Torque ripple

Lore & Background

Torque ripple is an effect seen in rotating electrical machines, referring to an oscillation or pulsation of the average output torque as the electrical machine operates under steady states. The combination between the winding distributed in slots as a discrete magnetomotive-force source (electric loading) and the rotation-dependent magnetic reluctance (including magnetic saturation, rotor and stator reluctances) is the main reason for torque ripple. Due to the torque ripple, the speed of the shaft will oscillate as well, as the entire mechanical load on the test bench will expose the oscillations of mechanical and electrical power.

A common example under no load is 'cogging torque' due to slotting and saliency effects of the stator and rotor cores, which causes variations in the magnetic reluctance depending on the rotor position. Usually, for smooth operations, a reduced value of torque ripple is required and acceptable. However, for specific applications, such as electric impact wrenches, a higher torque ripple has some advantages to increase the torque dynamically more than the average torque value.

Step-skewing and continuous skewing are effective measures to reduce torque ripple, and so the cogging torque. Mathematically, it is possible to relate torque ripple to a part of vibration of rotating electrical machines, as the radial and tangential Maxwell stress tensors contain the same parts of spatial and time harmonics.

Reader's Guide

Torque ripple is notable because it directly affects the smoothness of operation in rotating electrical machines. The article describes it as an oscillation or pulsation of average output torque under steady states, caused by the discrete nature of windings in slots and rotation-dependent magnetic reluctance. This ripple leads to shaft speed oscillations, exposing mechanical and electrical power variations on a test bench. The article notes that while reduced torque ripple is generally required for smooth operations, certain applications like electric impact wrenches benefit from higher torque ripple to dynamically increase torque beyond the average value. The significance of torque ripple extends to vibration analysis, as it can be mathematically related to vibration through the radial and tangential Maxwell stress tensors, which contain the same spatial and time harmonics. Reduction methods such as step-skewing and continuous skewing are effective for minimizing torque ripple and cogging torque. The article does not provide dates, specific figures, or named researchers, but establishes torque ripple as a key performance factor in electric motor design, with trade-offs between smooth operation and dynamic torque requirements depending on the application.

The Physics Behind the Fluctuation

Torque ripple in reluctance motors is the periodic swing between peak and minimum torque output over a single revolution of the rotor. The phenomenon is rooted in the motor's core operating principle: the rotor, fabricated from soft magnetic material such as laminated silicon steel, carries no windings and instead depends on induced magnetic poles to interact with the stator's electromagnet poles. As the rotor travels between the fully unaligned position—where a rotor pole sits equidistant between two stator poles and magnetic reluctance is at its maximum—and the aligned position, where rotor and stator poles face one another directly and reluctance drops to its minimum, the driving magnetic force varies continuously. At low rotational speeds this variation becomes especially pronounced, generating audible noise and uneven motion. The ripple is an inherent trade-off of the reluctance principle itself: the same magnetic attraction that pulls the rotor into alignment also produces the recurring surges and dips that define the torque waveform.

Geometric and Commutation Strategies

Engineers combat torque ripple primarily through the geometric arrangement of stator and rotor poles. In switched reluctance designs, the rotor typically carries fewer poles than the stator, a deliberate choice that prevents all poles from aligning simultaneously—a dead position where no torque can be generated—and smooths the transition between unaligned and aligned states. The stator itself features multiple projecting salient poles, reminiscent of a wound-field DC motor, while the rotor's projections serve as passive magnetic poles. For sustained rotation, the energized stator field must lead the rotor, continuously pulling the nearest pole from its unaligned state toward alignment. Modern switched reluctance variants exploit electronic commutation, which allows precise timing of phase energization and yields significant advantages in starting behavior, speed regulation, and minimizing the very ripple that plagued earlier mechanical commutation approaches. The inductance of each phase winding shifts with rotor position because the underlying reluctance changes, adding a further layer of design complexity.

From Prohibitive Cost to Embedded Intelligence

For decades, the very feature that makes reluctance motors attractive—their high power density at low manufacturing cost—was offset by the difficulty of controlling them. Before large-scale integrated circuits became widely available, the electronics needed to manage the position-dependent inductance and tailor drive waveforms were prohibitively expensive, confining these motors to niche or specialized uses. The breakthrough arrived with advances in electromagnetic theory, computer-aided design tools, and the proliferation of inexpensive microcontrollers. These embedded systems now execute real-time control algorithms that continuously adjust the drive waveform based on rotor position and feedback from current and voltage sensors. This capability transformed reluctance motors from laboratory curiosities into practical workhorses, enabling smooth operation at low speeds where torque ripple was once a disqualifying flaw. A motor family long limited by control complexity now competes across a wide spectrum of industrial and consumer applications.

Where Reluctance Motors Now Spin

Despite the torque ripple challenge, reluctance motors have found homes in a remarkably diverse set of applications. They drive the control rod mechanisms in nuclear reactors, power hard disk drive motors, and serve in electric vehicles where high power density is critical. In the workshop, they turn drill presses, lathes, and bandsaws; in the home, they appear in certain washing machine designs and even in the small mechanisms of analog electric clocks and meters. The synchronous reluctance variant, with its equal stator and rotor pole counts and internal flux barriers, offers a different profile: minimal rotor losses, operation at synchronous speed without current-conducting parts, and the ability to run on sinusoidal voltage once started. High-powered versions have traditionally relied on rare-earth elements like neodymium and dysprosium, though a 2023 study demonstrated a dual-phase magnetic laminate alternative that produced 23 kW at 14,000 RPM with 94% peak efficiency—nearly six times the output of a comparable conventional rotor—while forgoing those costly materials entirely.

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