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Experiment Number-1, AIM-Determine the permeability of magnetic material by plotting its B-H, curve., “OR”, , AIM-To trace hysteresis loop for different magnetic materials., , Apparatus Required:, B-H curve tracer kit,, CRO, Patch Cords,, Trace paper Magnetic Materials: Nail,, Hack Saw Blade,, Ferrite Rod,, Steel rod, , Theory:, 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 ratio 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:

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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 ampereturns 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

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increase in the magnetic field strength due to an increase in the electrical current flowing, through the coil will have little or no effect., Retentivity, 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 for a coil to retain some of its magnetism within the core after the, magnetisation process has stopped is called Retentivity or remanence, while the amount, of flux density still remaining in the core is called Residual Magnetism, BR ., 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

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The Magnetic Hysteresis loop above, shows the behaviour 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, circulating around the core also reduces to zero. However, the coils magnetic flux will not, reach zero due to the residual magnetism present within the core and this is shown on the, curve from point a to point b., 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 rearranging the molecular magnets until the core becomes unmagnetised at point c., An increase in this reverse current causes the core to be magnetised in the opposite, direction and increasing this magnetisation current further will cause the core to reach its, saturation point but in the opposite direction, point d on the curve., This point 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 additive’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.

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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., , Procedure:, a) Connect one terminal of the magnetizing coil to point C of main unit and the other, terminal marked VI (6Volts ac).connect H to the horizontal input of the CRO and V to, vertical input of the CRO. Operate the CRO in X-Y mode., b) Connect the IC probe to the “IC” marked on main unit., c) Switch ON the kit. To get proper loop vary the resistance to the maximum value with, the help of knob P on the panel., d) With no specimen through the coil, adjust the horizontal gain of the CRO until a, convenient X deflection is obtained. Note down this reading as SH., e) Insert the magnetic specimen( a 5’ nail), through the magnetizing coil such that it, touches the probe at the centre .Make sure that sample is touching I C only and, conducting tracks should not be shorted in any case. Adjust the oscilloscope vertical gain, (Y gain) until a trace showing the B-H loop conveniently fills the screen. Note down this, reading as Sv. If the curve is back to front, reverse the connection of the magnetizing coil., f) Trace the area of the loop on butter paper from the screen of CRO and retrace it on, graph paper., g) Note down the X intercept Vx and Y intercept Vy from the graph paper. Calculate the, coercivity H using relation 1and retentivity B using relation 2.

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h) Measure the area of the loop with the help of graph paper . The energy loss = (𝟎.𝟓 𝐱 𝐍, 𝐱 v S 𝐱 SH 𝐱 𝐚𝐫𝐞𝐚 𝐨𝐟 𝐭𝐡𝐞 𝐥𝐨𝐨𝐩 ) ( 𝐑 𝐱 𝐋) Joules/second/unit volume Where Sv and SH, are vertical and horizontal sensitivities of the CRO for that particular setting of the gains., (i) Repeat the experiment by varying the a.c.voltage by connecting coil between points, C& V2(9Volts)andV3(12V).The part of the curve showing saturation changes in length., j) Repeat the experiment with different specimen and note your comments on the, properties of different materials., , Precautions:, (a) The specimen should touch the probe., (b) If the area of the loop is expressed in cm2 , the sensitivities should be expressed in, volts/cm. If the area of the loop is expressed in sq.meter the sensitivities should be, expressed in volts/meter. The length of the coil should be in meter., , Conclusion:-