I. Introduction Currently, most frequency converters produced by Chinese manufacturers use the common V/f control method, with only a few claiming to employ speed-free vector control technology. Foreign brands have largely achieved a three-in-one control philosophy integrating open-loop, closed-loop, and speed-free control. Of particular note is ABB's DTC control method, which has been successfully applied in its products. Domestic academia has done much research on variable frequency speed control systems and achieved certain results, but there is still a gap in engineering practice and accumulation compared to foreign countries. How to transform theoretical knowledge into practical application in existing industrial products in China and improve the performance and quality of Chinese variable frequency speed control products is a pressing issue worthy of research. This paper attempts to compare and explain the current V/f control and implementation technologies from the perspectives of control strategies and modulation techniques, for discussion and exchange. Typically, the system structure of a V/f frequency converter consists of three parts: control, modulation, and main circuit. The control and pulse width modulation parts are entirely implemented using software algorithms. This control method is derived from the steady-state model of AC motors and does not depend on motor parameters or their changes, thus making it simple to control and easy to implement. However, its speed range is relatively narrow, and it is only suitable for loads such as fans and pumps that do not have high requirements for speed regulation performance. In order to improve the load-carrying capacity and dynamic performance of the system at low speeds and meet the needs of actual industrial sites, it is necessary to improve and enhance the existing control methods and pulse width modulation strategies. Several key technologies are discussed below. Source: www.tede.cn II. Several Technologies in Control Strategies 1. Compensation Technology Compensation technology is essential in open-loop control. It includes torque compensation, slip compensation, and dead-zone effect compensation. At low frequencies, the voltage drop of the stator resistance is not negligible relative to the inverter output voltage and must be compensated; otherwise, the output voltage will be insufficient, and the motor will not move or its speed will drop significantly at low frequencies. Slip compensation is mainly designed to address the issue that the actual output speed of the motor will be lower than the set speed when the load is large. Both of these compensation methods can be implemented using simple fixed values. Improved methods include calculations based on the three-phase motor current, but this is only based on the current amplitude and is essentially scalar compensation. A more precise method involves vector decomposition of the three-phase motor AC current, incorporating motor losses into the calculation, resulting in better compensation. However, this method is computationally complex and dependent on certain motor parameters, presenting some difficulties in implementation. Dead-zone compensation technology plays a crucial role in open-loop control. It effectively improves the smoothness of the output current waveform and reduces harmonics, while also increasing the effective value of the output voltage and reducing motor current oscillations. Especially in environments requiring quiet operation, artificially increasing the carrier frequency without dead-zone compensation may prevent the motor from running even under no-load conditions at low frequencies. Currently, commonly used dead-zone compensation techniques include direct current zero-crossing compensation, current decomposition based on stator magnetic field orientation, dead-zone voltage pulse width compensation, and sensorless dead-time prediction compensation. Compensation methods based on current zero-crossing judgment are simple and easy to implement. However, due to the large noise component in the current waveform, load fluctuations, and any external interference, the zero-crossing judgment can be flawed. A dead-zone plateau at the zero-crossing point affects the low-frequency compensation effect, especially at higher carrier frequencies. Methods based on stator magnetic field orientation do not directly determine the current zero-crossing point. Instead, they decompose the stator current in a rotating coordinate system to obtain the relationship between the current vector angle and the dead-zone voltage vector, and then compensate accordingly. Combining this method with dead-zone voltage pulse width compensation further enhances the effect. The phase angle prediction dead-time compensation method is a fixed compensation method that eliminates the need for a current sensor. This method first predicts the current phase angle and then compensates for the dead time. The predicted angle can be set in software according to the inverter's output capacity or modified externally. The advantage of this method is that it eliminates the need for a current sensor, reducing cost and system size. However, the compensation does not adjust according to changes in the external load, thus reducing accuracy and dynamic performance. 