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Article

Experimental Evaluation of Acoustical Materials for Noise Reduction in an Induction Motor Drive

by
Ashish Kumar Sahu
*,
Abeka Selliah
,
Alaa Hassan
,
Moien Masoumi
and
Berker Bilgin
McMaster Automotive Resource Center (MARC), Hamilton, ON L8P 0A6, Canada
*
Author to whom correspondence should be addressed.
Machines 2024, 12(8), 499; https://doi.org/10.3390/machines12080499
Submission received: 6 June 2024 / Revised: 17 July 2024 / Accepted: 22 July 2024 / Published: 23 July 2024
(This article belongs to the Section Electrical Machines and Drives)
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Abstract

:
Electric propulsion motors are more efficient than internal combustion engines, but they generate high-frequency tonal noise, which can be perceived as annoying. Acoustical materials are typically suitable for high-frequency noise, making them ideal for acoustic noise mitigation. This paper investigates the effectiveness of three acoustical materials, namely, 2″ Polyurethane foam, 2″ Vinyl-faced quilted glass fiber, and 2″ Studiofoam, in mitigating the acoustic noise from an induction motor and a variable frequency inverter. Acoustic noise rates at multiple motor speeds, with and without the application of acoustical materials, are compared to determine the effectiveness of acoustical materials in mitigating acoustic noise at the transmission stage. Acoustical materials reduce acoustic noise from the induction motor by 5–14 dB(A) at around 500 Hz and by 22–31 dB(A) at around 10,000 Hz. Among the tested materials, Studiofoam demonstrates superior noise absorption capacity across the entire frequency range. Polyurethane foam is a cost-effective and lightweight alternative, and it is equally as effective as Studifoam in mitigating high-frequency acoustic noise above 5000 Hz.

