if you're adding an extra motor to help balance the phase it doesn't need connecting in any special way, connect it to the 3 phase anywhere you like and it'll work just fine
the bigger the motor the better - you need to be able to start it with your converter... but as there's nothing connected to it it doesn't take much to get it started
rule of thumb for an unloaded motor is it'll use about 20% of normal loaded current
(I just googled this to check, and I'm surprised the figure is so high, guess creating magnetic fields and flipping them around is more lossey than I thought)
so a 4hp motor should cost less than 10p/hour to leave it running doing nothing
the bigger the motor the better - you need to be able to start it with your converter... but as there's nothing connected to it it doesn't take much to get it started
rule of thumb for an unloaded motor is it'll use about 20% of normal loaded current
(I just googled this to check, and I'm surprised the figure is so high, guess creating magnetic fields and flipping them around is more lossey than I thought)
so a 4hp motor should cost less than 10p/hour to leave it running doing nothing