Abstract : Early general-purpose frequency converters mostly used open-loop constant voltage ratio control, resulting in unstable control curves. This paper focuses on analyzing and comparing the currently popular vector control and direct torque control theories, and elucidates the characteristics of vector control and direct torque control. Keywords : AC variable frequency speed regulation; vector control; direct torque control. Early general-purpose frequency converters mostly used open-loop constant voltage ratio (V/F two constants) control, which has the advantages of simple control structure and low cost, but the disadvantages of low system performance, making it more suitable for applications such as fans and pumps. Specifically, its control curve changes with the load, resulting in slow torque response, low torque utilization, and performance degradation and poor stability at low speeds due to stator resistance and inverter dead-zone effect. 1 Development of AC Variable Frequency Control Methods The transformation of the U/F control system of frequency converters has mainly gone through three stages. The first stage: a) Voltage space vector of basic magnetic flux trajectory (or magnetic flux trajectory method). This method takes the overall generation effect of three-phase waveforms as a premise and aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, generating two-phase modulation waveforms in one step. This method is called voltage space vector control; b) Frequency compensation control is introduced to eliminate steady-state errors in speed control; c) Based on the steady-state model of the motor, the phase current is reconstructed using DC current signals, thereby estimating the flux linkage amplitude, and feedback control is used to eliminate the influence of stator resistance on performance at low speeds; d) Closed-loop control of output voltage and current is implemented to improve voltage control accuracy and stability under dynamic loads, while also improving the current waveform to some extent. [b][align=center]For details, please click: AC Variable Frequency Speed ​​Control System Control Methods[/align][/b]