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CLOSE THIS BOOKIntroduction to Electrical Engineering - Basic vocational knowledge (Institut fr Berufliche Entwicklung, 213 p.)
6. Electrical Field
VIEW THE DOCUMENT6.1. Electrical Phenomena in Non-conductors
6.2. Capacity
VIEW THE DOCUMENT6.2.1. Capacity and Capacitor
VIEW THE DOCUMENT6.2.2. Behaviour of a Capacitor in a Direct Current Circuit
VIEW THE DOCUMENT6.2.3. Types of Capacitors

Introduction to Electrical Engineering - Basic vocational knowledge (Institut fr Berufliche Entwicklung, 213 p.)

6. Electrical Field

6.1. Electrical Phenomena in Non-conductors

Electrical phenomena also occur in non-conductors. This becomes clearly evident in lightning during thunder-storms. A lightning may occur between two clouds or between cloud and earth. The cause of a lighting is a sudden charge equalisation between differently charged clouds or between different states of charge of cloud and earth. The form of discharge usually is a forked lightning. The voltage involved in lightning is about 100 · 106 V, the current intensity about 50 kA. With a time of discharge of 1 ms, an energy of 1000 kWh is released. Unfortunately, advantage cannot be taken of this enormous quantity of energy. But we can protect ourselves from the dangerous effects of a lightning stroke with the help of modern technical means.

Charging with considerably lower energy takes also place due to friction between different materials, when different materials contact each other, electrons from one material can migrate to the other one. When the two materials a separated, they show different charges. Due to frequent repetition of these contacts and separations (as involved in friction), high differences in charge may occur so that discharge via a spark takes place. This phenomenon will occur only when the materials involved are extremely well insulating (in a high atmospheric humidity, many materials lose their high insulating capacity and the charges can flow off). Due to the low energy involved in the way of charging up described here, a primary danger is not given for man, but other dangers may occur due to the effects of fight or shock. Spark discharges may become dangerous when they occur in close vicinity of easily combustible liquids or explosive substances. For example, protective measures are necessary when petrol is pumped from a bulk lorry into a storage tank. To avoid spark discharge, bulk lorry and storage tank must be properly connected electrically conductive before petrol should be pumped. Force actions occur between charges. Dissimilar charges attract each other and correspondent charges repel each other (see also force actions between magnetic poles). In the printing industry this force action is disturbing. During the rapid passage of paper through the machine, the paper may be charged so that proper transport of the paper will be prevented. Similar phenomena occur in the textile industry.

Direction and intensity of the force action is described by field lines like in the magnetic field. In contrast to magnetic field lines, however, electrical field lines arise from and end in charges. The extent of the electrical field is three-dimensional. Fig. 6.1. shows a few typical courses of field lines.


Fig. 6.1. Characteristic patterns of electric field lines

When an electrical conductor is placed in an electrical field (Fig. 6.2.), the freely movable electrons are displaced. The side facing the negative charge is positively charged and the side facing the positive charge is negatively charged. This phenomenon is called electrostatic induction or influence.


Fig. 6.2. Influence in an electric field


Fig. 6.3. Electrical shielding

1 - Metal ring
2 - Field-free space

When a conductive ring is placed in an electrical field, in the interior of the ring, a field-free space is brought about (Fig. 6.3.). This phenomenon is called electrical shielding. It is used in practice to shield from interference fields. Complicated electronic measurements are token in Faraday’s cage (working room surrounded by a double wall of copper foil) or aerial lines and other signal lines are screened.

When the electrical field acts on a non-conductor (also known as dielectric), the not freely movable electrons can be displaced only insignificantly in the direction of the positive charge. This phenomenon is called dielectric polarisation (Fig. 6.4.).


Fig. 6.4. Dielectric polarisation

1 - Elementary particle of the dielectric

If dust particles are in an electrical field, they will be charged negatively or positively, depending on their composition, attracted by the electrode having the opposite charge and deposited there. Advantage is taken of this effect in flue-gas cleaning. Dust can be removed almost completely from flue gases by means of electrical filters. The consumption of electrical energy for 1000 m3 of flue gas to be cleaned is about 1 kWh. A voltage of about 50 kV is applied to the electrodes.

When relating the voltage between two charged plates to the distance between the latter, we obtain the field strength E.

E= I/l

[E] = V/m

(6.1.)

where:

E

electrical field, strength

U

voltage

l

distance between the plates

This simple method of calculating the field strength is only applicable to parallel field lines (in a homogeneous field). At points and edges, the field strength is considerably higher than in the vicinity of large-area electrodes.

When the field strength reaches a critical value, the dielectric is subjected to a flashover or breakdown and, hence, to a spark discharge. The field strength required for a breakdown is called breakdown field strength. It is a quantity which depends on material (see Table 6.1.). The breakdown field strength of air is considerably lower than that of strong insulating materials. Therefore, the distance between conductors carrying high-voltage in air must be larger than between these conductors sheathed with strong insulating materials. For example, a voltage of 330 kV can break through an distance in air of about 100 mm and through a rubber insulation of maximum 13.3 mm, however.

Table 6.1. Dielectric Strength of a Few Insulating Materials

Insulating material

Dielectric strength in kV/mm

air

3.3

paper

10

rubber

25

porcelain

15

paraffin

40

aluminium oxide

1000

Since at points and edges (small surface areas) a particularly high field strength is prevalent, flashovers preferably start from them. This phenomenon is utilised for lightning protection pointed metal rods are fastened to the highest point of buildings, the rods are connected with the ground in a properly conducting manner and so are capable of arresting lightings and carry them off to ground without any damage to the building. This point effect also entails remarkable disadvantages. Thus, charges are sparked off from lines carrying high-voltage; this leads to considerable energy losses. Therefore, it is necessary to enlarge the electrically effective surface of high-voltage lines and provide a smooth surface. An enlargement of the surface is obtained by bunch lines. The line is divided into 4 conductors which are combined into one stranded conductor by means of spacers (Fig. 6.5.). Potential rings are attached to Insulators. Great store should be set by carefully rounded edges and smooth undamaged surfaces.


Fig. 6.5. Sectional view of a stranded conductor for highest voltage

1 - Individual conductor
2 - Spacer
3 - Electrically effective surface

Brush discharges nevertheless occurring in extra-high voltage lines may cause luminous phenomena under certain atmospheric conditions which are called corona because of their ring shape.

Electrical phenomena also occur in non-conductors. They show different effects (lightning, spark discharge, force action). In electric filters, the force action is used for dust separation. Charges can be produced by a continuous contacting and separating (e.g. friction). Spark discharges can lead to uncontrolled actions of man due to fear, to fire and explosions. Spark discharges occur when the breakdown field strength is exceeded. Discharges primarily take place at points and edges; that is why in high-voltage engineering all conductive parts should be provided with large and smooth surfaces.

Questions and problems:

1. Quote examples of electrical phenomena in non-conductors!
2. How are charges brought about?
3. What are the facts described by the breakdown field strength?
4. Why have points and edges to be avoided in high-voltage engineering?

6.2. Capacity

6.2.1. Capacity and Capacitor

Figs. 6.2. to 6.4. show certain phenomena between two charged plates. Two plates provided with connections and separated by a dielectric are called capacitor (Fig. 6.6.). This component is capable of storing a certain charge when a certain voltage is present. This storage capability is called capacity of a capacitor.


Fig. 6.6. Design of a capacitor

1 - Conducting metal plates (electrodes)
2 - dielectric
3 - Connections

C = Q/U

[C] = (A·· s)/V = F

(6.2.)

where:

C

capacity

Q

charge

U

voltage

Since the unit 1 F (farad) is very great, the capacity of the capacitors manufactured only reaches fractions of 1 F. These fractions are designated by the prefixes specified by legal regulation:

1 pF = 1 picofarad = 10-12 F
1 nF = 1 nanofarad = 10-9 F
1 µF = 1 microfarad = 10-6 F

The storage capacity of a capacitor is dependent on the area of the electrodes, the distance between them and the type of dielectric.

