Variable-frequency Speed Control System With Reduced DC-link Capacitence

The power factor of the traditional variable frequency air conditioner is very low and thus it cause the serious harmonica pollution to power grid. In the same time due to the volume and price of DC-link electric capcacitor, the air conditional can not be light like other inverter in the future. Variable frequency and speed control system based on three phase PWM rectifier is been researched deeply in this paper and the control performance is been improved in both converter and inverter side.The mathmatical mode of variable-frequency speed control system based on PWM rectifier is been analysized. The feasiblity of reduced-capcitance is been proved and the reason of capacitance selection is been given. The system can’t be stable due to the reduced-capcacitance, therefore, a reduced capacitance strategy based on power feedfoward is been used which make sure the system can be stable with small-capacitance.The disadvantages of power feedforward strategy are the voltage ripple and the stablity. Thus a new control strategy is been proposed which based on virtual resistor. This strategy reduces DC-link capacitance further by increasing virtual resistor in dq0 axis. The simulation result of MATLAB/Simulink shows that compare to the old strategy, the new one can supress the voltage ripple and make sure the system work well in less DC-link capcitance.In order to guarantee the well performance of system with small capcitance, the basic thesis of sliding mode is been introduced and a sensorless speed control strategy based on sliding mode observer is been designed.Stator resistance identification is been used so that the compressor can be fit with the working environment.The simulation results proved the good disturbance rejection of the proposed strategy.Finally, experimental platform of variable-frequency speed control system based on small-capacitance is been designed. The early and simulation experiment of the proposed reduced-capcitance strategy is been done.The experiment results show that the sensorless speed control strategy is reliable.