Electric power systems are used for the transformation of other types of energy into electrical energy and the transmission of this energy to the point of consumption.
Electric power systems transform mechanical energy into electrical energy and supply this to the end user.
Electric as power is a very cheap way of transferring power.
Electric power can be generated from renewable source e.g. Hydro or Wind.
Alternating Current (AC) electricity is used because it can be transformed between voltage levels efficiently and easily as required.
This allows transmission lines from generator to operate a high voltage-low ampere and then local supplies at lower voltage higher ampere.
A typical generation system would consist of 6 stages:
I. The power generation station (1000V to 26000V 10000V)
II. Step up transformers to high voltage for long distance transmission (138000V to 765000V 133000V)
III. Transmission lines (National grid)
IV. Step down transformers at substations to lower the voltage for local transmission (69000V to 138000V - 10000V)
V. Transmission lines (Local grid)
VI. Local substation to supply the consumer network (240V)
Rotating magnets inside a series of field coils generates electricity. The rotational movement is provided by steam, fluid or wind.
Most of the world power is generated by steam derived from coal, oil, gas or nuclear power source. The power source heats the water into steam at high pressure, which turns the turbine of the generator. Little power is generated from Hydro, Wind or internal combustion engines.
The National grid is a normally high steel tower carrying multi cables with a tower every 250-500M in straight lines.
Local grid is normally on tall wooden poles with few cables space every 100M. In towns underground distribution is used for safety reasons.
A complete delivery system includes protection circuits against overload or short circuits and form factor correction.
Exercise 1
:
1. Are electric power systems used for the transformation of other types of energy into electrical energy?
2. What are used for transmission of electric energy to the point of consumption?
3. Into what type of energy do electric power systems transform mechanical energy?
4. Is electricity a very expensive way of transferring power?
5. Can electric power be generated from renewable source? Give an example, please.
6. Can alternating current (AC) electricity be transformed between voltage levels efficiently and easily as required?
7. Does rotating magnets inside a series of field coils generate electricity?
8. What are the sources of a rotational movement of generator?
9. Is most of the world power generated by steam derived from coal, oil, gas or nuclear power source?
10. How do we call high steel towers carrying multi cables with a tower every 250-500M in straight lines?
11. How do we call the tall wooden poles with few cables space every 100 meter?
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12. What does a complete delivery system include?
Exercise 2
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Exercise 4
:
1. Electric power systems are used for the transmission of electric energy
the point of consumption.
3. Electric power can be generated from the renewable source.
4. Alternating current is used because it can be transformed between voltage levels efficiently and easily.
5. A typical generation system would consist of 6 stages.
6. The rotating movement is provided by steam.
7. Coal, oil and gas are mostly used for generation of power.
8. Generally the rotating movement is provided by steam, fluid and wind.
9. Step down transformers are used at substations to lower the voltage for local transmission.
10. Step up transformers are used to high up voltage for long distance transmission.
Exercise 5
, :
1. Transform; energy; mechanical; electrical.
2. Source; generated; renewable; can be.
3. Electricity; alternating; current; efficiently; voltages; between.
4. Magnets; rotating; generated; electricity.
5. Steam; fluid; wind; provide; movement; rotational.
6. Steam; turbine; generator; turns.
7. High; steel; grid; national; towers; multi; cables.
8. Wooden; poles; local; grid; few; cables.
9. Protection; includes; system; delivery; circuits.
Exercise 6
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4. 6 .
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Exercise 7
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Unit 5
Text A
Electric Motors and Generators.
Electric motors and generators are used to convert mechanical energy into electrical energy, or electrical energy into mechanical energy, by electromagnetic means. A machine that converts mechanical energy into electrical energy is called a generator, and a machine that converts electrical energy into mechanical energy is called a motor.
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Two related physical principles underlie the operation of generators and motors. The first is the principle of electromagnetic induction discovered by the British scientist Michael Faraday in 1831. If a conductor is moved through a magnetic field, or if the strength of a stationary conducting loop is made to vary, a current is set up or induced in the conductor.