1. Introduction

The shift towards vehicle electrification has led to a focus on the acoustic noise generated by electric motors as it might impact the overall quality of the system, especially in noise-sensitive applications, such as electric propulsion. Noise characteristics of an electric motor are different from those of Internal Combustion Engines (ICEs) [1,2]. Electric motors generally produce lower noise levels compared to ICEs [3,4,5]. However, the noise generated by electric motors can be perceived as more irritating due to its high-frequency and tonal nature [6].
Acoustic noise sources in an electric motor can be broadly categorized as follows: (i) electromagnetic sources such, as electromagnetic forces and switching excitation; (ii) mechanical sources, such as shaft and bearing defects and eccentricity; and (iii) aerodynamic sources, such as fan and moving air due to rotor motion [7,8]. High-frequency tonal acoustic noise in electric motors is commonly attributed to radial electromagnetic forces [9,10,11]. Strategies for mitigating acoustic noise can be applied at the source, during transmission, or at the receiver end [12].
Primary approaches to mitigating acoustic noise at the source in electric motors typically involve enhancing electromagnetic design or implementing current control techniques [10,13]. Optimization in rotor and stator geometry, as well as pole pair configurations, have shown significant noise reduction. An 8-pole, 48-slot configuration is approximately 5 dB quieter than a 46-pole, 48-slot configuration in a permanent magnet synchronous motor (PMSM) [14].
The impact of rotor notches, stator and rotor skew, and rotor slot numbers on acoustic noise in electric motor motors has also been researched. Introducing notches on the rotor outer circumference has resulted in about a 5 dB noise reduction in a PMSM [15] and an induction motor (IM) [16]. Skewing the stator stack in a PMSM has led to an acoustic noise reduction of approximately 5 dB [17]. Varying the rotor slot number can also impact the acoustic noise level in an IM [18]. Current control techniques such as pulse width modulation (PWM) have been applied in a PMSM, achieving up to 25 dB(A) noise reduction [19], and in an IM to reduce overall acoustic noise level [20]. The radial force shaping method has been employed in a switched reluctance motor (SRM), and up to 20 dB noise reduction was achieved [21].
Mitigating acoustic noise along the transmission path in an electric motor can be achieved by structural design improvements, applying active and passive acoustic noise, vibration reduction techniques, and noise masking methods [12]. The frame thickness and rib pattern have been investigated for an SRM to achieve up to a 16 dB acoustic noise reduction [22]. Rotor and stator geometries have been investigated for acoustic noise reduction in [23]. Active noise and vibration reduction techniques are considered more suitable for low-frequency applications. The use of viscoelastic materials as passive vibration dampers has been explored for vibration reduction in an SRM, which can lead to a reduction of 5–10 dB in equivalent radiated power [24]. Acoustical material as a passive acoustic noise mitigation approach has been applied to electric motors [25,26].
Acoustic materials offer a distinct advantage that they can be applied without compromising the electromagnetic performance of the motor, a contrast to the potential impacts associated with electromagnetic design enhancements and the implementation of current control techniques. Moreover, applying acoustic material on an electric propulsion motor can be easier and less expensive compared to structural design modification and noise masking methods [12]. Additionally, acoustical materials are particularly well-suited for addressing high-frequency acoustic noise applications, making them an ideal choice for mitigating noise in an electric propulsion motor. A sound barrier and sound absorber used together as an encapsulation have been applied to an electric motor to achieve 5.7 dB(A) acoustic noise reduction at 6300 Hz [27]. Motor encapsulation has been applied in a Mercedes-Benz EQC to mitigate acoustic noise from an e-motor drivetrain [28]. Up to 13 dB(A) noise reduction is observed during the motor ramp-up condition using motor encapsulation of 32 mm thick glass-wool- and glass-fiber-reinforced plastic as the outer cover [29]. The effectiveness of an encapsulation depends on the thickness of the encapsulation and how well it covers the noise source [25].
The use of acoustical material enclosures presents a disadvantage for fan-cooled motors as it may negatively affect their thermal performance. However, this application could be beneficial for liquid-cooled motors as their thermal management does not rely on air circulation around the motor.
Acoustic noise calculation in an electric machine is a complex and non-linear multiphysics problem. It involves electromagnetic simulations for radial force calculation, mechanical simulations for modal and structural harmonic analysis of the stator and housing assembly, and acoustic simulations for noise prediction. The numerical approach has typically been preferred for acoustic noise calculation due to its higher accuracy. The finite element approach has been employed to investigate the temporal and spatial harmonics of electromagnetic radial forces in an electric motor [30]. The acoustic noise and vibration of electric motors due to electromagnetic forces have been investigated using the finite element method [31].
In this paper, the acoustic mitigation performance of three different acoustical materials is investigated for an induction motor and a variable frequency inverter. The acoustical materials considered for investigation are 2″ Polyurethane foam, 2″ Vinyl-faced quilted glass fiber, and 2″ Studiofoam. First, the acoustic noise from the induction motor drive at multiple speeds is measured. Then, the acoustic noise measurements are conducted with the application of the acoustical materials. The reduction in acoustic noise achieved due to the acoustical materials is then compared. Next, the effect of the acoustical material application on the thermal performance of the induction motor is investigated. Finally, the cost and operational temperature range for the acoustical materials are presented for a practical application.

2. Acoustical Materials

Acoustical materials reduce acoustic noise by creating impedance in the path of sound waves. Acoustical materials can be broadly categorized as sound absorbers and sound barriers.

