The DC machine armature windings are always of the closed continuous type of double-layer lap
or wave winding. For small machines, the coils are directly wound into the armature slots using
automatic winders. In large machines, the coils are preformed and then inserted into the armature
slots. Each coil consists of a number of turns of wire, each turn taped and insulated from the
other turns and form the rotor slot. Each side of the turn is called a conductor. The number of
conductors on a machine's armature is given by
Z = 2CNc----- (1)
Where Z = number of conductors on rotor
C = number of coils on rotor
Nc= number of turns per coil
Since the voltage generated in the conductor under the South Pole opposite the voltage generated
in the conductor under the North Pole, the coil span is made equal to 180 electrical degrees, i.e.,
one pole pitch. In a 2-pole machine 180 electrical degrees is equal to 180 mechanical degrees,
whereas in a 4-pole machine 180 electrical degrees is equal to 90 mechanical degrees.
Thursday, July 14, 2011
Coupled Inductors
When a steady current flows in one coil as in the left illustration, amagnetic field is produced in the other coil. But since that magnetic field is not changing, Faraday's lawtells us that there will be no induced voltage in the secondary coil. But if the switch is opened to stop the current as in the middle illustration, there will be a change in magnetic field in the right hand coil and a voltage will be induced. A coil is a reactionary device, not liking any change! The induced voltage will cause a current to flow in the secondary coil which tries to maintain the magnetic field which was there. The fact that the induced field always opposes the change is an example of Lenz' law. Once the current is interrupted and the switch is closed to cause the current to flow again as in the right hand example, an induced current in the opposite direction will oppose that buildup of magnetic field. This persistent generation of voltages which oppose the change in magnetic field is the operating principle of a transformer. The fact that a change in the current of one coil affects the current and voltage in the second coil is quantified in the property called mutual inductance.
Compound-Wound Generators
Compound-wound generators have a series-field winding in addition to a shunt-field winding, as shown in figure 1-17. The shunt and series windings are wound on the same pole pieces.
In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.
By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure 1-18, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.
In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding.
By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure 1-18, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are seldom, if ever, as perfect as shown.
COMPOUND MOTOR
A compound motor has two field windings. One is a shunt field connected in parallel with the armature; the other is a series field that is connected in series with the armature.
The shunt field gives this type of motor the constant speed advantage of a regular shunt motor. The series field gives it the advantage of being able to develop a large torque when the motor is started under a heavy load. It should not be a surprise that the compound motor has both shunt- and series-motor characteristics.
The shunt field gives this type of motor the constant speed advantage of a regular shunt motor. The series field gives it the advantage of being able to develop a large torque when the motor is started under a heavy load. It should not be a surprise that the compound motor has both shunt- and series-motor characteristics.
Commutator
It is the process of conversion of generated ac voltage into the armature of a dc generator to dc voltage at the terminal of the dc generator by use of pair of brushes and commutator.
OR It is the process of conversion of given dc voltage at the terminal of the dc motor to ac voltage in the armature windings in a dc motor by use of pair of brushes and commutator.
Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings.
In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other. For good commutation, the following points may be noted:
(i) If a motor does not have commutating poles (compoles), the brushes
must be given a negative lead i.e., they must be shifted from G.N.A.
against the direction of rotation of, the motor.
(ii) By using interpoles, a d.c. motor can be operated with fixed brush
positions for all conditions of load. For a d.c. motor, the commutating
poles must have the same polarity as the main poles directly back of
them. This is the opposite of the corresponding relation in a d.c.
generator.
Note. A d.c. machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a d.c. machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity.
OR It is the process of conversion of given dc voltage at the terminal of the dc motor to ac voltage in the armature windings in a dc motor by use of pair of brushes and commutator.
Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings.
In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other. For good commutation, the following points may be noted:
(i) If a motor does not have commutating poles (compoles), the brushes
must be given a negative lead i.e., they must be shifted from G.N.A.
against the direction of rotation of, the motor.
(ii) By using interpoles, a d.c. motor can be operated with fixed brush
positions for all conditions of load. For a d.c. motor, the commutating
poles must have the same polarity as the main poles directly back of
them. This is the opposite of the corresponding relation in a d.c.
generator.
Note. A d.c. machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a d.c. machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity.
Characteristics of DC Motor
Direct-Current Motors - DC motors are divided into three classes, designated according to the method of connecting the armature and the field windings as shunt-series and compound wound.
Shunt-Wound Motors - This type of motor runs practically constant speed, regardless of the load. It is the type generally used in commercial practice and is usually recommended where starting conditions are not usually severs. Speed of the shunt-wound motors may be regulated in two ways: first, by inserting resistance in series with the armature, thus decreasing speed: and second, by inserting resistance in the field circuit, the speed will vary with each change in load: in the latter, the speeds is practically constant for any setting of the controller. This latter is the most generally used for adjustable-speed service, as in the case of machine tools.
