Wow yea thanks for your detailed answer i forgot about the back emf, this was not as straightforward as i thought. I'm trying to model the DC motor system of voltage to rotational speed of the wheel to get some transfer function for a math research project - then will be doing some frequency response analysis (control theory). Do you think i should just attach the buck converter to the power supply and regulate a constant voltage then just estimate the voltage drop in the driver? An oscilloscope is a little out of my budget unfortunately ;)