Учебные задания по английскому языку для студентов 2 курса направления бакалавриата «Электроэнергетика и электротехника» 4 семестр

of an iron core with a coil (6)_________(accommodate) in slots. The ends of the coil

(7)______(connect) to the bars of a commutator switch (8)_______(mount) on the rotor shaft. Stationary graphite brushes (9)______(lead) to external terminals.

Suppose a direct-current supply (10)________(connect) to the armature terminals such that a current (11)______(enter) at the positive terminal. This current (12)_______(interact) with the magnetic flux (13)_______(produce) a counterclockwise torque, which in turn (14)________(accelerate) the rotor. When the rotor has turned about 120°, the connection from the supply to the armature coil

(15)_______(reverse) by the commutator. The new direction of the current in the armature coil is such as (16)______(continue) to produce counterclockwise torque. As the rotor (17)______(rotate) in a counterclockwise direction, a voltage proportional to the speed (18)________(generate) in the armature coil. While this coil voltage is alternating, the commutator action (19)_______(produce) a unidirectional voltage at the motor terminals with the polarity shown. The electrical input (20)___(be) the product of this terminal voltage and the input current. The mechanical output power (21)___(be) the product of the rotor torque and speed.

4. Write out the passage using one of the words from the list.

In a practical DC motor, the (1)_______winding (2)_______of a number of coils in (3)______, each spanning 1/p of the rotor periphery for p (4)_____. In small motors the number of coils may be as (5)______as six, while in large motors it may be as (6)______as 300. The coils are all connected in (7)______, and each

(8)_______is connected to a commutator bar. If current (9)______at the positive brush, the coil currents have the directions shown. All coils under the poles (10)________to torque production.

5. Complete each sentence (a-g) with one of the endings (1-7).

a.Some types of permanent-magnet commutator motors have…

b.If speed adjustment is desired,…

c.These coils can be provided…

d.A variable series resistor can be used…

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e.With maximum field current and thus maximum magnetic flux,…

f.For a given rated armature current, the available torque will…

g.The motor, however, will be able…

1.…with current from the same supply as for the armature or from a separate

supply.

2.…to provide the same mechanical power at a higher speed and lower torque.

3.…the permanent-magnet field can be replaced by iron poles with field coils.

4.…be reduced because of the reduced flux.

5.…to adjust the field current.

6.…no provision for speed control when attached to a constant-voltage supply.

7.…the generated voltage will equal the supply voltage at a minimum value of no-load speed.

6.Complete the text with words formed from the words in capitals.

7. Fill in the correct preposition.

DC commutator motors have been extensively used ___ steel mills, paper mills, robots, and machine tools where accurate control ___ speed or speed reversal, or both, are required. The field is supplied ___ a separate voltage source, usually ___

constant field current, or ___ permanent magnets. The armature is supplied ___ a source __ controllable voltage. The speed is then approximately proportional ___ the source voltage. Reversal ___ the armature supply voltage ___ a controlled rate reverses the motor.

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SPEAKING

Make a presentation on a particular type of a DC generator. Pay special attention to the texts of the module.

WRITING

Write a short description of the functionality and application of DC motors. Use the information given in the module texts and find additional information.

MODULE II

ASSESSMENT

Translate from English into Russian.

A typical small DC motor, such as those used in automobile fans, contains two poles made of ferrite permanent-magnet material. When higher torque is required, as, for example, in the starter motor of an automobile, stronger magnets such as neodymium-iron-boron may be employed. When the terminals of this motor are connected to a constant direct-voltage source, such as a battery, the initial current will be limited only by the resistance of the armature winding and the brushes. The torque produced by the interaction of this current with the field accelerates the rotor. A voltage is generated in the winding proportional to the speed. This voltage opposes that of the source, thus reducing the current and the torque. With no mechanical load, the generated voltage will rise to a value nearly equal to the source voltage, allowing just enough current to provide for friction torque. Application of a load torque slows down the rotor, decreasing the generated voltage, increasing the current, and producing torque to match the load torque.

With larger motors, the armature winding resistance is too low to limit the current on starting to a value that can be switched by the commutator. These motors are normally started with a resistance connected in series to the armature supply. This resistance is usually decreased in stages as the speed increases.

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MODULE III

UNIT ONE

ALTERNATING-CURRENT (AC) GENERATORS (ALTERNATORS)

READING

Read the text. Six sentences have been removed from it. Put a sentence from A-G in the correct gap to complete the text. There is one sentence you will not need.

A simple generator without a commutator will produce an electric current that alternates in direction as the armature revolves. (1)_______________________In its simplest form, an AC generator differs from a DC generator in only two particulars: the ends of its armature winding are brought out to solid unsegmented slip rings on the generator shaft instead of to commutators, and the field coils are energized by an external DC source rather than by the generator itself. Low-speed AC generators are built with as many as 100 poles, both to improve their efficiency and to attain more easily the frequency desired. (2)_______________________The frequency of the current delivered by an AC generator is equal to half the product of the number of poles and the number of revolutions per second of the armature.

It is often desirable to generate as high a voltage as possible, and rotating armatures are not practical in such applications because of the possibility of sparking between brushes and slip rings and the danger of mechanical failures that might cause short circuits. (3)______________________The principle of operation is exactly the same as that of the AC generator described, except that the magnetic field (rather than the conductors of the armature) is in motion.

