What Is Hysteresis?
The term “hysteresis” refers to a lagging effect. In the context of magnetism, hysteresis describes how a material’s response to magnetic forces lags behind the actual force itself. By understanding how to read a hysteresis curve, we can learn about a material’s reaction patterns concerning the different strengths of magnetic fields.
What Is Hysteresis Loop?
The hysteresis loop, otherwise known as the B-H curve or hysteresis curve, is a four-quadrant graph showing the relationship between magnetic flux density (B) and magnetizing force(H). Its vertical axis measures the material’s flux density (in Teslas or Mega Gauss), and its horizontal axis measures the external applied magnetizing force (in Amperes per meter). The Hysteresis loop also allows us to understand many magnetic properties, like retentivity, residual magnetism (or residual flux), coercive force, permeability, and reluctance.
A magnetic material’s Flux Density (B) will increase in the presence of an applied magnetic field (H). The magnetism will increase from zero to a maximum of point “a,” where it no longer responds to increases in the external magnetic field. This is called Positive Saturation Point, and any further growth in the external magnetic field beyond this point will not affect the material’s flux density.
Residual Magnetism (Br)
We next return to the left along the horizontal axis toward the origin. We find that the material releases some of its magnetism. Where the applied field has reached zero again, Flux Density (B) decreases to a point called b . Point ‘b’ explains that after removing magnetizing force (H), only a small amount of magnetism remains in this magnetic material – known as residual magnetism (Br). Here 0 – b is equal to the value of residual flux density due to the retentivity of the material.
The Coercivity of a material is the amount of (H) required to drive (B) to the zero line. In the hysteresis loop, as H decreases in the reverse direction along path b-c, residual magnetism (Br) decreases until it reaches zero at point ‘c.’ The negative value of H at this point is called coercive force (Hc).
As H increased in the negative direction, B began to reverse and follow paths c-d. At point d, magnetic saturation occurred again but in the opposite direction from before. This is where B and H reach their maximum values in the reversed directions of -Bm and -Hm, respectively.
Negative Residual Magnetism
If we decrease the value of H in this direction, B will again follow the path “d-e” and subsequently decrease. At point ‘e,’ where H gets zero-valued, but B remains with a finite value, this indicates residual magnetism (-Br) of magnetic core material in the opposite direction compared to the previous case.
If we turn back to point a from d, we will only be doing what we initially did – but in reverse. If the value of H reverses again, then the residual magnetism or the flux density (-Br) decreases once more and gets to zero at point ‘f’ after following path e-f. With a further increase of H, magnetic flux (B) starts increasing from zero and reaches its saturation level at point a after following path f-a.
The path a – b – c – d – e -f-a form s hysteresis loop. (Note: The shape and size of the loop depend on the material chosen.)
An excellent video tutorial is available here if you would like to study the hysteresis loop more in-depth.
The Significance Of Hysteresis Loop
The width of the hysteresis loop is directly correlated to the amount of losses. In other words, a narrower curve signifies lower losses.
The Hysteresis loop defines a substance’s retentivity and coercivity, making it easier to choose the suitable material for permanent magnets.
Residual magnetism can be easily determined from a B-H graph, thus simplifying the choice of electromagnet materials.
Hysteresis is a common phenomenon in many scientific fields, including chemistry, engineering, economics, and the biological sciences. For example, electromagnetism hysteretic loops are often seen in electronic devices such as inductors and transformers. Hysteresis also plays a vital role in economic dynamics and ecological systems.
B-H Curves in Hard and Soft Magnetic Materials
Soft Magnetic Materials
- Having a thin hysteresis curve
- Low remanence, easy demagnetization, and low coercivity.
- It can switch polarity frequently and maintain low electrical losses.
- Examples: Ferrous-nickel alloy, Ferrites Garnets
- Applications: Two examples of this are transformers and motor windings.
Hard Magnetic Materials
- Having a broad hysteresis curve
- High remanence, complex magnetization, and demagnetization
- If the polarity were to change frequently, the hysteresis losses would be incredibly high.
- Examples: Steel, carbon steel, chromium steel, tungsten
- Applications: Exert their magnetic field on soft magnetic materials because of their wide hysteresis curve.
Frequently Asked Questions on Hysteresis
What is a demagnetization curve?
A demagnetization curve offers a more thorough description of a magnet’s magnetic qualities than a standalone number, such as Pull Force or Surface Field. By providing data about the magnet’s strength, how difficult it is to demagnetize, and how the shape of the magnet (or its use in a magnetic circuit) affects performance, this information can be used to make improvements.
What is magnetic permeability?
Magnetic permeability is a value that expresses how well a magnetic material responds to an applied magnetic field.
If the dipoles within the material’s orientations line up with the external magnetic field quickly, it has high permeability. However, if the dipoles do not react well to an applied magnetic field, it is low in permeability.
How can we reduce hysteresis?
Smaller hysteresis loop areas lead to fewer hysteresis losses. As a result, materials like silicon or high-grade steel, which have small hysteresis loop areas, are used to construct the core of electrical machines.
What is magnetic flux density?
The magnetic flux density, or magnetic induction, measures the number of lines of force passing through a unit area. The SI unit for magnetic induction is the tesla (T).
What is magnetic energy?
The magnetic flux density at any point on the demagnetization curve of a permanent magnet multiplied by the magnetic field strength H is called the magnetic energy BH.
What is retentivity?
Retentivity is the ability of a magnetic material to remain magnetized after removing an external magnetic field.