Electrical Engineer: May 2015

Sunday 31 May 2015

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Monday 18 May 2015

PN Junction Diode

CONDUCTION IN SEMICONDUCTORS

07:57

At room temperature of 300°K, it requires an energy of EG = 1.12 eV to break covalent bonds in Silicon material and EG = 0.7 eV to break the covalent bonds in Germanium material and to produce some ‘electron–Hole pairs’. Even at room temperature, a few of the covalent bonds will be broken, leading to equal number of electrons and Holes in Conduction Band and Valence Band, respectively. Electrons...

CONDUCTIVITY AND RESISTIVITY OF SEMICONDUCTOR MATERIALS

07:56

The value of conductivity of a material gives us an estimate of the extent to which a material supports the flow of current through it. Electrical conductivity depends upon the number of electrons available in the conduction process. The concept of conductivity is useful in many engineering applications including medical electronics. J = nqμE Equation (2.17) derived in the previous section...

CURRENT DENSITY IN A CONDUCTING MEDIUM

07:55

Currents in metals are due to the movement of charge carriers ‘electrons’. where I is the current in Amperes and A is the cross-sectional area of conducting medium in metre2. Describing current density J as current per unit area has the advantage, since the dimensions of the conducting medium are not directly involved. Relation between current density and charge density ρ is described...

CONDUCTION IN CONDUCTORS AND SEMICONDUCTORS

07:53

Conduction in conductors and semiconductors Mobility μ: In good conductors like metals, free electrons exist in abundance. They are supposed to be accelerated under the influence of electric or magnetic field as per ballistic (dynamics) laws. But in practice it is found that the electrons move with a constant velocity proportional to the field. The reason for this is the random nature of the...

CONDUCTION (INVERSE OF RESISTANCE) IN INTRINSIC SEMICONDUCTORS

07:52

Purest semiconductor is known as intrinsic semiconductor. At 0°K, semiconductor behaves like an insulator, because energies of the order of EG cannot be acquired from an electric field. At room temperature, covalent bonds in the semiconductor may be broken into a few Hole–electron pairs, contributing to current flow through the material allowing the conductivityto increase. With respect to...

CLASSIFICATION OF MATERIALS

07:50

When voltages are applied, materials offer different values of electrical resistances to the passage of currents through them. On the basis of electrical resistances, materials are classified as conductors, semiconductors and insulators. In solids, available energy states for the electrons form ‘bands of energy levels’ instead of discrete energy levels in atoms. Conductors: Materials with adjacent...

ENERGY-BAND CONCEPTS OF MATERIALS

07:49

Energy-band Concepts of Materials The electron energy levels for a single free atom in a gaseous medium are discrete, since the atoms are sufficiently far apart. So the energy levels of individual atoms are not perturbed. The proximity of neighbouring atoms in solid media such as crystals does not appreciably affect the energy levels of inner shell electrons. But, groups of energy levels of...

ELECTRONIC CONFIGURATION OF A GERMANIUM ATOM

07:47

Fig. 2.3 Electron configuration of germanium atom Germanium semiconductor atom has ‘atomic number’ Z = 32. It has 32 positive charges in the nucleus and 32 electrons in various shells containing 2, 8, 18 and 4 electrons. Germanium atom is electrically neutral. Germanium semiconductor as a whole is electrically neutral. First, second and third orbits are completely filled. Fourth orbit...

ELECTRON CONFIGURATIONS OF SILICON AND GERMANIUM ATOMS

07:46

REVIEW OF SEMICONDUCTOR PHYSICS The electronics subject begins from the concepts of behaviour of charge carriers in electron devices and Integrated Circuits (ICs) under influence of electric fields. A model of an atom is shown in Fig. 2.1. The aspect of electron motion is analogous to the planetary motion in which the planets rotate round the sun. On similar lines, electrons move in closed...

Single Phase Transformer

LOAD SHARING BY TWO TRANSFORMERS

10:25

Let us consider the following two cases: Equal voltage ratios. Unequal voltage ratios. 1.39.1 Equal Voltage Ratios Assume no-load voltages EA and EB are identical and in phase. Under these conditions if the primary and secondary are connected in parallel, there will be no circulating current between them on no load. Figure 1.48 Equal Voltage Ratios Figure 1.48 shows two impedances in...

