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# Power Electronics Dr .

Mohamaed Tawfeeq
Tutorial Sheet No.9

DC-AC CONVERTERS

Problem 1
a) Draw the circuit diagram of a single-phase half-bridge inverter.
b) Sketch the output voltage waveform and show conducting devices.
c) Give the rms values of the inverter output (load) voltage.
d) Sketch one transistor current and give its average value for a purely resistive load.
e) What is the peak inverse voltage of the transistors?

Problem 2
For the single-phase bridge inverter circuit shown in Fig. 1, we have Vs =100V, load
resistance is R10, load inductance is L=25mH and output voltage frequency
f=50Hz. The control logic for power electronics devices is also shown on the same
figure .

Fig.1

(a) Determine the amplitude of the Fourier series terms for the square load voltage
waveform, (consider up to the 9th order harmonic)
(b) Compute the rms value of the output voltage waveform in terms of the harmonic
components.
(c) Find the total power absorbed by the load.

Problem 3
For the circuit shown in Fig. 2, we have V=220V, load inductance L=0.08H and
output voltage frequency is 50Hz.

## )a) What is the circuit name?

)b) Draw the output voltage and load current waveforms
)c) What is the value of the rms voltage?
)d) What is the maximum value of the load current Iomax?
e) What is the average value of the input current Is for a purely inductive load?

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Fig.2

Problem 4
For the circuit shown in Fig. 3, we have Vs=200V, load resistance R = 20 Ω and
output voltage frequency f=50Hz. The control logic for power electronics devices is
also shown on the same figure.

Fig.3

## (a) Draw the output voltage and load current waveforms.

(b) Determine the value of the r.m.s output voltage at the fundamental frequency
Vo1 and the output power Po.
(c) Calculate the average and peak currents of each transistor.
(d) Estimate the total harmonic distortion factor THD of the circuit.

Problem 5
A single-phase, full-bridge, voltage source inverter as shown in Fig.4 supplies square-
wave output at 50 Hz to an R-L load with R=9Ω and L= 28.66 mH . The transistor
switching scheme is complementary bipolar.

(i) Obtain Fourier series expression for the fundamental and higher order harmonic
output voltages up to 7th order harmonics in terms of the input d.c. voltage Vdc.

(ii) If the rms fundamental ac output voltage is 200V, what is the required dc input
voltage for the inverter?

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(iii) Obtain expressions for the load current waveform.
(iv) Calculate the output power in terms of harmonics when Vdc = 250 V.

Fig.4

Problem 6
A three-phase transistor voltage-source inverter supplies a 3-phase load, as shown in
Fig.5. The load consists of star connected resistance of 10Ω in each phase. The
inverter supply voltage is 200 V dc and each inverter switch conducts for 120.
(i) Sketch the switching signals for the six transistors.
(ii) Sketch the line-line voltage for one complete cycle of the output voltage
(iii) Sketch the line-line voltage for one complete cycle of the output voltage
(iv) Calculate the rms values of the first five harmonics in the line-line output voltage,
including the fundamental.
(v) Calculate the rms values of the first five harmonics in the line-neutral output
voltage, including the fundamental.

Fig.5

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