Optimal modulation strategy for active front-end inverter feeding induction motors

Torque ripple in inverter fed induction motors is a critical problem if high switching frequency cannot be achieved because of the drive power rating or when switching losses must be kept low. In that case, the torque ripple minimisation should be one of the goals of the control strategies. The paper presents a PWM technique that can be used to minimise the steady state torque ripple of variable speed induction motor drives when high dynamic is not required, i.e. scalar control. The torque ripple is expressed as function of the de bus voltage and of the notches introduced in the output voltage waveform of a VSI and it is minimised by finding out the optimum dc voltage value and modulation pattern. The control of the dc voltage does not increase the cost of the whole drive when a PWM ac/dc converter is already used to reduce the current harmonies and to improve the power factor. The proposed PWM strategy is compared with the space vector modulation and experimental results are reported and examined.