Abstract: Accurate online measurement of DC bus voltage, current, and output three-phase current signals is one of the essential conditions for designing high-performance frequency converter products. This paper compares and analyzes voltage and current detection schemes, provides several practical circuits with good reference value in frequency converter design, and gives corresponding experimental results. Keywords : Frequency converter, voltage and current detection 1. Introduction A frequency converter is a power control device that uses the switching action of power semiconductor devices to convert power frequency power into electrical energy of another frequency. Its most important features are high-efficiency driving performance and good control characteristics. Simply put , a frequency converter changes the motor speed by changing the frequency of the motor input voltage. From the motor speed formula, it can be seen that adjusting the frequency f of the motor input voltage can change the motor speed n. Currently, almost all low-voltage frequency converters adopt the main circuit topology shown in Figure 1. Part 1 is the rectifier, which converts AC to DC. Part 2 is the reactive power buffer DC link, where capacitors or inductors can be used as buffer elements. Part 3 is the inverter section, which converts DC to three-phase AC with adjustable frequency. Frequency converters that use capacitors in the intermediate links are called AC-DC-AC voltage-type frequency converters, and this is the main circuit topology widely used in general-purpose frequency converters. This article will focus on some issues that should be considered in the voltage and current detection design of this structure. Why does the frequency converter need to detect voltage and current during operation? This requires an explanation of the motor's structure and control characteristics: ① The torque of a three-phase asynchronous motor is generated by the interaction between the motor's magnetic flux and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux will be too large, the magnetic circuit will saturate, and in severe cases, the motor will burn out. Therefore, the frequency and voltage must be changed proportionally. That is, while changing the frequency, the inverter's output voltage must be controlled to maintain a constant magnetic flux in the motor, avoiding weak magnetic field and magnetic saturation. ② During inverter operation, the overload starting current is 1.2 to 1.5 times the rated current; the overcurrent protection current is 2.4 to 3 times the rated current (different overcurrent protection points are selected according to different load requirements); additionally, current closed-loop tripping and stall prevention functions are all related to the current during inverter operation. ③ To improve the inverter's output characteristics, dead-zone compensation is required. Several commonly used dead-zone compensation methods all require output current monitoring. ④ If the motor reduces the command frequency too quickly during operation, the motor will switch to generator mode. The regenerated energy is stored in the inverter's DC capacitor. Due to the capacitor's capacitance and voltage rating, timely and accurate voltage monitoring is necessary to provide the inverter with accurate and reliable information, enabling timely and effective overvoltage protection. Simultaneously, the power-on and power-off processes of the frequency converter require determining the current DC bus voltage status to decide the next action of the program. Given the importance of voltage and current detection, it is essential to employ accurate and effective methods for voltage and current detection in frequency converter design. Several methods are discussed below. 2. Design Schemes for Online Voltage Measurement Overvoltage or undervoltage in the frequency converter manifests primarily in the DC bus voltage value. Under normal circumstances, the frequency converter's DC voltage is the average value after three-phase full-wave rectification. If calculated using a 380V line voltage, the average DC voltage is... When overvoltage occurs, the energy storage capacitor on the DC bus will be charged, potentially damaging the inverter, rectifier, and filter capacitors in the main circuit. When the voltage rises to approximately 800V, the frequency converter's overvoltage protection function activates. Furthermore, the frequency converter cannot operate normally when undervoltage occurs (around 350V). For the frequency converter, there is a normal operating voltage range; voltages exceeding or falling below this range can damage the frequency converter. Therefore, online detection of the bus voltage is necessary. Three commonly used voltage detection schemes exist. 