Stall Torque
The highest torque a motor can produce, right up to the point where the rotor stops, and the value at the zero-speed end of the speed-torque curve. At this point the motor produces no mechanical work and turns all the electrical power it draws into heat. It is a short-duration limit value, not a continuous rating.
Since torque in a brushed DC motor is proportional to current, the stall torque equals the stall current multiplied by the torque constant. The rated torque, on the other hand, is usually a small fraction of the stall torque; the continuous operating point is chosen in the middle region of the curve.
In practice, stall torque comes up in two ways:
- Sizing limit: The worst-case torque required by the load must stay below the stall torque, otherwise the motor cannot start the motion at all.
- Mechanical strength: While current-sensing stop is working, the mechanism has to withstand the stall force for the duration of the detection. The shaft, gears and mounting brackets must be chosen for that force.
Thermally, stall is the most demanding condition: the motor draws the highest current while not turning to cool itself. That is why the current threshold is set so as to allow the system to stay in stall for a matter of seconds; a mechanism left in stall for minutes will burn the winding insulation.
Where this term is used: Locked rotor and thermal protection