What is Hysteresis?
The inability of molecules in a ferromagnetic material to rapidly change their magnetization in response to a change in an applied magnetic field is referred to as hysteresis.
What is Hysteresis Loss?
Hysteresis loss is caused by the magnetization and demagnetization of the core when current flows forward and backward. As the magnetising force (current) increases, so do the magnetic flux. When the magnetizing force (current) is removed, the magnetic flux diminishes gradually rather than abruptly. As a result, the flux density stays positive even when the magnetising force is zero. The magnetising force must be exerted in the negative direction to achieve zero flux density.
Types of Hysteresis
Hysteresis is classified into two. They are
- Rate-dependent hysteresis:- It occurs when there is a lag between input and output. Consider a sinusoidal input X(t) resulting in a sinusoidal output Y(t), where there is a phase lag φ.
- Rate-independent hysteresis: This type of hysteresis is found in systems that have a permanent memory of the past that persists even after the transients have passed.
The magnetic flux density and magnetising field strength are represented by the hysteresis loop. The loop is created by monitoring the magnetic flux emitted by the ferromagnetic substance while the external magnetising field is changed.
If B is measured for different values of H and the results are shown in visual representations, the graph will indicate a hysteresis loop.
- When the magnetic field strength (H) is increased from 0 to 1, the magnetic flux density (B) increases (zero).
- As the magnetic field is increased, the value of magnetism increases until it hits point A, which is known as the saturation point, where B is constant.
- When the value of the magnetic field decreases, so does the value of magnetism. However, when B and H are equal to zero, a substance or material that retains some magnetism is known as retentivity or residual magnetism.
- Magnetism reduces as the magnetic field shifts to the negative side. At point C, the material will be totally demagnetized.
- Coercive force is the amount of force required to remove the material’s retentivity (C).
- The cycle is repeated in the opposite direction, where the saturation point is D, the retentivity point is E, and the coercive force is F.
- The cycle is complete as a result of the forward and opposite direction processes, and this cycle is known as the hysteresis loop.
Hysteresis loss formula
To find the hysteresis loss:
Pb = η * f * V * Bmaxn
Pb = hysteresis loss (W)
η = Steinmetz hysteresis coefficient varies on the material we are using (J/m3)
Bmax = maximum flux density (Wb/m2)
n = Steinmetz exponent, which varies with material and ranges from 1.5 to 2.5
f = Magnetic reversals frequency per second (Hz)
V = volume of magnetic material (m3)
How to Minimize Hysteresis Loss?
The following methods can be used to control hysteresis loss.
Use a material that has a smaller hysteresis loop.
For example, silica steel has a very small area for hysteresis loss.
When the current flow is interrupted, use the material that has a zero/non-zero flux.
Increase the number of laminations that exist between the plates.
Advantages of Hysteresis Loop
- A smaller section of the hysteresis loop indicates less hysteresis loss.
- The hysteresis loop provides a substance with retentivity and coercivity. As a result, the heart of machines simplifies the process of selecting the proper material for making a permanent magnet.
- The B-H graph can be used to compute residual magnetism, making the material selection for electromagnets simple.