The losses occurring in a d.c. motor are the same as in a d.c. generator
(i) copper losses
(ii) Iron losses or magnetic losses
(iii) mechanical losses As in a generator, these losses cause
(a) an increase of machine temperature and
(b) reduction in the efficiency of the d.c. motor.
The following points may be noted:
(i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (Ia),
Loss in interpole winding = Ia 2× Resistance of interpole winding
Loss in compensating winding = Ia 2× Resistance of compensating winding
(ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses are nearly constant.
(iii) The mechanical losses (i.e. friction and windage) vary as the cube of the speed of rotation of the d.c. machine (generator or motor). Since d.c. machines are generally operated at constant speed, mechanical losses are
considered to be constant.
Monday, June 27, 2011
Ferromagnetic Materials
There are a number of crystalline materials that exhibit ferromagnetism (or ferrimagnetism). The table on the right lists a representative selection of them, along with their Curie temperatures, the temperature above which they cease to exhibit spontaneous magnetization (see below).
Ferromagnetism is a property not just of the chemical makeup of a material, but of its crystalline structure and microscopic organization. There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called Heusler alloys, named after Fritz Heusler. Conversely there are nonmagnetic alloys, such as types of stainless steel, composed almost exclusively of ferromagnetic metals.
One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid quenching (cooling) of a liquid alloy. These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low coercivity, low hysteresis loss, high permeability, and high electrical resistivity. A typical such material is a transition metal-metalloid alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component (B, C, Si, P, or Al) that lowers the melting point.
A relatively new class of exceptionally strong ferromagnetic materials are the rare-earth magnets. They contain lanthanide elements that are known for their ability to carry large magnetic moments in well-localized f-orbitals.
Ferromagnetism is a property not just of the chemical makeup of a material, but of its crystalline structure and microscopic organization. There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called Heusler alloys, named after Fritz Heusler. Conversely there are nonmagnetic alloys, such as types of stainless steel, composed almost exclusively of ferromagnetic metals.
One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid quenching (cooling) of a liquid alloy. These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low coercivity, low hysteresis loss, high permeability, and high electrical resistivity. A typical such material is a transition metal-metalloid alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component (B, C, Si, P, or Al) that lowers the melting point.
A relatively new class of exceptionally strong ferromagnetic materials are the rare-earth magnets. They contain lanthanide elements that are known for their ability to carry large magnetic moments in well-localized f-orbitals.
Ampere's Law
Ampere's Law
The magnetic field in space around an electric current is proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source. Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to thepermeability times the electric current enclosed in the loop.
The magnetic field in space around an electric current is proportional to the electric current which serves as its source, just as the electric field in space is proportional to the charge which serves as its source. Ampere's Law states that for any closed loop path, the sum of the length elements times the magnetic field in the direction of the length element is equal to thepermeability times the electric current enclosed in the loop.
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.
Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC electrical generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines. Alternators in power stations driven by steam turbines are called turbo-alternators
Alternators generate electricity using the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called thestator. The field cuts across the conductors, generating an induced emf (electromotive force), as the mechanical input causes the rotor to turn.
The rotating magnetic field induces an AC voltage in the stator windings. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other.
The rotors magnetic field may be produced by induction (as in a "brush-less" alternator), by permanent magnets (as in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotors magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding, which allows control of the alternators generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
An automatic voltage control device controls the field current to keep output voltage constant. If the output voltage from the stationary armature coils drops due to an increase in demand, more current is fed into the rotating field coils through the Automatic Voltage Regulator or AVR. This increases the magnetic field around the field coils which induces a greater voltage in the armature coils. Thus, the output voltage is brought back up to its original value.
Alternators in central power stations use may also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults.
Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC electrical generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines. Alternators in power stations driven by steam turbines are called turbo-alternators
Alternators generate electricity using the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called thestator. The field cuts across the conductors, generating an induced emf (electromotive force), as the mechanical input causes the rotor to turn.
The rotating magnetic field induces an AC voltage in the stator windings. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other.
The rotors magnetic field may be produced by induction (as in a "brush-less" alternator), by permanent magnets (as in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotors magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding, which allows control of the alternators generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
An automatic voltage control device controls the field current to keep output voltage constant. If the output voltage from the stationary armature coils drops due to an increase in demand, more current is fed into the rotating field coils through the Automatic Voltage Regulator or AVR. This increases the magnetic field around the field coils which induces a greater voltage in the armature coils. Thus, the output voltage is brought back up to its original value.
Alternators in central power stations use may also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults.
Air Core Transformer Working
Energy is also transported from one circuit to another with air core transformers. With air core transformers, two cable wire-like coils referred to as windings are enfolded onto some form of core substance.
In most circumstances, the wire coils are wound onto a rectangular cardboard-like structure which, in fact, the core substance is air resulting in the transformer being referred to as an air core transformer. In addition, with air core transformers, "all" of the current (electrical energy) is considered to be an exciting or electrifying current, and the current stimulates or induces a secondary voltage that is comparative to a mutual inductance or shared stimulation of transported energy.
A functioning air core transformer can be created easily by simply placing the windings very close to one another. With many air core transformers the coils are wound on a core substance created with material that has superior magnetic permeability. This high magnetic material within the core substance causes the magnetic field which is induced by the electrical current in the primary to become intensely stronger and therefore increases the effectiveness of the air core transformer.
As a result there are no power losses and the ratio of primary voltage to secondary voltage is identical to the ratio of the number of turns within the primary winding coil to the number of turns within the secondary winding coil.
In most circumstances, the wire coils are wound onto a rectangular cardboard-like structure which, in fact, the core substance is air resulting in the transformer being referred to as an air core transformer. In addition, with air core transformers, "all" of the current (electrical energy) is considered to be an exciting or electrifying current, and the current stimulates or induces a secondary voltage that is comparative to a mutual inductance or shared stimulation of transported energy.
A functioning air core transformer can be created easily by simply placing the windings very close to one another. With many air core transformers the coils are wound on a core substance created with material that has superior magnetic permeability. This high magnetic material within the core substance causes the magnetic field which is induced by the electrical current in the primary to become intensely stronger and therefore increases the effectiveness of the air core transformer.
As a result there are no power losses and the ratio of primary voltage to secondary voltage is identical to the ratio of the number of turns within the primary winding coil to the number of turns within the secondary winding coil.
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