The motor runs unloaded but stops when load is applied, what should I do?
Under load the current rises: either the supply collapses, the current limit trips or the continuous rating is exceeded. Diagnosis starts with current.
This picture is almost always about current. The motor draws little current unloaded and turns without trouble; when the load comes on, the current rises and one of three things happens: the supply collapses, the current limit trips, or the driver's continuous current capacity is exceeded. A single measurement narrows the diagnosis — measure the current drawn unloaded and under load and compare it with the driver's rating.
The order of checks
- Voltage collapse: Measure the voltage at the moment of loading at the driver terminal, not at the battery post. If there is a clear drop, the cable cross-section is insufficient, the terminal is loose or the battery is weak. In a 12 V system the target for total voltage drop is typically below 5%; that means 0.6 V.
- The current limit trips early: On the PT500 check the pot for the relevant direction (clockwise A1, reverse A2). The threshold has to be above the real running current.
- Capacity exceeded: If the current under load exceeds the driver's continuous rating, the answer is not an adjustment but correct sizing. The KS250 is defined for 15 A and the PT500 for 30 A continuous current.
- Mechanical resistance: Bearing arrangement, misalignment, lack of lubrication or binding in the guide raise the current needlessly. Disconnect the motor and turn the mechanism by hand to get a feel for it.
- Fuse character: A fast-acting type of fuse may be blowing on the starting surge; in that case the motor stops without ever getting going.
From symptom to cause
| Symptom | Most likely cause | Verification |
|---|---|---|
| The speed falls as the load comes on, then it stops | Supply collapse | Measure the terminal voltage at the moment of loading |
| It cuts hard, and starts again when the command is repeated | Current limit or AntiSwitch | Open the threshold pot step by step and retry |
| It runs for a while and cuts after warming up | Thermal limit | Measure the board body temperature at the end of the cycle |
| It never gets going and the fuse blows | Starting surge or a short circuit | Check the fuse type and the motor separately |
How it comes back after cutting is a clue as well: if it starts again as soon as the command is repeated, the current threshold is likely; if you have to wait for it to cool, a thermal limit is likely. The difference between a current limit and thermal protection decides which measurement you take.
Taking the measurement correctly
Read the current under the real load, with a clamp meter, around a single conductor of the motor cable. How DC motor current is measured needs particular care on a circuit driven with PWM, because not every clamp meter reads a chopped current correctly. On the voltage side, do the voltage drop test with a multimeter between the two ends of the line; a value read at a single point does not show you where you are losing the voltage.
Cases this list does not solve
If the motor itself is tired, both the current and the voltage can look normal: brush wear, a dirty commutator or a partial short in the winding stop the motor producing torque. In geared applications the problem is often not in the motor but in the gearbox. And the result obtained by enlarging the fuse is no solution; raising the fuse rating leaves the cable and the driver unprotected.
What to do
Work through it in order: terminal voltage, load current, threshold setting, mechanical resistance. If the continuous current you measure comes close to the driver's rating, stop playing with the settings and move up a model; a system with no safety margin left in it will stop again on the first hot day.
The full topic: DC Motor Driver Selection Guide