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Analysis of the principle of space voltage vector svpwm control
Space Vector Pulse Width Modulation (SVPWM) is a modern control technique that has gained widespread application in inverters, uninterruptible power supplies (UPS), and reactive power compensators. In recent years, with the continuous advancement of industrial technology and the increasing demand for high-power, high-quality inverters—especially in China—the development of SVPWM has become more crucial than ever. The progress in power electronics, microelectronics, and control theory has provided the necessary foundation for the maturity of inverter technology, enabling the emergence of advanced SVPWM methods. Since the SVPWM algorithm is closely related to the inverter's topological structure, it is essential to choose an appropriate control strategy based on the specific inverter configuration.
SVPWM is a relatively new control method that has been widely adopted in motor drives and power conversion systems. Unlike traditional Sinusoidal Pulse Width Modulation (SPWM), which modulates the pulse width based on a sine wave reference, SVPWM utilizes a space voltage vector approach to generate a more efficient and accurate output waveform. It uses six power switching devices in a three-phase inverter to create a pulse-width modulated signal that makes the output current as close as possible to a perfect sinusoidal waveform. This method focuses on achieving a circular rotating magnetic field, resulting in reduced torque ripple and improved motor performance. Additionally, SVPWM enhances the utilization of DC bus voltage and offers better compatibility with digital control systems.
Pulse Amplitude Modulation (PAM) is another modulation technique where the amplitude of the pulses in a pulse train is varied according to a specific rule to adjust the output level and waveform. PAM builds upon PWM by modifying both the pulse mode and width, allowing for a more precise control of the output waveform. By arranging the duty cycle in a sinusoidal manner, the output can be effectively filtered to achieve a smooth and stable result.
**Basic Principle of SVPWM**
The theoretical foundation of SVPWM is based on the principle of average equivalence. Within each switching cycle, the basic voltage vectors are combined such that their average value matches the desired voltage vector. As the voltage vector rotates, it enters a region defined by two adjacent non-zero vectors and zero vectors. The action time of these vectors is adjusted within a sampling period, enabling the inverter to approximate the ideal circular flux linkage. This process determines the switching states of the inverter and generates the corresponding PWM waveform.
Assuming the DC bus voltage is Udc, and the three-phase output voltages are UA, UB, UC, they form a rotating space vector U(t). This vector has a magnitude of 1.5 times the peak phase voltage and rotates counterclockwise at an angular frequency of ω = 2πf. Its projection onto the three-phase coordinate axes results in a balanced three-phase sine wave.
In a three-phase inverter, there are six switching devices arranged in bridge arms. The switching function Sx (where x = a, b, c) defines the state of each leg. There are eight possible combinations of these functions, including six non-zero voltage vectors and two zero vectors. Each combination corresponds to a specific voltage vector, which can be calculated based on the switching state.
For example, when Sa = 1, Sb = 0, Sc = 0, the resulting voltage vector is U4. The calculation shows that the line-to-neutral voltages are Uan = 2Udc/3, Ubn = -Udc/3, and Ucn = -Udc/3. Similarly, other combinations yield different voltage vectors, all with equal magnitude and spaced 60° apart.
To synthesize any arbitrary voltage vector within a sector, two adjacent non-zero vectors and one zero vector are used. The principle of volt-second balance ensures that the integral effect of the reference vector over the sampling period matches that of the synthesized vectors. This allows the inverter to produce a smooth, rotating voltage vector that closely follows the desired trajectory.
By continuously adjusting the switching states based on the reference voltage vector, the inverter achieves a high-quality, sinusoidal output. This technique not only improves the efficiency and performance of the system but also reduces harmonic distortion and enhances overall reliability.