Questions about the engine

First, you need to accurately determine with a multimeter whether it is neutral or ground. The neutral will ring through all three phase terminals and show the same, fairly low resistances (units to tens of ohms). You should not make phone calls with the engine housing. If it is grounding, there will be no phone call with the phases, and with the case there will be almost zero resistance. In no case should the neutral be connected anywhere, the grounding should be grounded (well, or left unconnected, if grounded anywhere).

Most likely, you have a two- or multi-speed motor, the frequency of rotation of which when powered from a 50Hz network can be changed in steps by switching the windings. Standard connection diagrams for different speeds for this motor can be found in the documentation for the motor, on the back of the cover or on the Internet. The frequency converter can operate with any of these configurations, but the rated motor speeds, voltages and currents will be different. Determine what speed range you need and which network you plan to use (single-phase or three-phase). Depending on this, connect the conclusions of the windings in the motor terminal box and output 3 wires to the frequency converter. The frequency converter itself is selected as usual, based on the nominal voltage and current of the output motor.