. There will be practically no increase in the flux density B.Value of the permeability at saturation is very small or zero. Therefore the inductance is reduced. Hence the design of the magnetic circuit should be made such that the core is not driven into saturation.
The electrical resistivity of the core material does not play a direct role as it has not to carry electric current. But when the core is subjected to alternating flux, The eddy currents are induced in the core. This gives rise to the eddy current loss. So if the resistivity of the core is high the values of the eddy current induced in the core is reduced. Therefore the eddy current loss is reduced to decrease further the value of the eddy currents the core used is laminated instead of solid.
As per the hysteresis loop when the magnetizing force H is increased there is an increase in the flux density B.At some point flux density is maximum and it is Bm . Now if we reduce H, B is reduced and when H becomes zero as the residual magnetism or retentivity of the material.
If we reverse the direction of the magnetizing force H the material will be magnetized in the reverse direction. To gain reverse direction the residual flux density is reduced. Then at some point, it will become zero. This point is known as the coercivity force. Hence coercive force is the magnetizing force necessary to cancel the residual flux.
Inverse of the permeability is called the reluctivity.
∴R=1/µ is R=H/B
It is the property of the material to oppose the magnetic flux through it. It is similar to resistance in an electric circuit.
OR S= l/µ0µrA. Unit of reluctance is AT/Wb
Product of the maximum flux density Bm and the maximum magnetizing force Hm is called the energy product. This is an important term for magnetic materials. This energy is given to the magnetic material to import magnetism or to be taken out from is to demagnetize it.
The temperature at which the magnetic material loses its magnetic properties is called the curic temperature. It is denoted by Tc.
This temperature is different for different magnetic materials.To determine curic temperature the temperature is increased keeping the magnetizing force H constant.Due to the increase in temperature.there is a decrease in permeability. At some temperature, the permeability becomes 1. This temperature is called the Curic temperature.