How to reduce electric motor noise?

How to Reduce Electric Motor Noise?

What Is Electric Motor Noise?

Electric motor noise is the airborne sound a rotating machine radiates to its surroundings. It is almost always a mixture of three physically distinct mechanisms, and a “quiet motor” is one where all three have been controlled — not just one. Treating only the bearing, for example, leaves electromagnetic hum or fan roar untouched.

The Three Physical Sources

• Electromagnetic noise — produced by the periodically varying radial force in the air gap. Stator and rotor slot combinations, fractional-slot windings and magnetic saturation create force waves that shake the lamination stack and the frame. Research shows rotor notching can cut overall sound pressure by about 5 dB(A) on a 5.5 kW induction motor.
• Mechanical / bearing noise — from rolling-element bearings, rotor imbalance, loose end covers, keyways and resonance of the frame or connected structure. A worn bearing with a 0.1 mm clearance increase is a classic source of a rhythmic knock.
• Aerodynamic / ventilation noise — from the cooling fan and windage, dominant in high-speed two- and four-pole machines. Blade-pass frequency and turbulent eddies scale strongly with tip speed squared.

Sound Power vs Sound Pressure

Standards quote sound power level L_WA (the energy the source emits, independent of the room), while field engineers measure sound pressure level L_p in dB(A) at a defined distance. A bare-number “55 dB” is meaningless unless you also know the distance, the weighting (A), the mounting and the room. Because the decibel scale is logarithmic, a 3 dB change is barely perceptible, 6 dB is “twice as loud” to the ear, and 10 dB is roughly a doubling of perceived loudness — which is why small design changes rarely move the needle but a single dominant source does.

How Electric Motor Noise Is Generated

The path from a rotating field to an audible tone follows a fixed chain: force excitation → structural vibration → radiation from the frame → airborne sound. Each link can be broken.

Electromagnetic Excitation

The radial Maxwell force density in the air gap contains harmonics at frequencies set by the slot-pole combination. When a force harmonic coincides with a structural natural frequency of the stator, even a small force produces a large response — resonance. Mitigations include skew (rotating the rotor laminations axially to phase-cancel slot harmonics, or a double-skew/herringbone pattern), remote slot combinations (large stator-to-rotor slot-count difference), sinusoidal windings, magnetic slot wedges and a larger, more uniform air gap. Analytical and FEM studies of PMSMs show magnet skewing is often the better wide-band fix versus simply narrowing slot openings (Lin et al., Journal of Sound and Vibration).

Mechanical Excitation

Bearings generate a characteristic tone at the ball-pass frequency. Imbalance injects a once-per-revolution force; looseness lets parts rattle; and foot or foundation resonance multiplies the whole signal. The fix chain is: high-precision balanced rotor → low-noise sealed bearings with the right grease → rigid, flat mounting → isolation where the structure is light.

Aerodynamic Excitation

The fan is an aerodynamic machine bolted to an electric one. Axial fans on TEFC motors are the usual culprit above ~1,500 r/min. Lower blade tip speed, more blades with unequal spacing (to spread the tonal energy), and smooth shroud surfaces all reduce the broadband hiss and the blade-pass tone.

IEC 60034-9 Noise Limits — How Loud Is “Normal”?

IEC 60034-9:2021 defines test methods and maximum A-weighted sound power levels (L_WA) for factory acceptance of network-supplied rotating machines. The table below gives representative 4-pole, 60 Hz, IC411 (fan-cooled) limits for common frame sizes — the values a purchaser can actually specify against. Grade A is the less restrictive (higher) limit; Grade B is the quieter, more demanding one harmonized with the stricter NEMA levels.

Electromagnetic vs Mechanical vs Ventilation — Comparison

Engineering Methods to Reduce Electric Motor Noise

Electromagnetic Noise Reduction

• Rotor / stator skew. A single skew of one stator slot pitch cancels a large part of the cogging and slot-harmonic radial force. Herringbone or step-skew patterns spread the cancellation across speed.
• Slot-pole combination. Choose remote slot counts and avoid low-order force waves. For a brushless DC motor, fractional-slot concentrated windings need careful selection to keep low space harmonics small.
• Controlled commutation (FOC). Field-oriented control smooths torque ripple, which directly lowers the fluctuating electromagnetic force. Sensorless control also removes Hall-sensor commutation steps that can buzz at low speed.
• Air gap & saturation. A slightly larger, uniform air gap and staying out of magnetic saturation reduce force amplitude. Tighter concentricity of stator bore and rotor OD keeps the gap even.

