Why SRM Noise Is Uniquely Hard to Suppress Without Torque Penalty

In a switched reluctance motor, acoustic noise originates from two sources that are electromagnetically coupled: tangential forces, which produce torque, and radial forces, which excite stator structural modes and generate vibration and noise. Because both forces 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 is the fundamental reason why SRM noise control is substantially more complex than in permanent magnet motor counterparts.

The noise-torque coupling is explicitly characterised by the Group of Electrical Engineering Paris (GeePs, 2019), which formulates the trade-off directly: direct radial force control must be carefully coordinated with average torque control (ATC) to avoid degrading torque output. 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 structural resonance dimension compounds the challenge. Multiphysics finite element analysis (FEA) by McMaster University (2023) confirms that acoustic noise amplification occurs when radial force harmonic frequencies coincide with stator natural frequencies. The 8/6 SRM studied in that work—a configuration representative of auxiliary pump drive sizes—required a validated multi-physics model before structural modifications could be optimised without penalising electromagnetic performance. According to standards bodies such as ISO, quantifying vibro-acoustic performance requires precisely this kind of coupled structural-electromagnetic modelling.

In auxiliary pump applications—cooling pumps, oil pumps, and hydraulic actuator pumps—motors operate continuously at moderate speeds under steady hydraulic load. The dominant noise concern is therefore tonal acoustic emission at specific radial force harmonic orders, rather than broadband noise. This makes harmonic-selective suppression strategies, whether through current shaping or structural design, more tractable than in traction-drive SRMs subject to wide speed variation.

Current Waveform Shaping: The Leading Approach for Noise-Torque Co-Optimisation

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 independently targets radial force harmonics while preserving the average torque-producing component of the current profile. Two independently developed commercial patent families—from Turntide Technologies and ENEDYM Inc.—represent the state of the art.

Turntide Technologies has filed a family of patents describing an SR drive with an acoustic noise reduction system that derives an optimum current waveform satisfying two simultaneous conditions: generating average torque that meets the torque demand while creating radial force with minimum amplitude at the desired harmonic orders. The system applies turn-on and turn-off angles at the optimum current waveform to improve system efficiency concurrently. 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 orders. This patent family spans US and WO jurisdictions, confirming the commercial significance of the approach.

“The optimum current waveform simultaneously satisfies average torque conditions and minimises radial force amplitude at targeted harmonic orders—a constraint that makes SRM noise control substantially more complex than in permanent magnet counterparts.”

ENEDYM Inc. (Canada, 2023) has developed a complementary but independently derived approach. The ENEDYM system 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. Crucially, the 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.

At the academic level, Illinois Institute of Technology’s 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. Research published by IEEE has consistently identified field reconstruction methods as among the most precise for decoupling electromagnetic force components in variable-reluctance machines.

The GeePs direct force control (DFC) with a reference current adapter (RCA) auto-tunes the current reference to balance the DFC and ATC objectives, demonstrating experimentally that the trade-off between radial force suppression and torque maintenance can be managed dynamically. The University of Modena (2023) takes a complementary optimization-based approach: linear regression trained on simulation data computes 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 and Drive-Level Interventions

For auxiliary pump applications where controller simplicity and cost are primary design constraints, PWM-level interventions offer a practical complement to—or substitute for—full current waveform optimisation. The core mechanism is spectral spreading: by varying the PWM carrier frequency, the acoustic energy that would otherwise concentrate at a fixed harmonic frequency is redistributed across a wider band, reducing the peak sound pressure level at any single frequency.

CSMC Technologies has patented a direct implementation of this principle (US, 2020): the carrier frequency of the PWM drive signal is continuously varied 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 the noise energy spectrally without requiring changes to the fundamental current profile or turn-on/turn-off angles. A continuation patent (CSMC Technologies, US, 2022) confirms ongoing commercial development of this approach.

Liaoning Technical University (2021) extends random PWM (RPWM) to selectively eliminate specific harmonic frequencies responsible for high electromagnetic vibration, 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. Ford Global Technologies has patented a closely related concept (US, 2018): a PWM controller that selects pseudorandom sequences pre-optimised for each operating region, defined in a torque-speed map, to simultaneously reduce audible noise and switching losses—a key advantage for pump applications where efficiency is also a primary design requirement. WIPO patent data confirms that PWM-level noise reduction techniques for electric drive systems have been among the most actively filed categories in automotive motor control over the past decade.

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—an early validation of the fundamental PWM-level intervention principle that underpins the more sophisticated CSMC and Ford approaches.

Structural Design and Topology Selection for Machine-Level Noise Decoupling

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. Three structural levers have been experimentally validated in configurations relevant to auxiliary pump drives: stator geometry modification, rotor pole topology selection, and air gap optimisation.

Stator Geometry: Back Iron, Auxiliary Slots, and Multiphysics Validation

McMaster University’s multiphysics study (2023) of the 8/6 SRM evaluates multiple structural techniques—including 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. Institutions such as IEEE have documented that multiphysics modelling is now considered a prerequisite for credible NVH optimisation of electric motor structures.

The double auxiliary slot (DAS) approach studied at 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.

Pole Topology: The 10/4 Configuration

Three Gorges University’s research on the 10/4 SRM topology (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 the stator excitation current. The net effect is simultaneously reduced radial electromagnetic force amplitude and preserved or improved electromagnetic torque—a rare case where structural modification yields co-benefits for both noise and torque without any drive-level intervention.

Air Gap Length: The Dominant Structural Parameter

Hebei University of Science and Technology’s parametric FEA analysis of a 55 kW 12/8 SRM for air compressor applications (2017)—a configuration sharing significant design similarity with pump drives—identifies air gap length as the dominant structural parameter affecting vibration amplitude. This finding provides a straightforward optimisation lever during motor specification that does not directly impair torque output. According to data published by OECD on electrification technology trends, air gap geometry optimisation is among the lowest-cost structural interventions available during the motor design phase.

Liquid Cooling Medium as Passive Radial Force Countermeasure

For applications where liquid cooling is already present in the pump assembly, Florida State University (2021) proposes immersing the stator in evaporative cooling liquid medium, using the 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—making it uniquely compatible with the physical environment of EV auxiliary pump assemblies.

Key Innovators and the Convergence Trend Shaping SRM Pump Drive Development

Analysis of the patent and literature dataset reveals a clear stratification of innovators by approach and commercial maturity, with a significant convergence trend emerging in the most recent filings (2021–2024).

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 and 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, representing the most commercially accessible approach for cost-driven pump drive designs.

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.

“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 achievable noise reduction without torque ripple or efficiency penalty.”

A significant trend observable across the dataset is the convergence of structural co-design and drive control optimisation. 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. This paradigm shift—documented across the Chongqing Jiaotong University DAS study, the University of Modena angular interval work, and the Turntide and ENEDYM patent families—maximises the achievable noise reduction without torque ripple or efficiency penalty. For EV auxiliary pump programme teams, this convergence signals that the highest-performance solutions will require co-development between motor design and drive software teams from the earliest specification stage.