What Are Torque Pulsations?

Torque pulsations are periodic fluctuations in the output torque of an electric motor or rotating machine, arising when electromagnetic, mechanical, or load-side forces vary cyclically over a shaft revolution. Also called torque ripple, these oscillations appear as deviations from the smooth, steady torque a motor ideally delivers. The phenomenon is present to some degree in virtually every motor type, from DC brushed machines to switched reluctance and permanent-magnet synchronous designs, and its magnitude directly affects mechanical stress, acoustic noise, energy efficiency, and the quality of motion in driven systems.

Torque pulsations have been a studied concern since the electrification of industrial drives in the early twentieth century. As motor speeds increased and precision demands grew in aerospace, robotics, and electric vehicle applications, the field's focus expanded from simple measurement to active suppression and design-level mitigation.

Causes and Mechanisms

The dominant sources of torque pulsations differ by motor type, but the underlying physics share a common thread: non-uniformity in the air-gap magnetic field. In permanent-magnet machines, cogging torque arises from the interaction between rotor magnets and the stator slot geometry, producing a torque component that varies at a frequency equal to the slot number times rotor speed. In induction motors fed by variable-frequency inverters, torque pulsations result from harmonic rotor currents excited by the non-sinusoidal inverter output voltage. In single-phase induction motors, a double-frequency pulsation is inherent because the stator field has both forward and backward rotating components. Mechanical imbalance, eccentricity, and shaft misalignment can add further periodic components at sub-synchronous or super-synchronous frequencies.

Measurement and Characterization

Torque ripple is quantified as the peak-to-peak torque variation divided by the mean torque, typically expressed as a percentage. IEEE Standard 1255 provides a uniform method for calculating and measuring torque pulsations during starting of synchronous motors, specifying instrumentation bandwidth, averaging periods, and reporting conventions. For inverter-fed drives, spectrum analysis of the instantaneous torque signal identifies the harmonic orders responsible, linking each component to a specific excitation source. Dynamometers with high-bandwidth load cells or contactless torque transducers are the preferred instrumentation, since mechanical flexibility in the drivetrain can attenuate higher-order components before they reach a sensor.

Mitigation Techniques

Mitigation strategies fall into two broad categories: design-level and control-level. At the design stage, fractional-slot winding configurations, skewed stator or rotor laminations, and optimized magnet pole-arc ratios all reduce cogging torque and winding harmonics before the motor is ever energized. At the control level, current harmonic injection, deadbeat predictive control, and iterative learning control algorithms compensate for residual ripple in real time. Research published through OSTI on direct-drive EV wheel motors demonstrated that combining rotor geometry optimization with a harmonic current injection strategy can suppress pulsation amplitudes by more than 80 percent without increasing average copper losses.

Applications

Torque pulsations are a design and operational concern across a wide range of fields, including:

• Electric vehicle traction systems, where drivetrain vibration and passenger comfort are directly affected
• Industrial servo drives for precision manufacturing, milling, and robotics
• Wind turbine generators, where periodic torque loading accelerates gearbox and bearing fatigue
• Ship propulsion systems, where pulsating shaft torque can excite structural resonances in the hull
• Starting sequences for large synchronous motors in compressor and pump stations