Английский язык для моряков / Труханова Англ для моряков

6.What is electromotive force?

7.What happens to free electrons when the electromotive force is applied to the ends of the wire?

8.What kind of current is called direct current?

9.What are the sources of direct current?

10.How can alternating current be converted into direct one?

11.How can Ohm's law be applied to a circuit section? To an entire circuit?

12.What is alternating current characterized by?

13.What does the strength of the current in A.C. circuits depend on?

14. When is pulsating current produced?

4. BASIC UNITS OF ELECTRICITY AND MEASURING DEVICES

There exist three basic electrical units in any electrical circuit: ampere, ohm and volt.

The ampere is an electrical unit for measuring the strength of electric current in a circuit. One ampere represents the amount of current generated by 1 volt acting through the resistance of 1 ohm. For measuring the intensity of an electric current in amperes a device called ammeter is used.

The ohm is an electrical unit for measuring the resistance or opposition to the flow of current. All substances show different resistances to the flow of electricity through them. With the increase in the temperature the resistance of all metals increases while the resistance of carbon, insulating materials, (electrolytic) solution decreases. One ohm represents such a resistance that a one-volt addition to the potential produces 1 ampere of current. The device used for measuring resistance is called ohmmeter. The most commonly used device for measuring insulation resistance is megohmmeter or "megger". It is usually employed for continuity, ground and short-circuit testing in general electrical power work.

The volt is an electrical unit measuring the external force applied to a circuit to overcome the opposition to the flow of current. This force is called voltage and is also referred to as electromotive force or electric pressure. The electromotive force that causes a current of 1 ampere to flow through a resistance of 1 ohm equals 1 volt. The device used for measuring voltage is called voltmeter.

One more important unit of electrical measurement is the watt - the unit of power: the power of 1 ampere of current pushed by one volt of electromotive force. Devices used for measuring delivery of electric energy in watts are called wattmeter and watthourmeter.

VOCABULARY

QUESTIONS

1.What is ampere?

2.What does one ampere represent?

3.What device is used for measuring the intensity of electric current?

4.What is resistance measured in?

5.What is the difference between the resistance of metals and that of carbon?

6.What does 1 ohm represent?

7.What is ohmmeter used for?

8.What device is used for measuring insulation resistance?

9.What testing is "megger" employed for?

10.How is the external force applied to a circuit to overcome the resistance measured?

11.What does 1 volt equal?

12.How is the power of electric current measured?

EXERCISES

I. Check yourself in your vocabulary.

What do you call:

1.the property of the molecules of iron to store energy in a field?

2.the study of electricity at rest?

3.the study of electricity in motion?

4.a piece of iron or steel which being magnetized retains its magnetism?

a)electrons;

b)positive charges;

c)ions.

7.When the E.M.F. is applied to the ends of the wire:

a)positive charges move in one direction;

b)free electrons move in random manner;

c)free electrons move in one direction.

8. According to Ohm's law applied to the entire circuit the strength of the current is;

a)inversely proportional to the e.m.f. and directly proportional to the resistance of the circuit;

b)directly proportional to the e.m.f. and inversely proportional to the resistance of the circuit;

d)inversely proportional to the e,m.f. and to the resistance of the circuit,

9. The strength of the current in A.C circuits depends on:

a) the resistance of the circuit and the voltage applied to it only;

b) the resistance of the circuit and the inductance and capacitance of the circuit section;

c) the resistance of the circuit, the voltage applied to it, the inductance and capacitance of the circuit

sections.

10. With the increase in the temperature the resistance of all metals:

a)decreases;

b)increases;

c)remains unchangeable.

III. Mind your Grammar.

Put the verbs in brackets in the correct form - Present or Past Simple Active.

1.The study of the nature of electricity (to begin) in the 18th century.

2.Rubbing glass with silk (to produce) static electricity.

3.Electromotive force (to make) electrons move through a conductor.

