ENG009 SCHOOL OF ENGINEERING BENG (HONS) ELECTRICAL & ELECTRONICS ENGINEERING INTERMEDIATE ELECTRICAL PRINCIPLES & ENABLING POWER ELECTRONICS MODULE NO: EEE5003 Date: 15 January 2018 Time: 10.00 12.00 INSTRUCTIONS TO CANDIDATES: There are six questions. Answer ANY FOUR questions. All questions carry equal marks. Marks for parts of questions are shown in brackets. Electronic calculators may be used provided that data and program storage memory is cleared prior to the examination. CANDIDATES REQUIRE: Formula Sheet (attached).
Page 2 of 13 Q1. a) Explain the implementation of a unity gain buffer amplifier with the aid of a circuit diagram. (8 marks) b) Determine the output voltage of a differentiator circuit as shown in Fig.1. Assume that the input voltage v1=3.5 cos (100 π t) volt and the time constant RC = 1.5 ms. Fig.1: A differentiator circuit c) Find voltage gain Av=Vo/Vin and input impedance of the Operational amplifier circuit shown in Fig.2. i. with the switch open (6 marks) ii. with the switch closed (6 marks) Fig.2: An Operational amplifier circuit Total 25 marks Please turn the page
Page 3 of 13 Q2. (a) A half-wave single-phase rectifier circuit is shown in Fig. 3 below. The following are given: Vs=230 V, f=50 Hz, diode forward voltage drop is assumed to be zero. Determine: i. The load mean voltage and current (8 marks) ii. The load current ac component and shape for (7 marks) 1. A pure resistive load of 8 Ω resistor and 2. An inductive load of 0.1 H inductance in series with the 8 Ω resistor Fig. 3: A half-wave rectifier circuit (b) In the three-phase inverter shown in Fig.4 below, the following are given: Line-to-line voltage=450 V, f=50 Hz, E=540 V, LS=0.6 mh. Ld is assumed very large to produce id(t)=id. Calculate the angles: i. α ii. γ if the power flow is 54 kw. Question 2 continues over the page Please turn the page
Page 4 of 13 Question 2 continued Fig.4 A three phase inverter circuit Total 25 marks Q3. (a) Draw a circuit diagram for a buck converter and derive an expression for V out V in defining all parameters used in this circuit. (10 marks) (b) For a single-phase inductive load (with parallel resistance and inductance). i. Prove that the active power is always positive, has an average value of V.I 2 cosθ and zero average reactive power both pulsating at double supply frequency (2ωt) where V and I are the peak values of the voltage and current and θ is the power factor angle of the load. (10 marks) ii. Draw the relevant waveforms for v, i, and p Total 25 marks Please turn the page
Page 5 of 13 Q4. (a) A balanced positive sequence Y-connected source with Van=180 20 V is connected to a -connected balanced load (12+j8) per phase. Calculate : i. The load phase currents. ii. The load line currents (7 marks) (b) A 3-phase motor draws 20 kva at 0.707 power factor lagging from a 380 V., 50 Hz source. Determine: i. The kilovolt-ampere rating of capacitors to make the combined power factor 0.95 lagging ii. The line currents before and after the capacitors are added. iii. The capacitance value in Farads if they are connected in delta configuration (4 marks) (4 marks) Assuming that the voltage and active power remain constant. Total 25 marks Please turn the page
Page 6 of 13 Q5 The datasheet of the BJT Transistor TIP2955/3055 was provided: (a) Find the absolute maximum power rating for the given TIP2955 NPN Power transistor (b) What is the maximum power that can be dissipated without a heat sink at ambient temperatures of +25 o C and +50 o C? (c) Determine the junction and case temperature if the transistor dissipates 2W at an ambient temperature of 35 o C. Assume that no heat sink is used. (d) Assuming that this transistor dissipates a continuous power of 20W is mounted on a heat sink with a 3 mil mica insulator with heat sink compound, where the insulator thermal resistance (θcs) is 0.36 o C/W. Determine the maximum heat sink thermal resistance required if we want to keep the junction temperature at least 60 C below its absolute maximum (to extend its lifespan) and also to keep the heat sink temperature below 70 C. Assume that the ambient temperature varies from +15 C to +40 C and natural convection. (10 marks) Total 25 marks Please turn the page
Page 7 of 13 Q6 (a) A venturi meter uses a mercury filled manometer to measure pressure difference. Air is flowing in the main pipe, and the diameter reduces from 25mm to 20mm. At a certain flow rate a height difference of 46mm is measured between the two sides of the manometer. Calculate the flow velocity in the 25mm pipe, the volumetric flow and the mass flow rate. Given that density for the mercury is 13600 kg/m 3 and air is 1.225 kg/m 3. i. Calculate the flow velocity in the 25 mm pipe. ii. iii. Determine the volumetric flow Find the mass flow rate (7 marks) (4 marks) (4 marks) (b) Explain: i. Explain the Archimedes principle (4 marks) ii. An unloaded pontoon weights 200,000N. It has a plan area of 12m long and 7m wide. It floats in sea water with a density of 1025 kg/m3. What is the depth of immersion? (6 marks) Total 25 marks END OF QUESTIONS
Page 8 of 13 Formula sheet These equations are given to save short term memorisation of details of derived equations and are given without any explanation or definition of symbols; the student is expected to know the meanings and usage. Converters: V d = 1.35 V LL cosα 3 ωl s I π d cos(α + u) = cos α 2 ωl s 2V LL I d
Page 9 of 13 γ = 180 (α + u) V ph = V 3, I ph = I for star connection, V ph = V, I ph = I 3 for delta connection S = 3VI V. A, P = 3VIcosθ W., Q = 3VIsinθ V. A. r Q C = 3VI C V. A. r, X C = V 3I C Ω
Page 10 of 13 Three-phase systems Delta to Star conversion: Star to Delta conversion: Gravity: Thermal resistance of the interface material: 9.81 m/s θ cs = (ρ)(t) A Output voltage of a differentiator circuit: Compressibility relationship: General manometer:
Page 11 of 13 Venturi meter: Force on a submerged wall: Drag coefficient: Flow through a small hole: Flow through a rectangular slit: Tank draining: Flow over a rectangular weir: Flow over a V notch weir: Poisseuille s Law: Darcy s Law:
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Page 13 of 13 Datasheet