TEXT 2. THEORY OF ALTERNATING CURRENTS

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

TEXT 9. ELECTRICAL MEASURING INSTRUMENTS. 17

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 ohmis 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 ampereis 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.

Thevolt 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 currentmay 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.