Understanding Torque Ripple in AC Motors

AC motors are often assumed to deliver smooth, continuous torque. But that’s rarely the case in practice. Small fluctuations in torque occur naturally as the motor rotates and electrical fields interact. Under normal conditions, those variations are minimal and harmless. When torque ripple increases, however, it can transmit pulsating forces into shafts, couplings, gearboxes, and driven equipment.

The challenge is that torque ripple is difficult to diagnose. It shows up indirectly through vibration, audible hum, inconsistent speed, or premature component wear, making it easy to misdiagnose any number of routine mechanical issues.

What torque ripple is (and where it comes from)

Torque ripple refers to periodic variations in the torque a motor produces during each electrical cycle. Instead of a steady output, the motor generates small peaks and valleys of force. These fluctuations are influenced by how the motor is powered, built, and controlled. Several factors often work together to amplify torque ripple, including:

• Electrical supply issues, such as voltage imbalance or harmonic distortion
• Motor design, including stator slotting, rotor geometry, and air-gap variation
• Drive control behavior, especially at low speeds or with poorly tuned parameters
• Operating conditions, such as light loading or speeds that excite mechanical resonance

Individually, these factors may cause only a minor ripple. Combined, they can create noticeable performance problems.

The role of power quality, motor design, and drives

Power quality plays a major role in torque smoothness. Voltage imbalance creates uneven magnetic forces inside the motor, while harmonics distort the magnetic field. Both conditions lead to uneven torque production that fluctuates with each rotation.

Motor construction also matters. Induction motors inherently experience torque pulsations due to the interaction between stator fields and rotor slots. Higher-quality motors are frequently designed to minimize these effects, while others may exhibit stronger ripple under identical conditions.

Variable frequency drives (VFDs) add another layer. When properly configured, VFDs can reduce torque ripple by precisely controlling current and magnetic flux. When misconfigured, they can make it worse, particularly at low speeds, where PWM switching effects and aggressive control settings introduce pulsations instead of smoothing them out.

When torque ripple becomes a problem

Torque ripple becomes an issue when it interacts with the mechanical system. Pulsating torque can excite resonances, create audible noise, or cause uneven motion that affects product quality or equipment life.

Common signs include vibration that changes with speed, humming or whining sounds, conveyor surging, and accelerated wear in couplings or gearboxes. Lightly loaded systems and applications that require smooth motion tend to be the most sensitive.

Reducing excessive torque ripple

Addressing torque ripple starts with identifying whether the source is electrical, mechanical, or control related. Improving voltage balance and reducing harmonic distortion often produce immediate benefits. On VFD-controlled systems, verifying motor data, adjusting carrier frequency, and refining control parameters can significantly smooth torque output.

In some cases, selecting motors with optimized designs or adding mechanical damping helps absorb remaining ripple before it reaches sensitive components.

An electrical issue with mechanical consequences

Torque ripple is a normal characteristic of AC motors; excessive ripple is not. When left unaddressed, it quietly contributes to vibration, noise, and premature wear that appear mechanical on the surface. Treating torque ripple as an electrical performance issue — not just a mechanical nuisance — gives maintenance teams a clearer path to longer equipment life and more reliable operation.