2. Current Oscillation Suppression Technology: When an AC motor is powered by PWM, under light load or no-load conditions, it may experience local instability across a wide frequency range due to various reasons. This results in significant fluctuations in current amplitude and changes in output frequency. Current oscillations can lead to false alarms due to overcurrent, preventing stable and reliable system operation. The causes of oscillation are multifaceted, but a common view is that it occurs during energy exchange between the motor and the inverter, and its occurrence is also closely related to the dead-zone effect. Compensating for the dead-zone effect can effectively reduce the oscillation amplitude, but it cannot fundamentally suppress it. One effective method is to adjust the actual output frequency or voltage accordingly when oscillation occurs, forming a simple negative feedback system through the current to suppress the oscillation. However, this method has limitations. Since the oscillation frequency range varies from approximately 5Hz to 30Hz for different motors, and current amplitude control is only a scalar, the control effect is poor, reducing system robustness. If the stator current is decomposed and the magnetic flux excitation current component affecting energy exchange is directly controlled, the suppression effect will be significantly improved. A more precise and effective method is to use intelligent control, but the algorithm is complex and difficult to implement on a general V/f control platform. 3. Simple Flux Vector Control Method: Ordinary V/f control is based on a steady-state motor model, ignoring the stator resistance voltage drop, and therefore cannot control the motor's state during dynamic processes. Because it is open-loop control, it is insensitive to load fluctuations or changes in motor parameters, resulting in low dynamic performance. The simple flux vector control method controls the motor current based on ordinary V/f control. Specifically, it calculates the torque current component and excitation current component by vector decomposition of the inverter output current, and then adjusts the voltage to match the motor current with the load torque, thereby improving low-speed torque characteristics. This method can provide 200% of the rated torque at 6Hz. Some motor parameters used in the vector calculation are pre-stored in the controller's RAM; for a given motor model, these parameters are essentially constants. 4. Sensorless Vector Control Technology: For high-performance AC speed control systems, speed closed-loop is essential, while speed closed-loop requires real-time motor speed. Currently, speed feedback is mostly detected using photoelectric pulse encoders, rotary transformers, or tachogenerators. Speed ​​sensors are relatively expensive, significantly increasing the system's hardware cost; they also have poor environmental adaptability, making them unsuitable for high-temperature or vibration-prone environments; and their signal transmission distance is limited, preventing reliable operation over long distances. Therefore, researching sensorless AC speed control systems is of great significance for improving system reliability, environmental adaptability, and further expanding the application range of AC speed control systems, and has become a research hotspot in the academic and engineering communities both domestically and internationally in recent years. The ultimate goal of sensorless control is to simultaneously and accurately estimate motor speed, rotor flux linkage, and motor parameters. There are various methods for estimating motor speed and flux linkage, including observation and estimation methods based on ideal models: open-loop flux linkage estimation and compensated flux linkage estimation; Model Reference Adaptive (MRAS) method; and closed-loop observer method. Methods based on non-ideal characteristics include: speed identification method using tooth harmonic signals; high-frequency injection rotor salient pole detection method; leakage inductance pulsation detection method; dq impedance difference orientation method; saturated salient pole detection method. There are two methods for detecting motor parameters: offline detection and online detection. There are several aspects that deserve special attention in the implementation of sensorless vector control technology, which have a very important impact on the system control performance and control accuracy. These aspects are: (1) Current and voltage signal detection and signal processing technology. The signal processing technology mainly focuses on how to effectively and accurately filter the detected current and voltage signals so as to reproduce the effective signal without producing amplitude attenuation and phase lag. Practical methods include simplified extended Kalman filters, morphological filters, etc. (2) Online adjustment of stator resistance. The stator resistance value changes greatly with the temperature rise during motor operation, and the maximum change can reach 150% of the rated value. How to detect the stator resistance online during operation and adjust the corresponding control quantity at the same time is very important for the system performance. (3) Dead-zone effect compensation technology (4) Problem of establishing an accurate dynamic motor model The motor parameters measured online or offline are only obtained at a certain moment. If the parameters change during operation, the motor model should also be changed accordingly to achieve the best control effect. At present, the model reference adaptive method is more commonly used in practical research. (5) Problem of inverter model reconstruction This technology is mainly proposed for operation at 0Hz in extreme cases. In this case, the saturation voltage drop of power devices and the time relationship of collector current must be considered. III. Several techniques of PWM modulation strategy Early PWM modulation methods were basically generated by hardware circuit simulation, mainly based on sinusoidal pulse width modulation. Later, it developed into