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The U/f-constant (Voltage/Frequency equals to constants) control strategy of three-phase AC motor was studied. SPWM (Sinusoidal Pulse Wave Modulation) algorithm was employed to design the open-loop speed control system of three-phase AC motor and implement through programming on DSP (Digital Signal Processor), TMS320F2812. The test results showed that the output signal of the control system is characteristic of sine wave with small harmonic wave, the rotor speed agreed well with the theoretical value. The purpose of precise speed control in the system can be achieved in range of 20-70 Hz.