Years ago I had a controller for one of these wound field motors that had no active controls in it. The stator had a fixed DC current generated by a bridge rectifier connected to the full mains voltage. I don't recall if there was a resistor or if the winding had an inherently high resistance - obviously you can't use its inductance when dealing with DC. The rotor current was controlled by a rotary knob. This turned either a variac or a rheostat, then the output was connected to the rotor via another bridge rectifier. The speed regulation of these machines is reasonably good in open loop, so it was adequate for the application which was moving a bogey on a large PTFE pipe convoluting machine.

It was a very simple controller that worked fine until finally the insulation on the ancient motor windings broke down and the torque vanished.