Experiment 3.8. Magnetization Curve and Hysteresis LoopofTheFerromagneticMaterialFerromagnetic materials are the materials that exhibit the tendency of magnetizationfor a short time even after the removal of an external magnetic field. This property iscalled hysteresis. Among all types of magnetic materials, ferromagnetic materials arestrongly attracted to magnetic fields. Such materials have a spontaneous netmagnetization at the atomic level, even when no external magnetic field is present.Most of the ferromagnetic materials are metals. Common examples of ferromagneticsubstances are Iron, Cobalt, Nickel, etc. Besides, metallic alloys and rare earthmagnets are also classified as ferromagnetic materials. Magnetite is a ferromagneticmaterial which is formed by the oxidation of iron into an oxide. It has a Curietemperature of 580°C. Magnetite has the greatest magnetism among all the naturalminerals on the Earth. There are wide applications of ferromagnetic materials in theindustry.They are widely used in devices like electric motors, generators,transformers, telephones, loudspeakers, and magnetic stripes on the back of creditcards.Experimental Objectives(1) Understand the fundamental concepts of magnetization curve, hysteresis loop,permeability,coerciveforce,andresidualmagnetism(2) Learn how to observe magnetization curve and hysteresis loop of a ferromagneticmaterial using an oscilloscope.(3) Measure permeability, saturation flux density, coercive force, and residualmagnetism.(4) Compare the hysteresis loops under different frequencies for differentferromagnetic materials.Experimental InstrumentsDH4516N apparatus of measuring magnetic hysteresis loop,digital oscilloscopeExperimentalPrinciples
Experiment 3.8. Magnetization Curve and Hysteresis Loop of The Ferromagnetic Material Ferromagnetic materials are the materials that exhibit the tendency of magnetization for a short time even after the removal of an external magnetic field. This property is called hysteresis. Among all types of magnetic materials, ferromagnetic materials are strongly attracted to magnetic fields. Such materials have a spontaneous net magnetization at the atomic level, even when no external magnetic field is present. Most of the ferromagnetic materials are metals. Common examples of ferromagnetic substances are Iron, Cobalt, Nickel, etc. Besides, metallic alloys and rare earth magnets are also classified as ferromagnetic materials. Magnetite is a ferromagnetic material which is formed by the oxidation of iron into an oxide. It has a Curie temperature of 580°C. Magnetite has the greatest magnetism among all the natural minerals on the Earth. There are wide applications of ferromagnetic materials in the industry. They are widely used in devices like electric motors, generators, transformers, telephones, loudspeakers, and magnetic stripes on the back of credit cards. Experimental Objectives (1) Understand the fundamental concepts of magnetization curve, hysteresis loop, permeability, coercive force, and residual magnetism. (2) Learn how to observe magnetization curve and hysteresis loop of a ferromagnetic material using an oscilloscope. (3) Measure permeability, saturation flux density, coercive force, and residual magnetism. (4) Compare the hysteresis loops under different frequencies for different ferromagnetic materials. Experimental Instruments DH4516N apparatus of measuring magnetic hysteresis loop, digital oscilloscope Experimental Principles
Causes ofFerromagnetismIn a ferromagnetic material in the unmagnetized state, atomic dipoles in small regionscalled domains are aligned in the same direction.The domains exhibit a net magneticmoment even in the absence of an external magnetizing field. However, the magneticmoments of neighboring domains are oriented in opposite directions. They cancel out,and therefore the net magnetic moment of the material is zero. On applying anexternal magnetic field, these domains align themselves in the direction of the appliedfield. In this way, the material is strongly magnetized in a direction parallel to themagnetizing field producing a strong magnetic effect.DBYJU'SV兴1A.RandomdomainorientatiorB.AftermagnetizationFigure3.8-1.causesofferromagnetismMagnetizationorB-HCurveFor ferromagnetic materials, the relationship between the flux density, B and themagnetic field strength, H can be defined by the fact that the permeability, μ is not aconstant but a function of the magnetic field intensity thereby giving magnetic fluxdensity as: B = μH = μou,H. The relative permeability, symbol μr was defined asthe ratio of the absolute permeability μand the permeability of free space μo (avacuum, μo=4π ×10-7T·m/A).When the magnetic material is totallydemagnetizedand isthensubjectedtogradually increasingmagnetizingforce,thenthemagneticfluxdensityinthematerial will be increased byalargerfactorasa resultof its relative permeability for the material compared to the magnetic flux density invacuum. By plotting values of flux density, B against the field strength, H we canproduce magnetization curvefor each type of core material used as shown below
