What is stall (locked-rotor) current, and why is it critical in driver selection?
Stall current is what is drawn with the rotor held still; with no back-EMF it rises to 5-8 times rated and has to fit inside the peak window of the driver.
Stall current is the current a motor draws while the rotor is mechanically held still. Since the rotor is not turning, no back-EMF is produced; the only thing limiting the current is the winding resistance, and the value shoots to a very high point set by I = V / R. In practice it is typically 5-8 times the rated current. If a DC motor driver cannot meet this surge, the first failure comes out in the driver.
The same event as the starting current, a different duration
As a motor starts up the rotor is also counted as being still for an instant, so in the first milliseconds the current is close to the stall value. As the speed rises the back-EMF grows and the current drops to its normal level. The only difference between them is duration: a start is short, whereas a stall lasts until you cut it. Knowing how many times over the starting surge runs is for that reason the first step of driver selection.
What it strains: the driver first, then the motor
- Thermal: since the loss grows with the square of the current, a stall builds up heat in a very short time both in the motor and in the driver. The surge has to fit inside 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 has to be set above the real working current and below the stall current. On the PT500 an independent threshold for the two directions is set with the A1 and A2 pots.
Saving the driver is not enough; the same current passes through the winding of the motor as well. Since the rotor is not turning, the air flow and the oil circulation inside stop too, so the heat produced builds up under the worst cooling condition. The winding insulation therefore starts to fatigue on the order of seconds, not minutes. By the moment you smell burning the damage has already been done; do not put a sustained stall to the test.
How you find out the stall current
If there is a data sheet it states it directly. If not, two ways remain. You can measure the resistance at the motor terminals with an instrument capable of low resistance measurement and divide it into the supply voltage; that is rough but gives an upper bound. The alternative is to hold the rotor for a brief moment and read the current with a clamp meter. When you apply the second method, fuse the line, hold the motor on the order of seconds and make it mechanically safe; a stall trial that drags on burns the winding.
What to do in which case
| Situation | Result | What to do |
|---|---|---|
| Stall current below the peak value of the driver | Suitable | Calibrate the threshold above the working current |
| Stall current above the peak value, duration short | Borderline | Go up to the next model or add a mechanical end stop |
| Working current and stall close to each other | Indistinguishable | The motor is undersized; review the sizing |
Where a current threshold is not enough
A sustained stall, that is the motor staying against the stop for minutes, destroys both the winding and the driver. For that reason every application has to have a termination mechanism: AntiSwitch, a limit switch or a timed cut-off. On systems whose mechanical end point is not distinct, do not rely on current-based termination alone; whether current sensing takes the place of an external limit switch depends on the geometry of the application. On systems where the load gets heavier with position the threshold is misleading as well. On geared systems the stall torque is also carried straight into the gearbox: even if the motor and the driver survive the surge, the weakest link of the mechanical chain may not.
At commissioning the order is this: measure the working current at the heaviest load, find out the stall value, and place the threshold between the two. If the protection cuts in early, look for mechanical jamming first, and make turning the pot up the last resort.
The full topic: Locked rotor and thermal protection