Eastern Mediterranean University



Eastern Mediterranean University

Department of Electrical and Electronic Engineering

EE 442 FINAL EXAM

Date : 1 / 7 / 2002 ANSWER ALL FOUR QUESTIONS

Duration : 2 hrs. EACH QUESTION IS 25 PTS.

1-) A single-phase fully-controlled bridge converter supplies power to a resistive load with resistance RL = 10 ( from a 240 V rms 50 Hz AC source , as shown below.

a) Sketch the output voltage vo ( take ( = 90(), and find its average value as a function of the firing angle ( ( show all the steps in your derivation).(15pts)

b) Sketch the current iT1 through T1 and find its rms value for ( = 90(. (10pts)

2-) Below is shown a step-down dc-dc converter with an LC filter at its output. Diode D is ideal. Switch S is operated with duty ratio k = 0.4 at the switching frequency fs = 10 kHz. Assume that the output voltage vo = Va is constant and also the output current io = Ia is constant. Given that Va = kVs , Vs = 100 V, L = 250 (H and C = 100 (F

a) Find the minimum and maximum values of the inductor current iL

for Ia = 10A. Show all your calculations. (15pts)

b) Sketch the source current is , and find its average value.(10pts)

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 = 50 Hz , with phase

shift ( between half-bridge output voltages vao and vbo . The load is an R-L load with

R=5 ( and L = 10 mH.

a) For (= ( / 2 sketch vo and find its rms value. (7pts)

b) For ( = 2( / 3 find the rms values of load current harmonic components with

orders n = 1, 3 , 5, 7.( i.e. find Ion,rms , n=1,3,5,7) (8pts)

c) Find approximately the total harmonic distortion (THD) of io using the results in

part (b).(10pts)

[pic]

4-) The three-phase half-bridge inverter shown below feeds a balanced (-connected resistive load, and is operated in the square-wave mode (pole voltages are square waves) at frequency f s .

a) Sketch line current ia (show all important current and time values) and find its rms value .(12pts)

b) Find the total average power supplied to the load (take R= 10().(13pts)

USEFUL INFORMATION :

1-) THD of a periodic waveform x (t):

[pic] Xn = n’th harmonic amplitude

2-) Fourier series of a quasi-square-wave

x

A

[pic]

-(/2 (/2

-(/2 (/2 (t

-A

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