Carrier Based Pulse Modulation for Symmetrical Drive

Modulating the output of a voltage source inverter (VSI) inherently causes current distortions on the load side due to the switching behavior of the inverter. This article describes a method to reduce these distortions for a drive setup with six phases. The setup consists of a six-phase machine connected to two three-leg, two-level VSIs. The phases within the machine are symmetrically arranged and spatially shifted by 60. The phases are grouped so that two conventional three-phase sets are formed, each fed by one inverter. The output of each inverter leg is modulated by comparing a triangle carrier waveform with the desired fundamental wave. Arranged in this way, the drive setup offers an additional degree of freedom: the phase shift of the two carrier waveforms.

It is shown that, depending on the machine parameters, there exists an optimal angle which reduces the harmonic losses of the drive for a certain amplitude of the fundamental wave. The resulting overall current distortion can even be lower compared to a conventional three-phase drive with a two- or three-level inverter topology. For three different modulation techniques the optimal phase shift angles are derived in an exact form: naturally, symmetrical regular and asymmetrical regular sampled pulse width modulation (PWM). An approximation with reasonable accuracy is achieved out of the exact solution. Simulations and experimental tests validate the theoretical considerations.