How much safety margin should I leave on a motor driver?
Measured continuous current should not exceed 70-80% of the driver's continuous rating: 10-12 A on the KS250, 21-24 A on the PT500. Peaks are not margin.
The continuous current measured under load should not exceed roughly 70-80% of the driver's continuous rating. On the KS250's 15 A rating that means an operating point of 10-12 A, and on the PT500's 30 A rating one of 21-24 A. The remaining margin is there for the temperature rise, mechanical ageing and seasonal changes in load; it is not comfort, it is a requirement of physics.
Why a margin is left
Because conduction loss grows as P = I² x R, a 20% increase in current means roughly a 44% increase in loss. On top of that the resistance of semiconductors rises with temperature; as the temperature climbs the loss grows, and as the loss grows the temperature climbs further. This loop explains why a module chosen at the limit runs without trouble for a while and then suddenly turns unstable. The second reason for a margin is the measurement itself: the current figure you take in the field holds for one day, one load and one temperature. Think of the margin as the room you set aside for the conditions you could not measure.
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. Thermal derating is precisely the name for this.
- Duty cycle: In a continuously running application the margin has to be larger than in one that runs a few times a day; heat builds up over time.
- Mechanical ageing: Bearings, slides and joints gain resistance over the years; the current on day one is not the current three years later.
- Season: In cold weather the grease thickens and the starting current rises; the first movement of a winter morning is the hardest cycle of the day.
Target operating point by model
| Model | Continuous rating | Target operating band |
|---|---|---|
| ST42 (per channel) | 4 A | Around 3 A |
| KS250 | 15 A | 10-12 A |
| PT500 | 30 A | 21-24 A |
In practice: if the measured continuous current is 12 A you are at the upper limit of the KS250; if a hot environment, continuous operation or mechanical ageing is expected, moving to the PT500 is safer. At around 8-10 A the KS250 runs comfortably.
Where the margin is worked out wrongly
Do not count the peak ratings as safety margin; they are only for short surges. The KS250's 45 A peak is a 5 second limit and the PT500's 40 A maximum a 15 second one, and the module may go into protection once those times are up. Mixing the two values up is the most frequent way of making a marginal selection look generously specified on paper. In applications where the rotor can lock, a percentage calculation is not enough on its own; there a current limit and a mechanical stop have to come into play. The percentage rule is also stricter than necessary for a movement that runs very briefly and very rarely: on a hatch mechanism that runs for a few seconds a day the module never gets the chance to warm up, and there the starting surge is what decides.
What to do
Measure the current on the hardest cycle, on a hot day and, if you can, with aged mechanics; do not select on the strength of the measurement from the best day. Divide the result by the model's continuous current: if the ratio goes above 0.8, step up a model; if it stays below 0.7, you are comfortable. Measure this ratio again once a year after installation; how the current drifts over time is the earliest sign that mechanical maintenance is due.
The full topic: DC Motor Driver Selection Guide