There's a couple of Application Bulletins on the site of your motor supplier that help explain (essentially what everyone else above has mentioned - the current and back EMF can damage your arduino):
See driving motors with PWM section here:
There's a couple of Application Bulletins on the site of your motor supplier that help explain (essentially what everyone else above has mentioned - the current and back EMF can damage your arduino):
See driving motors with PWM section here: