So everybody, thank you very very much for all your kind help and the usefull opinions!
Finally we managed to start the compressor
(after 3 days...
, with the existing setup, just a little change, but no cost at all...
So here is how we did it, just for your interest if somone will have a similar problem:
We disconnected the centrifugal switch between the two capacitors, and replaced it with a push button. So now I have to contact the capacitors with the push button for 3 seconds and the compressor is running.
Looks like the original centrifugal switch connected the two capacitors (start and run capacitor) just for 2 seconds, which was enough to start the compressor from shore power
, but wasn't enough to start it from the generator
. So now I connect them for 3 seconds, which is enough for the start.
My only doubt now as if this way I will damage the compressor or the generator
Thank you again!