Text 4. Conductors and Insulators

Conductors are materials having a low resistance so that current easily passes through them. The lower the resistance of the material, the more current can pass through it.

The most common conductors are metals. Silver and copper are the best of them. The advantage of copper is that it is much cheaper than silver. Thus copper is widely used to produce wire conductors. One of the common functions of wire conductors is to connect a voltage source to a load resistance. Since copper wire conductors have a very low re­sistance a minimum voltage drop is produced in them. Thus, all of the applied voltage can produce current in the load resistance.

It should be taken into consideration that most materials change the value of resistance when their temperature changes.

Metals increase their resistance when the temperature increases while carbon decreases its resistance when the temperature increases. Thus metals have a positive temperature coefficient of resistance while carbon has a negative temperature coefficient. The smaller is the tem­perature coefficient or the less the change of resistance with the change of temperature, the more perfect is the resistance material.

Materials having a very high resistance are called insulators. Cur­rent passes through insulators with great difficulty.

The most common insulators are air, paper, rubber, plastics.

Any insulator can conduct current when a high enough voltage is applied to it. Currents of great value must be applied to insulators in or­der to make them conduct. The higher the resistance of an insulator, the greater the applied voltage must be.

When an insulator is connected to a voltage source, it stores electric charge and a potential is produced on the insulator. Thus, insulators have the two main functions:

to isolate conducting wires and thus to prevent a short between them and

to store electric charge when a voltage source is applied.

1. Find answers to these questions in the text above:

1. What materials are called conductors?

2. What is the advantage of copper compared with silver?

3. What is the most common function of wire conductors?

4. Why is a minimum voltage drop produced in copper conductors?

5. What is the relation between the value of resistance and the temperature in carbon?

6. What materials are called insulators?

7. What are the most common insulators?

8. What are the two main functions of insulators?

2. Complete the sentences using the correct variant:

1. Insulators are materials having a) low resistance.

b) high resistance.

2. Current passes through conductors a) easily.

b) with great difficulty.

3. Copper and silver are a) common conductors.

b) common insulators.

4. Air, paper and plastics are a) common insulators.

b) common conductors.

5. In case a high voltage is applied to. a) it does not conduct current

an insulator b) it conducts current.

6. Insulators are used a) to store electric charge.

b) to reduce voltage.

c) to prevent a short between conducting

wires

8. Carbon decreases its resistance a) when the temperature in­creases.

b) when the temperature de­creases.

9. Metals have a) a positive temperature coefficient of

resistance

b) a negative temperature coef­ficient of

resistance

Text 5. Electric generators and motors

A device for converting mechanical energy into electric energy is called a generator. The function of a motor is just the reverse, that is, it transforms electric energy into mechanical-energy. The enormous energy of steam engines, gas engines, and water turbines can now be transformed into electricity and transmitted many miles. The generator has revolutionized modern industry by furnishing cheap electricity.

The essential parts of a generator are: a) the magnetic field, which is produced by permanent magnets or electromagnets; and b) a moving coil of copper wire, called the armature, wound on a dram.

D. с generators are used for electrolytic processes. Large d. c. generators are used in certain manufacturing processes, such as steel making. Generators of small capacities are used for various special purposes, such as welding, automobile generators, train lighting, communication-systems, etc. –

Text 6. D.c. electric motors

There is a wide variety of d. c. and a. c. motors. There are shunt motors, series motors, synchronous motors, induction motors, single-, two-, and three-phase motors. They are used to drive various machines.

Direct-current motors are of three principal kinds, and are named according to the manner in which their field coils are connected to the armature. They are named respectively: series, shunt, and compound.

In the series motors the field windings and armature are connected in series with each other. All the current which passes through the armature passes through the field coils. The field windings are therefore composed of a few turns of thick wire. Starting under heavy load, a series motor will take a large current to provide the huge torque required.

The field coils of shunt motors are connected direct across the brushes, hence they have the foil voltage of the mains applied to them. The shunt motor may be called a constant speed motor, and is suitable for driving machine tools, lathes,. wood-working machines and any machines requiring a steady speed.

A compound motor has both shunt and series field windings and therefore partakes of the nature of both types of motors.

Text 7. A.c. electric motors

Motors for alternating-current circuits may be either single-phase or polyphase (two- or three-phase). They may again be divided into two kinds, named respectively: I. Synchronous; II. Non- or asynchronous, ordinarily called induction motors.

The most widely used a. c. motor is the induction motor shown in Fig. 2. It has two main parts: a) the stationary winding or stator, which sets up a rotating magnetic field, and b) the rotating part of the motor, i. e. the rotor. The rotor of a commercial a. c. motor consists of an iron core with large copper bars placed in slots around the circumference and connected at both ends to copper rings. This is called a squirrel-cage rotor. When a rotor is placed in a rotating magnetic field, a large current is induced tin it.

A. c. motors are exactly similar in construction to a.c. generators and may be called inverted alternators, since the same machine may be used as either a generator or a motor.

Synchronous motors are very suitable for large powers, where the machine can be started up without load, and once started run for long periods.

For supplying direct-current power networks, the supply comes first from an alternating-current source and is converted to direct current by synchronous converters or motor-generator sets.

1. Put English equivalents to the following:

широкое разнообразие

переводной двигатель

управлять различными механизмами

двигатель постоянного тока

трех основных видов

катушка

в соответствии с образом

комбинированный

проходит через арматуру

моток

тяжёлые нагрузки

требуется огромное вращение

двигатель постоянной скорости

неизменная скорость

двигатель переменного тока

названных соответственно

закрепленная катушка

вращательная часть

окружность, периферия

медные кольца

преобразовывается в постоянный ток

Text 8. Transmission Lines

A power system is an interconnection of electric power stations by high voltage power transmission lines. Nowadays the electricity is trans­mitted over long distances and the length of transmitting power lines varies from area to area.

A wire system is termed a power line in case it has no parallel branches and a power network in case it has parallel branches.

According to their functions, power lines and networks are subdivided into transmission and distribution lines.

Transmission lines serve to deliver power from a station to distribu­tion centres. Distribution lines deliver power from distribution centres to the loads.

Lines are also classed into: 1) overhead; 2) indoor; 3) cable (under­ground).

Overhead lines include line conductors, insulators, and supports. The conductors are connected to the insulators, and these are connected to the supports. The greater the resistance, the higher are the heating losses in the conducting wires. In order to reduce the losses, a step-down trans­former can be used.

Indoor lines include conductors, cords, and buses. The conductor may include one wire or a combination of wires not insulated from one an­other. They deliver electric current to the consumers.

As to underground lines, they are used in city areas. Accordingly, they are used in cities and towns, and in the areas of industrial enterprises.