2.1. Sound Absorbers

Sound absorbers can be classified as follows: (i) resonant sound absorbers such as metamaterials; (ii) fibrous sound absorbing material such as glass fiber and cotton fiber; and (iii) foam sound absorption such as Polyurethane foam and Melamine foam. Resonant absorbers function based on the Helmholtz principle [32], utilize multiple cavities to trap sound waves, induce resonance, and dissipate energy. Their application is suitable for low-frequency narrow-band acoustic noise. Energy dissipation in fibrous and foam sound absorbers occurs mainly in three ways, namely, (i) friction between the air molecules in the pores, (ii) compression and expansion of air in the pores, and (iii) resonance of air molecules in the pores [33], and their application is appropriate for mid- and high-frequency range [34,35].
The efficiency of a sound absorber is quantified by its absorption coefficient, representing the ratio of absorbed sound energy to incident sound energy. The sound absorption coefficient, α , is mathematically expressed as (1), where E a , E i , and E r are the absorbed, incident, and reflected sound energy, respectively. The sound absorption coefficient has a maximum value of 1. A higher value indicates greater efficiency in absorbing sound. Equation (2) shows that porosity, tortuosity, and flow resistivity collectively play a significant role in determining the acoustic performance of the sound absorber [36].
α = E a / E i = ( E i − E r ) / E i .
d 2 p d x 2 = s c 0 d 2 p d t 2 + r h c 0 2 ρ 0 d p d t .
Porosity, ρ , is the measure of the void volume compared to the total structure volume; flow resistivity, r, is the ratio of pressure gradient to flow velocity; and tortuosity, s, describes the complexity of the porous structure. In (2), p, x, and t denote the sound pressure, position, and time of a sound wave, respectively. The absorption coefficient is minimal at lower frequencies but experiences a substantial increase at higher frequencies when the thickness of the material designed for sound absorption is at least a quarter of the wavelength of the corresponding sound wave [37]. The sound absorption coefficient is influenced by an increase in temperature, as the acoustic speed and viscosity of air change, resulting in a decrease in the acoustic absorption coefficient at lower frequencies [38,39].

2.2. Sound Barriers

A sound barrier is a high-density, non-porous limp material which does not allow the sound energy to pass through it. Sound barriers can be made of stiff materials, such as steel and polycarbonate, or loaded mass materials, such as loaded Polyvinyl Cloride (PVC) and loaded Polyurethane (PU) [40]. The term loaded here refers to the use of a filler material that increases the density, and the term mass refers to materials that provide limpness. Limpness is defined as the quality of being soft and flexible. The effectiveness of a sound barrier is determined by its sound transmission loss (STL). Mathematically, STL is calculated using (3), where τ is the ratio of transmitted sound energy to incident sound energy. The unit of STL is dB, and the STL of a sound barrier typically increases with frequency [41]. A higher STL indicates better efficiency of the sound barrier.
S T L = 10 log ( 1 / τ ) .
Porosity, limpness, and density of a sound barrier are the critical parameters that affect its STL [40]. The presence of holes allows sound energy transmission through sound barriers, leading to a decrease in STL. A flexible or limp sound barrier does not transmit its inherent vibrational energy, making it a better choice for encapsulating structural noise sources. Increased density introduces resistance to sound energy, resulting in an increase in the STL of a sound barrier. The STL of sound barriers typically increases with frequency [40,41].

3. Acoustic Noise Measurement Setup

3.1. Acoustical Material Properties

The effectiveness of acoustical material on high-frequency noise is investigated on an induction motor mounted on a test bench and its wall-mounted variable speed inverter. Three acoustical materials are considered for this study: (i) 2″ Polyurethane foam (Acoustical material 1); (ii) 2″ Vinyl-faced quilted glass fiber (Acoustical material 2); and (iii) 2″ Studiofoam (Acoustical material 3). The Polyurethane foam is sourced from Aearo Technologies, the Vinyl-faced quilted glass fiber is supplied by Sound Seal, and the Studiofoam is supplied by Auralex.
Figure 1 shows the cross-section of the three materials. Polyurethane foam is a foam sound absorber, and quilted glass fiber and Studiofoam are fibrous sound absorbers. Acoustical Material 1 and Acoustical Material 3 have a uniform cross-section, whereas the cross-section of Acoustical Material 2 is non-uniform along the length.
Figure 2 and Table 1 compare the sound absorption coefficient of the three acoustical materials based on their datasheets [42,43,44]. Material 1 shows a low absorption coefficient at 125 Hz and 250 Hz, followed by an increase that remains high at higher frequencies. Material 2 shows a low absorption coefficient at 125 Hz and a high absorption coefficient at 250 Hz and 500 Hz and, subsequently, a decrease. Material 3 displays a low absorption coefficient at 125 Hz and 250 Hz, with an increase that remains high at higher frequencies, surpassing Material 1. At high frequencies, Material 3 is reported to have an absorption coefficient higher than 1, which is theoretically not possible. This anomaly is attributed to the edge effect or diffraction effect observed during experimental absorption coefficient measurements [45].
Notably, Material 2 provides a noise barrier capability as well. Table 2 shows the STL for Material 2. The STL increases with an increase in frequency, but the absolute value of STL is low, leading to a low sound transmission coefficient (STC) of approximately 19 dB. STC is defined as the average value of STL. According to the National Research Council of Canada, a sound transmission coefficient (STC) exceeding 50 dB is crucial for realizing the effectiveness of sound barriers [46].
The datasheet typically provides the sound absorption coefficient and sound transmission loss for frequencies of 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. Acoustical Material 2 demonstrates superior sound absorption coefficients between 250 Hz and 500 Hz. Acoustical Materials 1 and 3 exhibit better sound absorption coefficients above 500 Hz, with Acoustical Material 3 outperforming Acoustical Material 1. Additionally, Acoustical Material 2 has notable sound transmission loss characteristics due to the Vinyl quilt.

