Magnetic Hysteresis

In this article, we will discuss magnetic hysteresis. We know that the magnetic battery produced by an electromagnetic coil is the amount of magnetic field or force lines produced in a given area, which is more commonly called "Flux Density".

We also know from previous lessons that the magnetic power of an electromagnet depends on the number of rotations of the coil, the current passing through the coil or the type of core material used. If we increase the current or number of bandages, we can increase the magnetic field strength, Symbol H.

Previously, relative permeability was defined as symbol μr, μ absolute permeability and μo (a vacuum) ratio of permeability of free space, and this was given as a constant. However, the flux density, the relationship between B and magnetic field power, H, can be defined by the fact that relative permeability is not constant at μr, but magnetic field density has a function, and thus magnetic flux density is given as follows: B = μ H.

Then the concentration of magnetic flux in the material, the intensity of magnetic flux in the vacuum will be increased by a larger factor as a result of its relative permeability for the material compared to μoH, and for an air-core coil this relationship is given as follows:

Magnetic Hysteresis

Therefore, the ratio of flux density to field strength (B/H) for ferromamannetic materials is not constant but varies with flux density. However, for air core coils or any non-magnetic media core such as wood or plastic, this ratio can be considered a constant, and this constant, μo, is known as the permeability of the free space, (μo = 4.π.10-7 H/m).

By drawing the values of flux density (B) against the receiving force (H), we can produce a series of curves for each type of core material used, called Magnetization Curves, Magnetic Hysteresis Curves, or, more commonly, B-H Curves, for each type of core material used.

Magnetization or B-H Curve

Magnetic Hysteresis
Magnetization or B-H Curve

The above set of M magnetization curves represents an example of the relationship between B and H for soft iron and steel cores. But all types of core material will have their own magnetic hysteresis curves. You may notice that the flux density increases in proportion to the field force until it reaches a certain value. As the field strength continues to increase, it cannot increase any more and reaches almost the same level.

This is because there is a limit to the amount of flux density that can be produced by the core, since all areas in the iron are perfectly aligned. Further increase will have no effect on the M value, and the point in the graph where the flux density reaches its limit is also known as Magnetic Saturation. It is also known as the Saturation of the Core and in our simple example it is above the saturation point of the steel curve. begins at a rotation of about 3000 amps per meter.

Saturation occurs because, as we remember from our previous magnetism training involving Weber's theory, the random random regulation of the molecular structure within the core material changes as small molecular magnets within the material become "sorted".

As the magnetic field force (H) increases, these molecular magnets are increasingly aligned until they reach perfect alignment by producing the maximum flux density, and any increase in magnetic field force due to an increase in electric current flowing from the coil will have little effect. or have no effect.

Permanence
Let's say that we have an electromagnetic coil with high field power due to the current passing through it, and the ferromagnetic core material reaches saturation point, maximum flux density. Now, if we turn on a switch and remove the magneting current that flows from the coil, we wait for the magnetic field around the coil to disappear when the magnetic flux drops to zero.

However, magnetic flux does not disappear completely, as the electromagnetic core material retains some of its magnetism even when it stops flowing in the current coil. The ability of a coil to keep part of magnetism in the nucleus after the magnetization process has stopped is called Persistence or residue, while the amount of flux density that still remains in the nucleus is calledresidual magnetism, BR.

This is because some small molecular magnets do not turn into a completely random model, and yet show the direction of the original magnet area, giving them some kind of "memory". Some ferromagnetic materials have high permanence (magnetically hard). This makes them perfect for producing permanent magnets.

While other ferromomanetic materials have low persistence (magnetically soft), they are ideal for use in electromagnets, solenoids or relays. One way to reduce this residue flux density to zero is to reverse the direction of the current flowing from the coil, making the magnetic field force H negative. This effect is calledCoercive Force, HC. Reducing the magnetism current to zero once again will produce a similar amount of residual magnetism, but in the opposite direction.

