Abstract : This paper introduces the variable frequency control principle of a brushless doubly-fed motor. A variable frequency control system for a brushless doubly-fed motor, controlled by a commercial frequency converter, was modified using a THS320F2812DSP as the core. The hardware structure and software flowchart are given. Experimental results show that the system conforms to the speed regulation principle of a brushless doubly-fed motor. Keywords : brushless doubly-fed motor; Siemens frequency converter; variable frequency control; DSP 1 Introduction As a new type of speed-regulating motor, the brushless doubly-fed motor has a simple structure and reliable operation. When combined with a variable frequency speed control device to form a speed regulation system, it requires a small frequency converter capacity and can easily achieve various operating modes. Furthermore, it is easy to manufacture and has low cost, thus showing great development potential. 2 Variable Frequency Speed ​​Regulation Principle of Brushless Doubly-Fed Motor Figure 1 shows a simplified structural diagram of a squirrel-cage brushless doubly-fed motor. The stator is equipped with two sets of three-phase symmetrical windings with different pole numbers. One set is connected to a three-phase power frequency (frequency fp) with Pp pole pairs and is called the power winding; the other set is connected to a variable frequency power supply (frequency fc) with Pc pole pairs and is called the control winding. The rotor cage adopts a self-closing loop structure. According to the operating principle of the brushless doubly-fed motor, the steady-state speed expression of the motor is: (Full text available for download: Design of a Brushless Doubly-Fed Motor Variable Frequency Speed ​​Control System)