A novel overmodulation technique for space-vector PWM inverters

In this paper, a novel overmodulation technique for space-vector pulsewidth modulation (PWM) inverters is proposed. The overmodulation range is divided into two modes depending on the modulation index (MI). In mode I, the reference angles are derived from the Fourier series expansion of the reference voltage which corresponds to the MI. In mode II, the holding angles are also derived in the same way. The strategy, which is easier to understand graphically, produces a linear relationship between the output voltage and the MI up to six-step operation. The relationship between those angles and the MI can be written in lookup tables or, for real-time implementation, can be piecewise linearized. In addition, harmonic components and total harmonic distortion (THD) of the output voltage are analyzed. When the method is applied to the V/f control of an induction motor, a smooth operation during transition from the linear control range to the six-step mode is demonstrated through experimental results.