Then, as with an AC source, a Magnetic Hysteresis cycle of the ferromamannetic nucleus can be produced by constantly changing the direction of the magneting current passing through the coil in a positive direction in a negative direction.

Magnetic Hysteresis

Magnetic Hysteresis
Magnetic Hysteresis Cycle

The above Magnetic Hysteresis cycle graphically shows the behavior of a ferromary nucleus because the relationship between B and H is not linear. Starting with an uncontected nucleus, both B and H will be at zero and 0 points on the magnetization curve.

If the magnetization current is raised to a positive value, the magnetic field force increases linearly with H, i, and the flux density B will increase as shown by the curve from point 0 to point a as it heads towards saturation.

Now, if the magneting current in the coil is reduced to zero, the magnetic field around the core will also decrease to zero. However, the magnetic flux of the coils will not reach zero due to the residual magnetism present in the nucleus, and this is shown on the curve from point a to point b.

In order to reduce the flux density at point b to zero, we need to reverse the current passing through the coil. The magnetizing force that must now be applied to reset the flux density is called "Force Force". This force reverses the magnetic field and rearranges the molecular magnets until they become non-magnetized at core c point.

An increase in this reverse current causes the nucleus to magnetize in the opposite direction, and further increasing this magnetization current causes the nucleus to reach saturation point but in the opposite direction, d-point on the curve. This point is symmetrical to point b. If the magnetism current is reduced again to zero, the residues in the nucleus will be equal to the previous value, but vice versa at point e.

Again, turning the magnetizing current flowing from the coil in a positive direction this time will cause the magnetic battery to reach zero, f point on the curve and to increase the magnetization current in a positive way as before, reaching saturation at the point of the nucleus.

The B-H curve then follows the a-b-c-d-e-f-a path, alternating between a positive and negative value, such as the magnetizing current flowing from the coil, the cycle of an AC voltage. This pathway is called the Magnetic Hysteresis Cycle.

The effect of magnetic hysteria indicates that the magnetization process of a ferromary nucleus, and therefore the flux density, depends on which part of the curve the ferromamanyetic nucleus is magnetized, since this depends on past circuits that give the nucleus a form of "memory". Then ferromamanyetic materials have memory, as they remain magnetized after the external magnetic field is removed.

However, soft ferromamanyetic materials such as iron or silicon steel have very narrow cycles of magnetic hysteresis, resulting in very small amounts of residues magnetism, making them ideal for use in relays, solenoids and transformers, as they can be easily magnetized and magnetically removed.

Since a coercive force must now be applied to overcome this magnetism, work should be done to close the cycle of hysteresis while the energy used is dispersed as heat in the magnetic material. This heat is known as the loss of hysteresis, the amount of loss depends on the compelling force value of the material.

By adding additives such as silicon to iron metal, materials with a very small coercive force with a very narrow cycle of hysteresis can be made. Materials with narrow cycles of hysteresis are easily magnetized and dementia-free and known as soft magnetic materials.

Magnetic Hysteresis Cycles for Soft and Hard Materials

Magnetic Hysteresis
Magnetic Hysteresis Cycles for Soft and Hard Materials

Magnetic Hysteresis causes wasted energy to be distributed in the form of heat with wasted energy in proportion to the field of the magnetic hysteresis cycle. Hysteresis losses will always be a problem in AC transformers, where the current is constantly changing direction, and therefore the magnetic poles in the nucleus will cause losses as they constantly change direction.

Rotating coils in DC machines will also cause hysteresis losses as they alternately pass north through the south magnetic poles. As already mentioned, the shape of the cycle of hysteresis depends on the nature of the iron or steel used.

In the next tutorial on electromagnetism, we will look at Faraday's Electromagnetic Induction Act and see that by moving a wire conductor within a fixed magnetic field, it is possible to induce an electric current in the conductor, which produces a simple generator.