Main structural elements of a D. C. Machine

A direct-current machine consists of two main parts, a stationary part,

usually called the stator, designed mainly for producing a magnetic flux, and a rotating part, called the armature or the rotor. The stationary and rotating parts should be separated from each other by an air-gap. The stationary part of a d.c. machine consists of main poles, designed to create the main magnetic flux; commutating poles interposed between the main poles; and a frame. It should be noted here that sparkless operation of the machine would be impossible without the commutating poles. Thus, they should ensure sparkless operation of the brushes at the commutator.

The main pole consists of a laminated core the end of which facing the armature carries a pole shoe and a field coil through which direct current passes. The armature is a cylindrical body rotating in the space between the poles and comprising a slotted armature core, a winding inserted in the armature slots, a commutator, and a brush gear.

The frame is the stationary part of the machine to which are fixed the main and commutating poles and by means of which the machine is bolted to its bedplate. The ringshaped portion which serves as the path for the main and commutating pole fluxes is called the yoke. End-shields or frame-heads which carry the bearings are also attached to the frame.

Of these main structural elements of the machine the yoke, the pole cores, the armature core and the air-gap between the armature core and the pole core are known to form the magnetic circuit while the pole coils, the armature windings, the commutator and brushes should form the electric circuit of the machine.

II. Translate the following phrases, using the given variants of translation:

to consist – состоять: to consist of a stationary part and a rotating part;

separated – отдельный изолированный: the stationary and rotating parts

should be separated from each other by an air gap;

to serve – служить в качестве чего-либо: the ringshaped portion or yoke

serves as a path for the main and commutating pole fluxes.

III. Join the beginnings and the ends

IV. Arrange synonyms in pairs and memorize them:

a) to consist of; to be separated from; to create; to be interposed between; to pass; to rotate; в) to be divided with; to produce; to introduce into; to permeate; to roll; to revolve; to comprise.

V. Write out the names of the machine parts and describe their operational characteristics

UNIT 12

I. Read the text

Beginnings Ends

The stationary parts of a d. c. machine are....

a laminated core the end of which carries a pole shoe and a field coil.

The two main parts of a direct current machine are....

main poles, commutating poles and a frame

The main pole consists of.... A stationary part or stator and a rotating part, called the armature or the rotor.

THE ALTERNATOR

The alternator is an electric machine for generating an alternating current by a relative motion of conductors and a magnetic field. The machine usually has a rotating field and a stationary armature. In a synchronous alternator the magnetic field is excited with a direct current. The direction of an induced e.m.f. is reversed each time when a conductor passes from a pole of one polarity to a pole at another polarity. Most machines of this type are used for lighting and power, but there are alternators with a revoking armature and a stationary field. They are used in small sizes mostly for special purposes.

Any electrical machine is reversible. When a machine is driven by a source of mechanical power, it works as a generator and delivers electrical power. If it is connected to a source of electrical power, it produces mechanical energy, and operates as a motor. The alternator may also be operated as a motor.

The a-c.generator, or alternator, does not differ in principle from the d. c. generator. The alternator consists of a field structure and an armature. The field structure is magnetized by a field winding carrying a.direct current. An electromotive force is generated in tine winding of the armature. In alternators the field is usually the rotating element and the armature is stationary. This construction has a number of advantages. Only two rings are needed with a rotating field. These rings carry only a relatively light field current, at a voltage generally of 125, and seldom exceeding 250. The insulation of such rings is not difficult. A stationary armature requires no slip rings. The leads from the armature can be continuously insulated from the armature winding to the switchboard, leaving no bare conductor. The alternator with a rotating field may be further divided into the vertical and the horizontal types.

The vertical type is usually applied for large water-wheel generators where it is desirable to mount the water turbine below the generator. The more common horizontal type is used with diesel and steam engine drive. A lowspeed alternator of this type is suitable for a diesel engine drive, a high speed alternator is suitable for a steam turbine drive.

II. Form nouns, denoting devices with the help of the suffix – or. Translate them To alternate, to commute, to conduct, to generate

III. Read the text and write out the key words, characterizing the alternator

IV. Translate the following word combinations paying attention to the Participle 2

The leads from the armature can be continuously insulated from….., the vertical type of alternator applied for large water-wheel generator; alternators with a revoking armature and a stationary field used in small sizes mostly for special purposes; a machine driven by a source of mechanical power; the direction of an induced e. m. f. …

V. State 5 questions to the text

VI. Points for discussion:

1. The structure of the alternator;

2. The application of the alternator.

UNIT 13

I. Read the text

THE INDUCTION MOTOR

An induction motor like any other motor consists of a stationary part, the stator, and a rotating part, the rotor. The rotor of an induction motor is not connected electrically to the source of power supply. The currents which circulate in the rotor conductors are the result of voltage induced in the rotor in the magnetic field set up by the stator. The rotor is fitted with a set of conductors in which currants flow. As these conductors lie in the magnetic field produced by the stator, a force is exerted on the conductors and the rotor begins to revolve.The operation of the motor depends upon the production of a rotating magnetic field. The speed at which the field of an induction motor turns is called the synchronous speed of the field or of the motor.

