What is Hysteresis loop? What is Hysteresis Loss?

Understanding Hysteresis loop, Hysteresis Loss

In this post let us see what is meant by hysteresis loop and what is meant by hysteresis loss....
Before that it is highly recommended to read about terms related to magnetic circuits.
Please click here to know more about the Magnetic Circuit Terminology

What is Hysteresis loop?

Consider a completely demagnetized ferromagnetic material. [Completely Demagnetised material is the one in which B=H =0].
It will be subjected to increasing values of magnetic field strength H and the corresponding flux density B measured.
The result is shown in the below figure by the curve o-a-b.
Hysteresis_LoopAt the point b, if the field intensity (H) is increased further the flux density(B) will not increase any more.
This is called saturation ( the material is said to be saturated). In this figure b-y is called as saturation flux density.

Now if the field intensity (H) is decreased, the flux density (B) will follow the curve b-c.
When field intensity (H) is reduced to zero, flux remains in the iron.
This is called as remanent flux density or remanence.
It is shown in the figure as o-c.

  • Now if the H is increased in the opposite direction, the flux density decreases as will.
  • It will decrease until the point d. Here the flux density (B) is zero. 
  • The magnetic field strength (points between 0 and d) required to remove the residual magnetism, i.e. reduce B to zero. It is called the coercive force.
  • Now if the H is increased further in the reverse direction causes the flux density to increase in the reverse direction till the saturation point ( here point-e).
  • If H is varied backwards from 0x to 0y, the flux density(B) follows the curve e-f-g-b, similar to curve b-c-d-e.
  • From the figure the clear that the flux density changes lag behind the changes in the magnetic field strength.
  • This effect is called Hysteresis.
  • The closed figure b-c-d-e-f-g-b is called the Hysteresis Loop (or the B/H Loop).

Hysteresis loss:

What is meant by Hysteresis loss?

A disturbance in the alignment of the domains (Domains is nothing but groups of atoms. Domains are of microscopic size, but are large enough to hold from 1017 to 1021 atoms) of a ferromagnetic material causes energy to be expended in taking it through a cycle of Magnetization.
(If the domain fields in the ferromagnetic material line up, the material is magnetized; if they are randomly oriented, then the ferromagnetic material is not magnetized.)
This energy appears as heat in the specimen and is called the Hysteresis Loss.

The energy loss associated with hysteresis is proportional to the area of the hysteresis loop.

  • The area of a hysteresis loop varies with the type of material.
  • For hard materials the hysteresis loop area is large and thus the hysteresis loss also more. 
  • Hysteresis loop for hard material has a high remanence (0-c) and a large coercivity (0-d). Hysteresis_Loss_hard
  • For soft materials the hysteresis loop area is small and thus the hysteresis loss are less.
  • Hysteresis loop for soft steel has a large remanence and small coercivity. 
  • For hard materials the area and thus the energy loss, is much greater than the energy loss for the soft materials.
  • Hysteresis loss for ferrite (oxides of aluminium, iron, cobalt, magnesium, nickel and mangenese is very small.Hysteresis_Loss_ferrite

The hysteresis loop for AC excited devices  is repeated every cycle of alternating current.
Thus a material having large hysteresis loop area is unsuitable as the energy loss more.
The Silicon steel has a small hysteresis loop area, so less hysteresis loss.
So it is suitable for transformer cores and rotating machine armatures.

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