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

Check pronunciation of the following words:

excitation, separate a., amplifier, series, shunt, determine, cumulatively, differentially, occur, undesirable.

Types of D.C. Generators

The d.c. current in the field coils is called the 'excitation current', and may be supplied either from a separate d.c. voltage source, or by using the d.c. output of the generator itself. If the field is supplied with current from an external source, the generator is said to be 'separately-excited'; but if some of the generator output is used to supply the field current, it is said to be 'self-excited'. In a self-excited generator the field coils may be connected either in series with the armature coils ('series'), in parallel with the armature coils ('shunt'), or partly in series and partly in parallel with them ('compound').

Separately-excited d.c. generators have two circuits: the field current, consisting of the field coils connected across a separate d.c. source, and the armature circuit, consisting of the armature coil and the load resistance. In the generators of this type the field is independent of the armature, since it is supplied with current from either another generator (exciter), an amplifier or a battery. The separately-excited generator is frequently used in automatic motor control systems. In such systems the field current is controlled by an amplifier, and the output of the generator supplies the current which drives a motor.

In a series generator the field coils are connected in series with the armature so that the whole armature current flows through both the field and the load. If the generator is not connected across a load, the circuit is incomplete and no current will flow to excite the field. The series field winding contains relatively few turns of wire.

A shunt d.c. generator has its field winding connected in shunt (in parallel) with the armature. Therefore the current through the field coils is determined by the terminal voltage and the resistance of the field. The shunt field windings have a large number of turns and require a relatively small current to produce the necessary field flux.

A compound generator is a combined series and shunt generator. There are two sets of field coils – one in series with the armature, one in parallel. One shunt coil and one series coil are always mounted on a common pole piece. If the series field is connected so that its field aids the shunt field, the generator is called cumulatively compound. If the series field opposes the shunt field, the generator is called differentially compound. Compound generators were designed to overcome the drop in terminal voltage which occurs in a shunt generator, when the load is increased. This voltage drop is undesirable where constant voltage loads such as lighting systems are used.

Exercises

I. Translate the following words and phrases:

from English into Russian: separate, amplifier, supply, output, shunt, relatively, determine, aid, series, occur, cumulatively compounded;

from Russian into English: катушка возбуждения, генератор независимого возбуждения, внешний источник, требовать, генератор смешанного возбуждения, противодействовать, нагрузка, незамкнутый.

II. Complete the sentences using the words and word combinations from the text:

1. The d.c. current in the field coils is called the.... 2. If the field is supplied with current from an external source, the generator is said to be.... 3. Separately-excited d.c. generators have two circuits: the field circuit and the.... 4. Self-excited generators use part of the... to supply excitation current to the field. 5. The series field winding contains relatively... turns of wire. 6. If the series field is connected so that its field aids the shunt field, the generator is called....

III. Make up and write down questions to the following sentences, beginning from the words given in brackets:

1. In a series generator the field coils are connected in series with the armature. (How) 2. The fact that the series generator's output voltage is proportional to its armature current makes it useful for some special applications. (What) 3. In a separately-excited generator the field is independent of the armature, since it is supplied with current from either another generator (exciter), an amplifier or a battery. (Why) 4. There are two sets of field coils in a compound generator – one in series with the armature, one in parallel. (How many) 5. In a shunt generator the drop in terminal voltage occurs when the load is increased. (When)