What is stall (locked-rotor) current, and why is it critical in driver selection?
Stall current is the current a motor draws while the rotor is mechanically held still. Because no back-EMF is produced, the current is limited only by the winding resistance and rises to typically 5-8 times the rated value. The driver must handle this pulse within its peak window, and the AntiSwitch threshold must be set between this value and the running current.
When the rotor stops, the back-EMF produced by the motor goes to zero. Only the winding resistance remains; the current shoots to a very high value determined by I = V / R. Physically the situation at start-up is the same, the only difference being that it is brief.
For driver selection it has two consequences:
- Thermal: Because the loss grows with the square of the current, a stall builds heat in both the motor and the driver in a very short time. The pulse must fit within the peak window of the driver: KS250 45 A / 5 seconds, PT500 40 A / 15 seconds.
- Calibration: AntiSwitch uses exactly this rise in current. The threshold must be set above the actual running current and below the stall current. On the PT500 the independent thresholds for the two directions are set with the A1 and A2 pots.
In practice a sustained stall (the motor staying against the stop for minutes) destroys both the winding and the driver. For that reason every application must have a termination mechanism: AntiSwitch, a limit switch or a timed cut-off. In systems where the mechanical end point is not distinct, do not rely on current-based termination alone.
The full topic: Locked rotor and thermal protection