4.The experiment held yesterday (to prove) that these substances (to show) different resistances.

5.If you want to measure insulation resistance, you (to use) "megger".

6.Voltage (to overcome) the opposition to the flow of current.

7.Many scientists (to investigate) electric phenomena in the 19th century.

8.Electrons (to move) under the influence of e.m.f.

9.Ohm's law (to give) the possibility to measure electric current in a circuit.

10.After some experiments the scientists (to define) the law of measuring three basic electrical units.

11.Charges in motion (to give rise) to a magnetic field.

12.Michael Faraday (to discover) magnetic induction.

13.When the current (to stop), the magnetic field also (to disappear).

14.Loudspeakers and electric motors (to be) examples of application of magnetism.

15.The principal utilization of electricity (to increase) rapidly with the development of telegraph in 1844, electric motors in 1887 etc.

UNIT 2.

SHIP'S DIRECT CURRENT ELECTRIC MACHINES

I. THE CONSTRUCTION AND THE PRINCIPLE OF OPERATION

D.C. machines converting mechanical energy into electrical one are referred to as D.C. generators. Those, on the other hand, which convert electrical energy into mechanical one, are spoken of as D.C. motors. D.S. machines are reversible and, therefore, identical in construction. They are composed of two main parts - the stationary part which is called the frame provided with the main poles and the commutating poles, and the rotating part which is said to be the armature, its windings and commutator being positioned on it.

The frame is made of steel in the form of a closed magnetic conductor, The main poles for the purposes of decreasing the losses are assembled of sheet steel laminations. Mounted on them, the field windings are built of copper isolated conductors. The main poles are intended to produce the main magnetic flux.

The commutating poles are also made of steel and arranged midway between the main poles. Their windings as well as those of the main poles are built of copper conductors. The commutating poles with the windings are designed to ensure non-sparking operation of an electric machine.

The armature makes up a cylindrical core made of sheet steel laminations, a two-layer winding being fixed in their slots. The former is built of sections made of isolated copper conductors.

The commutator is constructed of separate copper bars isolated from each other and from the frame. The section leads of the armature winding are connected to the bars. The commutator is designed for the conversion of an alternating E.M.F. induced in the armature winding into a direct one.

The brushgear is an element designed for collecting the current from the armature winding and leading it to the latter. It is composed of brushes, brush-holders, brush-studs, brush-rockers and current collecting bars.

The commutator and the brushgear are the most essential parts of a d.c. machine, its reliable operation being dependent on their condition.

The operative principle of a D.C. machine is built upon the laws of electromagnetic induction and electromagnetic force. As the armature is rotated in the magnetic flux of the main poles by a drive motor, the E.M.F. is induced in the armature winding. In the loaded machine, the e.m.f. brings into existence the current

with which it coincides in direction. This current interacting with the magnetic flux produces the electromagnetic torque directed in opposition to that produced by a prime mover. As this takes place, the machine operates as a generator. The mechanical power consumed from the drive motor is converted into the electrical one and is given up to the mains.

When a d.c. machine is connected to the electrical source of supply, the current is generated in the armature winding. Interacting with the magnetic flux of the poles, it produces the electromagnetic torque which brings into rotation the armature. The e.m.f. directed in opposition to the current direction is induced in the armature winding. In this case the machine works as a motor. The electrical power consumed from the mains is converted into the mechanical one.

QUESTIONS

1.What type of D.C. machine is referred to as a D.C. generator?

2.What type of D.C. machine is spoken of as a D.C. motor?

3.What are the two main parts of a D.C. machines?

4.What is the frame composed of?

5.Is the construction of the commutating poles identical to the one of the main poles?

6.Are the commutating poles intended for the same purposes as the main poles?

7.What does the armature make up?

8.What is the commutator designed for?

9.What are the elements the brushgear is constructed of? What is it designated for?

10.What laws is the operative principle of a D.C. machine built upon?

