Efficiency of a voltage sourced inverter with controllable intermediate DC-voltage

The efficiency of a low cost three-phase PWM (Pulse-Width-Modulated) VSI (Voltage Source Inverter) is evaluated in this paper based on the fact that a controllable DC-link is available. The study was conducted considering that a VSI should supply a three-phase low-voltage induction motor, with rated power of 500 kW supplied with a industrial voltage of 690 V. Results presented in this paper are based on computer simulations and analytical expressions for calculation of losses. The goal is to improve the overall efficiency on the inverter part of the ASD (Adjustable-Speed-Drive), especially at low speed region without harming the current and torque quality, which should be kept within
acceptable values.
The circuit topology is a conventional two level IGBT-based three-phase voltage source inverter. A two-level natural sampling PWM with sine-wave modulation was used in order to generate the control pulses for the IGBT’s and hence allowing the output voltage level to be varied. Scalar control of the inverter output voltage was implemented in order to allow the maximum air gap flux in the motor.
Using an ideal (without ripple) and controllable intermediate DC-link voltage stage, several computer simulations are accomplished and waveforms are used to evaluate the inverter performance. The objective of the so-called open-loop simulations is to verify how the circuit behaves and to have a simple indication on trend of the results.
The final results clearly show that by lowering the DC-link voltage, the switching losses of the inverter can be reduced, especially at low speeds. Also torque quality and total harmonic distortion of the output current are slightly improved.