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Friday, November 30, 2018

Hysteresis loop or B-H curve and Hysteresis loss


What is Hysteresis loop or B-H curve?

Hysteresis loop gives information about the magnetic properties of a material. By studying hysteresis loop all the magnetic properties related information of a material can be easily traced out.

In another word’s we can define Hysteresis loop as, “When a ferromagnetic material is magnetized in a one direction, it will not come back to zero magnetization when the applied magnetizing field is taken out. It must be driven back to zero by a magnetizing field in the opposite or reverse direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop (in the form of curve graph) called a hysteresis loop.

The absence of re-traceability of the magnetization curve (H) is the property called as hysteresis and it is associated with the presence of magnetic domains in the material.

A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). This is the reason it is also called as the B-H curve. Below figure shows an example of hysteresis loop;

Hysteresis loop or B-H curve
Hysteresis loop or B-H curve
Below points explains the Hysteresis loop or B-H curve;

  • The loop is produced by measuring the magnetic flux (B) of a ferromagnetic material when the applied magnetizing force is changed (H).
  • A ferromagnetic material which has been never before magnetized or demagnetized ferromagnetic material will trail the dashed line (see the figure) as magnetizing force (H) is increased.
  • The dashed line shows that, the larger the quantity of current applied (H+), the stronger the magnetic field in the component (B+).
  • At "a" point nearly all of the magnetic domains are aligned and an extra increase in the magnetizing force will generate very little increase in magnetic flux.
  • The magnetic saturation point has been reached for the material.
  • When magnetizing force (H) is decreased to zero, the curve will move or change from "a" point to "b" point.
  • At this point, we can notice that some magnetic flux leftovers in the material even though the magnetizing force (H) is zero. This is called as the point of retentivity on the graph and shows the remanence or level of remaining magnetism in the material. Some of the magnetic domains stay aligned, but some magnetic domains lose their alignment.
  • With the application of magnetizing force in reverse direction, the curve moves to "c" point, where the flux has been decreased to zero. This point is called as coercivity point on the curve or loop. The reversed magnetizing force has reversed plenty of the domains, so that the remaining flux within the material is zero.
  • To remove the residual magnetism from the material a force has to be apply, this required force is called as the coercive force or coercivity of the material.
  • In the negative direction when magnetizing force is increased, the material will become again magnetically saturated or material under goes in saturation, but in the opposite or reverse direction i.e. towards "d" point.
  • Decreasing magnetizing force (H) to zero brings the curve to "e" point. The available level of remaining magnetism is equal to that achieved in the other direction.
  • Increasing magnetizing force (H) back in the positive direction will return or bring back the magnetic flux (B) to zero.
  • We can notice that, the curve did not return back to the beginning or origin of the graph because some magnetizing force is needed to remove the remaining or residual magnetism.
  • Now the curve in the graph will take a diverse or different path from “f” point back to the saturation point, here at this point it with complete the loop.
Below image shows B-H curve measurement on Oscilloscope;
B-H curve
B-H curve measurement on Oscilloscope

Advantages of Hysteresis loop or B-H curve

The outcome of magnetic hysteresis loop shows;

  • The magnetisation process of a ferromagnetic core.
  • The part of the curve the ferromagnetic core is magnetised decides flux density because this depends on the circuits previous history which gives the core a form of “memory”.
  • Ferromagnetic materials have memory because they stay magnetised after the external magnetic field has been taken out.
  • Relays, solenoids and transformers can be easily magnetised and demagnetised because, they are made up of Soft ferromagnetic materials such as silicon steel or iron, which have very narrow magnetic hysteresis loops resulting in very small amounts of residual magnetism.
  • Residual magnetism can be overcome by a coercive force; energy which is in use is dissipated as heat in the magnetic material. This heat is known as hysteresis loss, the material’s value of coercive force decides the amount of loss.
  • A very small coercive force can be made that have a very narrow hysteresis loop by adding additive’s to the iron metal such as silicon. Magnetisation and demagnetisation of soft magnetic materials with narrow hysteresis loops are easy.
B-H curve for Soft and Hard Material
Hysteresis loop for Soft and Hard Material

Applications of Hysteresis

There are varieties of applications of the hysteresis in ferromagnets. Many of the applications make use of their capability to hold a memory; like magnetic tape, computer hard disks and debit cards - credit cards. In these applications, hard magnets which have high coercivity like chromium and iron are required so the memory is not easily removed. Let understand it in detail;

Because of presence of magnetic domains in the material the magnetization curve is not re-traceable (which is termed as hysteresis). After re-orientation of magnetic domains, it will take some magnetizing field or energy to turn them back again. This characteristic of ferromagnetic materials is useful as a magnetic "memory". Some configurations of ferromagnetic materials will maintain a forced magnetization forever and are useful as "permanent magnets". The magnetic memory features of chromium and iron oxides are useful in audio tape recording and also for the magnetic storage of data on computer hard disks.

Soft magnets which have low coercivity for example iron oxide is used for the ferrite cores in electromagnets. The low coercivity decreases that energy loss related with hysteresis. This low energy loss at the time of hysteresis loop is the main reason of using soft iron for electric motors and transformer cores.

Hysteresis loss

As current flows in the forward and reverse directions the magnetization and demagnetization of the core happens which result in Hysteresis loss.

When we apply external magnetizing force to a material and as we increase the magnetizing force (current), the magnetic flux also increases, but when the magnetizing force (current) is decreased, the magnetic flux decreases gradually and not at the same rate. So, when the magnetizing force touches zero, the flux density didn’t come to zero and still has a positive value. Now the magnetizing force must be applied in the negative direction so the flux density reaches zero.

The link between the magnetizing force (H) and the magnetic flux density (B) is shown on a hysteresis loop or curve. The energy required for completing a full cycle of magnetizing and de-magnetizing is shown by the area of the hysteresis loop. Also this area of the loop characterizes the energy lost during this magnetisation process.

Below is the equation for hysteresis loss;
Where;

Pb = hysteresis loss (W)

η = Steinmetz hysteresis coefficient, depending on material (J/m³)

Bmax = maximum flux density (Wb/m²)

n = Steinmetz exponent, ranges from 1.5 to 2.5, depending on material

f = frequency of magnetic reversals per second (Hz)

V = volume of magnetic material (m³)

The hysteresis loss results in wasted energy which is proportional to the area of the magnetic hysteresis loop.

In AC transformers, hysteresis loss is always a problem where the current is continually changing the flow of direction and by this the magnetic poles continually flows in reverse direction and causes loss in the core.

Conclusion

Hysteresis loop provides information about the magnetic properties of a material. It is important that the B-H hysteresis loop is as small as possible so loss will be less because shape of B-H curve decides the loss. Bigger the area then more is the loss and vice-versa. The shape of hysteresis loop depends upon the nature of the material used i.e. iron or steel.

More details on Ferrite you can find in my previous blog;

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Hi, welcome to my blog, “Power Electronics Talks”.

I am Alok Pandey, an Electronics Engineer. I am passionate about Power Electronics and latest Technology. By profession I am design and application engineer and play with circuits.

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