Slip ring (wound rotor) 3 phase motor, what VFD should I use?
1) Is just shorting it ok?


Just short the rings, there is no reason to add resistors. The best sorting method is to remove the brush assembly and braze a piece of copper between the rings. If you are concerned about rotor balance, use two pieces on opposite sides of the rotor.


  Do I need a dv/dt filter?


The winding insulation in an old wound-rotor motor may not be as good as modern motor insulation. Even with 10 meter leads, there could be high voltage peaks. However the motor is wound for 380 V and you are only using 220 V, so that will help. My guess is that a dv/dt filter is not necessary.


  4) I got told that I would need a 2,2kW VFD due to that it is a slip ring motor, is this true?


You should select the VFD based on the motor nameplate current. You may need extra current for high starting current re 6).


  5) Would a VFD with built-in brake unit (resistor) of 50% nominal current be good enough?


Unless the existing motor has electrical braking, don't worry about it. The gearbox and other friction will probably provide fast enough stopping.


  6) ...use sensorless vector instead of a V/F VFD?


Yes. A wound rotor motor provides high starting torque. A sensorlesss vector drive may help. However the VFD may have difficulty tuning for the wound rotor motor. It is difficult to predict how much torque you will be able to get out of it. You may need to oversize the VFD quite a bit.


  7) I don't know how I came up with this info, but somewhere I read I should ground the rotor in some special case. I was shocked and now i'm just becoming insecure. Am I true that this is weird? And you should just ground the stator.


Do not ground either winding. Sometimes there can be bearing failures due to high frequency current leaking through the bearings to ground. I don't think that is often a problem, but I don't know much about it.


  8)...linear curve instead of an S-curve..?


S curve acceleration provides a softer start, less of a jerk. You probably don't need S curve. You should not be using a very fast acceleration and will probably not have a problem with linear acceleration.