IV. Read the statements and define if they are true or false (according to the text content). If the statement is false, correct it.

1. Separately-excited generators have one circuit. 2. In the automatic motor control systems the field current is controlled by an amplifier. 3. In a self-excited generator the field coils may be connected only in series with the armature coils. 4. The shunt field windings require a relatively small current to produce the necessary field flux. 5. If a generator is not connected across a load, the circuit is incomplete and no current will flow to excite the field. 6. If the series field is connected so that its field opposes the shunt field, the generator is called cumulatively compounded.

V. Speak on the different types of d.c. generators.

Text 2С

Ø Read and translate the text, using the given vocabulary. Learn the words and word combinations by heart.

Alternators

The a.c. generator is the most important means for the production of electrical power. A.c. generators, or alternators, vary greatly in size depending on the power they are required to supply. Regardless of size, however, all electrical generators, whether d.c. or a.c., depend for their operation on the action of a coil cutting through a magnetic field, or of a magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic field, a voltage will be generated. In order that relative motion may take place between a conductor and a magnetic field, all generators are made up of two mechanical parts – a rotor and a stator.

It is known that in a d.c. generator the revolving part is always the armature. In an a.c. generator, however, this is not usually true. There are two types of alternators: the revolving-armature type and the revolving-field type.

The revolving-armature type alternator is similar in construction to the d.c. generator in that the armature rotates through a stationary magnetic field. But in the d.c. generator the e.m.f. generated in the armature windings is converted into d.c. by means of a commutator; whereas in the alternator the generated a.c. is brought to the load unchanged by means of slip rings. The revolving-armature alternator is only found in alternators of small power rating and is not generally used.

The revolving-field type of alternator has a stationary armature winding and a rotating field winding. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load. Fixed connections are much more easily insulated than would be slip rings at very high voltages; so high-voltage alternators are usually of the rotating field type.

Since a major part of all electrical power generated is a.c., many motors are designed for a.c. operation. A.c. motors can, in most cases, duplicate the operation of d.c. motors and are less troublesome to operate. A.c. motors are particularly well-suited for constant-speed applications, since the speed is determined by the frequency of the a.c. applied to the motor terminals. A.c. motors are also made, however, which have variable speed characteristics within certain limits.

A.c motors can be designed to operate from a single-phase a.c. supply or from a multi-phase a.c. supply. Whether the motor is single-phase or multi-phase, it operates on the same principle. This principle is that the a.c. applied to the motor generates a rotating magnetic field, and this rotating field causes the rotor of the motor to turn.

A.c. motors are generally classified into two types: the synchronous motor and the induction motor. The synchronous motor is an alternator operated as a motor, in which a.c. is applied to the stator and d.c. is applied to the rotor. The induction motor differs from the synchronous motor in that it does not have its rotor connected to any source of power. Of the two types of a.c. motors mentioned, the induction motor is by far the most commonly used.

Exercises

I. Translate the following words and phrases from the text:

a) from English into Russian: revolving, regardless, by means of, fixed, slip ring, troublesome, induction motor, particularly, case;

b) from Russian into English: генератор переменного тока, вращаться, нагрузка, изолировать, подходить, частота, синхронный двигатель, отличаться, однако, предназначать.

II. Fill in the necessary preposition: at,into,on,between,from,within.

1. A.c. generators, or alternators, vary greatly in size depending ... the power they are required to supply. 2. In order that relative motion may take place ... a conductor and a magnetic field, all generators are made up of two mechanical parts – a rotor and a stator. 3. Fixed connections are much more easily insulated than would be slip rings ... very high voltages. 4. Whether the motor is single-phase or multi-phase, it operates ... the same principle. 5. A.c. motors are generally classified ... two types. 6. The induction motor differs ... the synchronous motor. 7. A.c. motors have variable speed characteristics ... certain limits.

III. Here are the answers. Make up questions, write them down.

1. There are two types of alternators: the revolving-armature type and the revolving-field type.

2. A voltage will be generated as long as there is relative motion between a conductor and a magnetic field.

3. All generators are made up of two mechanical parts – a rotor and a stator.

4. A.c. motors are particularly well-suited for constant-speed applications, since the speed is determined by the frequency of the a.c. applied to the motor terminals.

5. The induction motor differs from the synchronous motor in that it does not have its rotor connected to any source of power.

IV. Read the statements and define if they are true or false (according to the text content). If the statement is false, correct it.

1. All electrical generators, whether d.c. or a.c., depend for their operation on the action of a coil cutting through a magnetic field, or of a magnetic field cutting through a coil. 2. In a d.c. generator the revolving part is always the winding. 3. The revolving-armature alternator is found in alternators of large power rating and is widely used. 4. It is possible to design a.c. motors to operate from a multi-phase a.c. supply. 5. A.c. motors can hardly duplicate the operation of d.c. motors and are troublesome to operate.

V. Speak on:

§ a.c. generator, its construction and principle of operation;

§ a.c. motors and their different types.

Supplementary Texts

Text 1

Ø Read and translate the text, give a title to it. Before reading check the meaning of these words in a dictionary:

Exercises

I. Complete the sentences, choosing the words and phrases from the right column: