It took me quite a while to build a working wire drive that can provide constant speed, (almost) constant tension and a small footprint, but I think this design can be used for further construction.
The new wire is placed on the front spool which is braked by a Nema 23 stepper with shorted windings. The braking and with it the tension increases with the wire speed and should be otherwise quite constant. If needed I could also replace it with a mechanical brake, but using the shorted motor is likely the most simple solution.
From the spool the wire is guided over a load cell which measures the tension before the wire drive for showing its value in gramms on a 7 segment display.
From there it is guided through the water bath (not shown in the video) and to a stainless steel roll which is connected to the EDM circuit.
From there it is guided to the wire drive which is made of two shafts with three V rollers placed on each which are both driven with a GT2 belt on the backside. The wire is wound about four turns onto it to increase the contact surface and prevent it from slipping. The wire drive pulls the wire from the new wire spool with a Nema 23 and determines the wire speed.
From there the wire is guided over another load cell that measures the tension after the wire drive to show it on a 7 segment display and also to detect when the tension has lowered to a setpoint where the old wire spool's Nema 23 stepper has to wind up the old wire to complete the wire drive.
Next up is building a X and Z axis to move the workpiece against the wire.