If you use a single 0.4 ohm resistor (or 0.39 ohm, which is the nearest standard value), then it will indeed dissipate around 14W when the motor stalls. If you use three 1.2 ohm resistors in parallel, then each one only takes one third of the 10A current, so they dissipate just under 5W each.
My mistake, you're quite right. The total dissipation in the 0.4 ohm resistor will be 4v * 10A = 40W and this gets split between the three 1.2 ohm resistors. The power dissipation in the motor will also be around 40W in the stalled condition.This demonstrates the advantage of active current limiting. The principle is to sense the current in the motor and adjust the PWM fast enough to keep the current below a set value, rather than use series resistors.
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