Effects of inductive and Capacitive Reactances

Inductive Reactance. The effect of inductive reactance is twofold: (1) It offers an opposition to the flow of current; (2) it causes the current to lag behind the voltage. With a perfect inductor, that is, one having no resistance, the current would lag the voltage by 90 electrical degrees, and no power would be taken from' the line because the coil stores up energy in the magnetic field during one half-cycle and returns this energy to the line in the following half-cycles. The current taken by a perfect inductor would be equal to its voltage divided by its inductive reactance, or I_L = E_L\X_L

Capacitive Reactance. The effect of capacitive reactance is twofold: (1) It offers an opposition to the flow of current, (2) it causes the current to lead the voltage. With a perfect capacitor, that is, one having no resistance, the current would lead the voltage by 90 electrical degrees, and no power would be taken from the line because the capacitor stores up energy in the electrostatic field during one half-cycle. The current taken by a perfect capacitor would be equal to its voltage divided by its capacitive reactance, or I_C = E_C\ X_C

The conditions just described are for perfect inductors and capacitors which require that they have no resistance.

7. Answer the following questions:

1. Is the effect of inductive reactance (capacitive reactance) twofold?

2. What is the first effect of inductive and capacitive reactances?

3. What is the second effect of inductive reactance (of capacitive reactance)?

4. Under what conditions would the current lag behind the voltage (lead the voltage)?

5. Why would no power be taken from the line?

6. What is the value of the current taken by a perfect inductor (by a perfect capacitor)?

8. Ask your fellow-student:

if the effect of inductive reactance is twofold,

if the capacitive reactance offers an opposition to the flow of current,

what causes the current to lag behind the voltage,

what causes the current to lead the voltage,

what perfect inductors and capacitors require.

9.

1. Describe two effects of inductive reactance upon the current in an a-c circuit.

2. Describe two effects of capacitive reactance upon the current in an a-c circuit.

3. Define a perfect inductor.

4. Define a perfect capacitor.

5. Describe the following figures:

10. Learn the dialogues by heart:

A.

Prof.: Is everything ready for today's laboratory classes?

Prof. Ass.: Sure enough.

Prof.: The students are supposed to experiment on inductive and capacitive reactances, if I'm not mistaken?

Prof. Ass.: Yes. The students will obtain voltage and current characteristics for a perfect inductor.

Prof.: And how about a perfect capacitor?

Prof. Ass.: Just the same.

B.

St. A: I have quite forgotten we are going to have laboratory classes in inductive and capacitive reactance.

St. B: You don't say so! This is a very important work and the task is very difficult.

St. A: And what is it?

St. D: We shall be given an hour to obtain voltage and current characteristics. The subject is capacitive and inductive reactances.

St. A: I see. The classes begin at 4.1 have some time to get ready for them.

Test Exercises

Write the number of the right choice or choose the right letter:

A.

1. The inductive reactance:

   1. causes the current to lead the voltage;

   2. causes the current to lag the voltage;

   3. is in phase with the voltage.

2. A perfect inductor has:

   1. strong resistance;

   2. no resistance at all;

   3. weak resistance.

В.

1. an opposition... the flow of current (1. in, 2. to, 3. at)

2. the coil stores ... energy (1. in, 2. out, 3. up)

3. to divide... (1.for, 2.on, 3.by)

4. to lag ...the voltage (1. after, 2. behind, 3. with)

5. to be equal... (1. for, 2. in, 3. to)

Part III

A.

Read the text without dictionary:

Parallel Circuits

Parallel circuits are used more frequently in electrical systems than are series circuits. In electronic equipment, series, parallel, and combination circuits are used. Because all inductors and capacitors have some resistance, it is not possible to set up a circuit containing pure reactances connected in parallel. However, in some inductors and capacitors, especially capacitors, the resistance is so low in comparison with the reactance that the resistance is assumed to be zero. Under these conditions, a circuit may be considered as if it would contain any combination of pure resistances and pure reactances connected in parallel. The characteristics of such a parallel circuit can best be illustrated by solving for the ' impedance, currents, power, etc., of a typical parallel circuit.

B.

Test Exercise

Choose the right letter.

1. Series, parallel and combination circuits are used in:

   1. radio

   2. telephone

   3. electronic equipment

C.