1) Transformer Scheme In Figure 2 , P represents the positive ( + ) DC bus voltage , and N represents the negative ( - ) DC bus voltage. [b] The power supply voltage of the inverter control circuit is generally obtained by using a switching power supply. Utilizing the characteristics of a switching transformer, a winding N4 (the number of turns is determined according to the actual circuit parameters) is added to the secondary side as a sampling output of the bus voltage. The primary voltage of the switching transformer is the bus voltage, while the secondary output voltage changes linearly with the change of the primary input voltage. This achieves both strong and weak current isolation and voltage reduction. After processing, this sampled signal can be sent to the DSP for A/D sampling to realize various protection functions. 2) Linear Optocoupler Scheme P represents the positive ( + ) DC bus voltage , and N represents the negative ( - ) DC bus voltage . In this method, the primary of the optocoupler receives a set of analog voltage signals to be measured, and the secondary outputs a pair of differential voltage signals. The input and output have a linear equivalent relationship within a certain range. In the design application, the input and output terminals of the optocoupler must be provided with isolated +5V power supplies, and the operational amplifier circuit must provide ±15V power. The DC bus voltage is connected to the input terminal of the optocoupler after being divided by resistors. The output signal linearly follows the change of the input signal. The output signal of the optocoupler is amplified by the amplifier circuit and then provided to the DSP for internal processing. Since the linear range of this optocoupler is small, the configuration of the input terminal resistor must ensure that the input signal is within the linear range of the optocoupler. 3) [b]Voltage Hall Scheme[/b] The voltage Hall sensor is used to measure the bus voltage. According to the requirements of Hall sensor use, a ±15V power supply must be provided, and the error of the power supply voltage shall not exceed ±5%. Since the current at the Hall sensor input terminal does not exceed 10mA, the input terminal resistor can be configured according to the range of the bus voltage and the heat requirements of long-term operation. The input and output of this voltage Hall sensor are isolated. Therefore, the output current signal of the Hall sensor is sampled by resistors R5 and R6 and converted into a voltage signal before being processed (such as filtering, amplification, etc.) and directly introduced into the DSP for real-time sampling and calculation. Different voltage ratings of voltage Hall sensors can be selected according to different bus voltage detection ranges. Table 1 is a comparison table of three different measurement schemes: Table 1. [table][tr][td=1,1,15%][align=center]Name[/td][td=1,1,12%]Measurement Voltage Range[/td][td=1,1,12%]Response Time[/td][td=1,1,12%]Detection Accuracy[/td][td=1,1,18%]Number of Power Supply Groups Required[/td][td=1,1,9%]Price[/td][td=1,1,17%]Isolation Voltage[/td][/tr][tr][td=1,1,15%]Transformer Scheme[/td][td=1,1,12%]< 2400V[/td][td=1,1,12%]< 100ms [td=1,1,12%]0.3% [td=1,1,18%]——- [td=1,1,9%]Cheap [td=1,1,17%]2400V/50HZ/1min [td=1,1,15%]Linear optocoupler solution [td=1,1,12%]< 2500V [td=1,1,12%]< 10us[/td][td=1,1,12%]0.1%[/td][td=1,1,18%]4 groups[/td][td=1,1,9%]Moderate[/td][td=1,1,17%]2500V/50HZ/1min[/td][/tr][tr][td=1,1,15%]Voltage Hall scheme[/td][td=1,1,12%]< 2500V[/td][td=1,1,12%]< 40us[/td][td=1,1,12%]±0.6%FS[/td][td=1,1,18%]2 groups[/td][td=1,1,9%]High[/td][td=1,1,17%]2500V/50HZ/1min[/td][/tr][/table][/align] 3 Design Schemes for Online Current Measurement The purpose of real-time inverter output current detection is mainly to prevent damage to the inverter when overcurrent occurs, and to provide actual feedback values ​​for dead-zone compensation and trip-free current closed-loop control. If the current detection is inaccurate or the error is too large, and the inverter can only perform protection and calculation based on its internal measurement results, malfunctions will occur. Therefore, current detection must be timely and accurate. There are two commonly used current detection circuits. 1) **[b]Hall Current Scheme[/b]** Hall current sensors are a new generation of current sensors that apply the Hall effect principle. They can measure DC, AC, pulsating, and various irregular waveform currents under electrically isolated conditions. Because the response time of a closed-loop Hall current sensor is less than 10µs, when a short circuit occurs, the Hall output current signal is converted into a voltage signal by a sampling resistor and promptly sent to the DSP. This blocks the PWM drive signal output within the IGBT's 10µs short-circuit safety time, ensuring reliable protection for the IGBT. Of course, like voltage Hall sensors, the power supply voltage required for normal operation of the current Hall sensor must be provided, and