C = eA/d

(6.3.)

where:

e 1)

dielectric constant

A

area of the electrodes

d

distance between the electrodes

1) e Greek letter epsilon

The material constant is usually stated for the dielectric inquestion in the form of the product of the absolute dielectric constant times the relative dielectric constant.

e = e0 · er

(6.4.)

where:

e0

absolute dielectric constant

er

relative dielectric constant

The absolute dielectric constant applies to vacuum and is

e0 = 8.86 · 10-12 (A·· s)/(V·· m)

Table 6.2. er of Some Insulating Materials

Insulating material

er

air

1

paper

2

transformer oil

2.5

rubber

2.7

porcelain

5

Epsilon (special ceramic compound for the production of capacitors)

up to 10,000

Like resistors, capacitors can be connected in series or in parallel. The total capacity obtained in this way is to be determined. Fig. 6.7. shows the series connection of two capacitors. The two capacitors have the same charge Q. The following holds:

QAB = Q1 = Q2
UAB = U1 + U2


Fig. 6.7. Capacitors connected in series

When dividing the voltage equation by the charge, we have

UAB/Q = U1/Q + U2/Q

After inversion, we obtain from equation 6.2.

1/C = U/Q

and, for the total capacity of a series connection of capacitors we have

1/Cequ = 1/C1 +1/C2

(6.5.)

This equation has the same structure as the equation for the determination of Requ of a parallel connection of resistors.

The parallel connection of two capacitors is shown in Fig. 6.8. The same voltage is applied to the two capacitors, and each capacitor has tored a charge in accordance with its capacity.


Fig. 6.8. Capacitors connected in parallel

Thus, we have

U = UC1 = UC2
QAB = Q1 + Q2

After division, we obtain

QAB/U = Q1/U +Q2/U

and with equation 6.2. we have

Cequ = C1 + C2

(6.6.)

This equation has the same structure as the equation for the determination of Requ of a series connection of resistors.

From the equations 6.5. and 6.6., the following general statement can be derived: In a series connection of capacitors, the total capacity is always smaller than the smallest individual capacity, and in a parallel connection of capacitors, the total capacity is always greater than the greatest individual capacity.

Example 6.1.

Two capacitors with a capacity of 470 nF and of 680 nF have to be connected in series and then in parallel. Determine the total capacity of each of the two types of connections!

Given:

C1 = 470 nF

To be found:

C in series connection and in parallel connection

Solution:

Series connection of C1 and C2

1/Cequ = 1/C1 +1/C2 = (C2 + C1)/(C1 · C2)
Cequ = (C1 · C2)/(C1 + C2)
Cequ = (470 nF · 680 nF)/(470 nF + 680 nF)
Cequ = 277.9 nF

Parallel connection of C1 and C2

Cequ = C1 + C2
Cequ = 470 nF + 680 nF
Cequ = 1150 nF
Cequ = 1/15 µF

In series connection, a total capacity of 277.9 nF is obtained while in parallel connection the total capacity is 1.15/µF.

6.2.2. Behaviour of a Capacitor in a Direct Current Circuit

An uncharged capacitor is connected to a direct voltage source according to Fig. 6.9. (switch position 1).


Fig. 6.9. Circuit for charging and discharging a capacitor

The terminal voltage of the uncharged capacitor is 0. In order that, between the plates of the capacitor, the voltage can be applied to which it is connected, the capacitor must be charged. This means that a charging current must flow at the instant of switching on. The intensity of the current at the instant of switching on is determined by the large difference between charging voltage and terminal voltage of the capacitor and the resistance. With increasing charge of the capacitor, the voltage of it increases and will reach the value of the charging voltage when the process of charging is finished. With increasing charge, the voltage difference between charging voltage and terminal voltage of the capacitor also drops and, consequently, the charging current also drops. At the end of charging, the voltage difference and the charging current are 0. A direct current is no longer allowed to flow now. This is also due to the design of the capacitor because a dielectric (insulating material) is between its two connections.

