Browsing: Basics of Electrical Engineering

Electric Energy

Electric energy used or produced is the product of the electric power input or output and the time over which this input or output occurs:

Electric energy is what customers purchase from electric utility companies. These companies do not use the joule as an energy unit, but instead use the much larger and more convenient kilowatt hour (kWh) even though it is not an SI unit. The number of kilowatt hours consumed equals the product of the power absorbed in kilowatts and the time in hours over which it is absorbed:

kWh = Power × time

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Introduction to Alternating Current (AC)

Most electrical power lines carry alternating current. Very little direct current is used for electroplating, electro-deposition, electric lighting etc.

There are many good reasons for this choice of Alternating Current over Direct Current for electric power transmission. AC  voltage can be stepped up or stepped down easily and without appreciable power loss, through the use of a transformer, but on the other hand direct current voltages cannot be changed without a considerable power loss. This is a very important factor in the transmission of electric power, since large amounts of power must be transmitted at very high voltages. At generating station the voltage is stepped up with the help of the transformers to very high voltages and sent over transmission line and at receiving end of the line the step down transformer step down the voltage to values which can be used for lighting and power.

Various kinds of electrical equipment require different voltages for proper operation, and these voltages can easily be obtained by using a transformer and AC power transmission line.

Since the power transmitted equals the voltage multiplied by the current (P = VI), and the size of the wire limits the maximum current which can be used, the voltage must be increased if more power is to be transmitted over the same size wires. Also, excessive current flow causes overheating of the wires, resulting in larger power loss, so that maximum current is kept as low as possible. The voltage, however, is limited only by the insulation of transmission line. Since the insulation can be easily strengthened, the voltage can be increased considerably, permitting the transfer of larger amounts of power with smaller wires and much less power load.

When current flows through a wire to reach the electrical device using power, there is a power loss in the wire loss in the wire proportional to the square of the current. Any reduction in the amount of current flow required to transmit power results in a reduction in the amount of power lost in transmission line.

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Inductor and Types of Inductors

A component called an inductor is used when the property of inductance is required in a circuit. The basic form of an inductor is simply a coil of wire. There are different types of inductors available for different types of applications. Factors which affect the inductance of an inductor include:

  • the number of turns of wire — the more turns the higher the inductance
  • the cross-sectional area of the coil of wire — the greater the cross-sectional area the higher the inductance
  • the presence of a magnetic core — when the coil is wound on an iron core the same current sets up a more concentrated magnetic field and the inductance is increased
  • the way the turns are arranged — a short thick coil of wire has a higher inductance than a long thin one.

symbol of InductorAn inductor is a component which offers a  high impedance to a.c. but very low to d.c.

Types of Inductors

The inductors, like resistors and capacitors, can be classified broadly as fixed and variable inductors. The different types of inductors are available for different applications and are explained below:

Filter Chokes

A filter choke is an inductor used in the filter section of d.c. power supply. It blocks the a.c. signal and allows the d.c. signals to pass through it. The two major ratings of a filter choke are the inductance and current rating.

types of inductors

Radio-Frequency Chokes

These are smaller in size as compared to filter chokes. Air-core coils for RF applications have very small values of inductance. For example, an RF coil for the radio broadcast band of 535 to 1605 KHz may have an inductance of 250 μH or 0.25 mH.

air core inductor

Variable Inductors

Variable inductors are used in radio frequencies. The winding is placed over a fiber or ceramics former and to change the inductance, a ferrite core is employed.variable inductors

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Inductance and Factors Affecting Inductance

Inductance is parameter of electrical engineering. In this article, we will discuss inductance and factors affecting inductance.

Inductance is the name given to the property of a circuit where by there is an e.m.f. induced in to the circuit by the change of flux linkages produced by a current change. When the e.m.f. is induced in the same circuit as that in which the current is changing, the property is called self inductance.

It is denoted by capital letter L.

When the e.m.f. is induced in a circuit by a change of flux due to current changing in an adjacent circuit, the property is called mutual inductance, M.

