ΕΛΛΗΝΙΚΗ ΔΗΜΟΚΡΑΤΙΑ
Ανώτατο Εκπαιδευτικό Ίδρυμα Πειραιά
Τεχνολογικού Τομέα
Ξενόγλωσση Τεχνική Ορολογία
Ενότητα: DC Generators
Παναγιώτης Τσατσαρός
Τμήμα Μηχανολόγων Μηχανικών ΤΕ
Άδειες Χρήσης
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Το παρόν εκπαιδευτικό υλικό υπόκειται σε άδειες χρήσης Creative Commons.
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Για εκπαιδευτικό υλικό, όπως εικόνες, που υπόκειται σε άλλου τύπου άδειας
χρήσης, η άδεια χρήσης αναφέρεται ρητώς.
Χρηματοδότηση
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Το παρόν εκπαιδευτικό υλικό έχει αναπτυχθεί στα πλαίσια του εκπαιδευτικού
έργου του διδάσκοντα.
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Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα στο Ανώτατο Εκπαιδευτικό
Ίδρυμα Πειραιά Τεχνολογικού Τομέα» έχει χρηματοδοτήσει μόνο την
αναδιαμόρφωση του εκπαιδευτικού υλικού.
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Το έργο υλοποιείται στο πλαίσιο του Επιχειρησιακού Προγράμματος
«Εκπαίδευση και Δια Βίου Μάθηση» και συγχρηματοδοτείται από την
Ευρωπαϊκή Ένωση (Ευρωπαϊκό Κοινωνικό Ταμείο) και από εθνικούς πόρους.
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1.
Σκοποί ενότητας ................................................................................................ 4
2.
Περιεχόμενα ενότητας........................................................................................ 4
3.
DC Generators .................................................................................................. 5
3.1
Structure of a dc generator .......................................................................... 5
3.2
Exercise and Practice ................................................................................. 7
3.2.1
Exercise A: Open questions ................................................................. 7
3.2.2
Exercise B: Making notes ..................................................................... 8
3.2.3
Exercise C: Identifying functions .......................................................... 8
3.2.4
Exercise D: Defining technical terms .................................................... 8
3.2.5
Exercise E: Identifying synonyms and autonyms .................................. 9
3
1. Σκοποί ενότητας
.
The aims of this unit are to:
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




Provide authentic text and vocabulary specific to the needs of students of
Mechanical Engineering
Encourage students to combine their knowledge of English with their
technical knowledge
Encourage students to find out facts about a topic
Help students to describe component characteristics
Help students to describe component functions
Help students match terms and definitions
2. Περιεχόμενα ενότητας
.
Contents of the unit



