From a purely technical perspective, the control method of a low-voltage frequency converter also indicates its technological school to some extent. We analyze the following control methods:
1. Sine pulse width modulation
Sinusoidal pulse width modulation (SPWM) is characterized by its simple control circuit structure, low cost, and good mechanical stiffness, which can meet the smooth speed regulation requirements of general transmissions and has been widely used in various fields of industry.
However, at low frequencies, this control method suffers from a significant decrease in maximum output torque due to the lower output voltage and the substantial influence of stator resistance voltage drop. Furthermore, its mechanical characteristics are ultimately less robust than those of a DC motor, resulting in unsatisfactory dynamic torque capability and static speed regulation performance. The system performance is also low, the control curve changes with load variations, torque response is slow, motor torque utilization is low, and performance degrades at low speeds due to stator resistance and inverter dead-zone effects, leading to decreased stability.
2. Voltage Space Vector Pulse Width Modulation (SVPWM)
Voltage Space Vector Pulse Width Modulation (SVPWM) is based on the overall generation effect of three-phase waveforms and aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulated waveforms in one step and controls them by approximating a circle with an inscribed polygon.
Further improvements were made after practical use, namely, the introduction of frequency compensation to eliminate speed control errors; the estimation of flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speeds; and the closed-loop control of output voltage and current to improve dynamic accuracy and stability.
3. Direct Torque Control (DTC) method
Direct torque control (DTC) technology largely overcomes the shortcomings of vector control and has rapidly developed due to its novel control concept, simple and clear system structure, and excellent dynamic and static performance. Currently, this technology has been successfully applied to high-power AC drives for electric locomotive traction.
Direct torque control analyzes the mathematical model of the AC motor directly in the stator coordinate system, controlling the motor's flux linkage and torque. It does not require converting the AC motor into an equivalent DC motor, thus eliminating many complex calculations in vector rotation transformation; it does not require mimicking the control of a DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling.
4. Matrix-based cross-sectional control method
Matrix AC-AC control VVVF frequency converters, vector control frequency converters, and direct torque control frequency converters are all types of AC-DC-AC frequency converters. Their common drawbacks include low input power factor, high harmonic current, the need for large energy storage capacitors in the DC circuit, and the inability to feed regenerated energy back to the grid, meaning they cannot operate in four quadrants. Therefore, matrix AC-AC frequency converters were developed to address these issues. Because matrix AC-AC frequency converters eliminate the intermediate DC link, they also eliminate the need for large and expensive electrolytic capacitors.
Because matrix AC-AC converters eliminate the intermediate DC link, they also eliminate the need for bulky and expensive electrolytic capacitors. They can achieve a power factor of 1, sinusoidal input current, and four-quadrant operation, resulting in high system power density. This technology is not yet mature; its essence is not to indirectly control quantities like current and flux linkage, but rather to directly control torque as the controlled variable.
