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Transcript

EEE3201 (F) / Page 1 of 4 INTI INTERNATIONAL UNIVERSITY BACHELOR OF ENGINEERING (HONS) IN MECHANICAL ENGINEERING EEE 3201: ELECTRICAL MACHINES FINAL EXAMINATIONS: JANUARY 2013 SESSION Instructions: This paper consists of SIX (6) questions. Answer any FOUR (4) questions in the answer booklet provided. All questions carry equal marks. Question 1 (a) (i) (ii) (iii) (iv) (v) Compare and contrast the following: Lap and wave winding Separately and self-excited dc machines Hysteresis and Eddy current loss Cumulative and differential dc generators Compensating and inter pole windings (2 marks each) (b) A short-shunt compound dc generator supplies a current of 100 A at a voltage of 220V.The resistances of the shunt field, series field and armature windings are 50Ω,0.025 Ω and 0.05 Ω respectively. The total brush drop is 2V and the iron and friction losses amount to 1 kW. Draw the equivalent circuit diagram of the short-shunt dc generator, specifying the direction of current, resistance values and with proper polarity of terminal voltage and generated voltage. (1 mark) (c) In relation to problem (b), determine the following: (i) Series winding voltage drop Vs (1 mark) (ii) Voltage across the shunt windings Vsh (1 mark) (iii) Shunt current Ish (1 mark) (iv) Armature current Ia (1 mark) (v) Generated voltage Eg (1 mark) (vi) Electrical losses (1 mark) (vii) Total losses (1 mark) (viii) Power output of the prime mover driving the generator (1 mark) (ix) Efficiency of the generator (1 mark) (c) Experiments 1 and 2 are about the no load and load characteristics of dc machines. Basing from your own learning experiences identify (separately, self-excited, cumulative, differential and series) the best dc machine in terms of their characteristics. (5 marks) Question 2 (a) (i) (ii) (iii) (iv) (iii) Briefly explain the following: Core type transformer Shell type transformer Open circuit test of a single phase transformer Short circuit test of a single phase transformer Ideal transformer (1 mark each) EEE3201 (F) / Page 2 of 4 (b) Two tests were performed on a 30-kVA, 3000/110 V transformer to predetermine its efficiency. The results were tabulated in table 2(b). Voltage (V) 3000 150 Open circuit test Short circuit test Current (A) Power (W) 0.5 10 350 500 Table 2(b): Data of open and short circuit test (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x) (c) (d) Determine the following: The rated high side current IHS The core loss in table 2(b) The resistance referred to high side RegHS The impedance referred to high side ZegHS The reactance referred to high side XegHS The complete approximate equivalent circuit showing all the resistances, reactances, voltages and currents. The vector form of the rated current at 0.8 p.f. lagging. The transformation ratio “a” The voltage at the high side VHS The voltage regulation at rated load, 0.8 p.f. lag. The efficiency at half load 0.8 p.f. lag. (1 mark) (1 mark) (1 mark) (1 mark) (1 mark) (2 marks) (1 mark) (1 mark) (2 marks) (1 mark) (2 marks) Given the efficiency of a transformer as: 1 I HS ReqHS VHS cos HS Wi VHS I Hs cos HS (1) where Wi - iron loss in watts, 2 (i) (ii) Prove that Wi = I HS RHS by finding d and setting it to zero. dI HS (4 marks) An 11,000/230-V, 150-kVA, 50-Hz, single-phase transformer has a iron loss of 1.4kW and full-load copper loss of 1.6kW. Determine the kVA load for maximum efficiency from the relationship: iron loss kVAmax kVAfull load x copper loss full load (2 marks) Question 3 (a) (i) (ii) (iii) (iv) (v) Briefly explain the following terms: Synchronous speed Rotor speed Squirrel cage induction motor Wound rotor induction motor Power across the air gap (2 marks each) EEE3201 (F) / Page 3 of 4 (b) (i) (ii) (iii) (iv) (v) (vi) (vii) A 100-kW (output), 3300-V, 50-Hz, 3-phase, star-connected induction motor has a synchronous speed of 500 rpm.The full-load slip is 1.8% and full load power factor 0.85. Stator copper loss = 2440 W, Iron loss =3500 W, and Rotational losses = 1200 W. Determine the following: Pmech (2 marks) Prcl (2 marks) Pgap (2 marks) Pin (2 marks) Line current IL (2 marks) Complete the power stages by inserting all the values computed. (3 marks) Efficiency of the motor (2 marks) Question 4 (a) (b) (i) (ii) (iii) (c) (i) (ii) (iii) (iv) (v) Briefly discuss the difference between salient and non-salient synchronous machine Give specific application of each. (5 marks) A three-phase Y-connected synchronous generator is rated 120 MVA, 13.2 kV, 0.8 PF lagging, and 60 Hz. Its synchronous reactance is 0.9 Ω per phase, and its resistance may be ignored. Determine the following: The armature current in phasor form (2 marks) The excitation voltage per phase (4 marks) The voltage regulation (2 marks) A 2400 V, 60-Hz, 3-phase, 6-pole, star-connected synchronous generator is connected to an infinite bus and is supplying 350 kW at a power factor angle of 24o.The stator has a synchronous reactance of 12 Ω per phase. Neglecting losses. Determine the: input torque to the alternator (2 marks) excitation voltage per phase; (3 marks) armature current (4 marks) active and reactive components of power (2 marks) Power factor (1 mark) Question 5 (a) Sketch the V-curves of a synchronous motor and briefly explain the V-curves of a synchronous motor. What does it represent? (5 marks) EEE3201 (F) / Page 4 of 4 (b) The V-curve shown in figure Q5(b) is for a 460-V, 350 Hp, 60-Hz, 1800 rpm cylindrical rotor synchronous motor. The machine is operating at less than rated load with an excitation voltage of 450 V/ phase. The synchronous reactance is 0.65 Ω per phase. Figure Q5(b) (i) (ii) (iii) (iv) (v) (vi) (vii) In reference to Figure Q5(b) , estimate the armature current at the given excitation voltage of 450V/phase. (2 marks) State what power factor does the motor operate (lag, leading or unity) (2 marks) Find the magnitude of the synchronous reactance drop Ia jXs (2 marks) Sketch the corresponding phasor diagram to scale (use 50, V = 1 cm scale) (3 marks) from the phasor diagram, find the power angle δ (2 marks) Determine the power factor of the motor and; (5 marks) Find the active and reactive components of input power (2 marks) The developed torque (2 marks) Question 6 (a) (i) (ii) (iii) (iv) (v) Briefly describe or explain the following terms: Sensing element Signal processing Signal conditioning Data representation Measurements (b) A potentiometer has a supply voltage of 15V and a resistance of 50KΩ. The fractional displacement of the wiper is 0.3. Sketch the diagram for the circuit. (2 marks) Find the Thevenin voltage and resistance for the circuit. (6 marks) (i) (c) (2 marks each) A parallel plate capacitive displacement sensor consists of two square metal plates 8cmx8cm , separated by a gap of 2mm.The space between the plates is completely filled by a slab with a dielectric constant of 6. If the permittivity of free space is 8.85 pF/m, find the capacitance of the sensor. (7 marks) - THE END - EEE 3201/(F)/Jan 2013/F.Roy Jr/18022013