tailieunhanh - High Performance Driver P1

Tham khảo tài liệu 'high performance driver p1', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | HIGH PERFORMANCE DRIVES 29 . Voltage source inverter VSI Variable voltage variable frequency operation of induction machines is realised utilising autonomous inverters in conjunction with a rectifier and a DC link circuit. The voltage source inverter VSI is the most frequently applied power supply source for speed control of induction motors. It can be operated in six-step mode or in PWM mode. Six-step operation will be considered first. Three-phase VSI contains three inverter legs. Input voltage for a three-phase VSI is provided by a three-phase or single-phase bridge rectifier with capacitor placed at the output. The capacitor provides smoothing of the DC voltage and for sufficiently large capacitance DC voltage at the rectifier output approaches a constant value. It will therefore be assumed that inverter input voltage is constant in all the subsequent analysis. Power circuit of a voltage source inverter is shown in Fig. . As the inverter itself controls only the frequency of the output voltage when operated with switching frequency equal to output fundamental frequency six-step mode a controllable rectifier must be used in order to provide control of the output voltage magnitude output voltage magnitude is proportional to the input DC voltage . Each switch in the inverter circuit is composed of two back-to-back connected semiconductor devices. One of these two is a controllable switch while the other one is a diode. The three inverter legs are controlled in such a way that leg voltages constitute three-phase system of square-wave voltages. This means that assuming that upper transistor in leg A is fired at time instant zero firing of upper transistor in leg B will take place after 120 degrees while firing of the upper transistor in leg C will be delayed for another 120 degrees. The conduction of each of the six semiconductor switches is again 180 degrees so that at any time three out of six switches are on and the remaining three switches are off. The