a hybrid control of analog and digital circuits. The current modulation technology is basically implemented directly by software algorithm. Software implementation has very obvious advantages: flexible program writing, convenient modification, multiple modulation strategies can be implemented under the same hardware conditions, and it is easy to maintain and has strong anti-interference ability. From initially pursuing a sinusoidal voltage waveform, to a sinusoidal current waveform, and then to a sinusoidal magnetic flux; from optimal efficiency and minimal torque ripple to noise elimination, the development of PWM control technology has undergone a process of continuous innovation and improvement. PWM control technology can be divided into four categories based on its control philosophy: constant-width PWM, sinusoidal PWM (SPWM), flux tracking PWM, and current tracking PWM. The recently proposed discontinuous SVPWM and random PWM methods will be highlighted here. 1. SPWM Method The SPWM method focuses on generating a frequency- and voltage-adjustable three-phase symmetrical sinusoidal power supply from the perspective of the motor's power supply. Specifically, a sine wave is used as the reference wave (called the modulation wave), and a series of equal-amplitude triangular waves (called the carrier wave) intersects the reference sine wave. The switching mode of the inverter is determined by their intersection point. To improve the output voltage amplitude of the inverter, a quasi-optimized PWM method, namely the third harmonic superposition method, has been proposed for the SPWM method. By injecting a certain proportion of the third harmonic into a sine wave, the amplitude of the modulating wave is greatly reduced. Without overmodulation, the fundamental wave amplitude can exceed the triangular wave amplitude, achieving a modulation coefficient greater than 1. Under this modulation method, the maximum modulation ratio can be increased to approximately 1.15, and the corresponding DC bus voltage utilization rate can be increased by up to 15%. 2. SVPWM Method: The flux tracking PWM method, also known as Voltage Space Vector Pulse Width Modulation (SVPWM), differs from the SPWM method in that it focuses on the motor's perspective, aiming to obtain a circular magnetic field. It uses the ideal flux linkage of the AC motor as a reference when powered by a three-phase symmetrical sinusoidal voltage. The actual flux linkage vectors generated by different inverter switching modes are used to track the reference flux linkage circle. The tracking result determines the inverter's switching mode, forming a PWM wave. The inverter's switching modes have eight space voltage vectors, among which V0 and V7 are zero voltage vectors. SVPWM not only reduces motor torque ripple and current waveform distortion, but also significantly improves DC bus voltage utilization compared to SPWM technology. Under this modulation mode, DC bus voltage utilization can be increased by up to 15%, and it is easy to implement digitally. 3. Discontinuous SVPWM Strategy (DHPWM) The discontinuous SVPWM method is a novel voltage space vector pulse width modulation strategy proposed in recent years. Foreign literature refers to it as the discontinuous SVPWM strategy (DHPWM), while some domestic articles call it the hybrid modulation strategy (HPWM) or low switching loss mode modulation. For continuous PWM modulation methods, the three-phase modulation waves are all located between the peak values ​​of their corresponding carrier waves. Therefore, the switching losses of the inverter are the same for all continuous PWM modulation methods and are independent of the phase angle of the load current. The simplest way to reduce switching losses is to make the switching devices not operate, or to minimize their operation in a cycle. In traditional SVPWM methods, the positions of the zero vectors (V0 and V7) are symmetrically present during pulse width generation. The conduction time of the zero vectors is equal, and their positions are fixed and cannot be changed. If the effective conduction vector time remains constant, the effective value of the synthesized space voltage vector will not be affected. Simultaneously, changing the positions of the zero vectors V0 and V7 in the pulse width distribution reduces the number of switching actions; this is the discontinuous SVPWM method. Different zero vector allocations and positions will result in different modulation effects. If a suitable zero vector is fixed within the sector sandwiched by three adjacent vectors, each group can have a 120° sector without switching within one cycle. Since the non-switching range for each phase is a continuous 120° area, it leads to inconsistent switching losses between the upper and lower bridge arms, resulting in significantly greater waveform distortion than SVPWM. If different zero vectors are selected in adjacent 60° intervals, there are three zero vector allocation schemes. In practical applications, each phase switching device should ideally switch in the region with lower load current, while larger load currents should be placed in the non-switching sectors. This not only reduces the number of switching actions but also effectively lowers the maximum switching current of the switching devices, thereby minimizing switching losses. This method can reduce the number of switching operations to one-third of the original SVPWM, greatly reducing switching losses. Simultaneously, because the insertion of the zero vector alters the inverter's freewheeling process, it also has a certain effect on suppressing current waveform oscillations and distortion. In engineering practice, implementing dead-zone compensation techniques during modulation using this method presents certain difficulties. One effective method is compensation for the effective conduction vector within each sector. 