The motor runs unloaded but stops when load is applied, what should I do?
This picture is almost always current related: when the load is applied the current rises, and either the supply collapses, or the current limit trips, or the driver's continuous current capacity is exceeded. Measuring the current drawn unloaded and under load and comparing it with the continuous rating of 15 A for the KS250 and 30 A for the PT500 narrows the diagnosis quickly.
Possible causes, in order of checking:
- Voltage collapse: Measure the voltage at the driver terminal at the moment of loading. 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 must be above the real operating current.
- Capacity exceeded: If the current under load exceeds the driver's continuous rating, the solution 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-blow type fuse may be blowing on the starting pulse; in that case the motor stops without ever starting.
Lengthening the ramp time (KS250 soft start, PT500 RMP pot) softens the starting pulse and gives a clear benefit in many cases where a limit is being exceeded.
Full topic: DC Motor Driver Selection Guide