As powertrains become increasingly electrified, the performance requirements placed on vehicle auxiliary systems are changing substantially. In vehicles powered by internal combustion engines, powertrain noise partly concealed the sound emitted by auxiliary equipment. In battery-electric vehicles, the much lower ambient noise floor makes these sources considerably easier to perceive. Thermal-management equipment, including HVAC blowers, cooling fans, and electric pumps, is therefore receiving growing attention from an acoustic standpoint.
Within such systems, the electric motor is a major contributor to both noise and mechanical excitation. Beyond broadband airborne sound, perceived acoustic quality can be strongly affected by tonal components caused by electromagnetic forces, nonlinear contact behavior, or structural resonances. These constraints must be addressed while maintaining high power density, since greater airflow or pumping capacity is expected from limited packaging space and under demanding thermal conditions.
This article presents an electric machine developed by Moving Magnet Technologies to enhance NVH performance. Its design uses a two-piece stator with a controlled mechanical interface, targeting a key source of unwanted excitation in segmented stator assemblies. The aim is to lower sound pressure throughout the full speed range while retaining a robust mass-production process that requires no supplementary damping elements.
2 Existing Technologies and Remaining Challenges
Most current thermal-management systems use brushless DC motors (BLDC) combined with integrated electronics. Although this arrangement offers benefits in efficiency, control, and system integration, it must also satisfy increasingly demanding targets for NVH, service life, electromagnetic compatibility, and cost.
Motor configurations commonly fall into two categories: outer-rotor and inner-rotor designs. Outer-rotor machines provide compact packaging and advantageous cooling, yet their acoustic performance can deteriorate, especially at elevated rotational speeds and in operating zones where tonal excitation becomes more pronounced. Inner-rotor machines can deliver better motor-level NVH behavior, but they may facilitate the transfer of structure-borne noise to neighboring components, making additional system-level countermeasures necessary.
To mitigate these effects, designers frequently add damping materials or isolation features such as resins, foams, elastomers, and mechanical decoupling elements. These solutions, however, increase total system cost, consume packaging volume, and add manufacturing complexity through operations such as dispensing, curing, rework, and additional testing. Their acoustic effectiveness may also vary with process dispersion and material aging.
Consequently, even mature solutions still face limitations when NVH, efficiency, and cost must be optimized simultaneously. In two-piece or segmented stators, mechanically generated micro-contacts and impulse-type excitations are a particularly important source of unwanted noise. To address this issue, a stator design was created to manage the contact mechanism, improve tolerance robustness, and optimize NVH without introducing extra materials or manufacturing stages.
3 Two-Piece Stator with a Controlled Contact Interface
Using a two-piece stator is an established way to simplify winding operations. Such assemblies generally comprise a toothed star section and a separate yoke section. Because manufacturing tolerances cannot be eliminated, small gaps may remain between these parts after assembly. When electromagnetic forces act on the structure, these clearances can generate micro-impacts, friction-related behavior, and nonlinear contact effects. The resulting impulse excitations can produce both broadband sound and distinct tonal components.
Conventional approaches either create a rigid joint, which transfers vibration directly, or retain mechanical clearance, which permits uncontrolled contact. Under changing temperature, speed, and load conditions, both approaches can lead to variations in acoustic behavior.
The concept described here introduces a controlled mechanical interface between the star block and the yoke block. Complementary shapes establish a defined point-contact zone maintained by elastic preload. Purpose-designed compliance in the star section keeps it continuously engaged with the yoke throughout all operating conditions.
Figure 1 Assembly of the stator components (© MMT)
This interface functions as a mechanical filter. By combining a smaller contact zone with preload, it avoids impulsive contact, limits friction-generated excitation, and permits controlled elastic relative displacement that attenuates structure-borne excitation. The design is also highly tolerant of production variation. Deviations produced during cutting and stacking are absorbed through elastic deformation of the star section, which reduces residual gaps by design. The controlled contact state stabilizes mechanical behavior across the operating envelope and lowers sensitivity to thermal expansion. Because the contact zone is limited, wear is also reduced, helping preserve acoustic performance over time.
Figure 2 Isthmus deformation as the tooth pivots (© MMT)
Production relies on standard industrial methods. Laminations may be manufactured with conventional punching tools or progressive dies, and both stator elements can be incorporated into the same stamping layout to improve material yield and cost efficiency. The stacking operation remains conventional, with no added parts or supplementary process stages. NVH enhancement is obtained solely through the geometry of the lamination stacks. The architecture also accommodates different winding approaches, including winding the open star section before final assembly or preassembling the winding unit to shorten cycle time.
