Cogging Torque
Cogging torque is position-dependent torque in a permanent-magnet motor caused by the rotor magnets seeking positions of lower magnetic reluctance relative to the stator teeth and slots, even when no stator current is commanded.
How it differs from torque ripple
Cogging torque repeats as the rotor moves through slot-and-pole alignments. It can contribute to low-speed torque pulsation, vibration, audible tones, and a notchy feel when an unpowered motor is turned slowly.
It is only one source of torque ripple. Under load, current harmonics, magnetic saturation, inverter dead time, commutation, sensing, and control errors can create additional periodic torque. Designers can reduce cogging through slot-and-pole selection, skew, magnet or tooth shaping, winding layout, and control compensation, but each measure interacts with torque density, efficiency, cost, and manufacturing.
For the wider set of motor-geometry choices, see Electric Motor Construction and Design Choices.