Why SRM Acoustic Noise Is Difficult to Reduce Without a Torque Ripple Penalty

In a switched reluctance motor, acoustic noise and torque ripple share the same electromagnetic root cause—making them uniquely difficult to decouple. Tangential forces produce torque; radial forces excite stator structural modes and generate vibration and noise. Because both force components derive from the same magnetic flux linkage, any control action that reduces radial force harmonics risks perturbing the tangential force distribution and thereby altering torque ripple—and vice versa. This coupling makes SRM noise control substantially more complex than in permanent magnet motor counterparts, where the two force components can be addressed more independently.

Research from SJB Institute of Technology (2021) identifies radial magnetic force and torque ripple as the two primary acoustic noise causes in SRMs, noting that radial force is strongly influenced by motor design while torque ripple is the primary controllable variable accessible through drive-level intervention. The Group of Electrical Engineering Paris (GeePs, 2019) formalises the noise-torque trade-off explicitly, observing that direct radial force control must be carefully coordinated with average torque control to avoid degrading torque output.

The structural resonance dimension compounds this challenge. McMaster University’s 2023 multiphysics finite element analysis (FEA) of an 8/6 SRM confirms that acoustic noise amplification occurs when radial force harmonic frequencies coincide with stator natural frequencies. A multi-physics model validated against measured acoustic data was required before structural modifications could be optimised without penalising electromagnetic performance. According to IEEE standards for electric motor characterisation, multiphysics validation is now considered essential for noise-optimised motor design.

In auxiliary pump applications specifically—cooling pumps, oil pumps, and hydraulic actuator pumps—motors operate continuously at moderate speeds under steady hydraulic load. The dominant noise concern is tonal acoustic emission at specific radial force harmonic orders, rather than broadband noise. This characteristic makes harmonic-selective suppression strategies more tractable than in traction-drive SRM applications subject to wide speed variation, and it is precisely the operating profile that the most advanced patent filings are designed to exploit.

Research on the 10/4 SRM topology from Three Gorges University (2017) confirms that increasing the number of stator poles redistributes radial electromagnetic force, reducing peak force amplitudes while simultaneously increasing air gap flux and electromagnetic torque—a rare case where a structural modification yields co-benefits for both noise and torque performance.

Current Waveform Shaping and Radial Force Harmonic Reduction: The Leading Control Strategy

Selective radial force harmonic suppression via optimised current waveforms is the most promising avenue for achieving SRM noise reduction without torque ripple penalty, because it targets the specific harmonic orders responsible for tonal noise while preserving the average torque-producing component of the current profile. The most commercially advanced work in this domain comes from Turntide Technologies and ENEDYM Inc., whose independently developed patent families address the same physical problem through complementary optimisation frameworks.

Turntide Technologies’ approach, documented across US and WO patent filings (2023), applies turn-on and turn-off angles at the optimum current waveform to improve system efficiency concurrently with noise reduction. The proportional relationship between specific radial force harmonics at stator teeth and the resulting acoustic noise at those harmonic frequencies allows the processor-based noise reduction application to systematically minimise sound pressure level at targeted harmonic orders. The dual-jurisdiction filing confirms the commercial significance of this approach for automotive drive suppliers.

ENEDYM Inc.’s complementary approach (Canada, 2023; India, 2024) evaluates a plurality of candidate current waveforms by computing a cumulative sound pressure level for each, derived from the harmonic SPL contributions expected at multiple orders. The waveform associated with the optimal cumulative SPL is then applied to the phase coils. Critically, the ENEDYM patent specification explicitly states that noise reduction is achieved without sacrificing other motor performance metrics—directly addressing the torque ripple penalty concern. This waveform selection framework is well-suited to auxiliary pump duty cycles where the operating point is relatively stable and an optimal waveform can be pre-computed and stored for each speed-load combination.

“Field reconstruction-based current shaping was identified as the most effective approach for vibro-acoustic reduction while maintaining electromagnetic performance”—Illinois Institute of Technology comparative study, 2021, evaluating four control candidates on a high-rotor pole SRM.

The Illinois Institute of Technology comparative study (2021) evaluates four control candidates on a high-rotor pole SRM: phase advancing, current shaping based on field reconstruction, and random hysteresis band with and without spectrum shaping. Field reconstruction-based current shaping was identified as the most effective approach for vibro-acoustic reduction while maintaining electromagnetic performance, validating the current waveform shaping paradigm through both simulation and experiment.