4 Benefits in Electric-Vehicle Thermal Management
Electric-vehicle thermal management is an especially demanding use case. Systems must cover a broad speed range, withstand thermal loading, and meet stringent NVH targets at the same time. In HVAC blowers, motor-generated tones often shape perceived sound quality at low rotational speeds, while aerodynamic noise gains importance as speed rises. Cooling fans and pumps add further structure-borne coupling paths to the complete system.
The proposed stator reduces noise at its origin by suppressing micro-contact and impulse excitation within the motor. Its preloaded interface maintains consistent contact throughout the temperature range and avoids clearance-related acoustic changes caused by thermal expansion. Removing additional damping materials also decreases packaging demand and process complexity. As a result, the motor can be integrated more readily into compact thermal-management modules while reducing system-level compromises among NVH, cost, and performance.
Table 1 summarizes how the concept can be applied to automotive thermal-management functions.
Table 1 Relevance of the concept to automotive thermal-management applications
5 Test Methodology and Measured Performance
The concept was assessed at both component and complete-system levels. Comparative testing against reference motors identified a substantial NVH improvement over the full rotational-speed range.
Motor-level testing used an acoustic arrangement in a low-noise environment with a specified microphone distance. Results were analyzed in the time-frequency domain. Further measurements were then performed after integrating the motor into an HVAC module, allowing the design to be evaluated under representative installation and boundary conditions.
Figure 3 HVAC motor comparison: proposed design versus market references (© MMT)
Figure 3 presents measurements obtained from HVAC motors. At motor level, the sound pressure of the proposed machine rises progressively with rotational speed and does not display pronounced tonal peaks. Its frequency spectrum shows lower tonal content, particularly around electromagnetic excitation orders. This response is consistent with reduced impulse-type contact excitation and weaker interaction between electromagnetic forces and structural eigenmodes.
By comparison, reference motors typically show either a higher baseline level from low rotational speeds onward or strong tonal components associated with mechanical contact effects and/or electromagnetic excitation. Depending on the motor architecture and operating point, sound-pressure reductions of more than 20 dB(A) are achievable. This result underlines the value of mechanically stabilizing the stator contact interface in NVH-sensitive applications.
Figure 4 Waterfall analysis of HVAC motors: proposed design versus market references (© MMT)
Tests performed with the motor installed in an HVAC module, shown in Figure 4, confirm the system-level benefit. At high rotational speeds, broadband aerodynamic noise increasingly dominates the overall signal, so the total sound pressure levels of the different solutions tend to converge. At low speeds, however, flow noise remains limited and motor-related tonal content becomes easier to detect. In this region, a reference motor produces clearly perceptible tones, whereas the proposed design remains acoustically unobtrusive. The resulting improvement in perceived sound quality lowers the likelihood of NVH-related customer complaints in operating conditions that are particularly relevant to vehicle occupants.
Alongside its acoustic performance, the motor provides high output power, supporting increased rotational speed without a corresponding rise in perceived noise.
Figure 5 E-fan motor comparison: proposed design versus market references, measured without an impeller (© MMT)
Figure 5 also reports the performance assessment of an e-fan motor. The machine combines low acoustic output with high power and efficiency. According to the available measurements, it achieves approximately 49 dB(A) SPL at 1300 W with efficiency above 91 %. This provides a balanced combination of NVH, efficiency, and power density without the compromises commonly encountered between noise, power, and efficiency in conventional designs. For e-fan and HVAC systems in particular, the result creates additional flexibility in the operating strategy, including greater speed reserve with reduced acoustic prominence.
6 Summary and Outlook
The two-piece stator concept shows that focused mechanical design can markedly improve electric-machine NVH performance. A defined point-contact interface combined with elastic preload controls the coupling between structural parts, suppresses impulse-type contact excitation, and maintains stable acoustic behavior despite temperature changes and manufacturing tolerances.
The same design is compatible with industrial-scale production because it uses established manufacturing operations and remains robust to process tolerances. Since no additional damping material or manufacturing step is needed, the concept can reduce cost, packaging requirements, and production risk.
This technology therefore offers a scalable, cost-effective route to thermal-management systems and provides greater design freedom for quiet, highly integrated drives. It may also be extended to other electrified-mobility applications and supports a system-level approach in which mechatronic and fluidic functions are combined within fluidtronic architectures.
Authors
Bouali ROUACHED
works as an R&D Engineer specializing in direct-drive motors at Moving Magnet Technologies S.A. in Besançon, France.
Arnaud HYPPIAS
serves as Development Manager for mechatronic and fluidtronic systems at Moving Magnet Technologies S.A. in Besançon, France.
Damien LAFORGE
is Business Unit Manager for mechatronic and fluidtronic systems at Moving Magnet Technologies S.A. in Besançon, France.