3.2. Experimental Setup

Figure 3 shows the experimental setup for acoustic noise measurement. The acoustic noise radiating from the induction motor drive is measured by a microphone, collected by a data acquisition system, and postprocessed with a signal analyzer. The microphone is positioned at a distance of 1 m from the induction motor, mounted on the test bench, and approximately 2 m from the variable frequency inverter affixed to the wall. The 11 kW induction motor has a 300 mm outer diameter and a 525 mm axial length and operates at 208 V with a current of 37.7 A. The experiments involve the use of the ACS800-11 variable frequency inverter from ABB. Table 3 lists the instruments used for acoustic noise measurement and their make and model.
Acoustical material application can be achieved through direct mounting on the motor surface, referred to as motor-mounted encapsulation, or by mounting it on the body structure surrounding the motor, referred to as body-mounted encapsulation [29]. Figure 4a,b show the overall dimensions of the frames designed for encapsulating the induction motor and inverter. A minimum gap of 50 mm is necessary between the air-cooled variable frequency inverter and the external encapsulation to ensure adequate cooling of the inverter under operational conditions. Furthermore, sufficient spacing must be maintained between the air-cooled induction motor and the encapsulation to ensure proper cooling of the motor. This critical aspect is addressed during the design of the frame for encapsulation mounting. Frames are 3D printed and made of plastic to keep them lightweight. Figure 4c,d show the cross-section of the two structural members of the frames. Structural Member 1 has 60 mm height and 20 mm width, and Structural Member 2 has 40 mm height and 20 mm width. Both members incorporate a hollow cross-section with a 10 mm wall thickness to keep them lightweight. The height of both structural members is bigger than their width to provide a larger bending strength against the weight of the acoustical materials. Structural Member 2 is used for the induction motor frame and vertical members of the inverter frame. Structural Member 1 is designed for the horizontal elements of the inverter frame, ensuring robust support for wall-mounted applications as acoustical material weight is greater for the inverter. Other smaller parts were also designed to connect the structural members.
Figure 5a shows the inverter motor frame employed on the induction motor as a proof of concept for body-mounted encapsulation. To ensure adequate cooling, the air-cooled variable frequency inverter requires a minimum of a 50 mm gap on all sides if external covering is applied. Figure 5b shows the inverter frame with sufficient clearance on all sides to apply acoustical materials and maintain appropriate cooling. Figure 5c–e show the induction motor drive encapsulated with the three acoustical materials. Double-sided adhesive tapes are applied to the adjacent edges of acoustical materials to prevent unintended acoustic noise leakage from openings.

