Hysteresis Loop | Magnetization Curve

What is a Hysteresis Loop?

A curve, or loop, plotted on B-H coordinates showing how the magnetization of a ferromagnetic material varies when subjected to a periodically reversing magnetic field, is known as Hysteresis Loop.

Hysteresis Definition

Hysteresis is the lagging of the magnetization of a ferromagnetic material behind the magnetizing force H.

By using a graph having B-H coordinates, we can plot the hysteresis characteristics of a given ferromagnetic material. Such a curve is plotted in the following figure and called a hysteresis loop. By periodically reversing a magnetizing force, we can plot the changing values of B within the material.

Hysteresis Loop Diagram

Actually, a hysteresis loop is a B-H curve under the influence of an AC magnetizing force. Values of flux density B are shown on the vertical axis and are in Tesla. Magnetizing force H is plotted on the horizontal axis.

In above figure, the specimen is assumed to be unmagnified, and the current is starting from zero in the center of the graph. As H increases positively, B follows the red dotted curve from origin to saturation point a, indicated by Bmax.

As H decreases to zero, the flux follows the curve ab and drops to Br which indicates the retentively or residual induction. This point represents the amount of flux remaining in the core after the magnetizing force is removed.

When H starts in the negative direction, the core will lose its magnetism, as shown by following the curve from point b to c. The amount of magnetizing force required to completely demagnetize the core is called the coercive force and is designated as –Hc in the figure.

As the peak of the negative cycle is approached, the flux follows the portion of the curve labeled cd. Point –Bmax represents saturation in the opposite direction from Bmax . From point d, the – H value decreases to point e, which corresponds to a zero magnetizing force. Flux –Br still remains in the core.

A coercive force of +Hc is required to reduce the core magnetization to zero. As the magnetic force continues to increase in the positive direction, the portion of the loop from point f to a is completed. The periodic reversal of the magnetizing force causes the core flux to repeatedly trace out the hysteresis loop.

Demagnetization or Degaussing

The process by which the magnetization within the ferromagnetic materials is reduced to zero by exposing it to a strong alternating magnetic field that is gradually reduced to zero.

To demagnetize any magnetic material, we must reduce its residual magnetism Br to zero. This can be done by connecting a suitable coil to a source of alternating current and placing it close to the object to be degaussed. Slowly moving the coil and object away from each other, causes the hysteresis loop to become progressively small. Finally, a point is reached where the loop is reduced to zero and no residual magnetism remains.

 From the hysteresis loop, we can conclude different magnetic properties of a material such as:

Reluctance– The opposition that a magnetic circuit presents to the passage of magnetic lines through it.

Retentivity– The ability of a ferromagnetic material to retain residual magnetism is termed its retentivity.

Residual Magnetism– The magnetism remaining after the external magnetizing force is removed.

Coercive Force – The magnetic field strength required to reduce the residual magnetism to zero is termed the coercive force.

Permeability– Permeability is the measure of the ease, with which magnetic lines of force pass through a given material.

Hysteresis Loop Applications and Importance

The hysteresis loop is important in various areas of science and engineering due to its valuable insights into material behavior and system dynamics. Some key applications of the hysteresis loop include:

Magnetic Materials: In magnetism, the hysteresis loop provides essential information about the magnetic properties of materials. It illustrates how the magnetic induction (B) responds to changes in the magnetizing force (H) during the magnetization and demagnetization processes. The loop’s shape and area reveal characteristics such as coercivity, remanence, saturation, and magnetic losses, which are crucial for understanding and designing magnetic systems and devices.

Ferromagnetic Materials: The hysteresis loop is particularly relevant for ferromagnetic materials that exhibit strong magnetization effects. It demonstrates the phenomenon of hysteresis, where the magnetic properties of a material lag behind changes in the applied magnetic field. This behavior is key to various applications, including magnetic storage devices (hard drives, magnetic tapes), transformers, motors, and generators.

Material Science and Engineering: The hysteresis loop is used in material science and engineering to study and analyze the behavior of materials under cyclic loading or varying inputs. It helps understand the elastic, plastic, and viscoelastic properties of materials, including stress-strain relationships, fatigue, and creep behavior. This information is crucial for designing materials and structures that can withstand repeated loading and ensure durability and reliability.

Electrical Circuits and Devices: Hysteresis loops are significant in electrical circuits and devices, especially those utilizing nonlinear components like diodes and magnetic cores. Understanding the hysteresis behavior enables engineers to design circuits that exploit nonlinear characteristics for applications like signal rectification, switching, amplification, and memory storage.

Control Systems: Hysteresis is an important consideration in control systems, particularly when dealing with sensors, actuators, and feedback mechanisms. Hysteresis in these components can introduce delays, nonlinearity, and instability, affecting the overall system performance. Characterizing and compensating for hysteresis effects allows for improved control and accuracy in various applications, such as robotics, automation, and precision positioning systems.

Material Testing and Calibration: The hysteresis loop serves as a basis for evaluating the performance and calibration of measurement instruments, especially those involving force, displacement, strain, or magnetic field measurements. By subjecting the instrument to known hysteresis loops, the accuracy and linearity of the measurements can be assessed, ensuring reliable and traceable results.

Overall, the hysteresis loop is significant in understanding the behavior of materials, devices, and systems that exhibit nonlinear responses or memory effects. It provides crucial insights into magnetic properties, material characteristics, system performance, and control considerations, enabling advancements in various fields of science, engineering, and technology.

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