[ Abstract ] AC drive systems have achieved performance comparable to DC speed control in terms of wide speed range, high speed stability, fast response, and four-quadrant operation. Variable frequency drives (VFDs) have entered a stage of high performance, multi-functionality, miniaturization, and low cost. This article, based on the author's practical application experience, summarizes some key issues involved in VFD capacity selection and proposes relevant methods for rationally selecting VFD capacity. [ Keywords ] VFD capacity selection, verification, introduction With the rapid development of power electronics, microelectronics, computer technology, and control theory, AC drive systems have achieved performance comparable to DC speed control in terms of wide speed range, high speed stability, fast response, and four-quadrant operation. In particular, variable frequency speed control, with VFDs as its core, is widely recognized as the most promising speed control method due to its excellent speed control performance. Currently, VFDs have entered a stage of high performance, multi-functionality, miniaturization, and low cost. To facilitate the rational use of VFDs, this article will briefly discuss the process of VFD capacity selection. 1. VFD Capacity Selection The selection of VFD capacity is an important and complex issue. The matching of inverter capacity and motor capacity must be considered. If the capacity is too small, it will affect the output of the effective torque of the motor. This will affect the normal operation of the system and even damage the equipment. If the capacity is too large, the harmonic components of the current will increase. This will also increase the investment in equipment. 1.1 Steps for selecting inverter capacity The selection of inverter capacity can be divided into three steps: (1) Understand the nature and variation of the load. Calculate the magnitude of the load current or draw the load current diagram I=f(t); (2) Pre-select inverter capacity and others; (3) Verify the pre-selected inverter. If necessary, verify the overload capacity and starting capacity. If both are passed, the pre-selected inverter capacity is selected; otherwise, start from (2) again until it is passed. Under the premise of meeting the requirements of production machinery, the smaller the inverter capacity, the more economical it is. 1.2 Selection of inverter capacity based on the load current of the motor Generally speaking, there are three ways to express the capacity of inverter: rated current, rated power of the motor and rated apparent power. Regardless of which method is used, it is still about the selection of the rated current of the inverter. The overload capacity of a frequency converter should be determined based on the actual situation and the current that the motor may absorb from the frequency converter. Frequency converters typically have two overload capacities: ① 1.2 times the rated current, which can last for 1 minute; ② 1.5 times the rated current, which can last for 1 minute. Furthermore, the allowable current of the frequency converter is inversely related to the process time. For example, 1.2 (1.5) times the rated current can last for 1 minute; while 1.8 (2.0) times the rated current can last for 0.5 minutes. This means that the current supplied to the motor for more than 1 minute (or 0.5 minutes) must be within certain limits. The overload capacity indicator is only meaningful for the motor during startup (acceleration); during operation, it is practically equivalent to not allowing overload. The following discusses how to select the frequency converter capacity based on the motor's load current. 1.2.1 One frequency converter supplies only one motor, i.e., one-to-one. After calculating the load current, three factors should also be considered: First, when using a frequency converter, the pulsation of the motor current is relatively larger than when using a mains frequency power supply. Second, the starting requirements of the motor. This refers to whether the inverter starts at low frequency and low voltage, or directly at rated voltage and frequency; thirdly, the relevant data in the inverter's instruction manual is based on tests conducted using the company's standard motors. It's important to note that motors designed and manufactured according to conventional methods may have performance differences, so an appropriate margin should be allowed when calculating the inverter's capacity. Read the full article: Inverter Capacity Selection Based on Different Motor Loads