4. Experimental Acoustic Noise Measurements

The effectiveness of acoustical materials is studied for inverter noise and for induction motor drive noise at multiple speeds. Figure 6 compares the contour plot of the inverter running with and without a cooling fan before an acoustical material is applied. These contour plots depict the sound pressure as a function of time and frequency. The temporal average of acoustic noise is considered for comparison. The acoustic noise peaks of 67 dB(A) at 400 Hz, 60 dB(A) at 800 Hz, and 59 dB(A) at 1190 Hz are observed from the inverter. In contrast, the noise peak from the inverter operating without a cooling fan is less than 50 dB(A), indicating that the primary source of noise in the inverter is the cooling fan. The acoustic noise peak at 400 Hz is the fundamental frequency of the fan noise, whereas acoustic noise peaks at 800 Hz and 1190 Hz are the higher-order harmonics of the fan noise.
Figure 7 compares the acoustic noise contour plot from the induction motor drive operating at 1000 RPM and 1500 RPM before an acoustical material is applied. The inverter fan is operational in these experiments. In addition to the acoustic noise peaks in Figure 6a, noise peaks of 61 dB(A) at 1560 Hz and 52 dB(A) at 5360 Hz occur at 1000 RPM, as shown in Figure 7a. At 1500 RPM, there are additional noise peaks of 61 dB(A) at 1570 Hz and 60 dB(A) at 4700 Hz occur, as shown in Figure 7b. Below 1500 Hz, the acoustic noise appears to be primarily originating from the inverter, whereas above 1500 Hz, it is dominated by induction motor noise. The switching frequency of the ACS800-11 variable frequency inverter from ABB fluctuates between 4000 Hz and 6500 Hz when operating up to 1500 RPM, resulting in higher acoustic noise observed within this frequency band. The acoustic noise bands between 1500 Hz and 2000 Hz, when operating up to 1500 RPM, could be attributed to radial force harmonics, which depend on the number of pole pairs, number of phases, stator slots, and rotor bars in the induction motor [47].
Figure 8 compares the acoustic noise from the induction motor drive operating at 1000 RPM and 1500 RPM with acoustical materials applied on them. It is evident that acoustical materials contribute to noise reduction across the entire frequency range, with a more pronounced impact at higher frequencies.
Figure 9 details the measured acoustic noise for the induction motor drive operating at 500 RPM, 1000 RPM, and 1500 RPM in bare condition and with the application of acoustical materials. The entire frequency range is segmented into frequency bands of 500 Hz to analyze, quantitatively, the acoustic noise reduction by acoustical materials. It is observed in Figure 6 and Figure 7 that the measured acoustic noise exhibits temporal variations; so, an average of the peak values of acoustic noise over time is compared for the three cases.
Acoustic noise measurements indicate that Acoustical Material 3 exhibits better performance than Acoustical Material 1 and Acoustical Material 2 up to 4500 Hz. This is due to the higher sound absorption coefficient of Acoustical Material 3 at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz, as depicted in Table 1. The sound absorption coefficient of Acoustical Material 2 is higher than Acoustical Material 1 at 250 Hz, 500 Hz, and 1000 Hz but is lower at 2000 Hz and 4000 Hz. It still outperforms Acoustical Material 1 up to 4500 Hz. This is due to the additional noise barrier capacity of Acoustical Material 2, as depicted in Table 2.
Beyond 4000 Hz, the acoustical properties of the materials are not provided in the datasheet. The acoustic noise measurements indicate that all three acoustical materials perform similarly from 4500 to 5000 Hz. This could be due to the presence of an external noise source, which has a lower noise level than the induction motor. Beyond 5000 Hz, Acoustical Material 1 and Acoustical Material 3 outperform Acoustical Material 2.
Figure 10 compares the acoustic noise reduction of the induction motor drive by individual acoustical materials at 500 RPM, 1000 RPM, and 1500 RPM. It is calculated by subtracting the measured noise level from the acoustical materials from the noise level in the bare condition. The performance of the acoustical materials remains consistent at 500 RPM, 1000 RPM, and 1500 RPM, with minor deviations observed between 1000 Hz and 3000 Hz. These deviations increase with speed, suggesting interference from an external noise source associated with shaft rotation, such as bearings or coupling. The Acoustical Material 1 reduces acoustic noise by 5–8 dB(A) at around 500 Hz and 26–31 dB(A) at around 10,000 Hz. Acoustical Material 2 reduces acoustic noise by 10–12 dB(A) at around 500 Hz and 22–23 dB(A) at around 10,000 Hz. Acoustical Material 3 reduces acoustic noise by 11–14 dB(A) at around 500 Hz and 27–30 dB(A) at around 10,000 Hz.
Acoustical Material 3 generally outperforms the others across the entire frequency range except for a few frequency bands. Acoustical Material 2 performs better than Acoustical Material 1 up to 4500 Hz, while Acoustical Material 1 performs better than Acoustical Material 2 beyond 5000 Hz. The acoustical materials prove to be effective in mitigating acoustic noise across the entire frequency range, and their performance improves as the frequency increases.

