CONTEST

TEXT 1. ELECTRICAL UNITS 1

TEXT 2. THEORY OF ALTERNATING CURRENTS 3

TEXT 3. DIFFERENCE BETWEEN A.C. AND D.C. 5

TEXT 4. TERMS AND DEFINITIONS 6

TEXT 5. PHASE AND PHASE DIFFERENCE 8

TEXT 6. INDUCTANCE 10

TEXT 7. HYSTERESIS 12

TEXT 8. ELECTRICAL MEASUREMENTS 13

16

TEXT 10. DIRECT-CURRENT METERS 19

TEXT 11. AUXILIARY EQUIPMENT 20

TEXT 12. WATTMETERS 23

TEXT 13. AMMETERS AND VOLTMETERS 24

TEXT 14. TRANSFORMERS 27

TEXT 15. CURRENT TRANSFORMERS 29

TEXT 16. VOLTAGE TRANSFORMERS 31

TEXT 17. CONSTRUCTION OF THE FIELD MAGNET 34

TEXT 18. POWERHOUSE AUXILIARY MOTORS 37

TEXT 19. SINGLE-PHASE MOTORS 41

TEXT 20. POLYPHASE INDUCTION MOTORS 42

TEXT 21. DIRECT-CURRENT MOTORS 44

TEXT22. GENERATORS 46

TEXT 23. ALTERNATING-CURRENT GENERATORS 48

TEXT 24. DIRECT-CURRENT GENERATORS 50

Text 1. Electrical units

Practical Units

The three practical units, the ohm, ampere, and volt, provide standards for comparison. They are defined as follows —

The ohm is the first primary unit, and the international ohm is defined as the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 gm. in mass, of uniform cross-sectional area and of length 106.300 cm.

The ampere is the second primary unit. The international ampere is the unvarying electric current which, when passed through a solu­tion of nitrate of silver in water, in accordance with a specification, deposits silver at the rate of 0.00111800 gm. per second.

The volt is the third primary unit and is the electric pressure which, when applied steadily to a conductor whose resistance is one international ohm, will produce a current of one international am­pere. Further, the international watt is the energy expended per se­cond by an unvarying electric current of one international ampere under an electric pressure of one international volt.

In addition to the practical construction of the ohm as defined above it may be derived in absolute measure as it has the dimensions of a velocity.

The original c.g.s. ampere was based on the magnetic effect of a current instead of the present electro-chemical effect.

M.K.S. Units

An alternative absolute system is based on the dimensions metre, kilogramme, and second. This is only one of many possible alterna­tives containing any multiple or sub-multiple of the metre and the gramme. It has the advantage over the c.g.s. system of simpler iden­tity between the absolute and the practical units in that it removes the powers of 10.

For the practical applications of the m.k.s. units a fourth unit is required in order to define all the practical units. This has been taken to be the unit of resistance, leading to the expression M.K.S. Q units.

But the present agreed fourth unit is the term ji₀, the permeability of free space, this being more fundamental. This applies of course to the m.k.s. system of electromagnetic units, providing M.K.S. u units. Elec­trostatic units are not used with the m.k.s. system.

Notes

engineering - техніка

ohm - ом

mercury - ртуть

ampere - ампер

nitrate - нітрат

volt - вольт

to expend - витрачати

Comprehension questions:

1. What are the three practical units used in electrical engineering?

2. Give the definition of ohm.

3. How can you characterize ampere?

4. What do we call the electric pressure?

5. Give the characteristics of M.K.S. units.

Text 2. Theory of alternating currents

Types of Electric Current

An electric current may be produced in a variety of ways, and from a number of different types of apparatus, e.g. an accumulator, a d.c. or an a.c. generator. Whatever the source of origin, the elec­tric current is fundamentally the same in all cases, but the manner in which it varies with time may be very different. There are following kinds of current which:

(a) represents a steady direct current (D.C.) of unvarying magnitude, such as is obtained from an accumulator.

(b) represents a D.C. obtained from a d.c. generator, and consists of a steady D.C. superimposed on which is a uniform ripple of relative­ly high frequency, due to the commutator of the d.c. generator. As the armature rotates the commutator segments come under the brush in rapid succession and produce a ripple in the voltage which is reproduced in the current.

(c) represents a pulsating current varying periodically between max­imum and minimum limits. It may be produced by adding a D.C. to an A.C. or vice versa. The d.c. component must be the larger if the current is to remain unidirectional. All the first three types of current are unidirectional, i.e. they flow in one direction only.

(d) represents a pure alternating cur­rent (A.C.) The current flows first in one direc­tion and then in the other in a pe­riodic manner, the time of each alternation being constant. In the ideal case the current varies with time accord­ing to a sine law, when it is said to be sinusoidal. Considering the time of a com­plete cycle of cur­rent (a positive half-wave plus a negative half-wave) as equal to 360° the instan­taneous values of the current are proportional to the sine of the angle measured from the zero point where the current is about to rise in the positive direction.

(e) represents a type of A.C. with a different wave form. Such an A.C. is said to have a peaked wave form, the term being self ex­planatory.

(f) represents an A.C. with yet another different wave form. Such an A.C. is said to have a flat-topped wave form.

Notes

alternating - змінний

current - струм

apparatus - апарат

d.c.= direct current - постійний струм

a.c. = alternating current - змінний струм

to plot - наносити на карту

to superimpose - накладати

accumulator - акумулятор

ripple - дзюрчання

voltage - напруга

sine - синус

instantaneous - миттєвий

peaked - гострокінцевий

Comprehension questions:

1. What types of electric current do you know?

2. What can you say about the production of an electric current?

3. Give the characteristics of A.C. and D.C.

4. What did you learn from the text about pulsating current?

5. Compare with your partner a pure alternating current and a type of A.C. with a different wave form.