MARINE ELECTRO-TECHNIQUE 1

Solution It should be noted that each time the conductors passes under a pole (whether N or S) it cuts a flux of 15 mWb. Hence, total flux cut in one revolution is 15 x 4 = 60 mWb. Since conductor is rotating at 600/60 = 10 r.p.s Time taken for one revolution is 1/10 = 0.1 second  average e.m.f generated = N d Volt. dt Here N = 1 ; dф = 60 mWb = 6 x 10-2 Wb ; dt = 0.1 second.  e = 1 x 6 x 10-2 / 0.1 = 0.6 Volt.

Example 2.6 The field winding of a d.c electromagnet is wound with 960 turns and has resistance of 50 Ω. When the exciting voltage is 230 V, the magnetic flux linking the coil is 0.005 Wb. Calculate the self inductance of the coil and the energy stored in the magnetic field.

Solution L = N henry I Current through coil = 230 / 50 = 4.6 A ф = 0.005 Wb N = 960 L = ( 960 x 0.005 ) / 4.6 = 1.0435 H. Energy stored = ½ L I2 = ½ x 1.0435 x 4.62 = 11.87 J.

Example 2.7 Two identical coils X and Y of 1,000 turns each line in parallel planes that 80% of flux produces a flux of 0.05 mWb in it and current flows in the Coil X is 5A, find the mutual inductance between X and Y.

Solution M = N21 I1 Flux produced in X = 0.05 mWb = 0.05 x 10-3 Wb Flux linked with Y = 0.005 x 10 -3 x 0.8 = 0.04 x 10-3 Wb. M = (1000 x 0.04 x 10-3) / 5 = 8 x 10-3 H

Example 2.8 The hysteresis loop of a sample of sheet steel subjected to a maximum flux density of 1.3 Wb/m2 has an area of 93 cm2, the scale being 1 cm = 0.1 Wb/m2 and 1 cm = 50 AT/m. Calculate the hysterisis loop in watts when 1500 cm3 of the same material is subjected to an alternating flux density of 1.3 Wb/ m2 peak value at a frequency of 65 Hz

Solution Where 1 cm = 0.1 Wb/m2 and 1 cm = 50 AT/m For area of 93 cm2 Loss = xy (area of B/H loop) j/m3/cycle. = 0.1 x 50 x 93 = 465 j/m3/cycle. Volume = 1500 cm3 = 15 x 10-4 m3 No of reversals / second = 65  Wh = 465x 15 x 10-4 x 65 j/s = 45.3 W. Note.The given value of Bmax = 1.3 Wb/m3 is not required for solution

Example 2.9 Calculate the hourly loss of energy in kWh in a specimen of iron, the hysteresis loop of which is equivalent in area to 250 j/m3 Frequency 50 Hz, specific gravity of iron 7.5kg/dm3 for weight of specimen 10 kg.

Solution Hysteresis loop = 250 j/m3/ cycle Mass of iron = 10 kg Volume of a specimen = 10/7.5 x 103 m3 = 10-2 / 7.5 m3 No. of cycles of reversals / hr = 60 x 50 = 3000.  Loss/hour = 250 x (10-2 / 7.5) x 3000 = 1000 j = 1000/(36x105) = 27.8 x 10 -5 kWh.

LMD13003 CHAPTER 2 BASIC ELECTRONICS

Model of atom The Bohr atom is a tool for visualizing atomic structure. •The nucleus is positively charged and has the protons and neutrons. •Electrons are negatively charged and in discrete shells. •The atomic number is the number of protons and determines the particular element. •In the neutral atom, the number of electrons is equal to the number of protons. Electron Proton Neutron

Atomic structure The outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and in metals they account for electrical and thermal conductivity. A neutral Si atom is shown. + Shell 1 Shell 2 Shell 3 There are 4 electrons in the valence shell. Is Si a conductor, insulator, or semiconductor? Semiconductor

• A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus. The nucleus has positively charged protons and neutral neurons. Negatively charged electrons move around the nucleus a much greater distance than is suggested by this simplified model. Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons

– Electrostatic Charge. • Electrons move from atom to atom to create ions. –positively charges ions result from the loss of electrons and are called cations –Negatively charge ions result from the gain of electros and are called anions

(A) A neutral atom has no net charge because the numbers of electrons and protons are balanced. (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion. (C) The addition of an electron produces a net negative charge and a negative ion.

Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing. Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair

• The charge on an ion is called an electrostatic charge. • An object becomes electrostatically charged by –Friction ,which transfers electrons between two objects in contact –Contact with a charged body which results in the transfer of electrons –Induction which produces a charge redistribution of electrons in a material

Charging by induction. The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb

– Electrical Conductors and Insulators. • Electrical conductors are materials that can move electrons easily –Good conductors include metals. • Electrical nonconductors are materials that do not move electrons easily –These are also known as insulators • Semiconductors are materials that vary in their conduction and nonconduction, sometimes conducting sometimes not conducting.

• Measuring Electrical Charges. – The magnitude of an electrical charge is dependent upon how many electrons have been moved to it or away from it. – Electrical charge is measured in coulombs. • A coulomb is the charge resulting from the transfer of 6.24 X 1018 of the charge carried by an electron

– The fundamental charge is the electrical charge on an electron and has a magnitude of 1.6021892 X 10-19 C – To determine the quantity of an electrical charge you simply multiple the number of electrons by the fundamental charge on an electron or: • q=ne • Where q is the magnitude of the charge, n is the number of electrons, and e is the fundamental charge.

Coulomb constructed a torsion balance to test the relationships between a quantity of charge, the distance between the charges, and the electrical force produced. He found the inverse square law held accurately for various charges and distances

• Measuring Electrical Forces. – Force is proportional to the product of the electrical charge and inversely proportional to the square of the distance. – Coulomb’s Law F = k q1q2 d2 • F is the force • k is a constant and has the value of 9.00 X 109 newton•meters2/coulomb2 (9.00 X 10 9 N•m2/C2) • q1 represents the electrical charge of object 1 and q2 represents the electrical charge of object 2 • d is the distance between the two objects.

• Force Fields. – The condition of space around an object is changed by the presence of an electrical charge. – The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge – All electrical charges are surrounded by an electrical field just like all masses are surrounded by gravitational fields.

– A map of the electrical field can be made by bringing a positive test charge into an electrical field. • When brought near a negative charge the test charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled. • You can draw vector arrows to indicate the direction of the electrical field • This is represented by drawing lines of force or electrical field lines –These lines are closer together when the field is stronger and farther apart when it is weaker.

A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that the test charge would experience

Lines of force diagrams for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.

• Electrical Potential. – An electrical charge has an electrical field that surrounds it. – In order to move a second charge through this field work must be done – Bringing a like charge particle into this field will require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated. • In both of these cases the electrical potential is changed.

Electric potential results from moving a positive coulomb if charge into the electric field of a second positive coulomb of change. When 1.00 joule of work is done in moving 1.00 coulomb of charge, 1.00 volt of potential results. A volt is a joule/coulomb.

– The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt • A volt is a measure of the potential difference between two points • electric potential =work to create . potential charge moved • PD=W •Q • The voltage of an electrical charge is the energy transfer per coulomb. – The energy transfer can be measured by the work that is done to move the charge or by the work that the charge can do because of the position of the field.

The falling water can do work in turning the water wheel only as long as the pump maintains the potential difference between the upper and lower reservoirs.

SEMICONDUCTOR BASICS

Definition What is it ?? * Semi = half • Conductor = material that carry electricity • Low resistivity => “conductor” • High resistivity => “insulator” • Intermediate resistivity =>“semiconductor”

Cont.. • A semiconductor is a material that has intermediate conductivity between a conductor and an insulator. • Semiconductor are available as either elements or compound

Element… • Basic element in a materials. - silicon and germanium • Each atom has its four nearest neighbors at the corners of a regular tetrahedron with the atom itself being at the center

Categories of Semiconductor ▪Two categories: - intrinsic and extrinsic ▪ An intrinsic semiconductor: chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes)

Cont.. • extrinsic semiconductor: an improved intrinsic semiconductor with a small amount of impurities. (added by a process, known as doping, which alters the electrical properties of the semiconductor and improve its conductivity)

Compound .. • Compound semiconductors- combination of two different elements. • Include InSb (indium antimonide), InAs (indium arsenide), GaP (galium phosphide), GaSb,GaAs, SiC, GaN • Eg.when put together as a compound, GaAs creates a zincblend lattice structure.

