Given a 3phase inverter let's say 400vac rms line to line (means 230VAC rms each line referred to neutral of generator).
After the 6 diodes converter stage there is a about 550VDC (approx -275VDC to +275VDC) after the condenser which straighten the humps. This is because the non-rms phases range from -325 to +325V.
Now, what does the process to reconstruct any of the three output phases produces? We have an about, negative to positive, 550VDC. It seems to me that you could recreate at any different frequency :
1) 3 phases with a waveform peak to peak of 550V, -275VDC to +275VDC, but this is about 195VAC rms
2) 3 phases with a waveform peak to peak of 1100V, -550DC to +550VDC, but this is about 400VAC rms
Than of course varying the pulse duration with the igbt you can obtain any voltage from 0 to the maximum. But what is it the maximum, that of case 1), case 2) or, in case I was totally wrong, other?

 

Generally the maximum RMS voltage you get out of an inverter is the same as you put in. From your example, case 2) is correct but a complicated way to say: 400V RMS in, max 400V RMS out

 

Your out put can't be greater than what you have in the DC link !
The PWM after the DC Link decides the current flows through the load in AC drives & not exactly the voltages.
Few manufacturers add smoothing filters, so that you see them as a smooth sine wave instead of the rapid on & off pulses.

 

Ok but it seems to me that now 400VAC rms is now each line and not line to line and this because, in reality, what is it totally unclear to me is the voltage reference for the output. This happens also because when I watch at tutarials or university handouts published on the net it seems to me that the alternate voltage in output is more or less -550 to +550 taking the 0 as the potential of the DCBUS negative.

 

Your out put can't be greater than what you have in the DC link !

yes of course. I was saying in another post that looking at univ. handouts I frequently saw the alternate square output voltage doing -Vdc to Vdc (rectangular not sine but anyway alternating).

If this i strue I have an alternate ranging from -550 to 550. What is it now this? A line voltage or a line to line voltage? If it was a line voltage it is much bigger than the incoming line. The point is that I cant understand what is the reference for such voltage. If you say that is a line to line then I see no more problem. But if it really is I cannot understand why.

The PWM after the DC Link decides the current flows through the load in AC drives & not exactly the voltages.
Few manufacturers add smoothing filters, so that you see them as a smooth sine wave instead of the rapid on & off pulses.

This sound strange to me because varying the speed (frequency) inverter absolutely needs to change the voltage (the less the freqyuency , the less the voltage )

 

The square output voltage is an artificial sign wave, to create the required RMS voltage. Generally if you are talking about a 3 phase system you would be referring to line to line voltage, there is not a single reference point for your output voltage, measure line to line RMS voltages. This is what happens with inverters in the real world but I am not sure if your handouts are trying to analyse real world applications or the theory of what happens inside an inverter.

 

It's me that I can't see why it is a line to line. Doesn't matter, my problem

 

The Speed is not only dependent up on Voltage but also frequency. The IGBT knows when to do that change over !
The drives Works on V/f (Don't get confused with sensor less vector control). The DC bus peak will be root 2 times AC rms.
If you oscilloscope across the out put you will see the same.

You may refer to Eurotherm Drives 605C manual which is easy to follow. (You can get them in google as well)

Since you did not share your design, I assumed that you are using a H Bridge.
Since you mentioned the voltage is switching between +VE & -VE DC, you are having a split capacitor design, instead of full H Bridge.

The below link may give you better idea
https://www.theengineeringprojects.com/2012/11/pure-sine-wave-inter-design-with-code.html

 

Ok thank you I'm reading them