New stator voltage controller for high speed induction machines fed by current-source inverters

Feeding modern high speed induction machines directly by voltage-source inverters leads to intolerable disadvantages such as stator winding stress and rotor overheating. Using current-source inverters avoids these disadvantages but then weakly damped resonant circuits have to be controlled within a wide frequency range. The basic stability problems of known modulation strategies for current-source inverters based on pulse-width modulation render the control of these resonant circuits even more difficult. In this paper a new stator voltage controller based on a time-discrete modulation method is presented, which solves this basic problem, as the resulting control loop is fundamentally stable. Hence the filter capacitors can be dimensioned almost without limitations, just according to the requirements of the drive. Because of its fundamental stability the presented stator voltage controller simplifies the design and dimensioning of the superimposed stator current and speed controller considerably, reducing the expenditure to control such a complex high speed drive system. Measurements taken on a model plant with an output power of 35 kW and a nominal speed of 30000 min^-1 prove the feasibility of the presented stator voltage control loop: both in steady-state operation and during transients the stator voltages and stator currents run almost sinusoidally.