Semiconductor Material

Doping.. • What is it ? Adding impurities into a pure semiconductor • Why doping? - it produces two types of semiconductor - n-type - p-type • Higher dopant concentration, more carriers (electrons or holes)

P-type • The addition of trivalent impurities creates deficiencies of valence electrons, called “hole”

N-type • The addition of pentavalent (5) or donor impurities contributes free electrons & greatly increasing the conductivitiy of the intrinsic semiconductor

Energy Band • Insulator has a very wide energy gap - greater amount of energy required to move the electron from the Valance Band to Conduction Band. • Semiconductor has a smaller enery gap - requires less energy to move an electron from the VB to the CB. • Conductor - there is no energy gap and the VB and CB overlaps. With no energy gap, it takes a small amount of energy to move electrons into CB. Conductors pass electrons very easily.

Bridge Rectifiers • The diode is an important tool in many kinds of electrical circuits. As an example, consider the bridge rectifier circuit shown in Figure 11.14. The bridge rectifier is set up so that it allows current to flow in only one direction through the resistor R when an alternating current supply is placed across the bridge. The current through the resistor is then a rectified sine wave of the form (11.10) • This is the first step in changing alternating current to direct current. The design of a power supply can be completed by adding capacitors and resistors in appropriate proportions. This is an important application, because direct current is needed in many devices and the current that we get from our wall sockets is alternating current. Figure 11.14: Circuit diagram for a diode bridge rectifier.

Zener Diodes • The Zener diode is made to operate under reverse bias once a sufficiently high voltage has been reached. The I-V curve of a Zener diode is shown in Figure 11.15. Notice that under reverse bias and low voltage the current assumes a low negative value, just as in a normal pn-junction diode. But when a sufficiently large reverse bias voltage is reached, the current increases at a very high rate. Figure 11.15: A typical I-V curve for a Figure 11.16: A Zener diode reference Zener diode. circuit.

Transistors • Another use of semiconductor technology is in the fabrication of transistors, devices that amplify voltages or currents in many kinds of circuits. The first transistor was developed in 1948 by John Bardeen, William Shockley, and Walter Brattain (Nobel Prize, 1956). As an example we consider an npn-junction transistor, which consists of a thin layer of p- type semiconductor sandwiched between two n-type semiconductors. The three terminals (one on each semiconducting material) are known as the collector, emitter, and base. A good way of thinking of the operation of the npn-junction transistor is to think of two pn-junction diodes back to back. Figure 11.22: (a) In the npn transistor, the base is a p-type material, and the emitter and collector are n-type. (b) The two-diode model of the npn transistor. (c) The npn transistor symbol used in circuit diagrams. (d) The pnp transistor symbol used in circuit diagrams.

Transistors ◼ Consider now the npn junction in the circuit shown in Figure 11.23a. If the emitter is more heavily doped than the base, then there is a heavy flow of electrons from left to right into the base. The base is made thin enough so that virtually all of those electrons can pass through the collector and into the output portion of the circuit. As a result the output current is a very high fraction of the input current. The key now is to look at the input and output voltages. Because the base-collector combination is essentially a diode connected in reverse bias, the voltage on the output side can be made higher than the voltage on the input side. Recall that the output and input currents are comparable, so the resulting output power (current × voltage) is much higher than the input power. Figure 11.23: (a) The npn transistor in a voltage amplifier circuit. (b) The circuit has been modified to put the input between base and ground, thus making a current amplifier. (c) The same circuit as in (b) using the transistor circuit symbol.

Field Effect Transistors (FET) • The three terminals of the FET are known as the drain, source, and gate, and these correspond to the collector, emitter, and base, respectively, of a bipolar transistor. Figure 11.25: (a) A schematic of a FET. The two gate regions are connected internally. (b) The circuit symbol for the FET, assuming the source-to-drain channel is of n-type material and the gate is p-type. If the channel is p-type and the gate n-type, then the arrow is reversed. (c) An amplifier circuit containing a FET.

Schottky Barriers • Here a direct contact is made between a metal and a semiconductor. If the semiconductor is n-type, electrons from it tend to migrate into the metal, leaving a depleted region within the semiconductor. This will happen as long as the work function of the metal is higher (or lower, in the case of a p-type semiconductor) than that of the semiconductor. • The width of the depleted region depends on the properties of the particular metal and semiconductor being used, but it is typically on the order of microns. The I-V characteristics of the Schottky barrier are similar to those of the pn-junction diode. When a p-type semiconductor is used, the behavior is similar but the depletion region has a deficit of holes.

Schottky Barriers Figure 11.26: (a) Schematic drawing of a typical Schottky-barrier FET. (b) Gain versus frequency for two different substrate materials, Si and GaAs. From D. A. Fraser, Physics of Semiconductor Devices, Oxford: Clarendon Press (1979).