5. Effect of Acoustical Material Enclosure on Motor Drive Temperature

The application of acoustical material enclosures can impact the thermal performance of induction motor drives. To assess this, the temperature of the outer surface of the induction motor housing is measured using a thermocouple, with readings taken every 2 min. Figure 11 presents a comparison of the outer surface temperature of the induction motor housing running at 1500 RPM under different conditions: without acoustical material and with Acoustical Material 1, Acoustical Material 2, and Acoustical Material 3.
Under bare conditions, the outer surface temperature of the induction motor housing increases from 22 °C to a steady state of 33 °C in 180 min. In contrast, when acoustical materials are applied, the outer surface temperature rises from 22 °C to approximately 45 °C in 260 min, approaching a steady state. The use of acoustical materials increases the outer surface temperature of the induction motor housing by about 10 °C.

6. Comparison of Application of Acoustical Materials

Apart from the acoustic noise performance of the acoustical material, cost, operational temperature range, weight, and ease of application are some of the critical parameters that need to be considered for a practical application. Table 4 provides a comparison of these properties for the acoustical materials.
The costs of 2″ Polyurethane foam (Acoustical Material 1), 2″ Vinyl-faced quilted glass fiber (Acoustical Material 2), and 2″ Studiofoam (Acoustical Material 3) are approximately $57, $85, and $241 per square meter. Acoustical Material 3 is approximately four times more expensive than Acoustical Material 1 and approximately three times more expensive than Acoustical Material 2. Whether for motor-mounted or body-mounted encapsulation, acoustical materials need to be flexible enough to adhere to the cylindrical motor housing or adapt to random body shapes. Acoustical Material 1 and Acoustical Material 2 exhibit flexibility, allowing easy wrapping around an electric motor or a body structure. In contrast, Acoustical Material 3, purchased of the shelf, is flat and rigid, restricting its potential applications.
The operational temperature range for Acoustical Material 1 and Acoustical Material 2 is −40 °C to 107 °C and −30 °C to 82 °C, as per their datasheet. The operational temperature range for Acoustical Material 3 was not unavailable. Generally, fibrous sound absorbers have an operational temperature range that can extend up to 500 °C and beyond [48]. Acoustical Material 2 is limited by the low operational temperature range of the Vinyl plastic used in the quilt. The outer surface temperature of electric propulsion motors is typically below the maximum operational temperature of the acoustical materials [49]. This feature makes these acoustical materials well-suited for use in electric propulsion motors, even when employed as motor-mounted encapsulation.
The surface density comparison shows that Acoustical Material 1 is approximately three times lighter than Acoustical Material 2 and eight times lighter than Acoustical Material 3. Acoustical Material 1, Acoustical Material 2, and Acoustical Material 3 weigh approximately 1.2 kg, 3.7 kg, and 9.3 kg, respectively, for the body-mounted encapsulation and approximately 0.2 kg, 0.6 kg, and 1.5 kg, respectively, for a motor-mounted encapsulation. The weight difference can be significant for weight-sensitive applications. The thickness of the acoustical material is one of the driving factors for its cost. In this study, all the acoustical materials used are 50.8 mm thick. Lower thickness costs less, so the thickness of the acoustical material can be optimized based on the specific requirements of the application.
Acoustic noise for the inverter is dominant below 2000 Hz. So, Acoustical Material 2 is most suitable for inverter acoustic noise mitigation, considering the acoustic performance, cost, weight, and ease of application. The induction motor emits a broad frequency range but is particularly dominant at around 1560 Hz, 4700 Hz, and 5360 Hz. Acoustical Material 3 outperforms the others across the entire frequency spectrum. However, its drawbacks, including higher cost, weight, and limited flexibility, make it less favorable. Instead, the flexible, lightweight, and cost-effective Acoustical Material 1 can be employed to mitigate the acoustic noise of the induction motor.
Exterior acoustic noise requirements for automotive applications are driven by regulations, which define the limit of the loudness of the noise. As per the United Nations Economic Commission for Europe (UNECE), the maximum exterior sound level for electric vehicles should be less than 75 dB(A) for vehicles falling under the M1 and N1 categories, measured from a distance of 2 m [50]. The M1 category pertains to vehicles designed for carrying passengers, typically comprising no more than eight seats (excluding the driver’s). On the other hand, the N1 category is designated for vehicles intended for transporting goods, with a total mass of less than 3.5 tonnes and a minimum of four wheels. Interior acoustic noise levels in automotive applications may fluctuate with speed, but they typically fall within the range of 65–75 dB [51,52]. The specific noise levels depend on the overall quality and standards targeted by different automotive brands, and the design of a body- or surface-mounted enclosure should consider these requirements.