When the capacitor is now discharged via a resistor according to Fig. 6.9. (switch position 2), at the first instant of discharge, a discharge current must flow which is only limited by the resistance. Since, due to the discharge, the terminal voltage of the capacitor drops, the discharge current must also drop in the course of time. After complete discharge, the terminal voltage and the discharge current have dropped to 0. The course taken by current and voltage during charging and discharging is shown in Fig. 6.10. Charging commences at time t1 and discharging at time t2. It is evident that charging and discharging currents suddenly reach their maximum value at the beginning of the charging or discharging process and then they reach the value of 0 after some time. Both in charging and in discharging, the voltage changes its value only slowly. There are no sudden voltage changes in capacitors.


Fig. 6.10. Behaviour or current and voltage during the charging and discharging of a capacitor tein = ton · taus = toff

Since the peak value of the discharge current is limited only by the discharge resistance, a capacitor should not be discharged via s short circuit. If this would occur, however, at the first instant of discharge, an extremely high current would flow for a short time which might cause the destruction of the capacitor or a fusing of the shorting bridge. As a capacitor retains its charge for some time after charging without external discharge, particular caution is necessary when working at installations containing capacitors. After disconnection from the mains, the fact that the capacitors are completely discharged must be checked or discharge via a resistor must be effected. In many installations, a discharge resistance is incorporated in order to avoid dangers to man.

During discharging, a capacitor acts as an electrical energy source (during a certain time, its terminal voltage drives a current). The energy stored in a capacitor is written as

W = C/2 · U2

(6.7.)

where:

W

energy

C

capacity

U

voltage

The energy that can be stored in a capacitor is relatively small. It is of advantage however, that it is available as a short-time energy release. Advantage of this effect is taken in a photoflash device and in some spot-welding equipment.

Example 6.2.

Which energy is stored in a capacitor of 47 µF charged up to 100 V?

Given:

U = 100 V
C = 47 µF

To be found:

W

Solution:

W = C/2 · U2
W = 23.5 · 10-6 (A·· s)/V · 104 V2
W = 0.235 Ws
W = 235 mWs

The energy stored in the capacitor is 235 mWs.

6.2.3. Types of Capacitors

For the different fields of application, a great variety of designs of capacitors is available. In heavy current engineering, primarily paper capacitors in a metal cup are used (Fig. 6.11.). Two metal foils and two paper strips are placed one upon the other in the way shown in the illustration and then properly rolled up. The two metal foils are attached to connections and the roll is mounted in a metal cup. Such a paper capacitor is also know as roll-type capacitor. The MP-capacitor (metal-paper capacitor) is designed in a similar manner; in this case, the foil is replace by a coat of metal which is produced by vapour deposition. These capacitors are smaller than paper capacitors of the same capacity.

Fig. 6.11. Encased capacitor


Fig. 6.11.a Design of the roll

1 - Connections of the foils
2 - Metal foil
3 - Paper strips


Fig. 6.11.b External view of the capacitor

1 - Metal enclosure
2 - Connections

Another advantage of MP-capacitors is the fact that, after a breakdown or puncture of the dielectric, the extremely thin metal coat in the close vicinity of the puncture evaporates and, thus, removing the short-circuit - that is why MP-capacitors are called “self-healing” capacitors.

For the practical use, the value of the capacity printed on the device and the rated voltage up to which the capacitor may be used have to be observed.

An arrangement consisting of two plates with a dielectric between them is called capacitor. The capacity of a capacitor is a measure of the charge which the capacitor is capable of storing at a certain voltage, and it is also dependent on the design. The total capacity in series connection and in parallel connection of capacitors is expressed by the equations 6.5. and 6.6.

When a capacitor is connected to a direct voltage, a current will only flow during charging and discharging. There are not sudden voltage changes in a capacitor. A charged capacitor can retain is charge for a longer period of time (danger!) and it should never be discharged via a short circuit. The capacitor may be used as an energy store.

In heavy current engineering, the paper capacitor arranged in a metal cup is sued. For use, pay particular attention to the value of the capacity and the rated voltage.

Questions and problems:

1. Describe the basic design of a capacitor and, in particular, the design of a paper capacitor!

2. Which property of the capacitor is described by the capacity?

3. Explain the course taken by current and voltage during charging and discharging of the capacitor!

4. Why should capacitors not be discharged via a short circuit?

5. What should be strictly observed when working at installations incorporating capacitors?

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