BH curve shows the graph only for increase in current, while if we plot the graph for increase in current and decrease in current then we get the loop this loop is called as hysteresis loop.
When we give current through the coil then B and H starts to rise from zero.
When we decrease the current the coil get demagnetize. The magnetization and demagnetization cycle is called as hysteresis.
But we observe from the loop that point 2 is not at zero position even the current is reduced to zero. It means it have some residual magnetism in material. This property is called as retentivity. (i.e. 0-2)
Now if we further increase the current in reverse direction by changing the polarities of source, the core will magnetize in reverse direction.
So the value of H required to wipe out residual B is called coercive force.
Now if we further increase the current in reverse direction the core will get saturated in reverse direction, the flux density is maximum at saturation level. After saturation level we cannot further increase the magnetic flux density.
The relation between magnetic field strength (H) and current (I) is H = NI/l, means magnetic field strength is directly proportional to current.
The relation between magnetic flux density (B) and voltage (V) the magnetic flux density is directly proportional to voltage.
The ratio of magnetic flux density and magnetic field strength is called as permeability.
The BH curve and hysteresis loop are useful for making magnetic calculations.
The material with less hysteresis loop is preferred for Electrical machines and Transformers.
For hard material hysteresis loop is thick or fat. For soft material hysteresis loop is thin. Hysteresis waste the energy in the form of heat and decreases the efficiency of system. The smaller hysteresis loop have smaller hysteresis losses.