Size the series (serial) resistor so that it drops the voltage to the rated motor voltage at the rated current (usually locked rotor).
Thus, if your motor was rated at 10v and you apply 20v and the peak current of the motor is 10 amps, using E=IR,
10=10*R
or
R=1 ohm
Watts=I*I*R
or
Watts(resistor)=10*10*1
Resistor wattage= 100 watts
Be sure to heat sink it as this is a LOT of power to dissipate |