how could i modify an AC motor to run on DC (main purpose is to control the direction)
Only universal motor can run on both AC and DC. If you want to modify an AC motor to run on DC, then at first you need to know constructional details and working principle of DC motor. You can read the working of a DC motor at the link given in the source.
It depends on the kind of ac motor you have. If you have a universal motor, then you can use it with both dc and ac, now if you have an induction monophasic motor, then you should turn your dc to ac by using an inverter, and if you have a triphasic motor, then i thing it wouldn't be worth the effort to turn the dc to triphasic, so get another motor. Besides, if you have a triphasic motor, just change two of the three wires you connect it to. Now, if you want to turn an ac motor to dc motor, then depending on the kind of motor that's the modification. For example, if you have an induction motor, these have no brusher, so what you would have to do is change the wiring of the rotor so you get a rotor that gets its magnetic field from an outern source and insert brushes and a conmutator so it becomes a dc motor. Hope this helps.