The GeePs direct force control (DFC) with reference current adapter (RCA) architecture (2019) auto-tunes the current reference to balance the DFC and average torque control (ATC) objectives, demonstrating experimentally that the trade-off between radial force suppression and torque maintenance can be managed dynamically. Optimization-based direct torque control is a further control-layer strategy: both an Indian patent (Dr. Padmaja Venugopal, 2021) and SJB Institute of Technology (2021) deploy the artificial raindrop algorithm to optimise PI controller gains within a direct torque control framework, minimising torque ripple as the route to minimum acoustic noise. The University of Modena (2023) uses linear regression trained on simulation data to compute optimal current supply angular intervals as a function of speed and electromagnetic torque in real time, demonstrating simultaneous reductions in torque ripple and RMS phase current without geometric modification of the machine.

PWM Carrier Frequency Modulation: A Low-Cost Drive-Level Intervention for Pump Applications

For auxiliary pump applications where controller simplicity and cost are primary constraints, PWM-level interventions offer a practical complement to—or substitute for—full current waveform optimisation. The core principle is spectral spreading: by continuously varying the PWM carrier frequency, the noise energy that would otherwise concentrate at a single resonance-exciting frequency is distributed across a broader band, reducing peak acoustic emission without altering the fundamental motor control architecture.

CSMC Technologies’ patented method (US, 2020 and 2022) continuously varies the carrier frequency of the PWM drive signal as the SRM’s operating period changes—specifically triggered by phase commutation events. By preventing the acoustic noise harmonic from dwelling at a fixed frequency that could excite a structural resonance, the approach spreads noise energy spectrally without requiring changes to the fundamental current profile or turn-on/turn-off angles. The continuation patent (2022) confirms ongoing commercial development of this approach.

The spectrum-spreading concept is academically validated across motor types. Liaoning Technical University (2021) extends random PWM (RPWM) to selectively eliminate specific harmonic frequencies responsible for high electromagnetic vibration in induction motors, rather than simply spreading all harmonics uniformly. This selective approach is directly applicable to SRM auxiliary drives where certain radial force harmonic orders at stator tooth-passing frequency are far more problematic than others. As documented by IEEE, selective harmonic elimination techniques in power electronics have matured significantly for motor drive applications since 2015.

Ford Global Technologies has patented a closely related concept (US, 2018) wherein the PWM controller selects pseudorandom sequences pre-optimised for each operating region—defined in a torque-speed map—to simultaneously reduce audible noise and switching losses. This dual benefit of noise reduction plus efficiency improvement is a key advantage for pump applications where both acoustic comfort and energy consumption are primary design requirements. LG Electronics’ earlier patent (KR, 2000) introduced PWM duty variation with dual-phase current control to prevent current change at the freewheeling section from generating noise, providing early experimental validation of the fundamental PWM-level intervention principle.

For pump drive designers evaluating PWM-based approaches, the key trade-off relative to current waveform shaping is noise reduction depth versus implementation complexity. PWM carrier frequency modulation requires no modification to the current control loop and is implementable on existing drive hardware via firmware update. However, it spreads rather than eliminates targeted harmonic energy, meaning the peak SPL reduction is generally lower than that achievable through selective radial force harmonic suppression—particularly at the specific harmonic orders most problematic in steady-speed pump operation. As WIPO patent trend data confirms, the most recent filings (2021–2024) combine both approaches, using PWM modulation as a first-stage broadband attenuator and current waveform optimisation for residual tonal suppression.

Structural Design for Noise-Torque Co-Optimisation at the Machine Level

Structural modifications to the SRM itself can reduce the sensitivity of the stator to radial force excitation, effectively decoupling the noise-torque relationship at the machine level and reducing the burden placed on the drive controller. The most comprehensive investigation directly targeting SRM acoustic noise through structural design comes from McMaster University’s Automotive Resource Centre (MARC), whose 2023 multiphysics study of the 8/6 four-phase SRM is the most experimentally validated structural investigation in the surveyed dataset.

The McMaster study evaluates multiple structural techniques—stator back iron thickness variations, rotor pole shape modification, and auxiliary slot introduction—and confirms experimentally that structural modification can reduce acoustic noise while maintaining electromagnetic performance, provided the multiphysics model adequately captures the coupling between electromagnetic force, structural dynamics, and acoustic radiation. This requirement for multiphysics validation before structural optimisation is consistent with guidance from IEC standards on electromagnetic compatibility and acoustic testing of rotating electrical machines.

