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Posted: August 11, 2011 07:03 pm
Member No.: 35,544
Joined: August 11, 2011
I have a doubt relating common mode created by a 3phase SPWM inverter.
In all the references, articles and waveforms that I have seen always the output common mode is shown as a "castle-like" waveform with 0Vdc mean value.
Reviewing from Mohan book, I found in figure 8-22 and in some discussion in same chapter that:
- phase-neutral SPWM voltages have a DC mean value equal for each phase.
- line voltages (actually the ones used) havenīt DC component, as it is cancelled out.
From that I get that if phase-neutrals have all the same DC mean value (that depends in duty cycle or "modulation index" of SPWM) then neutral to reference (aka "common mode") must have a DC component as it is not cancelled out (think in neutral of a star-connected virtual load).
Only case that I see where there isnīt DC level is if reference is midpoint of DC bus and modulation index is 100% (so phase-neutral is half positive half negative and effective duty cycle 50%).
I have these cases simulated with LTC spice and it works like this (at least in my circuit) BUT I am a little unsure as I havenīt seen anywhere a reference to the "DC component of common mode voltage" in inverters (and it is a very hot topic).
This DC component I see that is as much as the DClink voltage, and donīt know if can have any bad effect in loads (i.e. motors).
Any idea or discussion please will be welcomed
green is phase1-neutral voltage, blue is line12 voltage, red common mode to DClink-, modulation index 60%, initially DC bus is charged
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