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Consult the TEXTS FOR SUPPLEMENTARY READING and complete the information about electricity (Texts 34, 35, 36, 37). Be ready to discuss the information you have read.
19. Read the following text and give its brief summary:
History of Electricity Use
Thales of Miletus (640-546 B.C.) is credited with the discovery that amber when rubbed acquire the property of attracting light objects. The word electricity comes from «elektron» the Greek word for amber. Otto von Guericke invented the first static electric generator in 1675, while the first current generator was made by Alosio Galvani in 1780. But except for some supposed medicinal applications, electricity had little use.
Communication, the first of the great uses for electricity, began with the telegraph invented by Samuel Morse around 1840, to be followed by the telephone, radio and television. Thomas Edison added lighting in 1880, which was soon followed by working electric motors and electric heating. Most recently has the electronics and computer revolution come. In all electricity has fundamentally transformed the way we live.
As the practical uses for electricity grew and multiplied, so did the demand for its production. Edison built the first central power station and many power companies still bear his name. Growth in distribution lead to high voltage transmission and the interconnection of the modern power grid, with power plants sometimes located over a thousand miles from consumers. Quite recently the monopoly structure of the industry has begun to be dismantled in favor of competition among generators.
Coal-fired steam and water power were the first sources of energy used to make electricity commercially, later gas and oil were also burned to make steam, as well as fueling reciprocating engines. In the late 1960’s gas and oil fired combustion turbines, similar to jet engines, were introduced, as was nuclear power. Fossil fuel still accounts for most production of electricity, about 70%, with coal powering about 75% of the fossil fraction.
20. Translate the following text into Russian:
Text 10 B
Electric Power Generation
Generators and motors are very closely related and many motors that contain permanent magnets can also act as generators. If you move a permanent magnet past a coil of wire that is part of an electric circuit, you will cause current to flow through that coil and circuit. That’s because a changing magnetic field, such as that near a moving magnet, is always accompanied in nature by an electric field.
While magnetic fields push on magnetic poles, electric fields push on electric charges. With a coil of wire near the moving magnet, the moving magnet’s electric field pushes charges through the coil and eventually through the entire circuit. A convenient arrangement for generating electricity endlessly is to mount a permanent magnet on a spindle and to place a coil of wire nearby. Then as the magnet spins, it will turn past the coil of wire and propel currents through that coil.
If you take a common DC* motor out of a toy and connect its two electrical terminals to a 1.5 V light bulb or a light emitting diode (try both directions with an LED because it can only carry current in one direction), you'll probably be able to light that bulb or LED by spinning the motor's shaft rapidly. A DC motor has a special switching system that converts the AC* produced in the motor’s coils into DC for delivery to the motor's terminals, but it's still a generator. So the easiest answer to your question is: «find a nice DC motor and turn its shaft».
There is no fundamental limit to how much current a generator can handle, however, the characteristics of the generator’s wiring, its magnetic fields, and the machinery turning it all tend to limit its current capacity. A generator’s wires aren’t perfect and, as the current passing through the generator increases, its wires waste more and more power.
Like any wiring, a generator’s wires convert electric power into thermal power in proportion to the square of the current. Thus if you double the current in the generator, you quadruple the power loss. While this power loss and the resulting heat are trivial at low currents, they become serious problems at high currents. Increasing the current in the generator also affects its magnetic fields because currents are magnetic.
At a low current, the current’s magnetism can be ignored. But when a generator is handling a very large current, the magnetic fields associated with that current are no longer small perturbations on the generator’s normal magnetic fields and the generator may not perform properly any more.
Finally, a generator’s job is to transfer energy from a mechanical system to the electric current passing through it. As the amount of current in the generator increases, the amount of work that the mechanical system provides must also increase — the generator becomes harder to turn. There will always be a limit to how much torque an engine or crank can exert on the generator to keep it spinning and thus there will be a limit to how much current the generator can handle.
