Stability Analysis of Induction Motor Driven by Voltage Source Inverter without DC Link Components

The voltage source inverter without DC link components does not have electrolytic capacitor and reactor (or resistor). The main circuit consists of inverter, rectifier and AC filter. The inverter is made from power MOSFET modules to use PWM control. The rectifier is the three phase bridge connections of 120° duration, and each diode has the antiparallel connected GTO for the reverse current flow path. The AC filter is indispensable in point of the switchings and the waveform of the source currents. The 1.5 kW three phase induction motor is driven by this circuit. This paper describes the analytical method an the calculated results on the stability of this system. The DC link voltage of the inverter takes immediately the influence of the AC source voltages. The stability analysis is performed by means of state variable method in consideration of the AC source voltages. In the equation of the small displacement on the state variable from steady state, the stability is estimated by the dominant root in eigenvalues analysis. The stable and unstable characteristics with constant Vlj control of induction motor are similar to the conventional voltage source inverter. An experimental result and the calculation of the step response are shown, and it is obtained that the analysis is reasonable.