The lag or delay of a magnetic material known commonly as Magnetic Hysteresis, relates
to the magnetisation properties of a material by which it firstly becomes magnetised and then de-magnetised. We know that
the magnetic flux generated by an electromagnetic coil is the amount of magnetic field or lines of force produced within a
given area and that it is more commonly called "Flux Density". Given the symbol B with the unit
of flux density being the Tesla, T.
We also know from the previous tutorials that the magnetic strength of an electromagnet depends upon
the number of turns of the coil, the current flowing through the coil or the type of core material being used, and if we
increase either the current or the number of turns we can increase the magnetic field strength, symbol H.
Previously, the relative permeability, symbol μr was defined as
the product of the absolute permeability μ and the permeability of free space
μo (a vacuum) and this was given as a constant. However, the relationship between
the flux density, B and the magnetic field strength, H can be defined by the fact that
the relative permeability, μr is not a constant but a function of the magnetic field
intensity thereby giving magnetic flux density as: B = μ H. Then the
magnetic flux density in the material will be increased by a larger factor as a result of its relative permeability for the
material compared to the magnetic flux density in vacuum, μoH and for an air-cored
coil this relationship is given as:
So for ferromagnetic materials the ratio of flux density to field strength ( B/H )
is not constant but varies with flux density. However, for air cored coils or any non-magnetic medium core such as woods or
plastics, this ratio can be considered as a constant and this constant is known as μo,
the permeability of free space, ( μo = 4.π.10-7 H/m ).
By plotting values of flux density, ( B ) against the field strength,
( H ) we can produce a set of curves called Magnetisation Curves,
Magnetic Hysteresis Curves or more commonly B-H Curves for each type of core material used
as shown below.
Magnetisation or B-H Curve
The set of magnetisation curves, M above represents an example of the
relationship between B and H for soft-iron and steel cores but every
type of core material will have its own set of magnetic hysteresis curves. You may notice that the flux density increases
in proportion to the field strength until it reaches a certain value were it can not increase any more becoming almost level
and constant as the field strength continues to increase.
This is because there is a limit to the amount of flux density that can be generated by the core as all
the domains in the iron are perfectly aligned. Any further increase will have no effect on the value of M,
and the point on the graph where the flux density reaches its limit is called Magnetic Saturation also known
as Saturation of the Core and in our simple example above the saturation point of the steel curve begins at
about 3000 ampere-turns per metre.
Saturation occurs because as we remember from the previous
Magnetism tutorial which included
Weber's theory, the random haphazard arrangement of the molecule structure within the core material changes as the tiny molecular
magnets within the material become "lined-up". As the magnetic field strength, ( H ) increases these
molecular magnets become more and more aligned until they reach perfect alignment producing maximum flux density and any increase
in the magnetic field strength due to an increase in the electrical current flowing through the coil will have little or no effect.
Lets assume that we have an electromagnetic coil with a high field strength due to the current flowing through
it, and that the ferromagnetic core material has reached its saturation point, maximum flux density. If we now open a switch and
remove the magnetising current flowing through the coil we would expect the magnetic field around the coil to disappear as the
magnetic flux reduced to zero.
However, the magnetic flux does not completely disappear as the electromagnetic core material still retains
some of its magnetism even when the current has stopped flowing in the coil. This ability to retain some magnetism in the core
after magnetisation has stopped is called Retentivity or Remanence while the amount of flux
density still present in the core is called Residual Magnetism,
The reason for this that some of the tiny molecular magnets do not return to a completely random pattern
and still point in the direction of the original magnetising field giving them a sort of "memory". Some ferromagnetic materials
have a high retentivity (magnetically hard) making them excellent for producing permanent magnets.
While other ferromagnetic materials have low retentivity (magnetically soft) making them ideal for use
in electromagnets, solenoids or relays. One way to reduce this residual flux density to zero is by reversing the direction
of the current flowing through the coil, thereby making the value of H, the magnetic field strength
negative. This effect is called a Coercive Force, HC .
