What is Hysteresis Loop? Its Importance

A hysteresis loop is a graph in a four quadrant that shows the relationship between the induced magnetic flux density B and the magnetizing force H. We call this loop curve as the B-H loop as well. It is possible to calculate the the retentivity, residual magnetism, coercive force, permeability, and the reluctance from the hysteresis loop (B-H curve).

On application of alternating magnetic field to the material, the material magnetizes. The magnetization traces out a loop called a Hysteresis loop. The hysteresis loop is a four quadrant B-H graph. We can find the Hysteresis loss, retentivity and coercive force  using the hysteresis loop.  

Let us take the completely demagnetized ferromagnetic material. The completely demagnetized ferromagnetic material has B=H=0. N number of turns coil is wound on the ferromagnetic material. The coil receives DC supply. A variable resistance varies the current.

The magnetizing force H is proportional to the current and the number of turns.

H = NI/l
Where,  
N = Number of turns of coil
 = Effective Length of the coil

The magnetic flux density B always lags the magnetizing force H. This property of the magnetic material is known as hysteresis of a magnetic material.

Coercive Force

Coercivity is a measure of ability of the ferromagnetic material to remain demagnetized under external magnetic field. The coercive force is the opposite force that must be applied to the  magnetized material to completely demagnetize it. With application of negative magnetizing force(-H), the residual magnetic flux becomes zero. This negative magnetizing force is equal to the coercive force of the magnetic material.

Residual Flux Density

The magnetic material has certain value of magnetic flux per unit area when the magnetizing force H is zero.

Retenitivity Flux Density

Retentivity is the ability of the magnetic material to retain certain amount of magnetic field after removing of magnetizing force after achieving saturation.

What is a Hysteresis Loop?

Definition of Hysteresis

Hysteresis is the property of a magnetic material, and by this virtue the flux density (B) of this material lags behind the magnetizing force (H).

The hysteresis loop or B-H curve of the magnetic material is as given below.

Hysteresis loop
Step 1:

The origin point (o) of the B-H curve is where the both axis meet. At this point the current I=0 and magnetic field density B=0.

Step 2:

When we increase the current from zero to a particular value the magnetizing force(H) and magnetic field(B) sets up. The path followed by B-H curve is o-a.

Step 3:

The point ‘a’ is called saturation point on the B-H curve. At this point if we increase the current further, the magnetic flux density does not increase because all the magnetic poles had already perfectly aligned and there is no scope of further magnetization of the magnetic material. At the point ‘B’ the flux density is maximum(Bm) on the B-H curve. It is the magnetic saturation point.

Step 4:

Now, if we decrease the current, the magnetizing force(H) decreases and correspondingly magnetic flux density(B) decrease.However, the B-H curve  does not follow the previous path(o-a). It follow the path o-b path.

Step 5:

The point ‘b’ indicates that the H=0 but the magnetic flux density has positive value.This lagging of B behind H is called the hysteresis. The point ‘b’ shows that after removing of the current the magnetic material has magnetic flux density(B) and its magnitude is equal to ob. The residual flux o-b is due to retentivity of the magnetic material.

Step 6:

If the direction of the current is reversed the direction of the magnetizing force gets reversed.When the current is increased in the reverse direction the magnetizing force(-Hc) first bring the residual flux(Br) down to zero is called the coercive force.  

Step 7:

With increase in the current in the negative direction the magnetizing force increase in the negative direction and the magnetic flux density(B) also increase in the negative direction. The B-H curve follow the path c-f. The magnetic flux density does not increase beyond point f is called the saturation point in the reverse direction. At point f the B and H attains its maximum value in the reverse direction.

Step 8:

With decrease of the current in the negative direction the B-H curve follows f-g path and the magnetizing force attains zero value when current is zero but the magnetic material has the residual magnetic flux density(-Br). The o-g is the residual flux in the reverse direction.

Step 9:

If the direction of  H is again reversed by reversing the current the residual magnetism (-Br) reduce to zero at point d following the path g-d. If the current is increased the B-H curve follows the path d-a.
Thus the path  a-b-c-f-g-d-a forms a hysteresis loop. 

Importance of Hysteresis Loop

  1. Small area of hysteresis loop means less hysteresis loss.
  2. Residual magnetism,retentivity & coercivity can be calculated with hysteresis loop.

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