A hysteresis loop is a four quadrant graph that shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is also known as the B-H loop and hysteresis curve. By studying hysteresis loops, we can determine a number of magnetic properties of a material such as the retentivity, residual magnetism (or residual flux), coercive force, permeability and the reluctance.
The hysteresis loop is generated by measuring the magnetic flux coming from ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as magnetic force (H) is increased. As the line demonstrates, greater the amount of magnetizing force (H) /current is applied then, stronger the magnetic field (B) in the component.
Almost all of the magnetic domains are aligned at point "a" and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When magnetic force (H) is reduced to zero, the curve will move from point "a" to point "b". At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force becomes zero. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost their alignment).
As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material.
As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction at point "d". Reducing H to zero brings the curve to point "e". It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back to the saturation point where it with complete the loop.
Advantage of Hysteresis Loop
The main advantages of hysteresis loop has been listed below.
- Smaller hysteresis loop area means less hysteresis loss.
- Hysteresis loop provides the value of retentivity and coercivity of a material. Thus the way to choose perfect material to make permanent magnet, core of machines becomes easier.
- From B-H graph, residual magnetism can be determined and thus choosing of material for electromagnets is easy.
Magnetic Material Properties By Studying Hysteresis Loop
- Retentivity - A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, ability of a material to retain a certain amount of residual magnetic field when the magnetizing force (H) is removed after achieving saturation. It is the value of B at point b on the hysteresis curve.
- Residual Magnetism or Residual Flux - The magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.
- Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to return the magnetic flux to zero. It is the value of magnetizing force (H) at point c on the hysteresis curve.
- Permeability - A property of a material that describes the ease with which a magnetic flux is established in the component.
- Reluctance - It is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.
- Lower Permeability
- Higher Retentivity
- Higher Coercivity
- Higher Reluctance
- Higher Residual Magnetism
- Higher Permeability
- Lower Retentivity
- Lower Coercivity
- Lower Reluctance
- Lower Residual Magnetism