Hi Jazzcnc and Muzzer, Many thanks for your responses. Apologies as it looks like I have fallen into that trap of not providing enough information in my first post. The motor is a two speed motor and rated at 400v only so I was going to step up the voltage first then put 415v single phase into the 3 phase to 3 phase vfd. This is the configuration others have used but in their posts don't mention the vfd used. I know I cannot switch speeds when the motor is turning so this would need to be prevented within the circuit design. There was a thread I saw recently about fooling the vfd by creating the other phase using a capacitor but it didn't go in to sizing the capacitor or whether it had been successful. I would prefer not to go down the route of changing the motor if I can avoid it. Thanks again for your advice.