3-Phase Sine PWM Amp Voltage Range


Thread Starter



suppose you have sinusoidal 3-phase analog servo amplifier that receives 3 (or 2) sinusoidal (phase current) commands from digital controller.

If these 3 current commands are sent directly to be scanned by PWM triangular wave for PWM pulses generation, it seems that DC bus voltage is essentially underutilized due to premature voltage saturation - about 15% of clean sinusoidal voltage is lost meaning torque-speed area cut, additional harmonic loss etc.

Has somebody thought over this phenomenon? Any remedies?


Alex Ruderman


if I got it, you are talking about the following. Velocity (position) loop is closed within digital controller. Its output is 3(2) analog sinusoidal current commands sent to (analog) PWM servo amplifier that closes current loop.

Based on current command and current feedback, servo amp generates 3 sinusoidal phase voltage commands.

You right - if these 3 sinusoidal voltage commands are scanned by triangular wave to produce PWM pulses then sine voltage dynamic range loss is

(2/SQRT(3)-1)*100%=15.5% .

For example, if your DC bus voltage is 100V then you ideally utilize about 86.6V (loose 13.4V).

This phenomenon that you fairly call "premature voltage saturation" and that causes high speed torque loss, additional motor heating etc. was first understood for open loop inverters in early 1990's - there it was fixed by a 3rd harmonic (stored in a look-up table) injection technique to make voltage command "flatter".

The "remedy" for servo amp is a little bit more tricky to be explained here because contrary to open loop inverter you don't have in advance clearly defined voltage fundamental (3rd) harmonic.

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