As for how the current varies with load: the more current the load permits to pass through it, the more current will pass through the generator. Assuming that the generator is well built and has very little electric resistance, the load will serve to limit the current. The generator will then deliver just as much current as the load will permit. If the load permits more current, the generator will deliver more. As a result, the wires in the generator will waste more power as heat, the magnetic fields in the generator will become more complicated, and the device powering the generator will have to work harder to keep the generator turning.
Notes on the text
DC - direct current - ïîñòîÿííûé òîê
AC - alternating current - ïåðåìåííûé òîê
TEXT AND VOCABULARY EXERCISES
21. Find in the text the words or phrases which mean the same as:
22. Match each word in A with the Russian equivalent in B:
23. Work in pairs and decide whether these statements are true or false:
1. Generators and motors are very closely related and many motors that contain permanent magnets can also act as generators.
2. A changing magnetic field is never accompanied in nature by an electric field.
3. There is no fundamental limit to how much current a generator can handle.
4. A generator’s wires do not convert electric power into thermal power in proportion to the square of the current.
5. At a low current, the current’s magnetism can be ignored.
6. A generator’s job is to transfer energy from a mechanical system to the electric current passing through it.
7. As the amount of current in the generator increases, the amount of work that the mechanical system provides must also increase — the generator becomes harder to turn.
8. Wires in the generator will waste more power as heat, the magnetic fields in the generator will become more complicated, and the device powering the generator will have to work harder to keep the generator turning.
24. Fill in the gaps with the words from the box:
To solve the problem of sending power over long 1 ___, George Westinghouse developed a device called a 2 ___. The transformer allowed power to be efficiently transmitted over long distances. This made it possible to supply power to 3 ___ and businesses located far from the electric generating plant. Despite its great importance in our daily lives, most of us rarely stop to think how life would be like without power. Yet like air and water, we tend to take power for granted. Everyday, we use 4 ___ to do many functions for us — from lighting and heating/cooling our homes, to being the power source for televisions and computers. Power is a controllable and convenient 5 ___ of energy used in the applications of heat, light and power. Today, the United States (U.S.) electric power 6 ___ is organized to ensure that an adequate supply of power is available to meet all demand requirements at any given instant.
Check your answers on p. 280.
25. Give situations from Text 10B in which the following are used:
Changing magnetic field; generating electricity; generator’s wires; generator’s work
26. Look through the text about the electricity production in the USA. Fill in the gaps with the prepositions from the box:
Electricity Production in the USA
There are several thousand power generating and supply organizations including investor owned utilities, government - especially municipal - utilities, rural electric cooperatives and independent power producers. Utilities and cooperatives sell the electricity ___ consumers, so do power marketers ___ some states, where consumers can now choose their supplier. The electric power industry is changing ___ a big way, called deregulation. Competition is being added ___ many levels. The future, including the role ___ fossil fuel, is hard to predict.
About 30% of all fossil fuel consumed ___ the United States is used to make electricity. Conversely, most electricity, about 70%, produced ___ the US is generated using fossil fuels, especially coal. Typically, the coal ___ one or more mines is transported ___ railroad or barge to a steam generating plant. Turbine generators utilize the steam to generate electricity ___ high voltage. This electricity is transformed a higher voltage ___ transmission over a power network to industrial, commercial and residential users. Near the users it is transformed again, down to a low voltage, where it is distributed to the users via the familiar distribution system ___ poles and overhead wires.
Electricity is distributed ___ 3 kinds of utilities - investor owned, municipal and cooperative. Many IOUs and municipals generate some ___ the electricity they sell. Electricity is also generated by «generation and transmission (G&T) » coops that are owned ___ groups of distribution coops. Power is also generated ___ the federal government, especially from dams, and by an increasing number ___ independent power producers. Since deregulation began in 1992, a number ___ independent power marketing firms have emerged as well.
27. Read the following text, think of a suitable title for it and render it according to the following scheme:
By «waterpower» we assume that one means hydroelectric power. In that case, water from an elevated source enters a pipe and travels downhill to a generating plant. As the water descends, its gravitational potential energy (the stored energy associated with height and the earth’s gravity) becomes pressure potential energy (the stored energy associated with pressure) and kinetic energy (the energy of motion). By the time the water reaches the generating plant, it has enormous pressure and a modest speed.