11.How can the operative principle of a D.C. machine be described?

12.In what case does a D.C. machine operate as a D.C. generator?

13.Under what condition does a D.C. machine work as a D.C. motor?

14.What happens to the electrical power consumed from the mains when a D.C. machine works as a motor?

Fig. 2 Four-pole D.C. generator

2. D.C. GENERATORS AND THEIR CHARACTERISTICS

D.C. generators are provided with the armature winding and one к two field windings. Depending on the type of the armature winding and field winding interconnection D.C. generators may be recognized as those of separate excitation, of shunt excitation, of series excitation, and of compound excitation.

In a separately excited generator, the field winding obtains its supply from a separate current source. It is connected in series with the armature winding in a series-wound generator and in parallel - in a shunt-wound generator. A compound-wound generator has two field windings positioned on the main poles, one of them being connected IN shunt, the other - in series with the armature winding. The parallel field winding concentrates the current ranging from 1 to 6% of the rated armature current. It is made from copper conductors provided with a large number of turns of relatively small section. The series field winding carries the entire armature current and hence its conductors are of large section.

Shunt-wound generators, series-wound generators and compoundwound generators are self-excited; that is to say they don't require a separate current source for their excitation. The current supplying the windings is derived from the generator armature.

The generator properties depend upon the method of the generator excitation. They may be expressed by definite characteristics, that is, the relationships between the e.m.f., the voltage, the armature curtail and the excitation, all of them being responsible for the operation of и D.C. machine on different loads. The most important of the indicated magnitudes is the voltage which is dependent on the above mentioned excitation current, the armature current and the rotational speed.

The main characteristics which reveal the properties of D.C. generators may be described as follows.

No-load characteristic is defined as the relationship between the generator e.m.f. E and the excitation current Iex with the armature current I = 0 (curve 1, fig.2,a): E = f(Iex). This characteristic lets us judge] the saturation of the magnetic circuit and may be applied for plotting] the other characteristics.

Short-circuit characteristic (fig. 2,b) shows the relationship: between the armature current Ish and the excitation current with the short circuited armature and the voltage equal to zero: Ish = F (Iex). This char acteristic is plotted as a straight line since under short-circuit condition^ the generator magnetic circuit is practically not saturated.

External characteristic (fig. 2,c) is the relationship between the voltage U and the armature current I with the resistance in the excitation circuit Rex = const. The equation is U = f (I). The external characteristics plotted in fig. 2,с are those of a separately excited generator (1), of a shunt-wound generator (2), of a serieswound generator (3) and of a compound-wound generator when its field winding is either connected accordantly (4) or in opposition (5) to the armature winding. With the excess M.M.F. of a series winding the compound-wound generator voltage shows an increase when the armature current rises (6).

Regulation characteristic (fig. 2,d) makes up the relationship between the excitation current and the armature current with the generator voltage U = const: Iex = f (I). Fig. 2,d shows by graphical display the regulation characteristics of a shunt-wound generator (1), of a separately-excited generator (2) and of a compound-wound generator (3) with its windings connected accordantly.

Load characteristic indicates the relationship between the volt age U and the excitation current Iex: U = f (Iex) with I = const. The load characteristics of a separately-excited generator (2) and of a compound wound generator (3) with its windings connected accordantly appeal in fig. 2, a. As illustrated in the figure, curve 3 passes above the noj load curve (1). This may be explained by the action of the m.m.f. of series winding.

QUESTIONS

1.How many armature windings and field windings are D.C. generators provided with?

2.What types of D.C. generators depending on the way of the armature winding and field winding interconnection do you know?

3.What fact does a separately excited generator take its name from?

4.In what way is the field winding connected with the armature winding in a shunt-wound generator?

5.In what way is the field winding connected with the armature winding in a series-wound generator?

6.How many field windings does a compound-wound generator have? How are they connected to the armature winding?