Torque Ripple
Torque ripple is the periodic variation of a motor's instantaneous torque around its average or requested torque while the rotor turns.
Main sources
- Motor geometry: slotting, saliency, magnet shape, winding distribution, and magnetic saturation can create position-dependent harmonics.
- Current and voltage waveforms: inverter switching, dead time, commutation, and non-sinusoidal current can add electromagnetic torque variation.
- Control and measurement: rotor-angle error, current-sensor error, parameter mismatch, or limited control bandwidth can leave periodic disturbances uncompensated.
Cogging torque is one contributor in permanent-magnet motors, but the terms are not interchangeable. Cogging exists without commanded stator current, whereas torque ripple also covers load-dependent electromagnetic and control effects.
Why it matters
Mechanical inertia filters some high-frequency ripple, but remaining components can contribute to vibration, tonal noise, driveline oscillation, and less accurate low-speed torque. Designers reduce it through motor geometry, winding and magnet choices, current-waveform control, calibration, and structural isolation; no motor type or control label alone guarantees zero ripple.
For the deeper motor-construction trade-offs, see Electric Motor Construction and Design Choices.