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This thesis discusses the advantages and drawbacks of three different PWM techniques: These three methods are compared by discussing their ease of implementation and by analyzing the output harmonic spectra of various output voltages poles voltages, line-to-neutral voltages, and line-to-line voltages and their total harmonic distortion THD.
Three-Phase Sinusoidal PWM Inverter A six-step inverter is composed of six switches S1 through S6 with each phase output connected to the middle of each inverter leg as shown in Figure 2.
The output of the comparators in Figure 2. Two switches in each phase make up one leg and open and close in a complementary fashion.
That is, when one switch is open, the other is closed and vice-versa. As mentioned above, this is approximately Consequently, it provides a better utilization of the DC supply voltage. Under-modulation and Over-modulation Regions in Space Vector Representation SVPWM is accomplished by rotating a reference vector around the state diagram, which is composed of six basic non-zero vectors forming a hexagon.
A circle can be inscribed inside the state map and corresponds to sinusoidal operation. The area inside the inscribed circle is called the linear modulation region or under-modulation region.
As seen in Figure 3. The switch combinations can be represented as binary codes that correspond to the top switches S1, S3, and S5 of the inverter as shown in Figure 3.
These voltages are also called pole voltages. Switching Sequence of all Six Sectors The switching pulse patterns of six different sectors in Figure 3. It is obvious that in the odd sector the active state sequence is in ascending-descending order; whereas, it is in a descending- ascending order in an even sector.
The over-modulation range can be considered as one region or it can be divided into two regions see Figure 4. In this thesis, the over-modulation region is considered as two regions with two modes of operation depending on the modulation index values.
Modulation Index in Mode 2 At the end of mode 1, the component of the reference voltage changes to a piecewise linear waveform. When the modulation index is higher than 0. Both the reference magnitude and the phase angle have to be changed compared to the linear region.
The modified reference vector is held at a vertex of the hexagon in every sector for the rest of the switching period. It has the following blocks: A three-phase voltage set Va, Vb, and Vc of variable amplitude is compared in three separate comparators with a common triangular carrier waveform of fixed amplitude as shown in the same figure.
The maximum neutral voltages Van, Vbn, and Vcn in Figure 5. As seen in Figure 6. However, it has a higher fundamental voltage and a higher value of the 3rd harmonic. The contributions of the thesis are as follows: The thesis has provided a thorough review of the each techniques with a special focus on the operation of SVPWM in the under-modulation and over-modulation modes.
The thesis discusses the advantages and drawbacks of each technique. Their simulation results are compared and analyzed by plotting the output harmonic spectra of various output voltages, and computing their total harmonic distortion THD.
Recommendations for Future Work There are couple of interesting topics suggested for future research: Neural network implementation is very fast and can increase the switching frequency of power switches in the inverter. Many papers in the literature have reported that the dead time of space vector PWM has an influence on drive systems.
To circumvent this problem, it is important to research how to compensate for the dead time effect in order to increase the performance of drive systems.Back to Items of Interest Sub-Table of Contents.
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