Abstract: Through the analysis of the working principle of asynchronous motor's soft-starter, a hardware expansion of the soft-starter was made based on single-chip microcomputer Intel-89C51 to realize the set up and alteration of the motor operation parameters. By means of a close-circuit feedback system composed of a power factor measuring circuit and a soft-starter's thyristor voltage modulator, the single-chip microcomputer can make corresponding adjustment of the output voltage of the soft-starter according to the load change, enabling the motor to keep running at the set power factor so as to realize a constant power factor control of AC asynchronous motor and gain the goal of energy-efficient running.

Key words: Asynchronous motor, Soft start, Power factor, Energy-effient operation