Flux trajectory forming PWM modulation. A new approach to harmonic elimination in low switching frequency voltage inverters

摘要

It is shown in the paper that classical harmonic cancelling synchronous pulse-width modulation (where variation of the inverter output voltage is done in such a way that the phase switching angles are varied according to offline calculated curves) leads to a great increase of motor current active (torque) and reactive (flux) distortion. This is valid for angles calculated by Patel and Hoft (1974) and also for alternative angles calculated by Zach and Erdl (1985). To achieve the same harmonic elimination effect the authors propose to utilize a set of angles that assures a maximal value of the output voltage fundamental component and to use zero vectors for reducing the inverter output voltage. Zero vectors are distributed synchronously along the trajectory in the simplest possible way. The strategy is called "flux trajectory forming modulation" since, for a fixed number of harmonics cancelled, the shape of the stator flux trajectory does not change. The proposed method offers two basic advantages: simplicity (there is no need to use generators of nonlinear dependencies describing variation of phase voltage angles as a function of the output voltage first harmonic); and low motor current active and reactive distortion.