The unit of inductance is the henry H.

A circuit has an inductance of one henry when an e.m.f. of one volt is induced in it by a current changing at the rate of one ampere per second.

Induced e.m.f. in a coil of N turns,

inductance

Where is the change in flux in Webers, and dt is the time taken for the flux to change in seconds. Induced e.m.f. in a coil of inductance L henrys,

factors affecting inductance

Where dI is the change in current in amperes and dt is the time taken for the current to change in seconds. The minus sign in each of the above two equations remind us of its direction (given by Lenz’s law).

Factors Affecting Inductance

The greater the self-induced voltage, the greater the self inductance of        the coil and hence larger is the opposition to the changing current. According to Faraday’s laws  of electromagnetic induction, induced voltage   in a coil depends upon the number of turns (N) and the  rate of change of flux (dφ/dt)  linking the coil.  Hence,  the inductance  of a coil depends upon these factors, viz :

  • Shape and number of turns.
  • Relative permeability of the material surrounding the coil.           
  • The speed with which the magnetic field changes.

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Capacitance and Factor Affecting Capacitance

Static electric fields arise from electric charges, electric field lines beginning and ending on electric charges. Thus the presence of the field indicates the presence of equal positive and negative electric charges on the two plates. In this article we will discuss about capacitance and factors affecting capacitance

The property of a capacitor to store electric charge is called capacitance. The capacitance of a capacitor is defined as the amount of charge required to create a unit potential between its plates.capacitanceHence, capacitance is the charge required per unit potential difference.

Unit of Capacitance

The unit of capacitance is Farad. It is denoted by capital letter F.

One farad is defined as the capacitance of a capacitor which requires a charge a one coulomb to establish a potential difference of one volt between its plates.

One farad is too large for practical purposes. Hence much smaller unit like microfarad (μF), nanofarad (nF) and picofarad are generally employed.

We can also defined a capacitance, it is the property which opposes any change of voltage across it.

Factor Affecting Capacitance

The capacitance of a capacitor depends upon the following factors and given by the relation;capacitance and factors affecting capacitanceArea of plates

The greater the area of capacitor plates, the larger is the capacitance of the capacitor and vice-versa. It is because larger the plates, the greater the charge can hold for a given potential difference and this results, greater will be the capacitance.

Distance between the plates

The capacitance of a capacitor is inversely proportional to the distance between plates. The smaller the distance between two plates, the greater will be the capacitance and vice-versa. When the plates are brought closer, the electrostatics field is intensified and hence capacitance increases.

Relative permittivity of dielectric

The greater the relative permittivity of the dielectric material, the greater will be the capacitance of the capacitor and vice-versa. It is because the nature of dielectric affects the electrostatic field between the plates and hence the charge that accumulates of the plates.

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Capacitor and Types of Capacitors

A capacitor is a device that is capable of storing electrical energy (or electric charge). It is essentially consists of two conducting materials (say plates) separated by an insulating medium dielectric. The most commonly used dielectrics are air, mica, paper etc. A capacitor is generally named after the dielectric used (for example, air capacitor, mica capacitor, paper capacitor etc.  The conducting materials may be in the form of either circular (or rectangular) plates or be of cylindrical (or spherical) shape. 

How does a Capacitor Store Charge?

A Capacitor stores charge when it is connected to D.C. supply. One plate A is joined to the positive end of the supply and the other plate B to the negative end. There is a momentary flow of electrons from A to B. As negatively-charged electrons are withdrawn from A, it becomes positive and these electrons collect on B, it becomes negative. Hence, a potential difference is establish between plates A and B. The transient flow of electrons gives rise to charging current.

capacitor and types of capacitorsThe process of electrons flow continuous till potential difference across capacitor plates becomes equal to battery voltage. When the capacitor is charged to the battery voltage V, the current flows stop.

