Cogging Torque

The periodic pulsing torque from a permanent-magnet motor's magnets snapping toward stator teeth, even unpowered.

A permanent-magnet motor's stator core has slots cut into it, and as the rotor's magnets pass the “teeth” and “slots,” the difference in magnetic reluctance pulls the rotor toward the position of least reluctance — so even with no power applied, turning the motor by hand feels like a series of small detents, or notches. This is cogging torque, and it superimposes a periodic ripple on top of the output torque; it's most noticeable at low speed, where it causes vibration and noise and hurts fine force control and the feel of backdriving. Common fixes include skewing the slots or poles, optimizing the slot-to-pole ratio, or switching to a slotless design that eliminates it entirely; control software can also use a lookup table for feedforward compensation. It's a factor worth watching when selecting a joint motor and calibrating force control.

ExampleSlowly turning an unpowered brushless motor by hand and feeling a series of small detents is cogging torque.

Also called

Detent Torque

Related

Slotted vs. Slotless Brushless Motor · Brushless DC Motor · Permanent Magnet Synchronous Motor (PMSM) · Coreless Motor · Torque Control · Backdrivability

Sources

Cogging torque - Wikipedia