Glossary · Automation fundamentals, platforms and components
Cogging torque
Also known as: Detent torque, Resting moment
German: Rastmoment
In electric motors, cogging torque is the torque ripple produced by the interaction between the permanent magnets of the rotor and the slots or teeth of the stator, present even when no current flows. In stepper motors, the corresponding holding torque without current is called detent torque.
- Automation components
In one sentence
Cogging torque is the torque ripple from rotor magnets aligning with stator teeth, felt even without current; in steppers it is detent torque.
Example
When the engineer turns the unpowered servo motor shaft by hand, it moves in small, noticeable steps caused by cogging torque.
How it applies
- Engineering: Cogging torque causes speed ripple and vibration at low speeds, which matters in precise positioning and smooth motion. Motor makers reduce it with skewed slots or magnet shapes; drives can compensate part of it.
- Commissioning: Speed ripple at low speed can be mistaken for tuning problems. Check the motor data before retuning the control loop.
- Documentation: Motor data sheets state cogging or detent torque. The detent torque of a stepper motor is small and does not hold a load safely; instructions must not present it as a brake.
Cogging torque vs. holding torque
Cogging (detent) torque exists without current. Holding torque of a stepper motor is the torque it can resist while energized at standstill, and a holding brake is a separate mechanical component.