The induction motor is the simplest of the various types of electric motors and it has found more extensive application in industry than any other type. It is made in two forms – the squirrel cage and the wound rotor, the difference being in the construction of the rotor.

The stator of the induction motor has practically the same slot and winding arrangement as the alternator and has the coils arranged to form a definite number of poles, the number of poles being a determining factor in connection with the speed at which the motor will operate. The rotor construction, however, is entirely different.

The squirrel-cage rotor is a simpler form and has been used in many machines.

Instead of coils the winding consists of heavy copper bars.

The wound-rotor type has a winding made up of well-insulated coils, mounted in groups whose end connections are brought out to fill in rings. The purpose of this winding is to provide for variation in the amount of resistance included in the rotor circuit.

Provision for ventilation is made by leaving passageways through the core and frame, through which air is forced by fan vanes mounted on the rotor. In main cases the motors now built in as an integral part of the machine it is to drive.

There being no electrical connection between the rotor circuits of the induction motor and the stator circuits, or supply line, the currents which flow in the rotor bars or windings correspond to the induced voltages, the action being similar to that of a transformer with a movable secondary. With but a singlephase winding on the stator, however, the torques produced in the two halves of the rotor would be in apposition, and the motor would not start. With more than one set of windings two for a two-phase motor, three for a three-phase motor a resultant field is produced which has the effect of cutting across the rotor conductors and induces voltages in them. This field is considered to be revolving at uniform speed.

The term “revolving field” should not be taken to mean actual revolution of flux lines. The magnetic field from the coils of each phase varies in strength with changes in current value but does not move around the stator. The revolutions are those of the resultant of the three, or two, phases, as the case may be.

A motor with a single-phase winding is not self-starting but must be provided with an auxiliary device of some kind to enable the motor to develop a starting torque. The effect of the revolving field is the same as would result from actual revolution of a stator having direct-current poles. As voltages have been induced in the bars or windings of the rotor, currants start flowing as a result of these voltages, and a torque is produced which brings the motor up to speed.

II. Find in the text the English equivalents for the word combinations given below:

1) асинхронный двигатель; 2) неподвижная часть; 3) вращающаяся часть; 4) проводник; 5) одновременная скорость; 6) широкое применение; 7) паз; 8) механизм обмотки; 9) трансформатор; 10) вращающий момент.

III. Complete the following sentences according to the contents of the text

1. The Induction Motor is …….. of electric motors and is more extensively applied in industry than any other type.

2. The purpose of this winding is …….. for variation in the amount of resistance included in the rotor circuit.

3. The effect of …. is the same as would result from actual revolution of a stator having direct-current poles.

IV. Answer the following questions:

1. What parts does the induction motor consist of?

2. What are the names of its rotating and stationary parts?

3. What does the motor operation depend on?

4. How can the difference between stator and rotor construction be explained?

5. What does the term “revolving field” mean?

V. Translate the sentences from the text paying attention to the Nominative Absolute Participle Constructions:

1. The induction motor is made in two forms – the squirrel cage and the wound rotor, the difference being in the construction of the rotor.

2. The stator of the induction motor has practically the same slot and winding arrangement as the alternator and has the coils arranged to form a definite number of poles, the number of poles being a determining factor in connection with the speed at which the motor will operate.

3. There being no electrical connection between the rotor circuits of the induction motor and the stator circuits, or supply line, the currents which flow in the rotor bars or windings correspond to the induced voltages, the action being similar to that of a transformer with a movable secondary.

VI. Discuss the following points:

1) The construction of an induction motor;

2) Induction motor operation principle.

UNIT 14

I. Read the text

TYPES OF INDUCTION MOTORS

TEXT 1

SINGLE – PHASE MOTOR

The single-phase induction motor differs from poly-phase type principally in the character of its magnetic field, as an ordinary single-phase winding will not produce a rotating field, but a field that is oscillating, and the induced currents and poles produced in the rotor by this field will tend to produce equal torque in opposite directions, therefore, the rotor cannot start to revolve. However, if the rotor can in some manner be made to rotate at a speed corresponding to the frequency of the current in the stator windings then the reaction of the stator and rotor flux is such as to produce a torque that will keep the rotor revolving.

In practice the starting of single-phase induction motors is accomplished by three general methods applicable to small-sized motors only.

First: the split-phase method, in which an auxiliary stator winding is provided for starting purposes only, this winding being displaced from the main stator winding by 90 electrical degrees. It has a higher inductance than the main stator winding, thus causing the currant in it to lag far enough behind the current in the main winding to produce a shifting or rotating field during the starting period, which exerts a starting torque on the rotor sufficient to cause rotation.

When nearly normal speed has been reached the auxiliary winding is out of circuit by a switch and clutch in the motor, which operates automatically by centrifugal force, and the rotor continues to run as a single-phase motor. The starting torque of such motions being limited, they are frequently constructed with the rotor arranged to revolve freely on the shaft at starting until nearly normal speed is reached, at which time the load is pitched up by the automatic action of a centrifugal clutch.

Second: an auxiliary winding may be connected to the single-phase line through an external inductance and a switch (for disconnecting the auxiliary winding from the circuit after the motor has reached normal speed), the introduction of the inductance in the auxiliary winding splitting the phase as before.

TEXT 2