Causes of Ferromagnetism In a ferromagnetic material in the unmagnetized state, atomic dipoles in small regions called domains are aligned in the same direction. The domains exhibit a net magnetic moment even in the absence of an external magnetizing field. However, the magnetic moments of neighboring domains are oriented in opposite directions. They cancel out, and therefore the net magnetic moment of the material is zero. On applying an external magnetic field, these domains align themselves in the direction of the applied field. In this way, the material is strongly magnetized in a direction parallel to the magnetizing field producing a strong magnetic effect. Figure 3.8-1. causes of ferromagnetism Magnetization or B-H Curve For ferromagnetic materials, the relationship between the flux density, B and the magnetic field strength, H can be defined by the fact that the permeability, μ is not a constant but a function of the magnetic field intensity thereby giving magnetic flux density as: 𝐵 = 𝜇𝐻 = 𝜇0𝜇𝑟𝐻. The relative permeability, symbol μr was defined as the ratio of the absolute permeability μ and the permeability of free space μ0 (a vacuum, 𝜇0 = 4𝜋 × 10−7 T ∙ m/A ). When the magnetic material is totally demagnetized and is then subjected to gradually increasing magnetizing force, 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. By plotting values of flux density, B against the field strength, H we can produce magnetization curve for each type of core material used as shown below
2.01.8B-HCurvesfor Various MetalsSteel1.61.41.2MagneticSaturationg-1.0su0.8Iron0.6IE0.4Air0.210002000300040005000600070008000900010000MagneticFieldStrength-H(At/m)Figure 3.8-2. magnetization curves for air, iron and steelThe set of magnetization curves represents an example of the relationshipbetween B and H for soft-iron and steel cores but every type of core material will haveits own set of magnetic hysteresis curves. You may have noticed that the flux densityincreases in proportion to the field strength until it reaches a certain value where itcannot increase any more becoming almost level and constant even though the fieldstrength continues to increase. The point on the graph where the flux density reachesits limit is called Magnetic Saturation. As the magnetic field strength, H increasesthese molecular magnets become more and more aligned until they reach perfectalignment producing maximum flux density and any increase in the magnetic fieldstrength due to an increase in the electrical current flowing through the coil will havelittleornoeffect.PermeabilityIn magnetics, permeability is the ability of a material to conduct flux. The magnitudeof the permeability at a given induction is the measure of the ease with which a corematerial can be magnetized to that induction. As presented above, it is defined as theratio of the flux density, B, to the magnetizing force, H. The slope of themagnetization curve, at any given point gives the permeability at that point.Permeability can be plotted against a typical B-H curve, as shown in Figure 3.8-3.Permeability is not constant; therefore, its value can be stated only at a given value ofB or H
Figure 3.8-2. magnetization curves for air, iron and steel The set of magnetization curves 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 have noticed that the flux density increases in proportion to the field strength until it reaches a certain value where it cannot increase any more becoming almost level and constant even though the field strength continues to increase. The point on the graph where the flux density reaches its limit is called Magnetic Saturation. 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. Permeability In magnetics, permeability is the ability of a material to conduct flux. The magnitude of the permeability at a given induction is the measure of the ease with which a core material can be magnetized to that induction. As presented above, it is defined as the ratio of the flux density, B, to the magnetizing force, H. The slope of the magnetization curve, at any given point gives the permeability at that point. Permeability can be plotted against a typical B-H curve, as shown in Figure 3.8-3. Permeability is not constant; therefore, its value can be stated only at a given value of B or H
