Yes the rotor looks like that. I have shorted the rotor cables together at the moment. So it will act a bit like an induction motor. Maybe.
It's only a guess, but I doubt with the number of turns in the rotor windings and their resistance that there would be enough induced current to work well as an induction rotor. For that possibility, you need someone who has more experience in this area than I have, or at least has put more thought into it.
If i dont short the dc rotor cables and don't put any voltage on them it acts a bit like a transformer and ac voltage is generated and there is no rotation.
Right, because that's all it is... the rotor windings don't do anything useful, and the stationary rotor laminations are not effective enough to make it act as a synchronous reluctance motor.
In theory i just need to provide enough current to magnitise the rotor?
Yes, with AC power to the stator, and slip rings connecting the rotor winding, I assume that you need to provide a DC current for the rotor field. This will be like a shunt-wired stator winding in a brushed DC motor, which is typically called a "separately excited" or "SepEx" configuration in this forum. While there are controllers for SepEx motors - with a separate output for the field winding - they are for DC motors, so they would not be useful in this case.
I expect that the Renault controller provides only as much current to the rotor as is needed to maintain synchronous operation (which means increasing rotor current when the stator current is increased), and may reduce rotor current at high speed (to reduce the stator voltage needed to overcome back EMF).
Perhaps you could use a small DC controller, intended for a much smaller series-wound brushed DC motor, programmed for a suitable constant current, or controlling current in response to the same input (typically treated as a torque request) as the main controller?
Also I was hoping that the nutral cable could be left out insulated and i could just use the 3 phase cables.
Really just guessing here, but if the variable speed drive can handle a wye connection without using the neutral, then I suppose that should work. I have heard that it is common to power wye-wired systems with the neutral isolated, although perhaps not with motors. Again, we need someone with more relevant experience here.