Structure of a dc generator
Description and function of the components of a dc generator
Operation of a dc generator
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3. DC Generators
A dc generator is a machine which supplies power to a load circuit by converting
rotary mechanical energy into electrical energy. The mechanical energy might be
provided by some prime mover, such as a waterfall, steam, wind, gasoline or diesel
engine, or an electric motor, which must rotate at a definite speed in order to produce
the desired voltage.
A generator is rated as to the kilowatts it can deliver, which is the electrical power
capacity of a machine, without overheating at a rated voltage and speed. Other
ratings specified by the manufacturer on the nameplate of the generator, may include
current capacity, output voltage, temperature, and speed.
A dc generator can be made in a wide range of physical sizes and with various
electrical characteristics. Its use has declined rapidly since the development of
rectifiers, but there are still certain applications such as power systems for railroads
or earth-moving equipment where a dc generator is used.
3.1 Structure of a dc generator
A dc generator operates on the principle that flux is produced by its field windings
and the motion of the conductors which, when moved through a magnetic field, cut
the magnetic lines and therefore an emf is generated in the conductors. The flux is
established in the field yoke, pole cores, air gap and armature, all of which form what
is known as the magnetic circuit of a generator
The field yoke or frame, made of cast steel or fabricated rolled steel, acts as a
mechanical support for the pole cores - a series of alternate north and south
magnetic poles spaced around a circular periphery - which are usually made of steel
plates insulated from each other and riveted together, bolted to the yoke. The
surfaces of the pole cores, next to the air gaps, facing the armature are referred to as
pole faces. The spaces between the pole faces and the armature are the air gaps
and their length must be relatively small so as to make the magnetic reluctance
relatively small as well. The term armature, when used with respect to a dc machine,
is frequently used to describe the entire rotating arrangement of a dc machine, where
an emf is induced. It is rotated by an external mechanical force and the voltage
generated in the armature is then connected to an external circuit. Since it rotates, it
is also called a rotor. It consists of a cylindrical core of iron or steel laminations,
insulated from one another and assembled on a shaft in the case of small machines
and on a cast-steel spider in the case of large machines. The purpose of laminating
the core is to reduce the eddy-current losses. Slots are stamped on the periphery of
the laminations to provide a means of securing the armature windings. The armature
windings along with the brushes, field windings and commutator make up the electric
circuits of a dc generator The armature windings, which act as conductors of the
armature of a dc generator, consist of coils wound to their correct shape and size on
a form. They are completely insulated from the core with treated fabric (paper or
bonded mica flakes) and are mounted on the armature core by being securely
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slipped into place in the proper armature slots. They are then connected to the
proper split-ring commutator segments
Apart from a few special windings, armature windings can be divided into two groups,
depending upon the manner in which the wires are joined to the commutator, namely,
lap windings and wave windings, both being types of the drum type armature winding
used in modern machines. They are formed by placing the armature coils in slots on
the surface of a drum- shaped or cylindrical iron core.
A lap winding can be simplex (single) in which the ends of each coil arc connected in
series to adjacent commutator segments or multiplex (double or triple) in which there
are two or three separate sets of coils, each set connected in series. Both lap and
wave windings are closed-circuit windings, that is, they close upon themselves to
form a closed electric circuit. Both windings are formed by interconnecting a number
of separately insulated coils which have been laid in place in the armature core slots.
However, in lap windings the two ends of any one coil are taken to adjacent
commutator segments whereas in wave windings the two ends of each coil are bent
in opposite directions and taken to commutator segments some distance apart. Also,
a lap winding has as many paths in parallel between the negative and positive
brushes as there are of poles while a wave winding has only two paths in parallel,
irrespective of the number of poles.
The brushes arc carbon-made connectors resting on the face of the commutator.
They are stationary and spring-mounted to slide or 'brush' against the commutator.
Thus, brushes provide the sliding electrical connection between the armature coils
and the external circuit. The field windings or field coils are electromagnets producing
the flux cut by the armature. They are placed around the pole cores, forming the field
circuit which may be connected either in series or in parallel with the armature circuit.
Series field coils have few turns of wire of large cross-section and a relatively low
resistance whereas parallel or shunt field coils have many turns of wire of small
cross-section and a relatively high resistance.
The commutator consists of copper segments assembled side by side to form a
cylinder or ring, the segments being insulated from each other by thin mica sheets
and also from the shaft of the machine. Each segment has a slot to allow tor the ends
of the armature coils to be soldered into.
The commutator is used so that the voltage taken from the dc machine remains
constant in direction and in magnitude or else the commutator converts the
alternating current flowing in the armature of the dc machine into direct current. A
rotating armature coil passes through a magnetic field and as it is rotated at a
constant rotational speed by some mechanical means, the number of magnetic flux
lines through the coil changes continually. As a result, an alternating emf is induced
in that coil. The amount of emf induced depends on the rate at which the number of
flux lines is changing through the coil, and its direction is determined by Fleming's
right-hand rule: the First finger represents the direction of the magnetic Field and the
second finger represents the direction of the Current. The thuMb then will indicate the
direction of Motion and hence electroMotive force, provided that all three fingers are
extended at right angles to one another. On the other hand, when the coil edges are
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moving parallel to the flux lines so that the flux is not changing, no emf is generated
and the coil is said to be in the neutral plane.
A commutator is needed to enable a steady or direct voltage to be obtained from the
alternating emf generated in the rotating armature coil, as mentioned above. This
generated alternating emf causes an alternating current to flow first in one direction
and then the other. It is possible to convert this alternating current that is induced in
the armature into a form of direct current. This conversion of ac into dc may be
accomplished through the use of a split-ring commutator, which has two segments
insulated from each other and from the shaft of the machine on which it rotates. Each
commutator segment is connected to each end of the armature coil. The purpose of
the split-ring commutator is to reverse the armature coil connections to the external
circuit at the same instant that the current reverses in the armature coil. This reversal
must take place while the two commutator segments to which the coil is connected
are being short- circuited by a brush, a process being called commutation. Since the
armature coils lie in magnetic material, they are inductive and the reversal ot" the
current during commutation is opposed by an emf of self-induction or inductance
developed in the commutator segments that are shorted by the brushes. This emf of
self-induction in the armature coil, then, opposes the reversal of the coil current and
maintains a current flow around the short- circuited coil, thereby forming a spark that
can burn the surface of the commutator. Sparking can be reduced by inducing a
voltage equal and opposite to that caused by the change in current. This is
accomplished by the use of interpoles or commutating poles, which are small poles
placed between the main poles of a generator. Interpole windings are connected in
series with the armature windings. Thus, an increase in armature current creates a
stronger magnetic field around the interpoles which counteracts the main field
distortion created by the armature windings.
3.2 Exercise and Practice
3.2.1 Exercise A: Open questions

