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Download EE442 FE Spr 98-99 - faraday - Eastern Mediterranean University
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Date Duration Eastern Mediterranean University Department of Electrical and Electronic Engineering EE 442 FINAL EXAM : 23 / 6 / 1999 ANSWER ALL FOUR QUESTIONS : 2 hrs. EACH QUESTION IS 25 PTS. 1-) The three-phase fully-controlled midpoint converter shown below supplies power to a highly inductive load with resistance RL = 10 . The converter is fed from a 380 V rms ( line-to-line ) three-phase source. Assume that the load current IL is constant . (a) Derive an expression for the average load voltage Vdc in terms of the firing angle . (b) For = / 3 find the average load current IL . va vb vc T1 va = Vm sin wt vb = Vm sin (wt-2 /3) vc = Vm sin (wt-4 / 3 ) T2 I T3 L Load n 2-) In the resonant-pulse commutation circuit shown below, assume that the initial capacitor voltage is Vo = Vs . (a) The maximum load current to be commutated is Ia = 15 A. Find the maximum tq of the main thyristor T1 . (b) For Ia = 10 A, find the overvoltage on the capacitor after the commutation interval. D2 T1 + Vs _ L C + -V c T2 Ia D1 Load Dm Vs = 200 v , C = 0.5 F , L = 50 H 3-) The single-phase full-bridge inverter shown below is operated in the quasi-square-wave ( QSW ) mode ( phase displacement control ) at the frequency f = 100 Hz , with phase shift between half-bridge output voltages va and vb . The load is an R-L load with R = 10 and L = 10 mH . (a) For = 2 / 3 sketch vo and find its total harmonic dostortion THD . (b) For = 2 / 3 find approximately the rms value of the load current i0 by considering harmonics with order n 7. + Vs va = 200 V + io vo - vb load - 4-) The three-phase half-bridge inverter shown below feeds a balanced Y-connected load , and is operated in the square-wave mode ( pole voltages are square waves ) at frequency f s . (a) Sketch the line-to-line voltage vab ( indicate all voltage levels ) and find its fundamental amplitude. (b) With a purely inductive load L per phase , find the peak value of phase A load current ia . Show your derivation explicitly. + a Vs b c - ia n USEFUL INFORMATION : 1-) THD of a periodic waveform x (t): 1/ 2 1 2 THD = Xn X1 n 2 2-) Xn = n’th harmonic amplitude Fourier series of a quasi-square-wave x A /2 -/2 /2 -A -/2 t x(t) = 4A n sin cos n t 2 n n=1,3,5,.. 3-) LC resonant circuits : t=0 i C + vc L vc (t) = Vo cos t Vo = vc (0) i(t) = -Ip sin t Ip = Vo C / L _ L + Vs _ - C + vc i vc (t) = Vs + ( Io / C ) sin t i (t) = Io cos t