PARALLEL OPERATION OF SINGLE-PHASE TRANSFORMER

21:59

It is required to connect a second transformer in parallel with the first transformer if the load exceeds the rating of the transformer shown in Figure 1.46. The primary windings are connected to the supply bus bars while the secondary windings are connected to the load bus bars. During paralleling of the transformer, similar polarities of the transformers should be connected to the same bus...

SUMPNER’S TEST

21:42

To determine the rise of maximum temperature of a transformer, its load test is of utmost importance. Using suitable load impedance, small transformers can be put on full load. The full-load test of large transformers is not possible because considerable wastage of energy occurs and it is difficult to get a suitable load for absorbing full-load power. Sumpner’s test is used to put large transformer...

ALL-DAY EFFICIENCY

21:41

The ratio of output in watts to input in watts is called commercial efficiency of a transformer. Distribution transformers are used for supplying lighting and general networks. Distributiontransformers are energized throughout the day. Their secondaries are at no load most of the time in a day except during the hours of lighting period. Core loss occurs throughout the day. Copper loss occurs...

POLARITY TEST OF A SINGLE-PHASE TRANSFORMER

21:41

Polarity testing of transformers is vital before connecting them in parallel. Otherwise, with incorrect polarity, it is not possible to connect them in parallel. The rated voltage is applied to the primary and its two terminals are marked as A1 and A2, respectively, as shown in Figures 1.44(a) and 1.45(b), respectively. The secondary winding terminals are also marked as a1 and a2, shown in Figures...

EFFICIENCY OF A TRANSFORMER

21:40

Due to the losses in a transformer, its output power is less than the input power.∴ Power output = Power input – Total losses∴ Power input = Power output + Total losses = Power output + Pi + PCuThe ratio of power output to power input of any device is called its efficiency (η). Output power of a transformer at full-load = V2I2ftcosθ, where cosθ is the power factor...

LOSSES IN A TRANSFORMER

21:38

Two types of losses occur in a transformer:Core loss or iron loss occurs in a transformerbecause it is subjected to an alternating flux.The windings carry current due to loading and hence copper losses occur.1.32.1 Core or Iron LossThe separation of core losses has already been introduced. The alternating flux gets set up in the core and it undergoes a cycle of magnetization and demagnetization....

KAPP’S REGULATION

21:37

Kapp had designed a diagram shown in Figure 1.42 to determine the regulation at any power factor. The description of the construction of the diagram is shown below.Figure 1.42 Kapp’s DiagramLoad current (I2) is taken as a reference phasor. OA representing V2 is drawn at angle θ2 with I2. AB represents I2R02 drawn parallel to I2, whereas BC represents I2X02 drawn perpendicular to AB, i.e., I2. Here...

CALCULATION FOR VOLTAGE REGULATION

21:36

The voltage regulation up is expressed mathematically byPositive sign is for lagging power factor and negative sign is for leading power factor.1.31.1 Zero Voltage RegulationFor lagging power factor and unity power factor, 0V2 > V2. Therefore, we get positive voltage regulation. For leading power factor, V2 starts increasing. At a certain leading power factor, 0V2 = V2 and hence regulation becomes...

PER UNIT RESISTANCE, LEAKAGE REACTANCE AND IMPEDANCE VOLTAGE DROP

21:35

Full-load voltage of a transformer can be expressed as a fraction of the full-load terminal voltage.Let I1fl be the full-load primary current, I2fl be the full-load secondary current, V1 be the rated pri-mary voltage and V2 be the rated secondary voltage.Per unit resistance drop of atransformer∴ Per unit reactance drop of a transformerPer unit reactance drop of...

SCR

MULTIPLE CHOICE QUESTIONS ON SCR (SILICON CONTROLLED RECTIFIER) PART 1

23:31

Hello Engineers. Q.1 If the current of an SCR increases, the forward breakdown voltage will A. Decrease B. Not be effected C. Increase D. Become zero Ans: A Q.2 If the gate current of an scr is increased, the forward breakdown voltage will A. Increase B. Decrease C. Not be affected D. Become infinite Ans: C Q.3 After firing an scr, the gate pulse is remove. The current in the scr will A. Remain the same B. Immediately fall to zero C. Rise up D. Rise a little and...

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