Draw B-H curve for magnetic material and state its nature. State the significance of hysteresis loop. Also draw the hysteresis loop for hard steel and soft steel. 
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B-H Curve for Magnetic Material:

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Nature of B-H Curve: The B-H curve is concave up for low flux desities upto point A, for medium flux densities, it becomes straight (AB), for heigher flux densities curve concaves down (after point B) Then almsot becomes flat i.e. saturation occures ultimetely it is nonlinear curve. 


Significance of Hysteresis Loop: The shape and size of the hysteresis loop largely depends on the nature of the magnetic material. The choice of a magnetic material required for a particular application often depends on the shape and size of the hysteresis loop. The smaller the hysteresis loop area of a magnetic material, the less is the hysteresis loss. For example, the hysteresis loop area for silicon steel has very small, for this reason silicon steel is widely used for manufacturing of transformer cores and rotating machines which are subjected to rapid reversals of magnetism. The hysteresis loop for Hard Steel (large hysteresis loop area) indicates that the material has high retentivity and coercivity. Therefore hard steel is quite useful in making permanent magnets. But due to large area hysteresis loss is quite high. This is the reason hard steel is not used for construction of electrical machines. Hysteresis Loop for Hard Steel and Soft Steel:

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The phenomenon of magnetisation and demagnitisation of ferromagnetic material is known as hysterisis. It is observed that as the electric field increases magnetic filed(H) increases and therefore magnetic flux density (B) also increases, but when it decreases, B does not decreases at the same rate at which it was increased. The magnetic material does not get demagnetised it retains some magnetisation this is hystersis. As magnetic field (H) increases, the magnetic flux density (B) too increaes, but B stop increasing and reaches saturation. The curve OP is saturation curve when it decreases the curve doesnot follow the path, it follows different path PQ. That means rate of decrease of B is not same as rate of increase of B. When magnetic fleld (N) reaches zero H=0, that means B should be zero but B≠0, that means material does not get demagnatised there is some residual magnetism i.e. OQ (graph is Remanent flux density Br. When H is increased in reverse direction B also increases in reverse direction and again get saturated. The magnetic flux density (B) becomes zero when reverse magnetic force is applied that is called coercitivity (OR) graph.
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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: 


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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:

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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”.

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