B, μrABB-HA,-Hu0H,HFigure3.8-3.variation of relative permeability,μr along the magnetizing curve.RetentivityLet's assume that the ferromagnetic core material has reached its saturation point,maximum flux density.If we remove the magnetizing current flowing through the coilwe would expect the magnetic field around the coil to disappear as the magnetic fluxreduced to zero.However, the magnetic flux does not completely disappear as theelectromagnetic core material still retains some of its magnetism even when thecurrent has stopped flowing in the coil. This ability for a coil to retain some of itsmagnetism within the core after the magnetization process has stopped iscalled retentivity or remanence, while the amount of flux density still remaining in thecore is called Residual Magnetism, Br.The reason for this that some of the tiny molecular magnets do not return to acompletely random pattern and still point in the direction of the original magnetizingfield giving them a sort of “memory" Some ferromagnetic materials have a highretentivity (magnetically hard) making them excellent for producing permanentmagnets.While other ferromagnetic materials have low retentivity (magnetically soft)making them ideal for use in electromagnets, solenoids or relays. One way to reducethis residual flux density to zero is by reversing the direction of the current flowingthrough the coil, thereby making the value of H, the magnetic field strength negative.This effect is called a Coercive Force, He
Figure 3.8-3. variation of relative permeability, μr along the magnetizing curve. Retentivity Let’s assume that the ferromagnetic core material has reached its saturation point, maximum flux density. If we remove the magnetizing 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 magnetization 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 magnetizing 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
MagneticHysteresis LoopB B,B-H-HHH,H-B.BFigure3.8-4.magnetic hysteresis loopThe Figure 3.8-4 shows the behavior of a ferromagnetic core graphically as therelationship between B and His non-linear. Starting with an unmagnetized corebothB and Hwill beat zero,point Oonthemagnetization curve.Ifthemagnetizationcurrent, iis increased in a positive direction to some value the magnetic fieldstrength H increases linearly with i and the flux density B will also increase as shownby the curve from point O to point a as it heads towards saturation.Now if the magnetizing current in the coil is reduced to zero, the magnetic fieldcirculating around the core also reduces to zero. However, the coils magnetic flux willnot reach zero due to the residual magnetism present within the core and this isshown on the curve from point a to point b. To reduce the flux density at point b tozero we need to reverse the current flowing through the coil. The coercive forcereverses the magnetic field re-arranging the molecular magnets until the core becomesunmagnetized at point c. An increase in this reverse current causes the core to bemagnetized in the opposite direction and increasing this magnetization current furtherwill 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 magnetizing current isreduced again to zero the residual magnetism present in the core will be equal to theprevious value but in reverse at point e. Again reversing the magnetizing currentflowing through the coil this time into a positive direction will cause the magneticfluxto reach zero,pointfonthe curve and as beforeincreasing the magnetizationcurrent further in a positive direction will cause the core to reach saturation at point a
Magnetic Hysteresis Loop Figure 3.8-4. magnetic hysteresis loop The Figure 3.8-4 shows the behavior of a ferromagnetic core graphically as the relationship between B and H is non-linear. Starting with an unmagnetized core both B and H will be at zero, point O on the magnetization curve. If the magnetization 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 O to point a as it heads towards saturation. Now if the magnetizing 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 coercive force reverses the magnetic field re-arranging the molecular magnets until the core becomes unmagnetized at point c. An increase in this reverse current causes the core to be magnetized in the opposite direction and increasing this magnetization 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 magnetizing 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 magnetizing 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 magnetization current further in a positive direction will cause the core to reach saturation at point a