1. Sinusoidal Pulse Width Modulation (SPWM) Control Method
Its advantages include a simple control circuit structure, low cost, and good mechanical stiffness, which can meet the smooth speed regulation requirements of general transmissions, and it has been widely used in various fields of industry. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, resulting in a reduction in the maximum output torque.
Furthermore, its mechanical characteristics are ultimately not as rigid as those of a DC motor, and its dynamic torque capability and static speed regulation performance are not entirely satisfactory. Moreover, its system performance is not high, the control curve changes with the load, 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. Therefore, vector control variable frequency speed regulation was developed.
2. Voltage Space Vector (SVPWM) Control Method
It is based on the overall generation effect of three-phase waveforms, aiming 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 the circuit by approximating a circle with an inscribed polygon. After practical use, it has been improved by introducing frequency compensation to eliminate speed control errors; estimating the flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speeds; and closing the output voltage and current loops to improve dynamic accuracy and stability. However, the control circuit has many components and lacks torque regulation, so the system performance has not been fundamentally improved.
3. Vector Control (VC) Method
Vector control variable frequency speed regulation works by transforming the stator currents Ia, Ib, and Ic of an asynchronous motor in a three-phase coordinate system into equivalent AC currents Ia1 and Ib1 in a two-phase stationary coordinate system through a three-phase to two-phase transformation. Then, through a rotor field-oriented rotational transformation, these are further transformed into equivalent DC currents Im1 and It1 in a synchronous rotating coordinate system (Im1 is equivalent to the excitation current of a DC motor; It1 is equivalent to the armature current proportional to the torque). Then, mimicking the control method of a DC motor, the control quantities for the DC motor are obtained. After a corresponding inverse coordinate transformation, the asynchronous motor is controlled. Essentially, it equates an AC motor to a DC motor, independently controlling the speed and magnetic field components. By controlling the rotor flux linkage and then decomposing the stator current to obtain the torque and magnetic field components, orthogonal or decoupled control is achieved through coordinate transformation.
The introduction of vector control method was epoch-making. However, in practical applications, due to the difficulty in accurately observing rotor flux linkage, the significant influence of motor parameters on system characteristics, and the complexity of vector rotation transformation used in the equivalent DC motor control process, the actual control effect is difficult to achieve the ideal analysis results.
4. Direct Torque Control (DTC) method
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology largely solved the shortcomings of the aforementioned vector control and has rapidly developed due to its novel control concept, simple and clear system structure, and excellent dynamic and static performance. 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.
5. Matrix-based cross-sectional control method
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-type 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 a high system power density. Although this technology is not yet mature, it continues to attract numerous scholars for in-depth research. Its essence is not to indirectly control quantities such as current and flux linkage, but rather to directly use torque as the controlled variable. The specific method is:
(1) Control the stator flux linkage by introducing it into the stator flux linkage observer to achieve a sensorless speed mode;
(2) Automatic identification (ID) relies on a precise mathematical model of the motor to automatically identify motor parameters;
(3) Calculate the actual values corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and calculate the actual torque, stator flux linkage, and rotor speed for real-time control;
(4) Implement Band-Band control. Generate PWM signals according to the Band-Band control of flux linkage and torque to control the switching state of the inverter.
Matrix AC-AC converters have fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%). They also have high starting torque and high torque accuracy, especially at low speeds (including 0 speed), where they can output 150% to 200% torque.