This moving, high-pressure water is then sent through a fan-like turbine. As the water moves toward the low pressure beyond the turbine, it does work on the turbine’s rotating blades and its energy is transferred to those blades. The water gives up its energy and the turbine takes away this energy in its rotary motion. The turbine is attached to an electric generator, which uses moving magnets and wire coils to turn the turbine’s rotary energy into electric energy. The electric energy is carried away on wire to be used elsewhere. Overall, the water’s gravitational potential energy has become electric energy. How does an internal voltage regulator type auto alternator work and are they any better than an external regulator type?
An alternator is a device that uses rotary motion to generate electricity. As the car engine turns, it spins a magnet (the rotor) in the alternator and this spinning magnet induces electric currents in a set of stationary wire coils (the stator). The alternator’s ability to generate electric currents by spinning a magnet past stationary wires is an example of electromagnetic induction. Induction is a general phenomenon in which a moving or changing magnetic field creates an electric field, which in turn pushes electric charges through a conducting material. Overall, some of the engine’s mechanical energy is converted into electric energy.
The amount of energy given to each electric charge that flows through the wires in the stator depends on the speed with which the magnet turns and the strength of that magnet. Whether it's internal or external, the voltage regulator monitors this energy per charge — also known as the voltage — to make sure that it’s correct. If not, it adjusts the strength of the alternator’s magnet. It can do this because the alternator’s magnet is actually an electromagnet and its strength depends on how much current is flowing through its wire coils.
The voltage regulator carefully adjusts the current flowing through the electromagnet in order to obtain the proper output voltage from the alternator. Actually, the alternator itself produces alternating current, so a set of solid-state diodes converts this alternating current into direct current. A car's electric system, particularly its battery, operates on direct current. Since the alternator's operation is the same whether the voltage regulator is inside it or external to it, neither version should be better than the other.
How efficient are solar energy cells and windmills in producing energy for everyday use?
There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you’re probably most interested in their cost efficiency - in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants.
As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.
28. Read the text below to find answers to the given questions:
Text 10 C
More Facts about Electricity
1. What does a transformer do?
A transformer transfers power between two or more electrical circuits when each of those circuits is carrying an alternating electric current. Transfers of this sort are important because many electric power systems have incompatible circuits — one circuit may use large currents of low voltage electricity while another circuit may use small currents of high voltage electricity. A transformer can move power from one circuit of the electric power system to another without any direct connections between those circuits.
2. How does a transformer change voltage and how does it regulate the amperage?
A transformer’s current regulation involves a natural feedback process. To begin with, a transformer consists of two coils of wire that share a common magnetic core. When an alternating current flows through the primary coil (the one bringing power to the transformer), that current produces an alternating magnetic field around both coils and this alternating magnetic field is accompanied by an alternating electric field (recall that changing magnetic fields produce electric fields).
This electric field pushes forward on any current passing through the secondary coil (the one taking power out of the transformer) and pushes backward on the current passing through the primary coil. The net result is that power is drawn out of the primary coil current and put into the secondary coil current.
3. How does a transformer reduce voltage?
When you send an alternating current through the primary coil of wire in a transformer, that current produces a magnetic field in the transformer. Because the current in the primary coil is changing with time — it’s an alternating current—this magnetic field is changing and changing magnetic fields are accompanied by electric fields. In the transformer, this electric field pushes electric charges around the secondary coil of wire in the transformer.
Since these electric charges are pushed in the direction they are travelling, work is being done on them and their energies are increasing. However, in the transformer the secondary coil of wire has fewer turns in it than the primary coil of wire. As a result, the charges don't receive as much energy per charge (as much voltage) as the charges in the primary coil are giving up. This type of transformer, in which the secondary coil has fewer turns of wire than the primary coil, is called a step-down transformer and reduces the voltage of an alternating current.
4. What is the purpose of the iron core in a transformer?
The iron core of a transformer stores energy as power is being transferred from the primary circuit to the secondary circuit. This energy is stored as the magnetization of that iron. The transformer needs to store that energy for roughly one half cycle of the alternating current or about 1/120th of a second. The more iron there is in the transformer, the more energy it can store and the more power the transformer can transfer from the primary circuit to the secondary circuit.
