Unit-3
Magnetic Material
Module - 3 : Transformers
Contents: Magnetic materials, BH characteristics, ideal and practical transformer, equivalent
circuit, losses in transformers, regulation and efficiency. Auto-transformer and three-phase
transformer connections.
Classification of Magnetic Materials:
All materials can be classified in terms of their magnetic behavior falling into one of five
categories depending on their bulk magnetic susceptibility. The two most common types of
magnetism are diamagnetism and paramagnetism, which account for the magnetic properties of
most of the periodic table of elements at room temperature.
Diamagnetism: In a diamagnetic material the atoms have no net magnetic moment when there
is no applied field. Under the influence of an applied field (H) the spinning electrons presses and
this motion, which is a type of electric current, produces a magnetisation (M) in the opposite
direction to that of the applied field. All materials have a diamagnetic effect, however, it is often
the case that the diamagnetic effect is masked by the larger paramagnetic or ferromagnetic
term. The value of susceptibility is independent of temperature.
Paramagnetism: There are several theories of paramagnetism, which are valid for specific
types of material. The Langevin model, which is true for materials with non-interacting localised
electrons, states that eachatom has a magnetic moment which is randomly oriented as a result
of thermal agitation. The application of a magnetic field creates a slight alignment of these
moments and hence a low magnetisation in the same direction as the applied field. As the
temperature increases, then the thermal agitation will increase and it will become harder to
align the atomic magnetic moments and hence the susceptibility will decrease. This behaviour is
known as the Curie law.
Ferromagnetism: Ferromagnetism is only possible when atoms are arranged in a lattice and
the atomic magnetic moments can interact to align parallel to each other. This effect is explained
in classical theory by the presence of a molecular field within the ferromagnetic material, which
was first postulated by Weiss in 1907. This field is sufficient to magnetise the material to
saturation. In quantum mechanics, the Heisenberg model of ferromagnetism describes the
parallel alignment of magnetic moments in terms of an exchange interaction between
neighbouring moments. Weiss postulated the presence of magnetic domains within the
material, which are regions where the atomic magnetic moments are aligned. The movement of
these domains determines how the material responds to a magnetic field and as a consequence
the susceptible is a function of applied magnetic field. Therefore, ferromagnetic materials are
usually compared in terms of saturation magnetisation (magnetisation when all domains are
aligned) rather than susceptibility. In the periodic table of elements only Fe, Co and Ni are
ferromagnetic at and above room temperature. As ferromagnetic materials are heated then the
thermal agitation of the atoms means that the degree of alignment of the atomic magnetic
moments decreases and hence the saturation magnetisation also decreases.
Anti-ferromagnetism: In the periodic table the only element exhibiting antiferromagnetism at
room temperature is chromium. Antiferromagnetic materials are very similar to ferromagnetic
materials but the exchange interaction between neighbouring atoms leads to the anti-parallel
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
,alignment of the atomic magnetic moments. Therefore, the magnetic field cancels out and the
material appears to behave in the same way as a paramagnetic material. Like ferromagnetic
materials these materials become paramagnetic above a transition temperature, known as the
Néel temperature, TN. (Cr: TN=37ºC).
Ferrimagnetism: Ferrimagnetism is only observed in compounds, which have more complex
crystal structures than pure elements. Within these materials the exchange interactions lead to
parallel alignment of atoms in some of the crystal sites and anti-parallel alignment of others.
The material breaks down into magnetic domains, just like a ferromagnetic material and the
magnetic behaviour is also very similar, although ferrimagnetic materials usually have lower
saturation magnetisations. For example in Barium ferrite (BaO.6Fe2O3).
Magnetic hysteresis:
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
, 1. Once magnetic saturation has been achieved, a decrease in the applied field back to zero
results in a macroscopically permanent or residual magnetization, known as remanance, Mr.
The corresponding induction, Br, is called retentivity or remanent induction of the magnetic
material. This effect of retardation by material is called hysteresis.
2. The magnetic field strength needed to bring the induced magnetization to zero is termed as
coercivity, Hc. This must be applied anti-parallel to the original field.
3. A further increase in the field in the opposite direction results in a maximum induction in the
opposite direction. The field can once again be reversed, and the field-magnetization loop can be
closed, this loop is known as hysteresis loop or B-H plot or M- H plot.
Semi-hard magnets:
•The area within the hysteresis loop represents the energy loss per unit volume of material for
one cycle.
•The coercivity of the material is a micro-structure sensitive property. This dependence is
known as magnetic shape anisotropy.
