Ξενόγλωσση Τεχνική ΟρολογίαΕνότητα: DC Generators

Παναγιώτης Τσατσαρός

Τμήμα Μηχανολόγων Μηχανικών ΤΕ

ΕΛΛΗΝΙΚΗ ΔΗΜΟΚΡΑΤΙΑΑνώτατο Εκπαιδευτικό Ίδρυμα ΠειραιάΤεχνολογικού Τομέα

Άδειες Χρήσης

• Το παρόν εκπαιδευτικό υλικό υπόκειται σε άδειες χρήσης Creative Commons.

• Για εκπαιδευτικό υλικό, όπως εικόνες, που υπόκειται σε άλλου τύπου άδειας χρήσης, η άδεια χρήσης αναφέρεται ρητώς.

Χρηματοδότηση

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• Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα στο Ανώτατο Εκπαιδευτικό Ίδρυμα Πειραιά Τεχνολογικού Τομέα» έχει χρηματοδοτήσει μόνο την αναδιαμόρφωση του εκπαιδευτικού υλικού.

• 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

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1. Σκοποί ενότητας.

The aims of this unit are to:

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

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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 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

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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 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

field11. What factor determines the amount of emf generated?

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12. Why is NO voltage induced in a rotating coil as it passes through the neutral plane?

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 Material Construction Location1 Field yoke ---------2 stell Around circular periphery3 Pole faces Surfaces of pole cores4 Air gaps ---------- ----------5 on shaft or cast –steel

spider6 Armature windings ------------ on armature core, in

slots7 carbon8 Field coils electomagnets9 cylinder or ring10 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

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Look back in the reading text and find the terms that match the following definitions.

1. Maximum output or producing ability.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 Match each word in column A with its synonym in column B.

Column A Column B1 As to (line 6) a. Impressed, pressed onto2 Stamped (line 39) b. cancels3 Slide (line 69) c. in relation to4 Counteracts (line 125) d. move slowly over something

Now match each word in column A with its opposite in column B.

Column A Column B1 Declined (line 12) a. In relation to, depending on2 Assembled (line 36) b. Detached, separated3 bonded (line 45) c. Dismantled, broken up4 Irrespective of (line 67) d. Risen, increased

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