7. Conclusions

This paper investigates the effectiveness of acoustical materials as a method of mitigating high-frequency tonal noise from an electric motor at the transmission stage. A 2″ Polyurethane foam, a 2″ Vinyl-faced quilted glass fiber, and a 2″ Studiofoam are applied as wall-mounted encapsulation on an induction motor and a variable frequency inverter. All three materials effectively mitigate the induction motor noise due to electromagnetic radial force and the inverter switching frequency. The acoustical capability of the materials is evaluated up to 10,000 Hz to demonstrate that their performance improves with increasing frequency. Studiofoam has a significantly higher sound absorption coefficient compared to Polyurethane foam; however, the improvement in noise reduction above 5000 Hz is lower. While Studiofoam offers superior performance across a wide frequency range, it is less practical than Polyurethane and Vinyl-faced quilted fiber due to its higher cost, greater weight, and application difficulty. Vinyl-faced quilted glass fiber performs better than Polyurethane below 4500 Hz, whereas Polyurethane excels above 5500 Hz. Considering factors such as acoustic noise reduction, cost, weight, operational temperature, and ease of application, Vinyl-faced quilted glass fiber is more appropriate for inverter applications, while Polyurethane is more suitable for motor encapsulation. The future course of action for this project involves material testing to generate sound absorption coefficients and sound transmission loss data up to 10,000 Hz for the acoustical materials.

Author Contributions

A.K.S.: Methodology, formal analysis, investigation, and writing—original draft; A.S.: Investigation; A.H.: Investigation; M.M.: Investigation; B.B.: Review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was undertaken, in part, thanks to funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation of Research (CFI).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank The Modal Shop and Siemens for their support with experimental modal analysis instruments and Seimens Testlab software in this research.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ICEInternal combustion engine
PMSMPermanent magnet synchronous motor
SRMSwitched reluctance motor
PWMPulse width modulation
PVCPolyvinyl chloride
PUPolyurethane
STLSound transmission loss
STCSound transmission coefficient
UNECEUnited Nations Economic Commission for Europe