The current generated by the alternators described above rises to a peak, sinks to zero, drops to a negative peak, and rises again to zero a number of times each second, depending on the frequency for which the machine is designed.

(4)_________________ If, however, the armature is composed of two windings, mounted at right angles to each other, and provided with separate external connections, two current waves will be produced, each of which will be at its

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maximum when the other is at zero. (5)____________________If three armature windings are set at 120° to each other, current will be produced in the form of a triple wave, known as three-phase alternating current. A larger number of phases may be obtained by increasing the number of windings in the armature, but in modern electrical-engineering practice three-phase alternating current is most commonly used, and the three-phase alternator is the dynamoelectric machine typically employed for the generation of electric power. (6)____________________

A. Such current is known as single-phase alternating current.

B. Alternators driven by high-speed turbines, however, are often two-pole

machines.

C. Voltages as high as 13,200 are common in alternators.

D. Alternators are widely spread in a great number of countries.

E. Such alternating current is advantageous for electric power transmission , and hence most large electric generators are of the AC type.

F. Alternators are therefore constructed with a stationary armature within which revolves a rotor composed of a number of field magnets.

G. Such current is called two-phase alternating current.

VOCABULARY PRACTICE

1.Explain the words and word combinations in bold in the text.

2.Complete the text with words formed from the words in capitals.

SYNCHRONOUS GENERATORS

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INDUCTOR ALTERNATORS

Inductor alternator is a (1)__________kind of synchronous generator in which both the field and the (2)________winding are on the stator. In the (3)_________type of machine, the magnetic (4)_________is produced by direct current in a field

(5)________concentric with the shaft. In the (6)__________type, the field coils are in slots in the stator.

Voltage is generated in the output windings by (7)_________in the flux in

(8)__________stator teeth. These pulsations are produced by use of a

(9)________rotor, which causes the (10)__________of the air path from the rotor to each stator tooth to vary periodically with (11)__________.

Inductor alternators are useful as (12)____________generators. They also are useful in situations requiring high (13)___________, a feature achieved by their having no electrical (14)__________to the rotor.

4. Write out the passage with a suitable preposition in each space.

GENERATORS FOR MOTOR VEHICLES

Such vehicles as automobiles, buses, and trucks require a direct-voltage supply

___ ignition, lights, fans, and so forth. In modern vehicles the electric power is generated ___ an alternator mechanically coupled ___ the engine. The alternator normally has a rotor field coil supplied ___ current ___ slip rings. The stator is fitted

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___ a three-phase winding. A rectifier is used to convert the power ___ alternating

___ direct form. A regulator is used to control the field current so that the output voltage ___ the alternator-rectifier is properly matched ___ the battery voltage as the speed ___ the engine varies.

5. Replace the words underlined with the most appropriate adjective from the list.

a.In small ratings, the magnetic field of the synchronous generator may be provided by permanent magnets.

b.Samarium-cobalt magnet material can provide a magnetic flux density in the air gap similar to that produced with field windings.

c.The use of ferrite material may result in a great reduction in air-gap flux

density.

d.Apart from the density reduction, there will be a matching increase in generator dimensions.

e.Permanent-magnet generators are not complicated in that they require no system for the provision of field current.

f.This equipment is highly dependable.

UNIT TWO

AC MOTORS

READING

1.Scan the first paragraph of the article and state the disadvantages of synchronous motors.

2.Scan the second paragraph and list the types of induction motor.

3.Read the text and mark the statements as TRUE or FALSE. Correct the FALSE ones.

a. The synchronous motor operates exactly as a three-phase alternator.

b. The constant speed of a synchronous motor is conditioned by the various current waves in the armature.

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c.Synchronous motors are very useful in a number of devices.

d.A squirrel-cage motors cannot operate from a single-phase power source.

e.The rotor and the magnetic field of a squirrel-cage motor revolve at exactly the same speed.

f.Repulsion-induction motors operate irrespective of their starting torque.

g.Series-wound motors are advantageous in industrial application.

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 variation of the three waves of current in the armature causes a varying magnetic reaction with the poles of the field magnets, and makes the field rotate at a constant speed that is determined by the frequency of the current in the AC power line. 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 armature of the squirrel-cage motor consists of three fixed coils similar to the armature of the synchronous motor. 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

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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.

VOCABULARY PRACTICE

1.Find Russian equivalents to the words and word combinations in bold.

2.Find English equivalents to the following words and word combinations.

a.скольжение;

b.выпадать из синхронизма;

c.пусковой момент;

d.многофазный ток;

e.иметь сходство;

f.наоборот;

g.трёхфазное электропитание;

h.встроенный.

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3. Match the characteristics from the list with the motors they are

associated with. You can use each word more than once.

4. Complete the text with an appropriate form of each of the verbs from

the list.

The heat (1)__________by power losses in the conductors and iron parts of the machine, as well as the friction heat, must (2)_________by the cooling system

(3)_________the temperature of the motor. The main purpose of protection apparatus is (4)______damage to the most vulnerable part of the motor, the insulation on the windings. For low-power motors, a temperature-sensitive device is often

(5)_________inside the motor and (6)________to switch off the electric supply if the temperature (7)________its limiting value. With larger motors, temperature-sensitive detectors may (8)__________at one or more locations in the stator windings.

5. Complete the text with words formed from the words in capitals.

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