Geared DC Motor (Gearmotor)
A gearbox on the motor shaft cuts the speed and raises the torque. The driver is chosen by the real load current at the gearbox output, not the label.
A geared DC motor is a DC motor produced together with a gearbox coupled directly to the motor shaft. The gearbox reduces the speed and increases the torque in the same ratio; in this way a heavy load can be moved with a small, low-current motor. That is also why the driver is chosen not by looking at the label of the motor but at the real load at the gearbox output.
What determines the output torque
The output torque is roughly the motor torque multiplied by the reduction ratio and the gear efficiency. While the efficiency is typically in the 90 per cent band in a single stage, it can fall to the 50–70 per cent level in multi-stage or worm gearboxes. This loss is reflected straight to the driver as current; the motor draws more current to do the same job and the driver heats up more.
Do the calculation backwards from the output: divide the output torque you need by the ratio and the efficiency to find the motor torque, and go from there to the current with the torque constant. Choosing a driver by looking at the current measured off load is the most common mistake, because it never sees the current reached under load.
Three critical points when choosing the driver
- What is decisive is the continuous current: The peak value of the board is only for a short window. The distinction of 15 A continuous with 45 A peak on the KS250, and 30 A continuous with 40 A maximum on the PT500, says exactly this; do not confuse continuous current with peak current.
- Stall occurs more easily: On high-ratio gearboxes a mechanical stop is easy to reach and the locked rotor current climbs at once; current-sensing cut-out makes use of this behaviour.
- Gear backlash: On reversing there is dead movement equal to the backlash; a soft start ramp keeps that backlash from closing with a shock.
What changes with the type of gearbox
| Gearbox type | Behaviour in the field | What is decisive at the driver |
|---|---|---|
| Low ratio | Speed high; a change of load drops the speed noticeably | Continuous current and supply voltage drop |
| High ratio, multi-stage | Slow and strong; reaches a mechanical stop easily | Current threshold and peak current window |
| Worm | Efficiency low; on most types it cannot be back-driven | Heating margin and ramp time |
Choose the ratio not by the speed you want but by the hardest working point: wherever the load is heaviest and the friction highest, that is where the calculation is set. Grease that stiffens in winter or a seized bearing carries that point in the field above where it stood on paper; a choice left on the margin comes up against the current threshold on the first cold morning and the movement is cut off half way.
What the gearbox does not solve
A gearbox solves the torque problem, not the heat problem: in a gearbox of low efficiency the power lost turns into heat and warms both the motor and the driver. In work that needs positioning accuracy the gear backlash is a source of error on its own; tie the backlash not to an adjustment but to a movement plan that always approaches the target from the same direction. Worm gearboxes that cannot be back-driven may look as though they are holding the load, but this does not take the place of a brake; in lifting mechanisms the holding duty is given to a separate element.
What to do
Run the motor with its real load and measure two values separately with a clamp meter: the peak current at starting and the continuous current in steady running. Choose the driver by the continuous current, leaving a safety margin on top. On modules that have a ramp setting, setting the ramp to the shortest time at which the gear backlash closes quietly improves both the mechanical life and the starting current.
The context in which this term is used: DC Motor Driver Selection Guide