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
Magnetic Material
Module - 3 : Transformers
Contents: Magnetic materials, BH characteristics, ideal and practical transformer, equivalent
circuit, losses in transformers, regulation and efficiency. Auto-transformer and three-phase
transformer connections.
Classification of Magnetic Materials:
All materials can be classified in terms of their magnetic behavior falling into one of five
categories depending on their bulk magnetic susceptibility. The two most common types of
magnetism are diamagnetism and paramagnetism, which account for the magnetic properties of
most of the periodic table of elements at room temperature.
Diamagnetism: In a diamagnetic material the atoms have no net magnetic moment when there
is no applied field. Under the influence of an applied field (H) the spinning electrons presses and
this motion, which is a type of electric current, produces a magnetisation (M) in the opposite
direction to that of the applied field. All materials have a diamagnetic effect, however, it is often
the case that the diamagnetic effect is masked by the larger paramagnetic or ferromagnetic
term. The value of susceptibility is independent of temperature.
Paramagnetism: There are several theories of paramagnetism, which are valid for specific
types of material. The Langevin model, which is true for materials with non-interacting localised
electrons, states that eachatom has a magnetic moment which is randomly oriented as a result
of thermal agitation. The application of a magnetic field creates a slight alignment of these
moments and hence a low magnetisation in the same direction as the applied field. As the
temperature increases, then the thermal agitation will increase and it will become harder to
align the atomic magnetic moments and hence the susceptibility will decrease. This behaviour is
known as the Curie law.
Ferromagnetism: Ferromagnetism is only possible when atoms are arranged in a lattice and
the atomic magnetic moments can interact to align parallel to each other. This effect is explained
in classical theory by the presence of a molecular field within the ferromagnetic material, which
was first postulated by Weiss in 1907. This field is sufficient to magnetise the material to
saturation. In quantum mechanics, the Heisenberg model of ferromagnetism describes the
parallel alignment of magnetic moments in terms of an exchange interaction between
neighbouring moments. Weiss postulated the presence of magnetic domains within the
material, which are regions where the atomic magnetic moments are aligned. The movement of
these domains determines how the material responds to a magnetic field and as a consequence
the susceptible is a function of applied magnetic field. Therefore, ferromagnetic materials are
usually compared in terms of saturation magnetisation (magnetisation when all domains are
aligned) rather than susceptibility. In the periodic table of elements only Fe, Co and Ni are
ferromagnetic at and above room temperature. As ferromagnetic materials are heated then the
thermal agitation of the atoms means that the degree of alignment of the atomic magnetic
moments decreases and hence the saturation magnetisation also decreases.
Anti-ferromagnetism: In the periodic table the only element exhibiting antiferromagnetism at
room temperature is chromium. Antiferromagnetic materials are very similar to ferromagnetic
materials but the exchange interaction between neighbouring atoms leads to the anti-parallel
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
,alignment of the atomic magnetic moments. Therefore, the magnetic field cancels out and the
material appears to behave in the same way as a paramagnetic material. Like ferromagnetic
materials these materials become paramagnetic above a transition temperature, known as the
Néel temperature, TN. (Cr: TN=37ºC).
Ferrimagnetism: Ferrimagnetism is only observed in compounds, which have more complex
crystal structures than pure elements. Within these materials the exchange interactions lead to
parallel alignment of atoms in some of the crystal sites and anti-parallel alignment of others.
The material breaks down into magnetic domains, just like a ferromagnetic material and the
magnetic behaviour is also very similar, although ferrimagnetic materials usually have lower
saturation magnetisations. For example in Barium ferrite (BaO.6Fe2O3).
Magnetic hysteresis:
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
, 1. Once magnetic saturation has been achieved, a decrease in the applied field back to zero
results in a macroscopically permanent or residual magnetization, known as remanance, Mr.
The corresponding induction, Br, is called retentivity or remanent induction of the magnetic
material. This effect of retardation by material is called hysteresis.
2. The magnetic field strength needed to bring the induced magnetization to zero is termed as
coercivity, Hc. This must be applied anti-parallel to the original field.
3. A further increase in the field in the opposite direction results in a maximum induction in the
opposite direction. The field can once again be reversed, and the field-magnetization loop can be
closed, this loop is known as hysteresis loop or B-H plot or M- H plot.
Semi-hard magnets:
•The area within the hysteresis loop represents the energy loss per unit volume of material for
one cycle.
•The coercivity of the material is a micro-structure sensitive property. This dependence is
known as magnetic shape anisotropy.
KEE-101T/KEE-201T Basic Electrical Engineering GCET, Greater Noida