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Figure 1. Cross-section view of the investigated acoustical materials.
Figure 1. Cross-section view of the investigated acoustical materials.
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Figure 2. Sound absorbtion coefficeint of the acoustical materials [42,43,44].
Figure 2. Sound absorbtion coefficeint of the acoustical materials [42,43,44].
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Figure 3. Test setup to measure induction motor and variable frequency inverter acoustic noise.
Figure 3. Test setup to measure induction motor and variable frequency inverter acoustic noise.
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Figure 4. (a) Overall dimensions of induction motor frame; (b) overall dimensions of inverter frame; (c) cross-section of Structural Member 1; and (d) cross-section of Structural Member 2.
Figure 4. (a) Overall dimensions of induction motor frame; (b) overall dimensions of inverter frame; (c) cross-section of Structural Member 1; and (d) cross-section of Structural Member 2.
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Figure 5. Installation of acoustical materials: (a) induction motor with frame; (b) inverter with frame; (c) Acoustical Material 1 encapsulation; (d) Acoustical Material 2 encapsulation; and (e) Acoustical Material 3 encapsulation.
Figure 5. Installation of acoustical materials: (a) induction motor with frame; (b) inverter with frame; (c) Acoustical Material 1 encapsulation; (d) Acoustical Material 2 encapsulation; and (e) Acoustical Material 3 encapsulation.
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Figure 6. Acoustic noisecontour plot for the inverter without any acoustical material applied: (a) cooling fan on; (b) cooling fan off.
Figure 6. Acoustic noisecontour plot for the inverter without any acoustical material applied: (a) cooling fan on; (b) cooling fan off.
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Figure 7. Acoustic noise contour plot for the induction motor without any acoustical material applied: (a) 1000 RPM; (b) 1500 RPM.
Figure 7. Acoustic noise contour plot for the induction motor without any acoustical material applied: (a) 1000 RPM; (b) 1500 RPM.
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Figure 8. Acoustic noise comparison for the inverter and induction motor operating with and without acoustical materials application: (a) 1000 RPM; (b) 1500 RPM.
Figure 8. Acoustic noise comparison for the inverter and induction motor operating with and without acoustical materials application: (a) 1000 RPM; (b) 1500 RPM.
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Figure 9. Acoustic noise measurements for the inverter and induction motor at (a) 500 RPM, (b) 1000 RPM, and (c) 1500 RPM.
Figure 9. Acoustic noise measurements for the inverter and induction motor at (a) 500 RPM, (b) 1000 RPM, and (c) 1500 RPM.
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Figure 10. Acoustic noise reduction for the induction motor drive at 500 RPM, 1000 RPM, and 1500 RPM with application of (a) Acoustical Material 1, (b) Acoustical Material 2, and (c) Acoustical Material 3.
Figure 10. Acoustic noise reduction for the induction motor drive at 500 RPM, 1000 RPM, and 1500 RPM with application of (a) Acoustical Material 1, (b) Acoustical Material 2, and (c) Acoustical Material 3.
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Figure 11. Experimentally measured outer housing temperature of the motor drive with and without application of acoustical materials.
Figure 11. Experimentally measured outer housing temperature of the motor drive with and without application of acoustical materials.
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Table 1. Sound absorbtion coefficeint of acoustical materials.
Table 1. Sound absorbtion coefficeint of acoustical materials.
Frequency (Hz)Acoustical Material 1Acoustical Material 2Acoustical Material 3
1250.220.190.16
2500.380.990.46
5000.740.960.99
10000.650.81.12
20000.70.571.14
40000.850.331.13
Table 2. Sound transmission loss of Acoustical Material 2.
Table 2. Sound transmission loss of Acoustical Material 2.
Frequency (Hz)125250500100020004000
STL (dB)61115202532
Table 3. Instruments used for acoustic noise measurement and analysis.
Table 3. Instruments used for acoustic noise measurement and analysis.
InstrumentManufacturerModel
Data acquisition systemSeimensSCADAS mobile
MicrophonePCB Piezotronics378B02
Signal analyzerSeimensSimcenter Testlab
Table 4. Additional properties of acoustical materials.
Table 4. Additional properties of acoustical materials.
PropertyAcoustical Material 1Acoustical Material 2Acoustical Material 3
Name2″ Polyurethane foam2″ Vinyl-faced quilted glass fiber2″ Studiofoam
Typefoamfibrousfibrous
Cost ($/m2)5785241
FlexibileYesYesNo
Opearational temperature range (°C)−40 to 107−30 to 82-
Density (kg/m2)0.6842.125.37
Thickness (mm)50.850.850.8
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MDPI and ACS Style

Sahu, A.K.; Selliah, A.; Hassan, A.; Masoumi, M.; Bilgin, B. Experimental Evaluation of Acoustical Materials for Noise Reduction in an Induction Motor Drive. Machines 2024, 12, 499. https://doi.org/10.3390/machines12080499

AMA Style

Sahu AK, Selliah A, Hassan A, Masoumi M, Bilgin B. Experimental Evaluation of Acoustical Materials for Noise Reduction in an Induction Motor Drive. Machines. 2024; 12(8):499. https://doi.org/10.3390/machines12080499

Chicago/Turabian Style

Sahu, Ashish Kumar, Abeka Selliah, Alaa Hassan, Moien Masoumi, and Berker Bilgin. 2024. "Experimental Evaluation of Acoustical Materials for Noise Reduction in an Induction Motor Drive" Machines 12, no. 8: 499. https://doi.org/10.3390/machines12080499

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