CURRENT SOURCE INVERTER-FED AC MACHINE DRIVE SYSTEM

Authors:-swetha k; ch silpa

Abstract:-Current source inverter (CSI) is an attractive solution in high-power drives. The conventional gate turn-off thyristor (GTO) based CSI-fed ac machine drives suffer from drawbacks such as low-frequency torque pulsation, harmonic heating, and unstable operation at low-speed ranges. These drawbacks can be overcome by connecting a current-controlled voltage source inverter (VSI) across the motor terminal replacing the bulky ac capacitors. The VSI provides the harmonic currents, which results in sinusoidal motor voltage and current even with the CSI switching at fundamental frequency. This paper presents new five-level current-source inverters (CSIs). A current source inverter (CSI) requires a capacitor filter for the commutation of switching device as well as for attenuating switching harmonics. Hence, the CSI-fed ac machine has a second-order
system in the continuous time domain. The proposed strategy features the following: 1) an on-line operated PWM inverter, using instantaneous output capacitor voltage control based on space-vector modulation and 2) an additional inverter modulation index control loop, ensuring constant inverter modulation index and minimum dc-link current operation. The resulting additional advantages include the following: 1) fixed and reduced motor voltage distortion; 2) minimized dc-bus inductor losses; 3) minimized switch conduction losses; and 4) elimination of motor circuit resonances. Experimental results based on a digital signal processor implementation are given.