4. Random Pulse Width Modulation (RPWM) In AC drive systems powered by frequency converters, noise has long been a concern. In some low-noise environments, the noise emitted by frequency converters and motors is unbearable. Frequency converter noise is mainly caused by the pulse width modulation method used by the inverter. In general PWM methods, the inverter's power switches are turned on and off in a "deterministic" manner. While this control method can effectively suppress low-order harmonics in the voltage waveform, it generates some high-order harmonics with large amplitudes. These harmonics are mainly concentrated near one and two times the carrier frequency, producing significant noise and vibration. The recently emerging Random Pulse Width Modulation (RPWM) provides a completely new approach to solving the inverter noise problem. The basic idea of ​​random PWM is to replace the fixed switching mode in conventional PWM with a random switching strategy so that the harmonic spectrum of the inverter output voltage is evenly distributed in a wider frequency range, thereby suppressing noise and mechanical vibration. There are currently three feasible RPWM schemes: (1) Randomizing the switching frequency. In the traditional SPWM, the slope of the triangular carrier wave is randomly changed, so the number of switching cycles can be randomly changed, thereby achieving the purpose of random switching frequency. (2) Randomizing the pulse position. In this scheme, the random quantity is the position of the switching signal pulse in each on-off cycle. The simplest one is to randomly select two bits, one at the beginning and one at the end. Source: Power Transmission and Distribution Equipment Network (3) Random switching. The random wave is compared with the sinusoidal reference signal, and the comparison result forms a digital RPWM signal. Based on the existing space vector pulse width modulation technology, random PWM can be achieved by using the randomized pulse position method. In the above optimized SVPWM, the different zero vector positions are analyzed, which will reduce the switching loss of the system. If the optimized SVPWM is randomly modulated by random method, the pulse position will be randomly changed in each carrier cycle due to the different zero vector insertion positions, thereby achieving the purpose of random PWM modulation. Currently, a relatively simple and practical method uses only two fixed zero vectors for random switching. A random function generates two random states, 0 and 1. If it is 0, the zero vector V0 is applied at both ends of the switching cycle; if it is 1, the zero vector V7 is applied in the middle of the switching cycle. This method is essentially a random switching of two low-switching-loss modulations. The better the randomness of the states generated by the random function, the more modulation states are switched, and the better the continuous distribution of harmonic energy in the system, resulting in a better effect of random PWM. However, the difficulty of algorithm implementation and the real-time requirements of the controller will also increase. Using the RPWM method can effectively reduce the noise of the inverter system, and at the same time, convert some concentrated discrete high-frequency components into continuous and evenly distributed frequency components, reducing the electromagnetic interference propagating outward. 5. Overmodulation Technology Overmodulation technology is a technique mainly implemented based on Space Vector Pulse Width Modulation (SVPWM). For high-performance AC drive systems, how to make full use of DC voltage to obtain the maximum output electromagnetic torque is a very important factor. Especially in the field weakening stage, in order to obtain sufficient voltage, it is necessary to control the inverter to operate in the overmodulation range. Traditional SPWM control can only achieve 78.54% of the output voltage of a square wave, while Space Vector Pulse Width Modulation (SVPWM) can increase the output voltage to 90.69% of the square wave. To obtain a higher output voltage, the inverter must operate in the overmodulation region until it reaches the square wave condition. Currently, there are many space vector overmodulation methods proposed in academia, with varying algorithm complexity and effectiveness. However, output voltage vector modulation methods generally only have two overmodulation methods: dual-mode control, which divides the overmodulation region into two parts for separate modulation; and single-mode control, which treats the overmodulation region as a whole. Essentially, single-mode is just an engineering simplification of dual-mode, making it simpler to implement, but the resulting fundamental voltage is lower than that of dual-mode, and the harmonic content is higher. If the controller's processing speed and storage space are sufficient, dual-mode control can be used to improve the system's output characteristics. The relationship between angle and modulation ratio can be obtained offline and stored in RAM, or calculated online using a fitted curve. IV. Conclusion This paper discusses several techniques and methods for improving inverter performance through control and modulation strategies. These methods can all be implemented in software on a general-purpose hardware platform, which is beneficial for modularity and integration. This paper introduces and discusses some of the technical issues involved, and proposes some corresponding solutions and methods for further discussion.