Rotary Converter

[eddie49]

Yes, if you join the "230" terminal of the primary to the "0" terminal of the secondary, and feed 230v AC mains to the primary, you will get 420v out across the "0" terminal of the primary to the "190" terminal of the secondary.
[Note that when a transformer is wired as an autotransformer, pri-to-sec isolation is lost, so the output will be live.]

 

[Martin-Sy]

Yes, if you join the "230" terminal of the primary to the "0" terminal of the secondary, and feed 230v AC mains to the primary, you will get 420v out across the "0" terminal of the primary to the "190" terminal of the secondary.
[Note that when a transformer is wired as an autotransformer, pri-to-sec isolation is lost, so the output will be live.]

Yes, you can do as @eddie49 has said and will get 420V out .... but it's better to connect the 180V to the primary 0V and take the 420 from the primary 230V connection and the 0V secondary connection.
The diagram you showed in post 7 had this arrangement. Why ... well both give the same output voltage but in the former case the motor windings will be at a higher voltage to earth which stresses the insulation more.
Remember that when the motor is connected to a real mains 3 phase 415V supply each of the phases is only 240V above earth.

Also note that if you go for 420V the capacitors need to be rated for that (or higher) voltage.