In the steady-state case, it's a simple linear interpolation:
\$I_S\$ is the stall current.
\$\omega_{nl}\$ is the no-load speed.
\$I_{nl}\$ is the no-load current.
For a motor loaded such that it spins with a speed \$\omega_L\$, the current draw is:
$$I_L = (I_S- I_{nl}) (1-\frac{\omega_L}{\omega_{nl}}) + I_{nl} $$
For example, you have a motor with a 2.2A
stall current, 0.2A
no-load current, and let's assume the no-load speed is 1000 rpm
. Technically \$\omega\$ usually refers to rads/s, but here the units cancel out; the only important factor is what the relative speed of the motor compared to the no-load speed is.
At a 250 rpm load speed, the current draw is:
$$I_L = (2.2A - 0.2A) (1-\frac{250}{1000}) + 0.2A = 1.7A$$
Keep in mind that even though the steady-state current of the motor is 1.7A
, the peak current could be much more. Starting up the motor would draw close to the stall current of 2.2A, decreasing until the motor reaches steady state. Depending on how quickly the motor is able to get to steady state, this may be significant or not.
Looking at the datasheet, while it doesn't lie, it is definitely on the edge of misleading.
Note that the "max power" stated occurs at almost exactly half the unloaded RPM and half the stall current. This is indeed the "max power" point for such a motor, but the datasheet fails to mention that it is also the nominal 50% efficiency point, thus dissipating 12V*68A-337W = 479W in that tiny motor - destroying it, probably in minutes.
(Ideally, exactly half the power would be delivered, about 400W shaft and 400W heat, but the motor isn't ideal).
The motor is probably suitable for 100-150W continuous output and 200-250W short term.
So practically you must operate the motor at the upper end of the speed range, and if the speed falls below (say) 70% of the unloaded speed (or the current rises to 30% of the stall current) then - unless this is strictly temporary, like starting a heavy load or hitting a chilled spot while machining a cast iron surface, you need to cut the current and protect the motor.
Then the question of which side of the torque speed curve doesn't apply - unless the protection has tripped, you should be on the high speed side.
You can get circuit breakers that will allow short-term overcurrent. These are "motor rated" or Class C breakers for the AC motors used in most machine tools. I don't know of anything suitable for 12V DC though. I'd be looking for a 12V DC supply that can be set to trip if its output exceeds 40A for more than a couple of seconds. And as Olin says, if you want to monitor it yourself, measuring the current is definitely the way to go.
Best Answer
There are four major (electrical) parameters that are used by engineers when selecting a brushed dc motor: The torque constant (Nm/A), the "back-emf" constant (V/(rad/s)), the armature resistance (ohms), and the armature inductance (H). Normally the inductance is low enough that it is ignored. By some fluke of units it turns out that in the SI system the newton-meter/amp is numerically equal to the "back-emf" which is simply the volts/(radian/second) of rotation. When a motor spins it generates a voltage which is called "back-emf". This happens even when you are using the motor to generate torque by applying a voltage. The result is the faster it turns, the less current flows until an equilibrium is reached. More here:
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed§ion=SystemModeling
The result is you can estimate the voltage the motor will produce at a given rotational velocity (Volt/(radian/second)). However, the actual output of the motor is going to vary a ridiculous amount so it really can't be depended on. Your best bet is to find an appropriate dc-dc converter that takes the nominal voltage of the motor as the input and outputs your desired voltage.
http://www.cui.com/parametric-search/power/dc-dc-converters
Also don't forget about the armature resistance that will limit the total power available.