the power supply voltage error must not exceed ±5%. Simultaneously, when selecting a current Hall element, the linear range must meet the range of the IGBT's maximum operating current. In the three-Hall current scheme, the DC-side Hall sensor is used to detect bridge arm shoot-through faults and has high requirements for response performance. The two-phase current detection on the output side is used to complete dead-zone compensation, non-trip current closed-loop, overload, and overcurrent detection. The second three-Hall scheme in Figure 6 removes the DC-side Hall sensor; shoot-through protection is ensured by an intelligent drive optocoupler. In addition to implementing the functions of the two Hall sensors in Figure 5, the three output-side Hall sensors can also perform output phase loss detection. 2) Linear Optocoupler Scheme The inverter output current is sampled by a low-resistance, low-inductive, high-precision sampling resistor. The resulting voltage signal is isolated and amplified by a linear optocoupler before being sent to the DSP. The DSP processes the signal internally to protect the inverter. The specific circuit can be referenced from the linear optocoupler circuit in voltage measurement, only the input signal terminals are slightly different. This method is commonly used in low-power inverters. The selection of the sampling resistor value should balance the factors of minimum power consumption and maximum accuracy. 4. Performance Requirements for Voltage and Current Sensors in Inverter Design a) Electromagnetic Compatibility ( EMC ) Requirements: With the widespread use of power electronic devices such as inverters, electromagnetic interference (EMI) in systems is becoming increasingly serious. Corresponding anti-interference design techniques (i.e., electromagnetic compatibility EMC) have become increasingly important, requiring voltage and current sensors to have strong anti-interference capabilities. b) Power Supply Requirements: ±15V±5%. In practical applications, the accuracy and cleanliness of the power supply are required; otherwise, inaccurate measurement outputs or even sensor overheating and damage may occur. c) Temperature characteristics requirements: The operating environment temperature should be -10℃ to +70℃. As the temperature increases, the sensor output should be as less affected by temperature as possible. d) Linearity requirements : Different series of voltage and current sensors have different linearities. In high-performance frequency converter design, a linearity of ≤±0.1%FS is adopted, and the linear range should be greater than the maximum value of the measured current. e) Size requirements: The smaller the size, the better, and the performance should be stable. f) Response time requirements: Different series of voltage and current sensors have different response times. Generally, sensors with shorter response times are selected , such as Tr ≤1μS. 5. Experimental Waveform Comparison and Conclusions: When the DC bus voltage PN is approximately 300V, the DC bus voltage was tested using the transformer scheme, linear optocoupler scheme, and voltage Hall scheme respectively. The test waveforms are shown in the following figures: [align=center] [/align] After power-on, when a 60A DC current is directly applied to the primary side of the current Hall, the primary and secondary signal waveforms are observed as shown in Figure 10: After power-on, the primary and secondary signal waveforms of the current Hall during acceleration are observed as shown in Figure 11: Through practical use and comparison, it was found that current detection is still widely used in frequency converters because of the simple application principle, convenient signal processing, and a series of unique advantages of the device itself. As frequency converters develop towards higher voltage and higher power, voltage detection increasingly relies on the application of Hall or linear optocoupler methods. References : [1] Zhang Zhansong, Cai Xuansan. Principles and Design of Switching Power Supply [M]. Beijing: Electronic Industry Press. 1998. [2] Han Anrong. General-purpose Frequency Converter and Its Application [M]. Beijing: Machinery Industry Press, 2000. [3] Zeng Yuenan, Chen Linkang, Hu Jiehe. New Fully Digital PWM General-purpose Frequency Converter [J]. Power Electronics Technology, 1999, 33(6): 28-30. [4] Zhou Keliang, Kang Yong, Xiong Jian. Development of Space Vector Method SPWM Fully Digital Frequency Converter [J]. Power Electronics Technology, 1998, 32(1): 14-16. Author Introduction: Wang Jianyuan (1973-), male, doctoral candidate, research direction is power electronics and power transmission, frequency converter, switching power supply. Gou Yaxian (1968-), female, senior engineer, mainly engaged in the design of frequency converter and power electronic device. Wang Jianyuan's telephone: 13609192409 email: [email protected]