Torque Ripple
The periodic oscillation of the torque produced by the motor around its average value. In brushed motors the main source is commutation: as the brush changes segment, the effective number of windings and the current path change momentarily. At low speeds it is felt as vibration or judder.
The frequency of the ripple is the speed multiplied by the number of segments; its amplitude decreases as the segment count increases. A second source is on the driver side: the current ripple within the PWM period produces torque ripple in the same proportion.
Ways to reduce it:
- Raising the PWM frequency: Typically the 16-20 kHz band both removes the audible whine and lowers the current ripple. As the frequency rises, switching loss and heating rise too; a balance must be struck.
- Reduction ratio: A gearbox smooths the ripple on the output side and the mechanical inertia acts as a filter.
- Operating point: At very low duty ratios the current becomes discontinuous and the ripple becomes pronounced; the mid band should be preferred where possible.
In the field, torque ripple is usually noticed as rattling in gate or shutter movement at low speed. Not keeping the ramp longer than necessary shortens the time spent in the low-speed region and noticeably reduces this complaint; the shortest ramp that the mechanical shock allows is a good balance point.
The context in which this term is used: DC Motor Driver Selection Guide