The converse of this principle is that of electromagnetic reaction, first observed by the French physicist Andre Marie Ampere in 1820. If a current is passed through a conductor located in a magnetic field, the field exerts a mechanical force on it.
The simplest of all dynamoelectric machines is the disk dynamo developed by Faraday. It consists of a copper disk mounted so that part of the disk, from the center to the edge, is between the poles of a horseshoe magnet. When the disk is rotated, a current is induced between the center of the disk and its edge by the action of the field of the magnet. The disk can be made to operate as a motor by applying a voltage between the edge of the disk and its center, causing the disk to rotate because of the force produced by magnetic reaction.
The magnetic field of a permanent magnet is strong enough to operate only a small practical dynamo or motor. As a result, for large machines, electromagnets are employed. Both motors and generators consist of two basic units, the field, which is the electromagnet with its coils, and the armature, the structure that supports the conductors, which cut the magnetic field and carry the induced current in a generator or the exciting current in a motor. The armature is usually a laminated soft-iron core around which conducting wires are wound in coils
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Words and expressions
electric motor -
electric generator -
mechanical energy -
electrical energy -
electromagnetic means -
physical principle -
electromagnetic induction -
magnetic field -
conducting loop -
electromagnetic reaction -
pass through -
exert -
dynamoelectric machines -
force produced -
permanent magnet -
electromagnet -
basic units -
exciting current -
armature -
soft-iron core -
Exercise 1
:
1. By means of what devices mechanical energy is converted into electrical energy?
2. How a machine that converts mechanical energy into electrical energy is called?
3. How a machine that converts electrical energy into mechanical energy is called?
4. What physical principles underlie the operation of generators and motors?
5. Who was the first to discover the principle of electromagnetic induction?
6. Who was the first to observe the principle of electromagnetic reaction?
7. What is the simplest of all dynamoelectric machines?
8. Is the magnetic field of a permanent magnet strong enough to operate big practical dynamo or motor?
9. What kinds of magnets are employed for large machines?
10. What do both motors and generators consist of?
Exercise 2
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Exercise 3
:
1. Electric motors and generators are used to convert mechanical energy into electrical energy.
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2. A machine that converts mechanical energy into electrical energy is called a generator.
3. A machine that converts electrical energy into mechanical energy is called a motor.
4. The British scientist Michael Faraday discovered the principle of electromagnetic induction in 1831.
5. The French physicist Andre Marie Ampere first observed electromagnetic reaction in 1820.
6. The magnetic field of a permanent magnet is strong enough to operate only a small motor.
7. Electromagnets are used for large machines.
8. Motors and generators consist of two basic units.
9. The armature is usually a laminated soft-iron core.
Exercise 4
:
1. Energy; convert; mechanical; into; electrical; motors; used.
2. Generator; convert; energy; electrical; mechanical.
3. Induction; principle; discovered; scientist; Faraday.
4. Magnet; permanent; strong enough; small; operate; motor.
5. Machines; large; electromagnets; used.
6. Consist of; motor; basic units; field; armature.
7. Soft-iron; core; armature; made of.
8. Used; electric power systems; electric motors; electric generators.
Exercise 5
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2. .
3. .
4. .
5. .
6. .
7. .
8. .
Exercise 6
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Text B
DC Motors
In general, DC motors are similar to DC generators in construction. They may be described as generators run backwards. When current is passed through the armature of a DC motor, a torque is generated by magnetic reaction, and the armature revolves. The action of the commutator and the connections of the field coils of motors are precisely the same as those used for generators. The revolution of the armature induces a voltage in the armature windings. This induced voltage is opposite in direction to the outside voltage applied to the armature, and hence is called back voltage. As the motor rotates more rapidly, the back voltage rises until it is almost equal to the applied voltage. The current is then small, and the speed of the motor will remain constant as long as the motor is not under load and is performing no mechanical work except that required to turn the armature. Under load the armature turns more slowly, reducing the back voltage and permitting a larger current to flow in the armature. The motor is thus able to receive more electric power from the source supplying it and to do more mechanical work.
