Working with space vectors -

Working with space vectors

Consider the task of driving a three-phase motor along the lines of the following sketch:


Figure 1: Three phase motor drive using pulse width modulation.

Six power MOSFETs are driven with pulse width modulation (PWM) to provide effectively sinusoidal motor winding excitation. A closer look at the sinusoidal waves versus the PWM is as follows:


Figure 2: Expanded view of PWM waveform.

This works very nicely, but there is a further refinement that can be employed to reduce the peak drive amplitudes demanded of the MOSFETs for delivery of sinusoidal drive to the windings while using some given rail voltage.

Looking at the three drives from a vector standpoint, we can examine the issue of dynamic range versus rail voltage. If there is a vector in collision with the rail voltage limit as in middle level of the following sketch, that is bad news, but we can mitigate that if we dynamically move the vectors up and down as shown in the bottom level of the sketch.


Figure 3: Rail voltage limit issue and space vector compensation.

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