Lets take an iron core which has a coil wounded around it. And now we magnetize the iron core by supplying a current though the coil, and then even if you remove the current supply, there is still some magnetic flux in the iron core. When a material is magnetized, it will not relax back to zero magnetization when the magnetizing force (it’s the current in most cases) is removed.
Magnetic Hysteresis is the phenomenon that describes why is the magnetic flux density lags behind the magnetizing force, when we magnetize a material using an alternating magnetic field. In other words, when the magnetic flux density is lagging behind the magnetizing force, it’s known as the magnetic hysteresis.
In order to drive the magnetic flux density back to zero, we need to supply the magnetizing force in the opposite direction. So, when we apply an alternating magnetizing force, it creates a loop called hysteresis loop.
Hysteresis loop – BH Curve
Magnetic Hysteresis loop, also known as the BH curve shows the nature of magnetic hysteresis. Once the material is driven to saturation, the magnetizing field can then be dropped to zero and the material will retain some of the magnetization.
To understand how it works, lets take a solenoid, variable resistor and a DC power supply.
o to a
In first step, we increase the magnetizing force by increasing the amount of current flow through the coil. To do that, we slowly reduce the resistance of the variable resister. When the amount of current increases, the flux density B increase along with it. And then it comes to a saturation point which is ‘a’.
a to b
Now we reduce the current to zero though the solenoid coil by increasing the variable resistance. Even if the magnetic flux density decreases with the current, it doesn’t come to zero. So, it comes to a value ‘b’ as shown in the above image.
Energy dissipation due to magnetic Hysteresis
The energy dissipation during the cycle of magnetization and demagnetization of a magnetic material, is equal to the area under the curve of Hysteresis loop (B-H curve).
Importance of magnetic Hysteresis loop (BH Curve)
- We can calculate the energy dissipation due to Hysteresis using the area under the curve.
- Retentivity, coercivity, susceptibility, permeability also can be measured using curve.
- It’s more useful when selecting materials for magnetic cores, permanent magnets and electromagnets.
The net magnetic flux density B can be represented as,
B = μ0H + μ0M
H is the magnetizing force, and M is the flux density due to magnetized core.
Importance of Magnetic Hysteresis
- It’s used to store memories in data storage disks.
- To select materials for magnetic cores, permanent magnets and electromagnets.
- Hysteresis is the concept behind a lot of sensors.
Downsides of Magnetic Hysteresis
- The main downside is the energy dissipation due magnetic Hysteresis known as Hysteresis losses.
- Power transformers would be more efficient if there were no hysteresis lose.