Abstract : Variable frequency drives (VFDs) are increasingly widely used in various industries due to their high efficiency, energy saving, convenient adjustment, and excellent protection functions. This article introduces the types of VFDs, the working principle of the main electrical circuit, and the main protection functions of VFDs. Keywords : VFD, motor, protection function [b][align=center]Converter and Its Excellent Protection Functions Huang Jianlong (Nantong Shipping College , Nantong 226006, Jiangsu, China)[/align][/b ] Due to its advantages such as small size, light weight, high precision, rich functions, complete protection, high reliability, simple operation, and strong versatility, it has been widely used in industrial and agricultural production since its introduction. 2 Types of Frequency Converters There are many types of frequency converters, which can generally be classified as follows: 2.1 Classification according to the method of frequency conversion (1) AC-DC-AC frequency converter. This type of frequency converter first converts 50Hz AC power into DC power through rectification, and then filters it to convert the relatively smooth DC power into AC power with adjustable frequency. Depending on the filter device selected, it can be divided into two forms: capacitor filtering and inductor filtering. The voltage waveform of the DC circuit of capacitor filtering is relatively flat, the output impedance is very small, and the voltage is not easy to change abruptly. It is equivalent to a constant voltage source, so it is called a voltage-type frequency converter; the current waveform of the DC circuit of inductor filtering is relatively flat, the output impedance is very large, and the current is not easy to change abruptly. It is equivalent to a constant current source, so it is called a current-type frequency converter. The main advantage of this type of frequency converter is that the frequency adjustment range is relatively wide, so it is currently widely used. [b][align=center]For more details, please click: Frequency Inverters and Their Excellent Protection Functions[/align][/b]