Without any iron, the energy must be stored directly in empty space, again as a magnetization. But space isn't as good at storing magnetic energy as iron is so the iron increases the power-handling capacity of a transformer. Without the iron, the transformer must operate at much higher frequencies of alternating current in order to transfer reasonable amounts of power.
5. What is the difference between current and voltage?
Current is the measure of how many charges are flowing through a wire each second. A 1-ampere current involves the movement of 1 Coulomb of charge (6,250,000,000,000,000,000 elementary charges) per second. Voltage is the measure of how much energy each charge has. A 1-volt charge carries 1 Joule of energy per Coulomb of charge. To use water in a pipe as an analogy, current measures the amount of water flowing through the pipe and voltage measures the pressure (or energy per liter) of that water.
6. What is resistance?
Resistance is the measure of how much an object impedes the flow of electricity. The higher an object’s resistance, the less current will flow through it when you expose it to a particular voltage drop. To use the water analogy, resistance resembles a constriction in a pipe. The narrower the pipe (higher the resistance), the harder it is to push water through that pipe. If you keep the water pressure constant (constant voltage drop) as you narrow the pipes (increase the resistance), then less water will flow (the current will drop).
7. What causes large electric resistances?
Thin wires or wires made of poor conductors. Some metals are simply better at carrying current without wasting energy than other metals. It has to do with how often a charge bounces off of a metal atom and loses energy. Copper, silver, and aluminum are good conductors while stainless steel and lead are pour conductors.
Metals tend to become better conductors as you cool them and worse as you heat them. Semiconductors such as carbon (graphite) are poor conductors but have the reverse temperature effect. At low temperature they are poor conductors but become good conductors at high temperature.
8. How does hydroelectric power work?
Hydroelectric power begins with water descending from an elevated reservoir, such as a lake in the mountains. While it’s in the elevated reservoir, this water has stored energy — in the form of gravitational potential energy. As this water flows downward through a pipe, its gravitational potential energy becomes either kinetic energy or pressure potential energy or both.
By the time the water arrives at the hydroelectric power plant, it is either travelling very quickly or has an enormous pressure or both. In the power plant, the water flows past the blades of a huge turbine and does work on those blades. The blades are shaped somewhat like airplane wings and they «fly» through the moving water.
Since the blades are attached to a central hub, they cause this hub to rotate and allow it to turn the rotor of a huge electric generator. The rotor of the generator typically contains a giant electromagnet. The electromagnet turns within a collection of stationary wire coils and it induces electric currents in those coils. These electric currents carry power out of the generator to the homes or business that needs it.
Notes on the text
natural feedback process — åñòåñòâåííûé ïðîöåññ îáðàòíîé ñâÿçè
TEXT AND VOCABULARY EXERCISES
29. Find in the text the words or phrases which mean the same as:
30. Translate into Russian the following words and word combinations:
incompatible circuits; low voltage electricity; direct connections; secondary coil; primary circuit; power-handling capacity; frequencies of alternating current; resistance; water descending from an elevated reservoir; to store energy; hub; wire coils; to turn the rotor
31. Find in the text the synonyms to the following words:
32. Choose among the words in parentheses the one that corresponds to the text above to complete the sentences:
1. A transformer transfers power ___ two or more electrical circuits.
(a. between; b. in; c. within)
2. A transformer ___ move power from one circuit of the electric power system to another without any direct connections between those circuits.
(a. may; b. can; c. must)
3. The type of transformer, in which the secondary coil has ___ turns of wire than the primary coil, is called a step-down transformer.
