Definition
Magnetic Hysteresis Curve represents the relation between magnetizing force (H) and magnetic flux intensity for ferromagnetic material. Another name for Magnetic Hysteresis Curve is B-H curve or simply Hysteresis loop.
Magnetic Hysteresis Curve relates to the magnetization properties of a material by which it first becomes magnetized and then demagnetized. Thus Magnetic Hysteresis Curve introduces the lag or delay of a magnetic material. Also important to note that every type of core material will have its own set of magnetic hysteresis curves.
Formation of Magnetic Hysteresis Curve
The B-H Curve or Magnetic Hysteresis loop above shows non-linear behaviour of a ferromagnetic core graphically
Initializing with an unmagnetized core. Let us suppose both B and H will be at zero at origin.
When the magnetization current ‘i’ increases in a positive direction, the magnetic field strength H also increases linearly. As a result the flux density B also increases. Point ‘0 to a’ shows this behaviour. This heads towards saturation.
As soon as the magnetizing current in the coil reduces to zero. The magnetic field circulating around the core also reduces to zero. However, the magnetic flux will not reach zero. This happens because of the residual magnetism present within the core. Point ‘a to b’ shows this behaviour.
Retentivity
Here the new term comes into view i.e. Retentivity or remanence. Retentivity is the ability of a material to retain some of its magnetism within the core after the magnetization process has stopped. While Residual Magnetism is the amount of flux density remaining in the core.
Further, To reduce the flux density at point b to zero we need to reverse the current direction.
Coercive Force
Again a new term came into vision “Coercive Force”. We can define Coercive Force as the magnetising force which must be applied to null the residual flux density. This coercive force reverses the magnetic field so that the core becomes unmagnetised at point c.
As a result, the core gets magnetised in the opposite direction. upon increasing the reverse current. Further increasing this magnetisation current will cause the core to reach its saturation point but in the opposite direction. Point d on the curve is explaining this behaviour.
Point d is symmetrical to point b. If the magnetising current is reduced again to zero. The residual magnetism present in the core will be equal to the previous value but in reverse direction at point e.
Again reversing the magnetising current this time in a positive direction. This will cause the magnetic flux to reach zero point f on the curve. And as before increasing the magnetisation current further in a positive direction will cause the core to reach saturation at point a.
The path a-b-c-d-e-f-a formed as the magnetising current flowing through the coil keeps on changing between a positive and negative value. This path is known as B-H Curve or Magnetic Hysteresis Loop.
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