Abstract : With the development of new power electronic devices, AC variable frequency speed control technology, like computer control, has become one of the main hallmarks of modern AC drive speed control technology. Keywords : Variable frequency speed control, speed inverter, controller 1. Variable frequency speed control principle : 1.1 The basic principle of variable frequency speed control is that the motor speed is proportional to the input frequency. By changing the frequency value of the three-phase power supply, the motor speed can be changed. From the speed equation of the asynchronous motor, we can derive: n=60f1×（1-s）/p……………………① f1——Power supply frequency of the stator winding of the motor p——Number of pole pairs of the rotating magnetic field S——Slip From the formula, it can be seen that changing the power supply frequency f1 of the asynchronous motor can change the motor speed n, thus achieving the purpose of speed control. However, the increase or decrease of f1 will affect other parameters of the asynchronous motor, such as the input voltage u1, input current I1, and magnetic flux Ф in the stator winding. The voltage of each phase of the three-phase asynchronous motor is: u1≈E1=4.44 f[sub]1[/sub]N[sub]1[/sub]K[sub]1[/sub]Ф……………② U[sub]1[/sub]——Storter phase voltage E[sub]1[/sub]——Storter phase electromotive force N[sub]1[/sub]——Total number of turns in stator phase winding K[sub]1[/sub]——Number of turns in fundamental winding Ф——Air system flux per pole [b][align=center]For more details, please click: Variable Frequency Speed ​​Control Principle and Precautions[/align][/b]