Low-voltage general-purpose variable frequency drives have an output voltage of 380–650V, an output power of 0.75–400kW, and an operating frequency of 0–400Hz.
Their main circuits all use AC-DC-AC circuits. Their control methods have gone through the following four generations:
Sine Pulse Width Modulation (SPWM) Control Method
Its characteristics include a simple control circuit structure, lower cost, and good mechanical characteristics, meeting the requirements for smooth speed control in general drives. It has been widely used in various industrial fields. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, reducing the maximum output torque. Furthermore, its mechanical characteristics are not as robust as those of a DC motor, and its dynamic torque capability and static speed control performance are not entirely satisfactory. The system performance is not high, the control curve changes with load variations, the torque response is slow, and the motor torque utilization is low. At low speeds, performance deteriorates due to stator resistance and inverter dead zone effects, and stability worsens. Therefore, researchers developed vector control variable frequency speed regulation.
Voltage Space Vector Pulse Width Modulation (SVPWM) Control Method
This method is based on the overall generation effect of the three-phase waveform, aiming to approximate the ideal circular rotating magnetic field trajectory in the motor air gap. It generates the three-phase modulated waveform in one step, using an inscribed polygon to approximate the circle. After practical use, it has been improved by introducing frequency compensation to eliminate speed control errors; estimating the magnetic flux 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 torque regulation is not introduced, so the system performance has not been fundamentally improved.
Vector Control (VC) Method
Vector control variable frequency speed regulation involves 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 rotating transformation oriented by the rotor magnetic field, these are transformed into equivalent DC currents Im1 and It1 in a synchronously 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). The control method of a DC motor is then imitated to obtain the control quantities of the DC motor. Through corresponding inverse coordinate transformations, the control of the asynchronous motor is achieved. Essentially, it is equivalent to transforming the AC motor into a DC motor, and 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 the vector control method was groundbreaking. However, in practical applications, due to the difficulty in accurately observing the rotor flux linkage, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is complex, making it difficult to achieve the ideal analytical results in practice.
Direct Torque Control (DTC) Method
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control variable frequency technology. This technology largely solved the shortcomings of the vector control method mentioned above, and has developed rapidly 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 directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the motor's flux linkage and torque. It does not require transforming the AC motor into an equivalent DC motor, thus eliminating many complex calculations in the vector rotation transformation; it does not need to imitate the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
Matrix Converter Control Method
VVVF variable frequency, vector control variable frequency, and direct torque control variable frequency are all types of AC-DC-AC frequency conversion. Their common drawbacks include low input power factor, large harmonic currents, the need for large energy storage capacitors in the DC circuit, and the inability to feed regenerative energy back into the power grid, meaning they cannot operate in four quadrants. Therefore, matrix converters have emerged. Because matrix converters eliminate the intermediate DC link, they eliminate the need for large and expensive electrolytic capacitors. They can achieve a power factor of 1, a sinusoidal input current, and four-quadrant operation, resulting in high power density. Although this technology is not yet mature, it continues to attract many scholars for in-depth research. Its essence is not the indirect control of current, magnetic flux, etc., but rather the direct control of torque. The specific methods are:
Controlling the stator magnetic flux by introducing a stator magnetic flux observer to achieve a sensorless control method.
Automatic identification (ID) relies on an accurate motor mathematical model to automatically identify motor parameters.
Calculating the actual values of stator impedance, mutual inductance, magnetic saturation factors, inertia, etc., to calculate the actual torque, stator magnetic flux, and rotor speed for real-time control.
Implementing Band-Band control: Generating PWM signals based on Band-Band control of magnetic flux and torque to control the switching state of the inverter.
Matrix converters have fast torque response (<2ms), high speed accuracy (±2%, without PG feedback), and high torque accuracy (<+3%); they also have high starting torque and high torque accuracy, especially at low speeds (including zero speed), where they can output 150% to 200% torque.