What is Hysteresis Loop ?

Magnetic Hysteresis or Hysteresis loop is very important topic to understand Machine in Electrical Engineering, included in the basic of Electromagnetism. You can learn about a great deal of information can be learned about the magnetic properties of a material by studying its hysteresis characteristics.

What is Magnetic Hysteresis or Hysteresis Loop?

Magnetic Hysteresis, also known as Hysteresis Loop is a representation of magnetic flux density(B) versus magnetizing force(H) of ferromagnetic material. It is often referred to as B-H curve.

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

Hysteresis Loop is a four quadrant curve from where the hysteresis loss, coercive force and retentivity  of a material are obtained. An example of hysteresis Loop is shown below - 

The curvature of hysteresis is characteristic of the type of magnetic material and can be vary in size (i.e narrow or wide). This loop is formed by measuring the flux density(B) of a ferromagnetic material while the magnetizing force(H) is varying.

To understand Hysteresis loop or B-H curve , take a magnetic material as a core around which insulated wire is wounded. and that coil or wire is connected to a DC supply through a rheostat to vary the current I.

As Magnetizing force(H) is directly proportional to the current, I i.e Magnetizing force(H) also varies.

                                                         H=(N*I)/l

 N= No of turns of the coil, l = effective length of the coil. 

• Step -1 :

When supply current,I=0 then Magnetic Flux Density(B) and Magnetizing Force(H) doesn't   exist. which is shown as a point O. Then Magnetic dipoles are disoriented and the magnetic material portrays paramagnetism.

• Step - 2 :

When current is gradually increased from zero value, magnetizing force(H) and flux density(B) is adding to the material and follows the pathway (o-a) upto saturation point. So for a certain value of current the flux density(B) becomes maximum(Bₘ). The point indicates maximum flux density is called magnetic Saturation point. At this point all magnetic dipoles are oriented in the direction of magnetizing force(Hₘ).  This curve also called as virginal curve.  

   

                     

• Step - 3 :

From saturating point if you decrease the current , magnetizing force(H) is decreased along with decrement of  flux density(B) not following the previous path rather following the curve a-b .

• Step - 4 :

At point 'b' indicates H=0 and I=0 but some remnant magnetization remains. This lagging of B behind H is termed Hysteresis. point 'b' explains that after removing the magnetizing force (H), magnetism property with little value remains in the material and it is known as residual magnetism (Br) , and the value of residual flux density due to retentivity of the material indicated by o-b and the point 'b' is called as retentivity point.

• Step -5 :

If the current is reversed, the dirction of H also gets reversed. In reverse direction H increase following path b-c, decreases the value of residual magnetism (Br). and a certain negative value of H the residual value(Br) will be zero, at the point 'c'. This negative value of H at point 'c' is called coercive force(Hc). That means to remove the remnant magnetization, a coercive magnetizing force is applied in the reverse direction. 

So, The point in which flux density(B)=0 due to cancellation of dipoles moment acting in opposite directions is called as Coercivity point(-Hc).

• Step - 6 :

In negative direction if we increased H more, the flux density (B) follows path c-d. and at point 'd' magnetic saturation takes place in reverse direction with respect to previous case. At point 'd', B and H has maximum value (i.e. -Bₘ and -Hm) in opposite direction.

• Step -7 :

From point 'd' if we decrease the value of 'H' towards zero , again B decrease following the path d-e. At point 'e' there is a residual magnetism(-Br) in opposite direction with respect to previous case.

• Step - 8 :

If again the direction of current, I is reversed, the direction of H again reversed then the residual flux density (-Br) gets zero at point 'f' following the path e-f. Further increment of H, the value of B from zero to its maximum value or saturation point following f-a. 

The curve never go back to origin where B=0 and H=0. In order to rake up this point(0,0) the material will need to be demagnetized  (i.e. return to having paramagnetic behavior) by hitting the material against a surface , reversing the direction of H, or heating at Neel Temperature.

The path a-b-c-d-e-f-a is called Hysteresis Loop.

• Definition of Retentivity -

It's a measure of residual magnetic flux density corresponding to the saturation induction of a material. Its also defined as the degree to which the magnetic material gains its magnetism after magnetizing force is reduced to zero.

• Definition of Residual Flux or Residual Magnetism -

Residual Flux is certain amount of magnetic flux that remains in the material without presence of magnetizing force(H).

 Note :- The residual magnetism and retentivity are same when the material is magnetized to the saturation point. However, when magnetizing force did not reach the saturation level , the value of residual magnetism is lower than the retentivity value.

• Definition of Coercive Force - 

To make magnetic flux is zero, the amount of magnetizing force applied is called Coercive Force. Coercive Force is negative.
Note - The high coercivity of a material symbolizes that the memory(residual magnetism) is not easily erasable. 

Importance of Hysteresis Loop :

1. To construct electric devices that are subjected to rapid magnetism reversal or require memory storage, Hysteresis Loop is most important. Hysteresis loop is important in the memory capacity devices for audio recording or storage of data in computer disks.
2. Smaller hysteresis loop area means less hysteresis loss.
3. Hysteresis Loop helps to choose a perfect material to make permanent magnet as core of machine by providing the value of retentivity and coercivity of a material.
4. As residual magnetism can be determined from B-H curve so choosing of magnetic material is easy.