ELECTROMAGNETISM – PART – 06 - IMPORTANCE AND FACTORS AFFECTING HYSTERESIS LOOP

FERRITE

Magnetic materials are classified as

(i) Ferromagnetic material and (ii) Ferrimagnetic material.

Ferromagnetic material – Iron and its various alloys

Hard ferromagnetic material – permanent magnetic materials such as alnicos, chrome steels, certain copper-nickel alloys

Ferrimagnetic materials – mixed oxides of iron and other metals.

The oxide mixture is sintered i.e. heated to a steady temperature of 1300 degree centigrade which is maintained for several hours. The material is known as ‘ferrite’ is chemically homogenous and extremely hard.

Ferrite has typically maximum flux density of 0.3 to 05 T, as compared to 2.18 T for pure iron.

HYSTERESIS

The word hysteresis means lagging behind. The curve gets its name from the fact that the flux density (B) lags behind the magnetic flux intensity (H). B is the cause -  H is the effect.

The lagging flux density (B) behind the magnetizing force (H) in a magnetic material subjected to cycles of magnetization is known as magnetic hysteresis. 

HYSTERESIS LOOP

When a magnetic material is subjected to one cycle of magnetization, B always lags behind H so that the resultant B-H curve forms a closed loop, called hysteresis loop.

B-H CURVE

1. The dotted curve passing through tips of the hysteresis loops is the normal magnetization curve or B-H curve of the material.

2. In a B-H curve, the value flux density (B) at H is equal to zero is known as the residual flux density (B_r). [OR] When a field strength is reduced to zero the core is not completely demagnetized. There still remains a certain flux, called remanent flux density or residual flux density.

3. This remaining flux density in the core when H fell from the saturation value H to zero is called remanence of the core material.

4. The value of H to reduce flux density B_r to zero is called coercive force H_C.

5. The maximum possible value of residual flux (B_r) corresponding to deep saturation is known as RETENTIVITY and the maximum value of Ho is the COERCIVITY.  

FACTORS AFFECTING THE SIZE AND SHAPE OF THE HYSTERESIS LOOP

1. If the material is easily magnetized, the loop will be narrow.

2. If the material does not get magnetized easily, the loop will be wide.

3. Different materials will saturate at different values ‘B’ thus affecting the height of the loop.

4. The size and shape of the hysterias loop depend upon the nature of the material.

5. The size and shape of the loop also depend upon the initial state of the specimen.

IMPORTANCE OF HYSTERESIS LOOP  

1. Silicon steel has less hysteresis loop area. Due to less area, the hysteresis loss is less. Hence, it is widely used for making transformer cores and rotating machines.

2. Hard steel is large hysteresis loop area hence which has high retentivity and coercivity. Due to the large area of the loop, there is greater hysteresis loss. Hence, it is suitable for making permanent magnets.

3. Wrought iron has fairly good residual magnetism and coercivity. Hence, it is suitable for making cores of electromagnets.

4. Ferrite material is known as magnetic ceramic has square hysteresis loop. Hence it is suitable for switching circuits, as storage elements in computers, and in a special type of transformers in electronic circuits.

5. The magnetization curves for different ferromagnetic materials are shown in the figure. For economic reasons, magnetic circuits are designed with magnetic materials in a slightly saturated state.