ElectroBOOM explains Brushed DC Motor Control
I found this video very helpful. If you’re looking to understand what a controller actually does, this will help. The MG uses a series wound DC motor, which has brushes like his demonstration motors. The primary difference is that the permanent magnets in his motors are replaced by another coil in mine. The field coil (stator) is wired in series with the armature (rotor) coils.