Because the speed of rotation controls the flow of current in the armature, special devices must be used for starting DC motors. When the armature is at rest, it has virtually no resistance, and if the normal working voltage is applied, a large current will flow, which may damage the commutator or the armature windings. The usual means of preventing such damage is the use of a starting resistance in series with the armature to lower the current until the motor begins to develop an adequate back voltage. As the motor picks up speed, the resistance is gradually reduced, either manually or automatically.
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The speed at which a DC motor operates depends on the strength of the magnetic field acting on the armature, as well as on the armature current. The stronger the field, the slower is the rate of rotation needed to generate a back voltage large enough to counteract the applied voltage. For this reason the speed of DC motors can be controlled by varying the field current.
Words and expressions
similar in construction -
torque -
magnetic reaction -
revolve -
commutator - ;
connection -
field coil -
revolution of the armature -
induce -
back voltage -
under load -
constant -
resistance -
adequate -
gradually reduce -
manually -
automatically -
strength of the magnetic field -
rate of rotation -
Exercise 1
:
1. Are DC motors similar to DC generators in construction?
2. Is torque generated by magnetic reaction when current is passed through the armature of a motor?
3. Are the action of the commutator and the connections of motors and generators the same?
4. Does the revolution of the armature induce a voltage in the armature windings?
5. Is induced voltage opposite in direction to the outside voltage applied to the armature?
6. When does the back voltage rise until it is almost equal to the applied voltage?
7. Does the armature turn more slowly under load?
8. How does back voltage reduce a larger current flow in the armature?
9. What devices must be used for starting DC motors?
10. Does the speed at which DC motors operate depend on the strength of the magnetic field acting on the armature?
11. Is it right that the stronger the field, the slower is the rate of rotation?
12.How can the speed of DC motors be controlled?
Exercise 2
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Exercise 3
:
1. DC motors are similar to DC generators in construction.
2. The revolution of the armature induces a voltage in the armature windings.
3. Induced voltage is opposite in direction to the outside voltage.
4. When the motor rotates more rapidly, the back voltage rises.
5. Under load the armature turns more slowly.
6. The speed of rotation controls the flow of current.
7. When the armature is at rest, it has no resistance.
8. The usual means of preventing damage is the use of a starting resistance.
9. The speed of DC motor depends on the strength of the magnetic field.
10.Varying the field current can control the speed of DC motors.
Exercise 4
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1. Motors: similar; construction; generators; in general.
2. Torque; magnetic; generate; relation; armature; revolve.
3. Voltage; induce; winding; armature; revolution; voltage.
4. Rotate; motor; rapidly; voltage; back; equal; apply; rise.
5. Load; slowly; turn; under; armature.
6. Speed; control; rotation; current; flow; armature.
7. Magnetic; field; speed; operate; motor; depend; act; armature.
8. Speed; can be controlled; vary; current; field.
Exercise 5
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7. 220 .
Exercise 6
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Text C
Direct-Current (DC) Generators
If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution.
In older machines this reversal is accomplished by means of a commutator, a split metal ring mounted on the shaft of the armature. The two halves of the ring are insulated from each other and serve as the terminals of the armature coil. Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. Thus there is a flow of unidirectional current in the outside circuit to which the generator is connected. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage. The highest potential commonly developed by such generators is 1500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, diode rectifiers.
Modern DC generators use drum armatures that usually consist of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant. Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.
DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound generator has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under varying electrical loads. The series-wound generator is used principally to supply a constant current at variable voltage. A magneto is a small DC generator with a permanent-magnet field.
