How much safety margin should I leave on a motor driver?
A healthy approach is for the continuous current measured under load not to exceed roughly 70-80% of the driver's continuous rating. That means aiming for an operating point of 10-12 A on the KS250's 15 A rating and 21-24 A on the PT500's 30 A rating. This leaves margin for temperature rise, ageing and load variation.
Safety margin is not comfort, it is a requirement of physics. Because conduction loss grows with P = I² x R, a 20% increase in current means roughly a 44% increase in loss. Semiconductor resistance also rises with temperature; as the temperature rises the loss increases, and as the loss increases the temperature rises further still.
Take these into account when setting the margin:
- Ambient temperature: In a hot environment the same loss produces a higher internal temperature; widen the margin.
- Duty cycle: In a continuously running application the margin must be larger than in one that runs a few times a day.
- Mechanical ageing: Bearings, slides and joints gain resistance over time; the current on day one is not the current three years later.
- Season: In cold weather the grease thickens and the starting current rises.
In practice: if the measured continuous current is 12 A you are at the KS250's 80% upper limit; if a hot environment, continuous operation or mechanical ageing is expected, moving to the PT500 is safer. Around 8-10 A the KS250 runs comfortably. Do not count the peak ratings (KS250 45 A / 5 s, PT500 40 A / 15 s) as safety margin; they are only for short pulses.
Full topic: DC Motor Driver Selection Guide