Series-Wound DC Motors - This type of motor speed varies automatically with the load, increasing as the load decreases. Use of series motor is generally limited to case where a heavy power demand is necessary to bring the machine up to speed, as in the case of certain elevator and hoist installations, for steelcars, etc. Series-wound motors should never be used where the motor cab be started without load, since they will race to a dangerous degree.
Compound-Wound DC Motors - A combination of the shunt wound and series wound types combines the characteristics of both. Characteristics may be varied by varying the combination of the two windings. These motors are generally used where severe starting conditions are met and constant speed is required at the same time.
Shunt-Wound Motors - This type of motor runs practically constant speed, regardless of the load. It is the type generally used in commercial practice and is usually recommended where starting conditions are not usually severs. Speed of the shunt-wound motors may be regulated in two ways: first, by inserting resistance in series with the armature, thus decreasing speed: and second, by inserting resistance in the field circuit, the speed will vary with each change in load: in the latter, the speeds is practically constant for any setting of the controller. This latter is the most generally used for adjustable-speed service, as in the case of machine tools.
Series-Wound DC Motors - This type of motor speed varies automatically with the load, increasing as the load decreases. Use of series motor is generally limited to case where a heavy power demand is necessary to bring the machine up to speed, as in the case of certain elevator and hoist installations, for steelcars, etc. Series-wound motors should never be used where the motor cab be started without load, since they will race to a dangerous degree.
Compound-Wound DC Motors - A combination of the shunt wound and series wound types combines the characteristics of both. Characteristics may be varied by varying the combination of the two windings. These motors are generally used where severe starting conditions are met and constant speed is required at the same time.
Back EMF
The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF ) [1] is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is represented by Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. One practical application is to use this phenomenon to indirectly measure motor speed and position.[2] Counter EMF is a voltage developed in an inductor network by a pulsating current or an alternating current [1]. The voltage's polarity is at every moment the reverse of the input voltage.[1][3]
In a motor using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.[4]
To experience the effect of counter-electromotive force one can perform this simple exercise. With a window closed, lift the switch of an electric window in a car and hold it momentarily and notice the idle RPM drop. The electric motor in the door is stationary and therefore the inrush current will be very high; the alternator will try to provide for the large current which subsequently drags down the engine. As soon as the power window motor overcomes its inertia and starts spinning, back EMF will be produced, exerting less load on the alternator. Hence, the engine speed will return to normal.
In a motor using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.[4]
To experience the effect of counter-electromotive force one can perform this simple exercise. With a window closed, lift the switch of an electric window in a car and hold it momentarily and notice the idle RPM drop. The electric motor in the door is stationary and therefore the inrush current will be very high; the alternator will try to provide for the large current which subsequently drags down the engine. As soon as the power window motor overcomes its inertia and starts spinning, back EMF will be produced, exerting less load on the alternator. Hence, the engine speed will return to normal.
Armature Reaction in DC Motor
As in a d.c. generator, armature reaction also occurs in a d.c. motor. This is expected because when current flows through the armature conductors of a d.c. motor, it produces flux (armature flux) which lets on the flux produced by the main poles. For amotor with the same polarity and direction of rotation as is forgenerator, the direction of armature reaction field is reversed.
(i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that:
Armature reaction in a d.c. generator weakens the jinx at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a d. c. motor produces the opposite effect.
(ii) In case of a d.c. generator, with brushes along G.N.A. and no commutating poles used, the brushes must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a d.c. motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation.
With no commutating poles used, the brushes are given a forward lead in a d.c. generator and backward lead in a d.c. motor.
(iii) By using commutating poles (compoles), a d.c. machine can be operated with fixed brush positions for all conditions of load. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor (with consequent reversal of current), the polarities of commutating poles must be of opposite sign.
Therefore, in a d.c. motor, the commutating poles must have the same
polarity as the main poles directly back of them. This is the opposite of
the corresponding relation in a d.c. generator.
(i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that:
Armature reaction in a d.c. generator weakens the jinx at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a d. c. motor produces the opposite effect.
(ii) In case of a d.c. generator, with brushes along G.N.A. and no commutating poles used, the brushes must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a d.c. motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation.
With no commutating poles used, the brushes are given a forward lead in a d.c. generator and backward lead in a d.c. motor.
(iii) By using commutating poles (compoles), a d.c. machine can be operated with fixed brush positions for all conditions of load. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor (with consequent reversal of current), the polarities of commutating poles must be of opposite sign.
Therefore, in a d.c. motor, the commutating poles must have the same
polarity as the main poles directly back of them. This is the opposite of
the corresponding relation in a d.c. generator.
Monday, June 27, 2011
Losses in DC Motors
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.
(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.
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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