Cogging torque

Even when a cog or sprocket is not used, the uneven/bumpy impression of torque remains in the condition of a motor’s rotation.
Many motors use a slotted core. These motors tend to produce cogging. This torque ripple is generated by the interaction between the armature core and permanent magnets. As this torque is generated in addition to that due to the current, an uneven torque can result. Many motors produced today have low cogging due to research to reduce cogging by adopting various tooth shapes and magnetization methods of the magnet. A certain amount of cogging is desirable, however, in certain applications.
The figure below shows the measurement system for cogging torque.