Answer the following questions about the reading text.
1. How does a dc generator obtain the mechanical energy it needs?
2. What principle is used to convert mechanical motion into electrical energy in dc
generators?
3. What is the magnetic circuit of a dc generator composed of?
4. How does the armature operate in a dc generator?
5. Why is the core of the armature laminated?
6. Name the parts that form the electric circuits of a dc generator.
7. How are the wave windings joined to the commutator?
8. Why are lap and wave windings referred to as "closed-circuit windings"?
9. How are series field coils different from parallel field coils?
10. Explain why an alternating emf is generated in a coil that is rotated in a magnetic
field
11. What factor determines the amount of emf generated?
12. Why is NO voltage induced in a rotating coil as it passes through the neutral
plane?
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13. How does a split-ring commutator act in order to convert the ac induced in the
armature into dc?
14. How does the armature react to commutator action?
15. How do interpoles help to compensate for the main field distortion caused by the
armature reaction?
3.2.2 Exercise B: Making notes

In note form, fill in the blanks in the following table using information from the
text and the diagrams provided.
Component
Field yoke
1
2
3
4
5
Material
Construction
stell
Pole faces
Air gaps
6
7
8
9
10
----------
Armature windings
------------
Field coils
carbon
electomagnets
Location
--------Around circular periphery
Surfaces of pole cores
---------on shaft or cast –steel
spider
on armature core, in
slots
cylinder or ring
interpoles
3.2.3 Exercise C: Identifying functions

Search through the text and find which component performs each of the
functions described below. Note that some components may perform more
than one functions.
1. It serves as a means of connecting the brushes to the armature.
2. It provides the rotating element in a dc generator.
3. It turns rotary mechanical energy into electrical energy.
4. They eliminate the effects of armature reaction.
5. They connect the armature to an external circuit load in order to pick up or use
the induced emf.
6. They provide a means of support for the field coils.
7. It causes a generator to produce dc voltage rather than ac voltage at its output
terminals.
8. It serves as a mechanical support for the pole cores as well as serving as part of
the magnetic circuit.
9. They increase the cross-sectional area and thus reduce the magnetic reluctance
of the air gaps.
10. They provide the magnetic field for producing a voltage.
11. It provides mechanical energy to a dc generator.
12. They form contact with opposite parts of the commutator.
3.2.4 Exercise D: Defining technical terms

Look back in the reading text and find the terms that match the following
definitions.
1. Maximum output or producing ability.
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2. Listings indicating effectiveness of an engine in capacity, current, etc.
3. A small flat piece of metal or wood on which the ratings are marked.
4. A piece of magnetic material that connects two or more magnetic cores
permanently.
5. Formed in a mould.
6. Made by assembling parts, manufactured.
7. A supporting rod or bar in a machine that revolves in order to transfer power or
motion.
8. Framework with several leg-like extensions.
9. Narrow openings or holes.
10. Wire-wound spirals or loops of wire used as conducting elements.
11. Material processed with a special substance in order to be protected from
damage or be given special properties.
12. Any of the parts into which a body is separated.
13. Something that is made by cutting across it, usually at right angles to its axis.
14. The speed or degree something is changing in relation to something else.
15. A flash of light caused by an electric discharge, often accompanied by a cracking
sound.
3.2.5 Exercise E: Identifying synonyms and autonyms

1
2
3
4
1
2
3
4
Match each word in column A with its synonym in column B.
Column A
Column B
As to (line 6)
a. Impressed, pressed onto
Stamped (line 39)
b. cancels
Slide (line 69)
c. in relation to
Counteracts (line 125)
d. move slowly over something
 Now match each word in column A with its opposite in column B.
Column A
Declined (line 12)
Assembled (line 36)
bonded (line 45)
Irrespective of (line 67)
Column B
a. In relation to, depending on
b. Detached, separated
c. Dismantled, broken up
d. Risen, increased
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