High-voltage frequency converters refer to devices that drive motors with an input power of 6,000V or 10KV. High-voltage frequency converters generally offer the following options: 1. Direct high-voltage control (high cost): Currently, harmonic-free frequency conversion technology similar to that of Robicon (USA) is used. Low-voltage modules are connected in series to form a high-voltage output. Its advantage is extremely low harmonics, but it requires a dedicated input transformer, resulting in the highest initial cost. At low frequencies, the IGBT saturation voltage drop in series leads to lower efficiency. It is more suitable for driving large-capacity high-voltage motors in loads such as fans and pumps. 2. Three-level control (medium cost). Due to the low voltage withstand capability of power semiconductors, a series connection scheme is used to increase the output voltage. Similar to low-voltage inverter technology, the difference lies in the output voltage being doubled, resulting in lower output current harmonics and other advantages. It is more suitable for medium-voltage applications (690-3300V) and has a medium capacity. Because IGCT high-voltage power modules are also used, the voltage can be increased to 6,000V, but this is currently less common in the market. Three-level technology is currently used in Europe and America for subway drives, energy-saving operation of fans and pumps, and oil and mining applications. The voltage range is generally concentrated in the low and medium voltage range. Japan also uses three-level technology to produce and sell low-voltage inverters. High-low voltage control (low cost): High-low voltage control inverters use transformers to reduce high voltage to low voltage, and then purchase a low-voltage inverter to drive the low-voltage motor. This system has the most mature technology, the highest reliability, the highest operating efficiency, the lowest investment cost, and convenient maintenance services. It is most suitable for medium- and low-capacity motors (generally below 2,000KW). This solution is most suitable for new projects. High-low-high control is generally suitable for energy-saving retrofits of older equipment, particularly for upgrading existing high-voltage motors. This high-low-high control scheme is particularly suitable for high-voltage frequency converters below 2,000KW. This technology uses a transformer to reduce the high voltage to a low voltage, then purchases a low-voltage frequency converter, and finally steps the converter's output voltage back up to high voltage via another transformer to drive the high-voltage motor. Siemens of Germany has a significant sales track record in China, and the company also sources its input and output transformers from China for equipment exported to Europe and America, demonstrating the advantages of this solution. The advantages of high-low-high control include mature technology, high reliability, convenient maintenance, and low investment costs. Due to the widespread application of frequency converters, the demand for small and medium-capacity high-voltage motors has gradually decreased, mainly because frequency converters can effectively control starting current, operating efficiency, and power factor. Therefore, low-voltage drives should be used for small and medium-capacity high-voltage motors whenever possible. Currently, some European motor manufacturers are producing low-voltage motors up to 1,500KW, allowing users to purchase low-cost, low-maintenance, high-efficiency, and high-reliability frequency converters.