Power Electrical Past Paper
THE UNIVERSITY OF NEW SOUTH WALES
FINAL EXAMINATION
JUNE/JULY, 2001
ELEC4240/ELEC9240 POWER ELECTRONICS
1.
Time allowed: 3 (three) hours
2.
This paper has six questions. Answer any four.
3.
All questions are of equal value.
4.
A separate examination book should be used for each question.
5.
Candidates may bring only slide rules, drawing instruments and approved electronic calculators into the examination room.
6.
Answers must be written in ink. Except where they are expressly required, pencils may only be used for drawing, sketching or graphical work.
7.
This paper may be retained by the candidate.
ELEC4240/9240 - Power Electronics
1
See over for Question 1
QUESTION 1
The input current drawn for an ac source line connected to a power converter is indicated in figure Q1, together with the ac input voltage of the line to neutral of the supply. The current waveform is flat-topped quasi-square of amplitude 150A. The ac source voltage is given by v = Vmax sin ω t where Vmax is 340 V, ω = 2π f , where f is the supply frequency, at 50 Hz.
vs
150 A
120°
60°
−150 A
75°
120°
60°
is sec Figure Q1
(i)
Find the rms values of the first five harmonics in the input current, including the fundamental.
[8 marks]
(ii)
Calculate the input power drawn from each line of the ac source.
[6 marks]
(iii) Calculate the Total Harmonic Distortion (THD) in the input current waveform.
[6 marks]
(iv) Calculate the input power factor at which the converter operates.
ELEC4240/9240 - Power Electronics
2
[5 marks]
See over for Question 2
QUESTION 2
The single-phase, fully-controlled, converter circuit of figure Q2 operates from a 240V, 50
Hz supply. The ac source has a total source-inductance Ls = 1.5 mH. The load supplied by the converter is highly inductive, so that the load current may be assumed to be smooth and ripple-free. +
Ls
is
T1
Id
T3
L
FINAL EXAMINATION
JUNE/JULY, 2001
ELEC4240/ELEC9240 POWER ELECTRONICS
1.
Time allowed: 3 (three) hours
2.
This paper has six questions. Answer any four.
3.
All questions are of equal value.
4.
A separate examination book should be used for each question.
5.
Candidates may bring only slide rules, drawing instruments and approved electronic calculators into the examination room.
6.
Answers must be written in ink. Except where they are expressly required, pencils may only be used for drawing, sketching or graphical work.
7.
This paper may be retained by the candidate.
ELEC4240/9240 - Power Electronics
1
See over for Question 1
QUESTION 1
The input current drawn for an ac source line connected to a power converter is indicated in figure Q1, together with the ac input voltage of the line to neutral of the supply. The current waveform is flat-topped quasi-square of amplitude 150A. The ac source voltage is given by v = Vmax sin ω t where Vmax is 340 V, ω = 2π f , where f is the supply frequency, at 50 Hz.
vs
150 A
120°
60°
−150 A
75°
120°
60°
is sec Figure Q1
(i)
Find the rms values of the first five harmonics in the input current, including the fundamental.
[8 marks]
(ii)
Calculate the input power drawn from each line of the ac source.
[6 marks]
(iii) Calculate the Total Harmonic Distortion (THD) in the input current waveform.
[6 marks]
(iv) Calculate the input power factor at which the converter operates.
ELEC4240/9240 - Power Electronics
2
[5 marks]
See over for Question 2
QUESTION 2
The single-phase, fully-controlled, converter circuit of figure Q2 operates from a 240V, 50
Hz supply. The ac source has a total source-inductance Ls = 1.5 mH. The load supplied by the converter is highly inductive, so that the load current may be assumed to be smooth and ripple-free. +
Ls
is
T1
Id
T3
L