Probably a stupid thought - VFDs, Transformer & switched load

[m_c]

I already have an RPC, it's just that I'm considering an upgrade, and I'm curious as to how certain companies are creating digital converters with what appears to be a bog standard VFD.

[Jonathan]

Looks like you're question has been answered - it's not going to work, at least not directly.
A 'vector drive' (i.e. one that uses the field oriented control algorithm) senses a number of the phase currents and uses these to infer the position of rotor, it's field and the stator field. The controller then frequently modifies the currents to ensure the stator and rotor fields are always perpendicular, as this gets the most torque. It's hard to sense the motor phase currents accurately with a transformer in the way... There are plenty of other reasons not to do this, e.g. the transformer adds another time constant to the system.

I'm curious as to how certain companies are creating digital converters with what appears to be a bog standard VFD.

The basic power electronic circuit to convert from a fixed frequency supply to a single one is just a 3-phase bridge generally made from IGBTs. This circuit is used in all sorts of appliances, what differs is how it's controlled, so take a 'bog-standard' VFD, remove the control circuit and make your own, then (within reason) you can put whatever you want on the output. That's why for instance you can now get motor drives which can control induction motors and synchronous motors - the basic circuit inside them is the same, it's the control that differs.