B-H curve for magnetic material: The B-H curve is the graphical representation of relation between flux density (B) and applied field strength (H), with H plotted on the x-axis and B plotted on the y-axis. Typical B-H curve is as shown in figure below:
The B-H curve can be described by dividing it into 3 regions.
Region OA: For zero current, H = 0 and B is also zero. The flux density B then increases gradually as the value of H is increased. However B changes slowly in this region.
Region AB: In this region, for small change in H, there is large change in B. The B-H curve is almost linear in this region.
Region beyond B: After point B, the change in B is small even for a large change in H. Finally, the B-H curve will tend to be parallel to X axis. This region is called as saturation region.
Concept of leakage flux, useful flux and fringing:
Leakage flux: Some flux while passing through the magnetic circuit, leaks through the air surrounding the core. This flux is called as leakage flux.
Useful flux:- The flux in the air gap which is actually utilized for various purposes depending upon the application is called as useful flux
Fringing: When the magnetic flux passing or crossing an air gap tends to bulge outwards the iron ring, this effect is called as “Fringing”.