If this reverse current is increased further the flux density will also increase in the reverse direction
until the ferromagnetic core reaches saturation again but in the reverse direction from before. Reducing the magnetising current,
i once again to zero will produce a similar amount of residual magnetism but in the reverse direction.
Then by constantly changing the direction of the magnetising current through the coil from a positive direction to a negative
direction, as would be the case in an AC supply, a Magnetic Hysteresis loop of the ferromagnetic core can be produced.
Magnetic Hysteresis Loop
The Magnetic Hysteresis loop above, shows the behavior of a ferromagnetic core
graphically as the relationship between B and H is non-linear. Starting
with an unmagnetised core both B and H will be at zero, point
0 on the magnetisation curve.
If the magnetisation current, i is increased in a positive direction to some
value the magnetic field strength H increases linearly with i and the
flux density B will also increase as shown by the curve from point 0
to point a as it heads towards saturation. Now if the magnetising current in the coil is reduced
to zero the magnetic field around the core reduces to zero but the magnetic flux does not reach zero due to the residual
magnetism present within the core and this is shown on the curve from point a to point
To reduce the flux density at point b to zero we need to reverse the current
flowing through the coil. The magnetising force which must be applied to null the residual flux density is called a "Coercive
Force". This coercive force reverses the magnetic field re-arranging the molecular magnets until the core becomes unmagnetised
at point c. An increase in the reverse current causes the core to be magnetised in the opposite
direction and increasing this magnetisation current will cause the core to reach saturation but in the opposite direction,
point d on the cure which 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 at point e.
Again reversing the magnetising current flowing through the coil this time into a positive direction 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. Then
the B-H curve follows the path of a-b-c-d-e-f-a as the magnetising
current flowing through the coil alternates between a positive and negative value such as the cycle of an AC voltage. This
path is called a Magnetic Hysteresis Loop.
The effect of magnetic hysteresis shows that the magnetisation process of a ferromagnetic core and therefore the
flux density depends on which part of the curve the ferromagnetic core is magnetised on as this depends upon the circuits past history
giving the core a form of "memory". Then ferromagnetic materials have memory because they remain magnetised after the external magnetic
field has been removed. However, soft ferromagnetic materials such as iron or silicon steel have very narrow magnetic hysteresis loops
resulting in very small amounts of residual magnetism making them ideal for use in relays, solenoids and transformers as they can be
easily magnetised and demagnetised.
Since a coercive force must be applied to overcome this residual magnetism, work must be done in closing the
hysteresis loop with the energy being used being dissipated as heat in the magnetic material. This heat is known as hysteresis loss,
the amount of loss depends on the material's value of coercive force. By adding addictive's to the iron metal such as silicon, materials
with a very small coercive force can be made that have a very narrow hysteresis loop. Materials with narrow hysteresis loops are easily
magnetised and demagnetised and known as soft magnetic materials.
Magnetic Hysteresis Loops for Soft and Hard Materials
Magnetic Hysteresis results in the dissipation of wasted energy in the form of heat with
the energy wasted being in proportion to the area of the magnetic hysteresis loop. Hysteresis losses will always be a problem
in AC transformers where the current is constantly changing direction and thus the magnetic poles in the core will cause
losses because they constantly reverse direction.
Rotating coils in DC machines will also incur hysteresis losses as they are alternately passing north the
south magnetic poles. As said previously, the shape of the hysteresis loop depends upon the nature of the iron or steel used
and in the case of iron which is subjected to massive reversals of magnetism, for example transformer cores, it is important
that the B-H hysteresis loop is as small as possible.
In the next tutorial about Electromagnetism, we will look at Faraday's Law of
Electromagnetic Induction and
see that by moving a wire conductor within a stationary magnetic field it is possible to induce an electric current in the conductor
producing a simple generator.