Abstract: This paper elucidates the basic principle of U/F control variable frequency speed regulation, analyzes the typical topology of variable frequency speed controllers, and then introduces variable frequency speed controller systems designed with isolation and common ground schemes. In particular, it elaborates on the design of a common ground system using a single-power-supply intelligent power module, and presents the design differences between the two and the cost advantages of the common ground system. Finally, it provides an outlook on the application of a common ground system using a single-power-supply intelligent power module. 1 Introduction With the development of computer technology, microelectronics technology, and power electronics technology, the application of variable frequency speed regulation technology has developed rapidly. The application fields of variable frequency speed regulation control technology are becoming increasingly widespread, such as textile machinery, mining machinery, industrial control, and variable frequency home appliances. Variable frequency speed regulation technology, as a major means of energy saving, consumption reduction, process improvement to enhance product quality, environmental improvement, and technological progress, has gained widespread acceptance. The successful application of variable frequency speed regulation technology in home appliances has greatly improved people's quality of life. The enormous market potential of variable frequency speed control products is becoming increasingly apparent. However, due to the late start of research and application of variable frequency speed control technology in my country, especially the lagging commercialization of variable frequency technology products, high-end variable frequency products are basically monopolized by foreign brands. Therefore, domestic variable frequency products face both huge domestic market demand and intense competition from both domestic and international markets, particularly in terms of price and performance. Consequently, low cost and high performance have become the goals pursued by variable frequency products. This paper will describe the design of a variable frequency speed controller system, analyzing a traditional isolated scheme and a common-ground system design using a single-power-supply intelligent power module. It will demonstrate that the common-ground system variable frequency speed controller system designed with a single-power-supply intelligent power module can significantly simplify hardware circuit design and reduce system hardware costs. 2. Variable Frequency Speed ​​Regulation Principle Among various AC asynchronous motor speed regulation technologies, such as voltage regulation, pole changing speed regulation, cascade speed regulation, slip speed regulation, and variable frequency speed regulation, variable frequency speed regulation has the following advantages: (1) good smoothness and high efficiency during speed regulation; (2) large speed regulation range and high precision; (3) low starting current, no impact on the system and power grid, and significant energy saving effect; (4) easy to realize process automation. Therefore, variable frequency speed regulation technology is currently the most widely used speed regulation technology. In small and medium power variable frequency speed regulation systems, the most commonly used variable voltage variable frequency speed regulation is called U/F control, and the corresponding variable frequency speed controller is the voltage source variable frequency speed controller (VSI). According to the knowledge of motors, the speed of an asynchronous motor is related to the power supply frequency as follows: Where: n—motor speed (r/min); p—number of magnetic pole pairs; s—slip (%); f—power supply frequency (Hz). It can be seen from equation (1) that changing the power supply frequency can change the motor speed. In addition, according to the electromotive force formula of the asynchronous motor, the applied voltage is approximately proportional to the product of frequency and magnetic flux. That is, U∝E≈C1fφ (2) where C1 is a constant. Therefore, we have: φ∝E/f≈U/f (3) If the applied voltage remains unchanged, the magnetic flux φ will change with the frequency. For example, if the frequency f decreases, the magnetic flux φ will increase, causing magnetic circuit saturation, increasing excitation current, decreasing power factor, and overheating of the core and coil, which is obviously not allowed. Therefore, the voltage must be reduced at the same time as the frequency is reduced, which requires coordinated control of frequency and voltage. In addition, in many cases, in order to keep the maximum torque generated by the motor constant during speed regulation, it is also necessary to keep the magnetic flux constant, which is also achieved by coordinated control of frequency and voltage. By changing the power supply frequency of the asynchronous motor, the motor speed can be arbitrarily adjusted to achieve smooth stepless speed regulation. 3 Composition of the variable frequency speed regulation control system The variable frequency speed regulation control system generally consists of three parts: drive control part, feedback part and drive object. The drive control section is the core of the entire speed control system, generally referred to as the variable frequency speed controller. The feedback section feeds back relevant quantities of the driven object, such as speed and current, to the control section to achieve closed-loop control. The main driven object is the electric motor, including stepper motors and brushless DC motors. A schematic diagram of the variable frequency speed control system is shown in Figure 1. If the control accuracy requirement is not very high, the feedback section in Figure 1 can be omitted. Since the specific driven objects in the system vary, the feedback section also differs in application design and will not be discussed in detail here. This section mainly discusses the core part of the variable frequency speed control system, namely the drive control section. Depending on the speed control system, the drive control section can be designed as a general-purpose speed controller, such as a general-purpose frequency converter, or as a dedicated speed controller, such as a weft feeder used in the textile industry, but their basic structure is the same or similar. Because different design methods can be used in the specific design process, the complexity of the hardware circuit varies greatly. The topology and design method of the variable frequency speed controller will be described below. Figure 1. Schematic diagram of variable frequency speed control system structure. 4. Variable frequency speed controller design. 4.1 Typical topology. The power circuit of the variable frequency speed controller for asynchronous motors or brushless DC motors generally adopts a typical AC-DC-AC topology, as shown in Figure 2 (Figure 2 only shows the full-bridge rectifier circuit for single-phase AC). It consists of a rectifier section, a filter section, and an inverter section. When it is working, the three-phase or single-phase AC power is first rectified by the bridge rectifier into a pulsating DC voltage with a frequency twice that of the grid voltage. After the pulsating DC voltage is smoothed and filtered, a stable DC voltage with smaller fluctuations is obtained. Then, under the control of the microcontroller, the inverter converts the DC power back into a three-phase AC power supply with adjustable voltage and frequency, which is output to the motor that needs speed control, thereby realizing the speed control of the motor. Figure 2. Typical topology of AC-DC-AC variable frequency speed controller. 4.2 Isolation scheme design. Early variable frequency speed controllers adopted discrete component design, including the design of the inverter circuit, which made the hardware structure complex, reduced reliability, and correspondingly long product development cycle. With the development of power electronic devices, especially high-power switching devices such as IGBT modules, the design of inverter circuits has been simplified to some extent. However, to meet production needs, IGBT modules with higher integration and stronger functions have been introduced, namely Intelligent Power Modules (IPMs). They have many advantages such as small size, multiple functions, and ease of use, and are suitable for inverter designs of different power ratings. Using intelligent power modules to design variable frequency speed controllers can greatly simplify hardware design, shorten product development cycles, and improve system performance. IPM modules released at different stages have different internal design technologies and integrated protection functions. Early 6-unit IPM modules required four independent isolated power supplies for driving the IGBT modules: one independent power supply for each of the three upper bridge arms, and one power supply shared by the three lower bridge arms. This increased the size and design complexity of the system switching power supply. In addition, the system control section, i.e., the MCU or DSP control circuit and the power circuit section, required electrical isolation to ensure the safety of the control circuit. The signals requiring isolation mainly include: the six PWM signals for inverter control; the fault protection signals output by the IPM module; and voltage detection signals. The following is a schematic diagram of the variable frequency speed controller designed with an isolation scheme, as shown in Figure 3. Figure 3: Variable Frequency Speed ​​Controller Designed with Isolation Scheme. Because the reference power grounds of the power supply and control section of the system are different, the implementation of current and voltage detection is complex. Generally, Hall current sensors and voltage transformers are used, making it difficult to reduce hardware costs and leading to a decline in product market competitiveness. 4.3 Common Ground Scheme Design. Due to the development of power device integration technology, some manufacturers, such as Mitsubishi, Toshiba, and Fairchild, have adopted bootstrap power supply technology and launched single-power IPMs, which only require one drive power supply to the IPM module. This makes it possible to achieve a common ground for the system. The following uses Mitsubishi's single-power module PS21255-E as an example to introduce its new features. The PS21255-E is Mitsubishi's fourth-generation IGBT power module, used for DC-AC power conversion to drive small-power (1.5kW) motors. It mainly integrates functions such as drive, protection and system control, and specifically includes the following features: (1) The three-phase IGBT inverter bridge has gate drive circuit and protection circuit; (2) The upper bridge arm driver power supply uses bootstrap power supply technology; (3) All drives share a power supply; (4) It contains a fast level shifter circuit with high voltage isolation; (5) The control signal does not require optocoupler isolation, etc. The biggest feature of the fourth-generation IPM module is that it realizes single power supply drive. Its core technology is that the drive power supply adopts bootstrap power supply technology and level shifter circuit. Its schematic diagram is shown in Figure 4. Among them, the bootstrap power supply is composed of R1, D1 and C1 in Figure 4, and IGBT1 and IGBT2 form one phase bridge arm. It can be seen from Figure 4 that the charging circuit of the bootstrap power supply is R1, D1, C1, IGBT2 and N. This single power supply drive technology provides the conditions for the system to adopt a common ground scheme design. Figure 4 shows the principle of the bootstrap power supply and level shifting circuit. The advantage of using a common ground scheme is that it greatly simplifies the design of the hardware circuit. Figure 5 shows a schematic diagram of the variable frequency speed controller designed based on a single-supply IPM module using a common ground scheme. As can be seen from Figure 5, the variable frequency speed controller designed with a common ground scheme reduces the number of power supply paths for optocouplers and switching power supplies used for PWM signal isolation; it simplifies the current detection and voltage detection circuits. The entire system hardware design is greatly optimized in terms of cost and performance, meeting the requirements of low cost and high performance. Figure 5 Variable frequency speed controller designed with a common ground scheme 5 Conclusion Variable frequency speed control technology is a very mature speed control technology. Due to its unique advantages in speed control, it is widely used in many speed control fields, and the variable frequency speed controller is a typical application. The two design schemes mentioned in this paper are both representative: the isolation scheme is relatively mature and widely used, but the hardware cost is relatively high; the common ground scheme based on a single-supply IPM module is more attractive, and can minimize hardware costs while meeting performance requirements, greatly improving the product's market competitiveness. Moreover, the common ground solution has been successfully applied to the development of industrial frequency converter products and has been commercialized. Practice has proven that designing a variable frequency speed controller using the common ground solution is feasible and reliable, and is well worth promoting and applying.