Rating of Capacitors

Capacitors are rated in Farad

Types of Capacitors

Some Types of capacitors are explained in this article. These are given below:

Paper Capacitors

Paper capacitors are most common of all capacitors. For the construction of these capacitors two metal foils are separated by paper impregnated with a dielectric material such as wax, plastic or oil are rolled into compact cylinder. Connecting leads are attached to the metal plates.

 paper capacitorsCharacteristics of paper capacitors

Range :- 0.0001 μF to 2 μF

Working Voltage : – upto 2000 volts D.C.

Mica Capacitors

These capacitors consist of alternate thin sheets of metal foils separated by mica thin sheets. Alternate metal sheets are connected together and brought out as one terminal for one set of plates, while the opposite terminal connects to the other set of plates. The whole construction is encased in a plastic housing or moulded in Bakelite case.

types of capacitors mica capacitorsCharacteristics of mica capacitors

Range :- 2.5 pF to 0.05 μF

Working Voltage: – 500 to 2500 volts

Ceramic Capacitors

The ceramic is a dielectric material made from earth fired under extreme heat. Titanium oxides are used to obtain very high value of dielectric of ceramic materials. The ceramic capacitors may be disc type or tubular type.

ceramic capacitors

Characteristics of ceramic capacitors

Range :- 2.5 pF to 0.22 μF

Working Voltage: – 500 to 2500 volts

Electrolytic Capacitors

An electrolytic capacitor contains two aluminium electrodes. Between the two electrodes , absorbent gauze soaks up electrolyte to provide the required electrolysis that produces an oxide film at the positive electrodes when d.c. voltage is applied. The oxides film acts as an insulator and forms a capacitance between positive aluminium electrodes and the electrolyte in the gauze separator the negative aluminium electrode simply provides a connection to electrolyte. These are two types wet type and dry type.

electrolyte capacitorsCharacteristics of electrolytic capacitors

Range :- 1 pF to 2000 μF

Working Voltage: – 500 to 2500 volts

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Power Factor Improvement

Usually, the power factor of power system lies between 0.8 and 0.9 lagging. However, sometimes its value falls below 0.8. In such cases it is desirable to take special steps to improve the power factor for economical reasons. In this article, we will discuss about power factor improvement equipments. This can be achieved by employing the following equipment :

  • Static Capacitors
  • Synchronous Condensers
  • Phase Advancers

Static Capacitors

We know that static capacitor takes current which leads the applied voltage by nearly 900. Therefore, if capacitors is connected across an inductive load resultant quadrature component of current of the whole combination will be the difference of leading component of capacitor current and lagging component of load current. For 3-phase induction motor, the capacitors can be connected in delta or in star. However, practically capacitors are only connected in delta because in that case the size of each capacitor will be one third to that of the capacitors when they are connected in star. Static capacitors are invariably employed for power factor improvement in factories.power factor improvement static capacitorsAdvantages

  • They have small losses, hence no energy consumption.
  • They require almost no maintenance.
  • They can work under ordinary atmospheric conditions.
  • They can easily installed as they are light and require no foundation.

Disadvantages

  • They have smaller life.
  • They are damaged quickly when the voltage exceeds the rated value.
  • The damaged capacitor can not be repaired.

Synchronous Condensers

A synchronous motor takes a leading current when over-excited and, therefore, behaves like a capacitor. An over-excited synchronous motor when run without any mechanical load and soley for the purpose of power factor correction is called synchronous condenser.

 To improve the power factor an over-excited synchronous motor is connected in parallel with an industrial load operating at lagging power factor. The leading wattless component of current taken by synchronous motor partly or completely neutralizes the lagging wattless component of the original load current and thus overall power factor is improved.power factor improvement synchronous condenserThese method is used to improve the power factor of the load above 500kVA. Below this load static capacitor are more economical for the improvement of the power factor.

Synchronous condensers are used for the power factor improvement of transmission lines. Therefore, sysnchronous condensers are installed at the receiving sub-stations.

Advantages

  • By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This result, power factor can be controlled easily.
  • The motor windings have high thermal stability to short circuit current.