The double auxiliary slot (DAS) approach from Chongqing Jiaotong University (2021) modifies local tooth profiles of both stator and rotor to redirect magnetic flux lines, reducing radial magnetic flux density in the air gap. The study investigates the influence of turn-on angle, turn-off angle, and initial rotor position on both radial electromagnetic force and maximum torque simultaneously, establishing that torque output can be preserved while radial force is reduced through combined geometrical and commutation angle optimisation. This dual-lever approach—structural plus control co-optimisation—is particularly relevant for custom pump motor designs where geometric freedom exists during the product development phase.

The 10/4 SRM pole configuration from Three Gorges University (2017) demonstrates that increasing stator pole count from the conventional 8/6 to 10/4 increases both the number of torque-producing commutation events per revolution and the air gap flux, while early turn-off angle control reduces stator excitation current. The net effect is simultaneously reduced radial electromagnetic force amplitude and preserved or improved electromagnetic torque. For auxiliary pump applications demanding compact form factor and low noise, the 10/4 topology merits consideration at the motor specification stage.

For pump motor configurations where liquid cooling is already present in the assembly, Florida State University (2021) proposes immersing the stator in evaporative cooling liquid medium, using fluid pressure to counteract radial electromagnetic force directly. In pump motor configurations where coolant or hydraulic fluid is already present in or around the motor housing, this concept translates naturally to a system-level passive vibration suppression solution with no torque impact—an approach uniquely compatible with the pump system integration context.

Key Innovators, Patent Landscape, and the Convergence Trend

Analysis of the patent and literature data reveals a clear stratification of innovators by approach and commercial maturity, with a significant convergence trend in the most recent filings toward joint optimisation of motor geometry, commutation angles, and current waveform as a unified design variable set.

Commercial Patent Leaders

Turntide Technologies is the most active patent filer specifically targeting SRM acoustic noise reduction, with multiple active US and WO patents on selective radial force harmonic reduction via optimised current waveforms. Their approach is notable for explicitly optimising turn-on/turn-off angles concurrently with waveform shaping, targeting both noise and efficiency simultaneously. ENEDYM Inc. (Canada/India) has a complementary patent family emphasising cumulative SPL minimisation across harmonic orders as the optimisation criterion, with explicit claims of no sacrifice to other motor performance metrics. ENEDYM’s approach appears particularly well-suited to auxiliary applications with defined duty cycles. CSMC Technologies Fab2 holds two active US patents on PWM carrier frequency variation for SRM noise reduction, representing the most commercially accessible approach for cost-driven pump drive designs.

Academic Research Leaders

McMaster University (MARC) leads academic structural design research, with the 2023 multiphysics study of the 8/6 SRM being the most experimentally validated structural investigation in the dataset. Illinois Institute of Technology leads academic power electronics control comparison research for high-rotor-pole SRMs. GeePs (France) contributes the DFC+RCA control architecture specifically designed for automotive SRM NVH. SJB Institute of Technology and University of Modena contribute optimisation-based direct torque control and angular interval strategies respectively.

“The most recent patents and papers (2021–2024) increasingly treat motor geometry, commutation angles, and current waveform as jointly optimised variables rather than independent design choices—a paradigm that maximises the achievable noise reduction without torque ripple or efficiency penalty.”

Recommended Approach Selection by Application Constraint

• Maximum noise reduction, no torque ripple penalty: Selective radial force harmonic suppression via optimised current waveforms (Turntide Technologies / ENEDYM Inc. patent approaches), validated by Illinois Institute of Technology field reconstruction-based current shaping.
• Low implementation cost, firmware-only intervention: PWM carrier frequency modulation triggered by phase commutation (CSMC Technologies approach), with pseudorandom sequence pre-optimisation per operating point (Ford Global Technologies approach).
• Custom motor design with geometric freedom: Combined structural modification (DAS slots, rotor pole shaping, air gap optimisation) plus commutation angle co-optimisation (McMaster University / Chongqing Jiaotong University approaches).
• System-level integration in liquid-cooled pump assemblies: Stator immersion in cooling medium as passive radial force countermeasure (Florida State University approach), with zero electromagnetic torque impact.
• Novel topology selection at specification stage: 10/4 pole configuration for simultaneous radial force reduction and torque preservation (Three Gorges University).

The convergence of these approaches in the most advanced 2021–2024 filings—particularly the explicit joint optimisation of geometry, commutation angles, and current waveform—suggests that the next generation of commercial SRM auxiliary pump drives will deploy layered noise reduction strategies rather than relying on any single technique. For engineering teams evaluating SRM adoption, understanding the full patent landscape across all three approach categories is essential to both freedom-to-operate analysis and competitive technology positioning. PatSnap’s innovation intelligence platform, used by over 18,000 customers across 120+ countries, provides the patent search, citation mapping, and technology landscape tools needed to navigate this rapidly evolving space.