bori02082014:
After researching more about DC motor, if you suddenly stop the motor, there will be back EMF that can hurt the power source.
Ah, no, that's wrong - suddenly stopping the motor drops the back EMF to zero, leading to a high current
demand (stall current levels can be 10 times maximum continuous rating).
Back-driving the motor so it spins faster than the power source can spin it unloaded will mean there's
too much back EMF for the power source.
back EMF is basically proportional to speed of rotation, the constant is called the motor constant.