Should I buy a 15A driver or a 30A driver, and how do I decide?
The continuous current under load and the starting surge decide: near 15 A, or on a long start, you need to move to the PT500 with its 30 A continuous.
The decision is made by looking at the real continuous current of the motor under load and at the starting surge; the power on the label is not enough on its own. If the measured continuous current comes close to the rating of the 15 A KS250, or if the starting surge strains 45 A for longer than 5 seconds, move to the PT500, which gives 30 A continuous and 40 A maximum (15 s). The difference between them is not only current; the ingress protection rating and the size come into the decision too.
The three measurements that settle the decision
- The continuous current under load. Run the motor with its real load and measure it with a clamp meter. Note not the momentary highest value but the steady running value. Measuring how many amps the motor draws is the only solid basis for this decision.
- The starting surge. The starting current is typically 5-8 times the rated current and lasts 0.5-3 seconds. A ramped start typically brings this surge down to 2-4 times the rated current. How many times over the starting current runs depends on the inertia and the load of the motor.
- The duty cycle. A gate running continuously at short intervals and a shutter running a few times a day are not the same. As the cycle gets more frequent the board does not find time to cool and the effective capacity falls.
KS250 or PT500
| Criterion | KS250 | PT500 |
|---|---|---|
| Continuous current | 15 A | 30 A |
| Peak / maximum | 45 A (5 s) | 40 A (15 s) |
| Ingress protection rating | IP45 | IP67 |
| Size and weight | 120x40x46 mm, 110 g | A larger body |
| Where it suits | Measured current markedly below 15 A, inside a panel | Current high, cycle frequent or outdoor use |
There is a detail in the table that stands out: the maximum current of the PT500 looks lower than the peak value of the KS250, but its window is longer. The difference between continuous current and peak current is exactly this; a peak value cannot be read apart from its duration.
Why a safety margin is essential
Catalogue values hold for a particular ambient temperature and mounting condition. In an enclosed panel, in the middle of summer or next to a hot surface, the same board carries less current; reducing the current capacity when the ambient temperature rises is not a preference but an obligation. If the measured current is close to the limit, the system works on the bench and stops on site. Boards working close to the limit also cut out thermally more often and their life is shortened.
What a bigger driver does not solve
Making the driver bigger does not correct a mechanical problem. A jamming guide, a misaligned shaft, a dry bearing or a worn gearbox goes on raising the current; a big driver only postpones the failure. In the same way thin cable, a loose terminal and a wrongly chosen fuse are not solved with current capacity. If the motor can lock up mechanically, the value to look at is the stall (locked-rotor) current; this value can go far above the limit of both models and the protection has to be planned separately.
What to do
Run the motor with its real load, measure the continuous current and the starting surge, and note the cycle frequency. If the measured value is markedly below 15 A and the board will stay inside a panel, the KS250 is enough. If you are approaching the limit, if the cycle is frequent or if the board will sit outdoors, move to the PT500; changing the module later means redoing the cabling, the mounting and the setting.
The full topic: DC Motor Driver Selection Guide