Speed-Torque Curve
The straight line showing the relationship between the torque a brushed DC motor produces at constant voltage and the speed at which it turns. At one end is the no-load point where torque is zero and speed is highest, at the other end the stall point where speed is zero and torque is highest. The change in between is approximately linear.
The current curve is drawn on the same graph as well: current rises linearly together with torque, while power peaks in the middle of the curve. The efficiency peak sits slightly to the left of the power peak, in the region closer to the no-load speed.
When choosing the operating point, pay attention to these regions:
- No-load end: High speed, low current. Only meaningful in mechanisms that run unloaded.
- Middle region: The area suitable for continuous operation. Nominal torque is generally in the 20–30 percent band of the stall torque.
- Stall end: Thermally the most demanding region; you can stay there only for a matter of seconds.
Lowering the supply voltage shifts the curve downward: the no-load speed decreases proportionally and the stall torque decreases in proportion to the voltage. That is why if you drive a 24 V motor with 12 V both the speed and the lifting capacity are halved. Cutting the average voltage with PWM has the same effect.
The context in which this term is used: DC Motor Driver Selection Guide