dc42:
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.
Sorry for the late reply, I was trying to ask a friend of mine for advice but he hasn't been able to get back to me. Looks like I'll need to go with active limiting as the resistor route isn't going to work.
I've looked up Active Current Limiting on Google, though there seem to be a few different things of varying complexity. Could you point me in the right direction of what I need to do for active limiting?
Thanks again.