What amperage model should I choose for a DC motor driver?
Driver current is chosen at least 1.5 times the continuous current measured under load; 2 times with frequent starts. Inrush reaches 3-5 times continuous.
The continuous current rating of the driver should be at least 1.5 times the continuous current of the motor measured under load. In applications that stop and start frequently, or that start while lifting the load, take this factor as 2. The value on the motor nameplate is the starting point, but the selection is not made according to it: at the moment of start-up (inrush) the current rises to 3–5 times the continuous value, and the driver has to see that surge as well.
Why the nameplate current is not enough on its own
The nameplate gives the current of the motor at the nominal point. In the field the motor does not run at that point: mechanical resistance, misalignment, grease consistency and the load itself change the current. The right starting point is measuring the current under load with a clamp meter. The current measured with no load is markedly below the real working current; a selection made from that measurement comes out small and the driver reaches its limit on the first hot day.
The application type determines the factor
| Application | Factor | Why |
|---|---|---|
| Constant load, infrequent starts (greenhouse screen, ventilation hatch) | ×1.5 | The start-up surge repeats rarely |
| Frequent stop-start, starting under load (crane, lift, levelling leg) | ×2 | The surge is renewed on every cycle |
| Mechanism liable to jam (slide, door, guide) | According to the stall current | When the rotor locks, the current goes through the roof |
How many times the rated current the inrush will be depends on the type of the motor, on inertia and on the load; that is why the factor is not a guess but a safety margin.
Continuous current or peak current?
You see two values in the catalogue, and the selection is always made according to the continuous value. KS250 is specified as 15 A continuous / 45 A peak, PT500 as 30 A continuous / 40 A maximum, and ST42 for 4 A per channel and 8 A in total. The peak value is only for surges of limited duration; it cannot be used as a continuous operating point. Confusing the difference between the two values is the most frequent way of making a selection look larger than it is. When you read the peak value, read its duration as well: if the surge exceeds that duration, the module either goes into protection or ages.
Where this rule is not enough
The factor of 1.5 holds for normal operation with the rotor turning freely. Where the movement runs up against its end stop, where the guide jams or freezes over, the motor stops but goes on drawing current; this stall current is far above the calculation with the factor and can only be met with a current limit or with a mechanical limitation. Nor does the factor take the place of protection on a driver that has no instantaneous overcurrent protection: without protection, the start-up surge wears out the output stage over time. Another limit is two-channel use; on modules that drive two motors separately, such as ST42, the total value has to hold as well as the per-channel value, and if you run both channels at full load at the same time the per-channel figure misleads you.
What to do
Measure the current of the motor under load, on the most demanding cycle and if possible on a hot day; multiply the resulting value by the factor. Do not trust a single measurement, repeat the movement a few times and take the highest continuous value as the basis. If the result falls between two models, choose the larger one; the price difference of the driver stays small next to the cost of replacing a module working at the limit and of stopping the machine. For a model comparison, look at the differences between ST42, KS250 and PT500.
The full topic: DC Motor Driver Selection Guide