How is current calculated from motor power (W)?
The rough calculation is I = P / V: 180 W at 12V is about 15 A, at 24V about 7.5 A. If the label power is mechanical output, the real current is higher.
The rough calculation is a single line: I = P / V. On that basis 180 W at 12V comes to roughly 15 A, the same power at 24V to roughly 7.5 A, and 360 W at 12V to roughly 30 A. This figure is the starting point when choosing a DC motor driver; the type of power printed on the label and the efficiency of the motor both pull the real current upwards.
Settle two questions before you calculate
The first is whether the value on the label is mechanical output power or electrical input power. If output power is stated, the motor's own losses have not been counted; the current actually drawn is, because of efficiency, noticeably higher than the number you arrive at. The second is that the calculation holds only for steady operation. At starting the current rises to typically 5-8 times the rated value; because that surge is repeated at every switch-on and every change of direction, it has to be assessed together with the driver's peak window.
If you have a choice of supply voltage
For the same power, 24V draws half the current of 12V. Halving the current directly shrinks the cable cross-section, the fuse rating and the heating at the terminals; that is why a thinner cable is enough for the same job on a 24V system. If the motor can run on both 12V and 24V and a 24V line already exists on the vehicle, the safety margin on the driver side grows at no cost. Do not do this by exceeding the motor's label voltage: running a motor wound for 12V at 24V does not lower the current, it makes the motor faster and hotter. The choice of voltage is settled together with the motor itself.
Compare the result with the product ratings
| Calculated continuous current | Suitable module | Limit |
|---|---|---|
| Two motors, up to 4 A per channel | ST42 | 8 A in total; speed and ramp adjustable |
| Below the 15 A band | KS250 | 15 A continuous, 45 A peak |
| Above 15 A | PT500 | 30 A continuous, 40 A maximum |
If the calculated result comes close to the driver's continuous rating, no safety margin is left. On paper a 180 W motor at 12V sits exactly at the limit of the KS250; in that case either moving to 24V or stepping up to the PT500 is the safer choice. Do not confuse continuous current with peak current; the peak value holds only within a short window, it is not a permanent operating point.
Where the calculation is not enough
- The label gives no power, only a current. No calculation is needed; use the current on the label directly, do not convert it to power and back again.
- The load changes with position. In crane, slide, hatch and lever applications what decides is not the average current but the current at the heaviest point.
- The motor is old. A motor with tired bearings or worn brushes does the same job at a higher current; the label no longer represents reality.
- There are frequent reversals. Heat builds up not only from the continuous current but also from how often the surges come; step up a model in applications like this.
- The motor drives a pump or a gearbox. Here the current is set not by the motor itself but by the element it drives; as the pressure or the load rises, so does the current. A calculation made from a single operating point misleads.
Measurement takes the place of calculation
Run the system with the heaviest real load and measure the current on the supply line with a clamp meter; watch the moment of starting as well. If the continuous value you measure stays below the driver's continuous current and the starting surge fits inside the peak window, the selection is right. If measurement is not possible, use the calculation but step up a model: enlarging the driver costs less than replacing a burnt module.
The full topic: DC Motor Driver Selection Guide