(a. smaller; b. greater; c. fewer)
4. The transformer needs to ___ the energy for roughly one half cycle of the alternating current.
(a. store; b. pass; c. deliver)
5. Resistance is the measure of how much an object impedes the flow of ___.
(a. gravity; b. electricity; c. magnetism)
6. Copper, silver, and aluminum are ___ conductors.
(a. good; b. poor; c. bad)
7. Hydroelectric power begins with water ___ from an elevated reservoir, such as a lake in the mountains.
(a. ascending ;b. climbing; c. descending)
8. The electromagnet turns ___ a collection of stationary wire coils and it induces electric currents in those coils.
(a. beneath; b. within; c. above)
33. Read the following text, think of a suitable title for it and render it according to the following scheme:
The electricity you receive comes from a distant power plant. A generator in that power plant produces a substantial electric current of medium high voltage electric charge. This current is alternating, meaning that its direction of flow reverses many times a second — 120 reversals per second or 60 full cycles of reversal (over and back) in the United States. This alternating electric current flows through the primary coil of wire in a huge transformer at the power plant, where it produces an intense alternating magnetic field. When a magnetic field changes with time, it produces an electric field and, in the transformer, this electric field pushes electric charges around a second coil of wire in the transformer, the secondary coil.
The effect of this transformer is to transfer power from the current in the primary coil of the transformer to the current in the secondary coil of the transformer. Thus the generator’s electric power moves along to the current passing through the secondary coil of the transformer. However, the secondary coil has far more turns of wire than the primary coil and this gives each charge passing through that coil far more energy than the charges had in the primary coil. Although the current passing through that secondary coil is relatively small, it acquires an enormous voltage by the time it leaves the secondary coil. The transformer has produced this high voltage power needed for efficient power transmission to a distant city.
This high voltage electric current passes through the countryside on high voltage transmission wires. The value of using a small current of high voltage charges is that wires waste power in proportion to the square of the electric current they are carrying. Since the current in the transmission wires is small, they waste relatively little power. When this current reaches your town, it passes through a second transformer, which transfers its power to yet another electric current.
This current is large and, because it passes through a coil that has few turns of wire, it acquires only a medium high voltage when it flows through the secondary coil of the new transformer. Electricity from this second transformer flows toward your neighborhood through medium high voltage wires.
Finally, near your home there is a third and final transformer that extracts power from the medium high voltage current and transfers that power to a very large current that acquires a low voltage when it flows through the secondary coil of the final transformer. It is this very large current of low voltage charges that flows through appliances in your home and those of your neighbors. That final transformer is often visible as a large gray drum on a utility pole or a green box in someone’s yard.
Consult the TEXTS FOR SUPPLEMENTARY READING and complete the information about the generation of electricity (Text 38, 39, 40, 41). Be ready to discuss the information you have read.
35. Fill in the gaps with the prepositions/conjunctions from the box:
Electricity is measured ___ units ___ power called watts. It was named ___ honor James Watt, the inventor ___ the steam engine. One watt is a very small amount ___ power. It would require nearly 750 watts to equal one horsepower. A kilowatt represents 1,000 watts. A kilowatthour (kWh) is equal ___ the energy ___ 1,000 watts working ___ one hour.
The amount ___ electricity a power plant generates ___ a customer uses ___ a period ___ time is measured ___ kilowatthours (kWh). Kilowatthours are determined ___ multiplying the number ___ kW's required ___ the number of hours of use. For example, ___ you use a 40-watt light bulb 5 hours a day, you have used 200 watts ___ power, ___ 0.2 kilowatthours of electrical energy.
36. Read the text below to find answers to the given questions:
Text 10 D
1. How does an electric motor work?
An electric motor uses the attractive and repulsive forces between magnetic poles to twist a rotating object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic and their magnetic poles are initially arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator’s south poles and vice versa.
The rotor thus experiences a twist (what physicists call a torque) and it undergoes an angular acceleration — it begins to rotate. But the magnets of the rotor and stator are not all permanent magnets. At least some of the magnets are electromagnets. In a typical motor, these electromagnets are designed so that their poles change just as the rotor’s north poles have reached the stator’s south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator’s south poles and it continues to experience a twist in the same direction.
2. How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it positive and negative?
An electric current is itself magnetic — it creates a structure in the space around it that exerts forces on any magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire — the current’s magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the wire up into a coil, the magnetic field takes on a more familiar shape.
The current-carrying coil effectively develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your right hand, then your thumb points to the north pole that develops at one end of the coil.