Retell the text

УРОК III. ELECTRICAL POWER SYSTEMS

Word and Phrase List

average value - среднее значение

battery, n. - батарея

battery cell - элемент батареи

communication, n. - связь

continuous current - непрерывный, постоянный ток

core, n. - сердечник

delta connection - соединение треугольником

effective value - действующие, эффективное значение

efficiency, n. - коэффициент полезного

действия, (к.п.д.)

electromotive force - электродвижущая сила, (э.д.с.)

extensive, adj. - широкий, обширный

generator, n. - генератор

а-с generator, n. - генератор переменного тока

alternator, n. - альтернатор

d-c generator, n. - генератор постоянного тока

induce, v. - индуктировать, вызывать

induction, n. - индукция

electromagnetic induction - электромагнитная индукция principle of electromagnetic - принцип электромагнитной

induction индукции

instantaneous value - мгновенное значение

load, n. - нагрузка, груз

machine, n. - машина, устройство

magnet, n. - магнит

motion, n. - движение

numerical value - численное значение

output voltage - напряжение на выходе

per cent, n. - процент

power factor - коэффициент мощности

power input ' - подводимая мощность

power output - мощность на выходе

power plant - электростанция

power supply - энергоснабжение

power-transmission systems - системы передачи энергии ratio, n. - соотношение

relation, n. - отношение

restate, v. - вновь формулировать

set of values - ряд значений

supply, v. - снабжать, питать

system, n. - система

electrical power system - энергетическая система

polyphase system - многофазная система

single-phase system - однофазная система

two-phase system - двухфазная система

transformer, n. - трансформатор

the fundamental transformer - основной, фундаментальный

(иногда: обычный

трансформатор)

volt-amperes - вольтамперы

voltage, n. - напряжение

to step down voltage (or current) - понижать напряжение (ток)

to step up voltage (or current) - повышать напряжение (ток)

wave, n. - волна

sine wave - синусоидальная волна

winding, n. - обмотка

the primary winding - первичная обмотка

the secondary winding - вторичная обмотка

wye, n. - тройник

Part I

1. Read and translate the following words and word combinations:

power supply, continuous current, induction, battery, generator, voltage, frequency, characteristic, average, sine wave, effective value, relation, power factor , delta, wye, transformer, instantaneous value.

2. Form as many word combinations as you can :

a)

These new a-c Alternating voltage Current The circuit

characteristics

Of a-c systems Of d-c system

b)

This The instantaneous The maximum The average The accepted The effective The numerical

value

Obtained from table of emf

c)

d-c four-wye wye- four electrical single- poly- two- three- three-

power wire connected delta phase power phase phase phase wire phase

system

3. Change the following word combinations using the pattern:

reading of a meter => meter reading

1. the characteristic of a circuit

2. the polarity of the magnet

3. the connection in series

4. the frequency of a power system

5. a system of communication

6. the value of the voltage

7. the value at any instant

8. the effect of heating

Part II

1. Choose a word or phrase of the same meaning:

A period of 180 electrical degrees, one complete set of values, the highest value in a cycle, the volt-amperes, an a-c generator with one coil, the efficiency of a transformer, alternation, a cycle, the maximum value, the apparent power, an alternator, the ratio of the power supply output to the power input.

2. Put in the missing prepositions or adverbs if necessary:

The alternator provides four phases ... output ... the common wire and each ... the other line wires. A four-phase output can also be obtained ... tell four combinations ... line wires ... proper design, a standard value ... output voltage could be obtained, (of, from, of, from, of, by, of)

3. Make questions using the pattern:

The voltages across the line wires are equal

to the sum of two phase voltages =>

What are the voltages across the line wires equal to?

1. At position I, the voltage is equal to zero.

2. The maximum value of a sine-wave voltage is equal to 500 volts.

3. The average value is equal to 2/π times the maximum.

4. The product of volts and amperes is equal to the power of watts.

5. The voltages across the line wire are equal to the individual phase voltages.

6. The current in the line wires in equal to 1.73 times the current in an individual phase.

4. Answer the questions (see suggestions in brackets):

1. How is the transformation ratio defined? (отношение напряжений первичной и вторичной обмотки от тока нагрузки)

2. What do you call the external characteristics of the transformer? (зависимость напряжения вторичной обмотки к мощности первичной обмотки)

3. What do you call the efficiency of the transformer? (отношение мощности вторичной обмотки к мощности первичной обмотки)

4. How are alternating voltages and currents continually varying? (изменения дают синусоидальную волну)

5. What is the product of volts and amperes equal to in d-c circuit if the voltage and current are constant in value? (произведение равно мощности в ваттах)

5. Translate the following words and word combinations into Russian:

трехфазная система, генераторы переменного тока, коэффициент мощности, действующие (эффективное) значение, мгновенное значение, средняя величина, характеристика тока, период, частота, к.п.д., электромагнитная индукция, простой генератор, повышающий (понижающий) трансформатор, постоянный (переменный) ток, якорь, катушка.

6. Read and translate the text with the dictionary: