Abstract: This paper introduces a variable frequency speed controller based on the 8051 microcontroller, and uses its application in a constant pressure water supply system as an example to illustrate the hardware composition and software implementation of the controller. Keywords: Microcontroller; Variable frequency drive; Asynchronous motor; 1 Introduction Variable frequency speed control technology is a new technology that has developed in the last 20 years. With the increasing development of power electronics technology and the maturity of PWM control technology, the principle that the motor speed and the frequency of the input power supply are linearly related is utilized. 50 Hz AC power is rectified and inverted to convert it into a frequency-adjustable power supply to supply the asynchronous motor, achieving the purpose of speed regulation. The variable frequency speed controller composed of a microcontroller can realize starting from a low frequency (1-2 Hz) to 50 Hz, eliminating the impact on the motor caused by direct starting at the previous 50 Hz power frequency, extending the motor's service life. At the same time, because the output voltage of the inverter can be adaptively adjusted, the load motor can operate below the rated voltage, which not only saves energy but also extends the motor's service life. The following section uses its application in a constant pressure water supply system as an example to illustrate the hardware composition and software implementation of this controller. [b][align=center]For details, please click: Design of a Microcontroller-Based Motor Variable Frequency Speed ​​Control Controller[/align][/b]