3. In a three-phase induction motor, there is a rotating magnetic field in the stator, which induces a rotating magnetic field in the rotor. Those two magnetic fields will interact together to make the rotor turn. Is the interaction attractive or repulsive?
The magnetic interaction between the stator and the rotor is repulsive — the rotor is pushed around in a circle by the stator’s magnetic field; it is not pulled. To see why this is so, imagine unwrapping the curved motor so that instead of having a magnetic field that circles around a circular metal rotor you have a magnet (or magnetic field) that moves along a flat metal plate. As you move this magnet across the plate, it will induce electric currents in that plate and the plate will develop magnetic poles that are reversed from those of the moving magnet — the two will repel one another. That choice of pole orientation is the only one consistent with energy conservation and is recognized formally in «Lenz’s Law».
For reasons having to do with resistive energy loss and heating, the repulsive forces in front of and behind the moving magnet don’t cancel perfectly, leading to a magnetic drag force between the moving magnet and the stationary plate. This drag force tends to push the plate along with the moving magnet. In the induction motor, that same magnetic drag force tends to push the rotor around with the rotating magnetic field of the stator. In all of these cases, the forces involved are repulsive — pushes not pulls.
4. How does a fan motor work?
A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of these electromagnets carries an alternating current that it receives from the power line and its magnetic poles fluctuate back and forth as the direction of current through it fluctuates back and forth.
These electromagnets are arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different rates.
5. How does an electromagnetic doorbell work?
When you press the button of an electromagnetic doorbell, you complete a circuit that includes a source of electric power (typically a low voltage transformer) and a hollow coil of wire. Once the circuit is complete, current begins to flow through it and the coil of wire becomes magnetic. Extending outward from one end of the coil of wire is an iron rod. When this coil of wire — also called a solenoid — becomes magnetic, so does the iron rod. The iron rod becomes magnetic in such a way that it’s attracted toward and into the solenoid, and it accelerates toward the solenoid. The attractive force diminishes once the rod is all the way inside the solenoid, but the rod then has momentum and it keeps on going out the other side of the solenoid. It travels so far out of the solenoid that it strikes a bell on the far side — the doorbell!
The rod rebounds from the bell and reverses is motion. It has traveled so far out the other side of the solenoid that it's attracted back in the opposite direction. The rod overshoots the solenoid again and, in some doorbells, strikes a second bell having a somewhat different pitch from the first bell. After this back and forth motion, the rod usually settles down in the middle of the solenoid and doesn’t move again until you stop pushing the button. Once you release the button, the current in the circuit vanishes and the solenoid and the rod stop being magnetic. A weak spring then pulls the rod back to its original position at one end of the solenoid.
6. How do electric/magnetic linear drives work?
Linear electric motors are very much like rotary electric motors — they use the forces between magnetic poles to push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle, a linear motor uses these forces to push a carriage along a track.
Both the carriage and the track must contain magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed. By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the track smoothly and continuously.
7. What is the difference between a single-phase electric motor and a three phase motor?
To keep the center component or «rotor» of an electric motor spinning, the magnetic poles of the electromagnets surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic poles. With single-phase electric power, producing that rotating magnetic environment isn’t easy. Many single-phase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of the other electromagnets.
The result is magnetic poles that seem to rotate around the rotor and that start it turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the starting process is clearly rather complicated in a single phase motor. In a three phase motor, the complicated time structure of the currents flowing through the three power wires makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general, three phase motors start more easily and are somewhat more energy efficient during operation than single phase motors.
8. Does the monorail at Disneyland and the metro in D.C. run on the idea of direct current motors?
Those trains probably run on AC motors, because then they can use transformers to transfer power between circuits. Most likely, these trains use induction motors. To reverse the direction of the train, the engineer reverses the direction in which magnetic poles in the motors’ stators circle the motors’ rotors. When the poles reverse directions, the rotor has to reverse its direction, too, so that it chases those poles around in a circle.
TEXT AND VOCABULARY EXERCISES
37. Find in the text words or phrases which mean the same as:
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