Disadvantages

  • There are considerable power losses in the motor.
  • The maintenance cost is high.
  • Below 500kVA, it becomes costly.
  • It produces noise.

Phase Advancers

Phase advancers are used to improve the power factor of induction motors. The low power factor of an induction motor is due to the fact that motor draws exciting current that lags behind the supply voltage by 900. If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and power factor of the motor can be improved.

Advantages

  • The exciting ampere turns are supplied at the slip frequency, therefore, lagging kVAR drawn by the motor are considerably reduced.
  • Phase advancer can be conveniently used where the use of synchronous motors is inappropriate.

Disadvantages

  • Phase advancers are not economical for motors below 200 H.P.

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Electric Power

Electric power is the rate at which energy is expanded. It is denoted by either P or p.

If one joule of energy is expended in transferring one coulomb of charge through the circuit, then the rate of energy expenditure in transferring one coulomb of charge per second through the circuit is one watt. So, the SI unit of electric power is watt or joule per second.

This absorbed power must be proportional both to the number of coulombs transferred per second or per second or current and to the energy needed to transfer one coulomb through the element or voltage.

electric power

In practice, watt is often found to be inconveniently small, consequently the unit kilowatt is used.

1kW = 1000 watts

1MW = 1000kW

All appliances and devices  used in domestic purpose as well as industrial purposes are rated in watt. For example, 100 watt lamp, 5 kW motor, etc

Important formulae

According to Ohm’s law V = IR

P = VI

P = V2/R

P = I2R

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Electric Cell

An electric cell is source of electrical energy. The cell gives dc current. The emf developed and current supplied by a cell is very small. The electrical energy can be stored in cell.

Definition of Electric Cell

A cell is source of emf in which chemical energy is converted into electrical energy.

Construction

  • An electric cell basically consists of two electrodes of different material, so that different potentials are built up when chemical action takes place on them.
  • An electrolyte such as an acid, alkali or salt solution so that chemical action takes place between two electrodes. The solution must be capable to react chemically with the two electrodes.

electric cell

  • When two electrodes are immersed in the electrolyte, due to chemical action between electrodes and electrolyte, a potential difference established between two electrodes.

E.M.F developed in a Cell

The magnitude of emf developed in cell depends upon the following.

  • Nature or material of the plates used as electrodes of the cell.
  • Type of the electrolyte used in the cell.

Types of Cells

Electric cells can be divided into two types.

Primary Cell

The cell in which chemical action is not reversible, called primary cells. Voltaic cell, Daniell cell, Silver Oxide cell, Dry cell etc are the examples of Primary Cell.

In these types of cells, once the cell  discharge,it cannot be recharged because chemical action, in this case, is not reversible and the cells cannot be recharged. This makes Primary cells expensive source of electrical energy and that is why these cells are rarely used in commercial applications.

Secondary Cell

The cell in which chemical action is reversible, called secondary cell. Lead acid cell, Nickel-iron alkaline cell, etc are the examples of secondary cells. These cells are rechargeable. In these cells, chemical action is reversible and cells can be recharged. While recharging, electrical energy is converted in the cell itself.

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Types of Electric Circuits

An electric circuit can be divided into four types that are given below:

  • Closed circuit
  • Open circuit
  • Short circuit
  • Earth or leakage circuit

Closed Circuit

The complete path for the flow of electric through the load is called a closed, such as the glowing of electric lamps, heating of a press, etc.  Current flows only when the circuit is closed.types of electric circuits closed circuitsOpen Circuit

If any one of the supply wires is disconnected or the fuse burns out, then the current will not flow through the bulb. The circuit is then called an open circuit.Short Circuit

If the supply mains are connected directly by a piece of wire without any load, it is called a short circuit. Since, in this circuit the value of the current is much greater than in the closed circuit, fuse gets blown off.Earth or Leakage Circuit

If any wire of supply mains touches the body of an appliance, then it is called earth circuit or leakage circuit.

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