1. Introduction Variable frequency drive (VFD) technology arose to meet the need for stepless speed regulation of AC motors. Since the 1960s, power electronic devices have undergone a development process, including SCR (Signal Recognition Transistor), GTO (Gate Turn-Off Thyristor), BJT (Bipolar Junction Transistor), MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), SIT (Stationary Induction Transistor), SITH (Stationary Induction Thyristor), MGT (MOS-Controlled Transistor), MCT (MOS-Controlled Thyristor), IGBT (Insulated Gate Bipolar Transistor), and HVIGBT (High Voltage Insulated Gate Bipolar Transistor). The updating of these devices has promoted the continuous development of power electronic conversion technology. Starting in the 1970s, research on pulse width modulation variable voltage variable frequency (PWM-VVVF) speed regulation attracted considerable attention. In the 1980s, the optimization of PWM modes, the core of VFD technology, attracted great interest, leading to the development of numerous optimization modes, among which the saddle-shaped wave PWM mode showed the best performance. Beginning in the latter half of the 1980s, VVVF frequency converters from developed countries such as the United States, Japan, Germany, and the United Kingdom entered the market and gained widespread application. 2. Frequency Converter Control Methods: Low-voltage general-purpose frequency converters 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 adopt AC-DC-AC circuits. Their control methods have gone through the following four generations. 2.1 Sinusoidal Pulse Width Modulation (SPWM) control method (U/f=C): This method is characterized by a simple control circuit structure, low cost, and good mechanical stiffness, meeting the smooth speed regulation requirements of 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 ultimately not as rigid as those of a DC motor, and its dynamic torque capability and static speed regulation performance are not satisfactory. 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. Therefore, vector control variable frequency speed regulation was developed. 2.2 Voltage Space Vector (SVPWM) control method: This method 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 system by approximating a circle using an inscribed polygon. After practical application, it has been improved by introducing frequency compensation to eliminate speed control errors; estimating flux linkage amplitude through feedback to eliminate the influence of stator resistance at low speeds; and closing the output voltage and current loop 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. 2.3 Vector Control (VC) Method Vector control variable frequency speed regulation involves transforming the stator currents Ia, Ib, and Ic of the 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 magnetic field-oriented rotational transformation, these are equivalent to 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 of the DC motor are obtained. After a corresponding inverse coordinate transformation, the asynchronous motor is controlled. Essentially, it equates the 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, the torque and magnetic field components are obtained. Through coordinate transformation, orthogonal or decoupled control is achieved. The proposal of the vector control method is of epoch-making significance. However, in practical applications, due to the difficulty in accurately observing rotor flux linkage, the system characteristics are greatly affected by motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is complex, making it difficult for the actual control effect to achieve the ideal analysis results. 2.4 Direct Torque Control (DTC) In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency converter technology. This technology largely solves the shortcomings of the above-mentioned vector control and has been rapidly developed with 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 directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the motor flux linkage and torque. It does not require the AC motor to be equivalent to a DC motor, thus saving many complex calculations in the vector rotation transformation; it does not require the control of a DC motor to be imitated, nor does it require the simplification of the mathematical model of the AC motor for decoupling. 2.5 Matrix AC-AC Control Method VVVF frequency converter, vector control frequency converter, and direct torque control frequency converter are all types of AC-DC-AC frequency converters. Their common drawbacks are 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. To address these issues, matrix AC-AC converters were developed. Because matrix AC-AC converters eliminate the intermediate DC link, they also eliminate the need for large 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 methods are as follows: — Stator flux linkage is controlled by introducing a stator flux linkage observer to achieve a sensorless speed control; — Automatic identification (ID) relies on a precise motor mathematical model to automatically identify motor parameters; — Calculate the actual values ​​corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., to calculate the actual torque, stator flux linkage, and rotor speed for real-time control; — Implement Band-Band control to generate PWM signals based on flux linkage and torque Band-Band control to control the inverter switching state. Matrix AC-AC inverters 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%~200% torque. 