Words and expressions
stationary field poles -
half of each revolution -
steady flow -
unidirectional -
each revolution -
metal ring -
shaft of the armature -
two halves of the ring -
serve as the terminal -
armature coil -
brush -
connecting the coil -
external wire -
reverses its direction -
outside circuit -
sparking between brushes -
highest potential -
power electronic devices -
diode rectifier -
drum armature -
longitudinal slits - ,
loop - ,
high-intensity area -
electromagnetic poles -
interpoles - ()
distortion in magnetic flux - ()
series-wound generator -
shunt-wound generator -
connected in parallel -
compound-wound generator -
constant voltage -
electrical load -
permanent-magnet field -
Exercise 1
:
1. How does the current in the armature move if it revolves between two stationary field poles?
2. What should be done to produce a steady flow of direct current from generator?
3. Is in older machines the flow of direct current provided by means of a commutator?
4. Is there a flow of unidirectional current in the outside circuit to which the generator is connected?
5. Are DC generators usually operated at fairly low voltages?
6. Why DC generators usually operate at low voltages?
7. Do drum armatures usually consist of a large number of windings?
8. Why the fields of modern generator equipped with four or more electromagnetic poles?
9. How DC generators are commonly classified?
10. Is the series-wound generator used mainly to supply a constant current at variable voltage?
Exercise 2
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Exercise 3
:
1. Armature revolves between two stationary field poles.
2. The current flow outside the generator once during each revolution.
3. Fixed brushes of metal or carbon are held against the commutator.
4. Each brush is in contact alternately with the halves of the commutator.
5. DC generators are usually operated at fairly low voltages.
6. Modern DC generators use drum armatures.
7. Fields of modern generators are usually equipped with four or more electromagnetic poles.
8. Smaller interpoles are added to compensate for distortions in the magnetic flux.
9. DC generators are commonly classified according to the method used to provide field current.
10. The series-wound generator is used principally to supply a constant current at variable voltage.
Exercise 4
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Text D
AC Motors
Two basic types of motors are designed to operate on polyphase alternating current, synchronous motors and induction motors. The synchronous motor is essentially a three-phase alternator operated in reverse. The field magnets are mounted on the rotor and are excited by direct current, and the armature winding is divided into three parts and fed with three-phase alternating current. The constant speed of a synchronous motor is advantageous in certain devices; however, in applications where the mechanical load on the motor becomes very great, synchronous motors cannot be used, because if the motor slows down under load it will fall out of step with the frequency of the current and come to a stop. Synchronous motors can be made to operate from a single-phase power source by the inclusion of suitable circuit elements that cause a rotating magnetic field.
The simplest of all electric motors is the squirrel-cage type of induction motor used with a three-phase supply. The rotating member consists of a core in which are imbedded a series of heavy conductors arranged in a circle around the shaft and parallel to it. With the core removed, the rotor conductors resemble in form the cylindrical cages once used to exercise pet squirrels. The three-phase current flowing in the stationary armature windings generates a rotating magnetic field, and this field induces a current in the conductors of the cage. The magnetic reaction between the rotating field and the current-carrying conductors of the rotor makes the rotor turn. If the rotor is revolving at exactly the same speed as the magnetic field, no currents will be induced in it, and hence the rotor should not turn at a synchronous speed. In operation the speeds of rotation of the rotor and the field differ by about 2 to 5 percent. This speed difference is known as slip. Motors with squirrel-cage rotors can be used on single-phase alternating current by means of various arrangements of inductance and capacitance that alter the characteristics of the single-phase voltage and make it resemble a two-phase voltage. Such motors are called split-phase motors or condenser motors (or capacitor motors), depending on the arrangement used. Single-phase squirrel-cage motors do not have a large starting torque, and for applications where such torque is required, repulsion-induction motors are used. A repulsion-induction motor may be of the split-phase or condenser type, but has a manual or automatic switch that allows current to flow between brushes on the commutator when the motor is starting, and short-circuits all commutator segments after the motor reaches a critical speed. Repulsion-induction motors are so named because their starting torque depends on the repulsion between the rotor and the stator, and their torque while running depends on induction. Series-wound motors with commutators, which will operate on direct or alternating current, are called universal motors. They are usually made only in small sizes and are commonly used in household appliances.
Words and expressions
polyphase alternating current -
synchronous motors -
induction motors -
three-phase alternator -
field magnets -
a series of heavy conductors -
arranged in a circle -
slip - ,
split-phase motors -
repulsion-induction motors - -
stator -
Exercise 1
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Text E
