How does the driver protect itself under overload?
Under overload the current rises fast; drivers with current sensing see it and cut the movement. The threshold sits between running current and stall current.
Under overload the motor current rises quickly; drivers with current sensing see that rise and cut or limit the movement. The AntiSwitch function on the KS250 and PT500 works on exactly this principle. Overload arrives in two forms: a sudden stall caused by mechanical jamming, or continuous high current caused by running above capacity. Current-threshold protection catches the first very quickly and the second as soon as the threshold is exceeded.
A current threshold and thermal protection are not the same thing
A current threshold is instantaneous: once the value is exceeded it cuts at once, and it does not look at temperature. Thermal protection looks at heat accumulated over time and engages on loads that stay below the threshold but last a long time. It matters to know the difference between the two protections; one does not stand in for the other. If the driver runs for hours just below its limit, the current threshold never trips, and the module still heats up and tires.
Two conditions for the logic to work
- The mechanics must withstand the stall force for the duration of the detection; otherwise damage occurs before the protection engages.
- There must be a distinguishable difference between the normal running current and the stall current. If the motor is already working at the edge of its capacity, the two values cannot be told apart.
For this reason the threshold is calibrated above the real running current but below the stall current. Set it too low and the movement is interrupted at normal load; set it too high and the protection does not do its job. On the PT500 the threshold for each direction is separate and is set independently with the A1 and A2 pots; that is a real advantage in applications where the load is heavy on the way up and light on the way down.
Commissioning: how you find the threshold in concrete terms
Run the system with the heaviest real load and measure the current drawn. Place the threshold clearly above that value and below the stall value. Once the setting is made, mark the pot position with the label kit supplied with the product; months later you will not remember where a pot that has drifted with vibration used to sit. It is worth repeating the current threshold calibration cold and loaded: cold grease and stiff bearings make the first movement harder.
From symptom to cause
| Symptom | Likely cause | What to do |
|---|---|---|
| The movement is interrupted at normal load | Threshold low, or the mechanics have stiffened | Check the bearings and the lubrication first |
| The protection engages late when it jams | The threshold is set high | Lower it against the measured running current |
| It cuts in the mornings but not during the day | Starting torque rising in the cold | Calibrate the threshold in cold conditions |
| The protection never engages and the driver heats up | Continuous overload | Review the sizing |
What the protection does not cover
Current-threshold protection does not stand in for a safety function. Do not expect it to stop the load falling or to protect a person or the mechanism; whether the need for external limit switches really disappears is assessed according to the application. If the overload is continuous and stays just under the threshold, the current threshold never sees it; in that scenario what decides is the duty cycle and the mounting temperature.
Nor should the system be assumed healthy because the protection engaged. The protection only tells you the current exceeded the threshold; it does not tell you no damage was done. If it trips repeatedly, check the mechanics first (jamming, bearing arrangement, lubrication), then review the sizing. Raising the threshold suppresses the symptom, it does not remove the cause.
The full topic: DC Motor Driver Selection Guide