Does the motor's torque drop when I reduce the speed?
In a brushed DC motor torque is proportional to current, and speed to the average voltage applied. On a switching driver, reducing the duty ratio reduces the voltage; the motor turns more slowly but keeps producing the same torque if it draws the same current. The limit is that cooling is reduced and heating increases at low speed.
Two relationships need to be separated:
- Speed ≈ the average voltage applied (minus the drop the current creates across the winding resistance).
- Torque ≈ armature current × the motor torque constant.
When you halve the duty ratio, the speed the motor would run at unloaded halves, but if the load demands more torque the motor draws more current and produces the torque. So low speed does not automatically mean low torque.
The real limits in practice are these:
- Cooling: In motors whose fan is turned by their own shaft, the airflow decreases at low speed; the temperature climbs more quickly at the same current. A heavy load held for a long time at low speed creates a thermal risk.
- Starting threshold: At a very low duty ratio the torque produced may not be enough to overcome friction; the motor draws a high current without being able to start.
- Current limit: The driver's continuous current rating (15 A on the KS250, 30 A on the PT500) in effect also limits the maximum torque that can be produced.
If high torque is needed over a wide speed range, the correct solution is not to cut the voltage but to change the gearbox ratio; gearing multiplies torque mechanically, independently of speed.
Full topic: DC Motor Driver Selection Guide