so 1 ohm would be the R in the time constant calculation. so if your measurement finds that the inductance is 1mH for instance, then that time constant would be 0.001S. one tenth of that would be 0.0001S, so the PWM frequency would be 1/0.0001 or 10khz. lower frequencies will work, but your losses will be higher. some of those losses will be as heat, some of those losses will be in the form of audio emitted from the motor. just out of curiousity, what is the manufacturer and model/part number of this motor. the inductance may be in the data sheet for the motor. that would save a lot of time and effort, although i admit letting you learn how to measure it yourself would teach you a lot. i keep a bunch of caps and inductors set aside for measuring unknowns with a sine wave generator and oscope.