Torque Constant (Kt)
The amount of torque a motor produces per ampere, given in Nm/A. The torque produced equals the armature current multiplied by this constant. This constant explains why torque changes when the speed is reduced with PWM and how the required current is calculated.
Example: if you want 1 Nm from a motor with a torque constant of 0.05 Nm/A, roughly 20 A of current is needed. The current that overcomes no-load friction is added to this calculation as well. In a geared arrangement, first divide the desired output torque by the ratio and the gear efficiency to find the motor torque, then convert it to current with this constant.
A frequent misunderstanding is the belief that torque also drops when the speed is reduced with PWM. The truth is this:
- Torque is determined by current, not by duty ratio. As long as the same current flows, the torque is maintained.
- When the duty ratio drops, the average voltage and therefore the speed drop; if the load is constant, current and torque stay largely the same.
- If the duty ratio is reduced too far, the applied average voltage is not enough to drive the current the load demands, and only then does the torque really fall and the motor stall.
That is why, in applications demanding high torque at low speed, the solution is not to cut the duty ratio but to increase the reduction ratio.
The context in which this term is used: DC Motor Driver Selection Guide