3. Reasonable selection of inverter control mode The control mode is the key to determining the performance of the inverter. Currently, there are many brands of low-voltage general-purpose inverters on the market, including about 50 types from Europe, America, Japan, and China. When selecting a frequency converter, do not assume that the higher the grade, the better. Instead, choose one based on the load characteristics and to meet the usage requirements, ensuring appropriate use and cost-effectiveness. The parameters listed in Table 1 are for reference during selection. 4. Selection and Related Issues of Torque Control Frequency Converters Based on their advantages of convenient speed regulation, energy saving, and reliable operation, frequency converters have gradually replaced traditional pole-changing speed regulation, electromagnetic speed regulation, and voltage regulation speed regulation methods. Several years after the introduction of PWM flux vector control frequency converters, frequency converters using DTC control technology appeared at the end of 1998. ABB's ACS600 series was the first generation of frequency converters using DTC technology. It can accurately control speed and torque in an open-loop manner, and its dynamic and static performance is superior to that of PWM closed-loop control. Direct torque control uses measured motor current and DC voltage as inputs to an adaptive motor model. This model generates a set of accurate actual torque and flux values ​​every 25μs. Torque comparators and flux comparators compare the actual torque and flux values ​​with the given torque and flux values ​​to determine the optimal switching position. This shows that it achieves precise control by measuring torque and magnetic flux, instantly adjusting the switching state of the inverter circuit, and thus adjusting the motor's torque and magnetic flux. 4.1 Selection Principles First, based on the machine's requirements for speed (maximum, minimum) and torque (starting, continuous operation, and overload), determine the maximum input power required by the machine (i.e., the minimum rated power of the motor). An empirical formula is P=nT/9950 (kW), where: P—required input power (kW); n—machine speed (r/min); T—maximum torque of the machine (N·m). Then, select the number of poles and rated power of the motor. The number of poles determines the synchronous speed, requiring the synchronous speed to cover the entire speed range as much as possible to maximize continuous load capacity. To fully utilize the equipment's potential and avoid waste, the motor can be allowed to exceed the synchronous speed for short periods, but must be less than the maximum allowable speed. Torque is taken as the maximum torque of the equipment under starting, continuous operation, overload, or maximum speed conditions. Finally, the inverter parameters and model are determined based on the principle that the inverter's output power and rated current are slightly greater than the motor's power and rated current. It is important to note that the inverter's rated capacity and parameters are specified for a certain altitude and ambient temperature, generally below 1000m altitude and below 40℃ or 25℃. If the operating environment exceeds these specifications, the derating factor caused by the environment must be considered when determining the inverter parameters and model. 4.2 External Configuration of the Inverter and Issues to Note 1) Selecting suitable external fuses to avoid damage to rectifier devices due to internal short circuits. After determining the inverter model, if there is no fast-acting fuse protecting silicon devices before the inverter's internal rectifier circuit, a compliant fuse and disconnect switch should be configured between the inverter and the power supply. Air circuit breakers cannot be used to replace fuses and disconnect switches. 2) Selecting the inverter's input and output cables. Select a three-core or four-core shielded power cable with a suitable conductor cross-section based on the inverter's power. In particular, the power cable between the inverter and the motor must be a shielded cable and as short as possible to reduce electromagnetic radiation and capacitive leakage current. When the cable length exceeds the inverter's allowable output cable length, the cable's stray capacitance will affect the inverter's normal operation; therefore, an output reactor must be configured. Control cables, especially I/O signal cables, must also be shielded. The length of the connecting cable between the inverter's peripheral components and the inverter itself must not exceed 10m. 3) Install an AC reactor or EMC filter on the input side. Depending on the power grid quality requirements of other equipment at the inverter installation site, if the inverter's operation affects the normal operation of these devices, an AC reactor or EMC filter can be installed on the inverter's input side to suppress electromagnetic interference caused by the switching of power devices. If the neutral point of the transformer in the power grid connected to the inverter is not grounded, an EMC filter cannot be used. When a frequency converter drives a motor with a voltage of 500V or higher, a du/dt filter must be configured on the output side to suppress inverter output voltage spikes and voltage variations, which helps protect the motor. It also reduces capacitive leakage current, high-frequency radiation from the motor cable, high-frequency losses in the motor, and bearing current. When using a du/dt filter, it is important to note that the voltage drop across the filter will cause a slight decrease in motor torque; the cable length between the frequency converter and the filter should not exceed 3 meters. 5. Conclusion: Selecting a frequency converter is a task that requires careful consideration. Currently, there are many types and specifications of low-voltage general-purpose frequency converters on the market. When selecting one, the actual load characteristics should be considered to meet the usage requirements, ensuring appropriate use and cost-effectiveness.