Bearing & Mechanical Noise Reduction

• Low-noise bearings. Specify precision-grade, greased-for-life ball or sleeve bearings with verified acoustic ratings. Cheap bearings are the most common avoidable noise source.
• Dynamic balance. Balance the rotor (and fan) to a low residual imbalance so the once-per-rev force is small.
• Rigid, flat mounting. A warped foot or loose bolt lets the frame resonate. Use a flat base, torque the feet, and isolate only where the structure is light (see motor flange dimensions for interface fit).

Ventilation Noise Reduction

• Fan design. Lower tip speed, optimized blade count and smooth shroud surfaces cut both the tone and the hiss.
• Speed. Halving fan speed can drop aerodynamic noise by ~12 dB(A) (tip speed squared). A VFD that slows the motor at part load quiets both motor and fan.
• Enclosures. A ventilated acoustic enclosure traps sound but must preserve cooling — never blanket a TEFC motor without an airflow path.

VFD & Control Fixes

• PWM switching frequency. Raising the carrier above ~3 kHz pushes magnetically excited noise above the audible band; 4 kHz → 16 kHz typically removes 8–10 dB(A). This is the single biggest field lever for VFD whine (NEMA MG 1 Part 30; IEC 60034-1 context, IEC 60034-25 for converter supply).
• Soft start / soft stop. Eliminates the inrush and commutation clunk at switch-on.
• Skip bands. Program the drive to avoid speeds where the forcing frequency meets a structural resonance.
• Shaft-current protection. PWM drives can induce shaft currents that pit bearings; insulated bearings or a shaft grounding ring prevent the resulting growl and premature failure.

Worked Example: Meeting a 60 dB(A) Target

A designer must keep an AGV drive module below 60 dB(A) at 1 m. The module combines three independent (incoherent) sources measured individually:

Incoherent sources add by energy (not by arithmetic). The combined level is:

Ltotal = 10 · log10(1052/10 + 1058/10 + 1055/10)

= 10 · log10(158,489 + 630,957 + 316,228) = 10 · log10(1,105,674) ≈ 60.4 dB(A)

Result: 60.4 dB(A) — it fails the 60 dB(A) target by 0.4 dB.

Decibel Addition Rules Every Designer Should Know

For a micro gearbox or precision planetary gearbox the same energy-sum rule applies: size the noise budget per source before you commit to a ratio.

Best Applications for Low-Noise Motors

• Medical & lab equipment — ventilators, surgical robots, infusion pumps, where patient comfort and clinician concentration demand <40 dB(A). FAULHABER and maxon document medical-grade BLDC solutions built around sensorless control and high PWM frequency.
• Automotive cabin — seat, liftgate and wiper actuators where the motor competes with a silent EV interior; 24 V BLDC platforms are specified below ~35 dB(A).
• Office & home appliances — printers, HVAC dampers, where broadband hum is a quality complaint.
• Robotics & automation — rehabilitation and service robots need near-silent joints; servo + low-backlash gearing is the usual path (servo fundamentals).
• HVAC & building systems — fans and pumps where the motor sits in occupied space.

Typical Product Noise Targets

Step-by-Step Selection of a Quiet Motor

Follow this sequence so noise is designed in, not bolted on:

1. Set the dB budget. Start from the product target at 1 m (e.g. 55 dB(A)), then split it across motor, gearbox and fan using the energy-sum rule above.
2. Pick the motor technology. Choose BLDC or brushed DC, servo or induction based on torque, speed and control needs; brushless removes the commutator noise floor.
3. Size bearings and balance. Specify low-noise bearing grade and a balance class suited to the speed (see SKF bearing guidance).
4. Choose the reduction. If you need a gearbox, weigh reducer types: helical and planetary are quieter than worm at the same ratio, while a direct-drive option removes gear mesh entirely.
5. Specify the drive. Set PWM carrier > 3 kHz, enable soft start and skip bands on the motor controller.
6. Verify to ISO 1680. Measure in a semi-anechoic chamber at the defined radius before sign-off; compare only like-for-like conditions.

Common Engineering Mistakes

• Chasing the quiet source. Adding a fan silencer while the gearbox dominates the dB sum (see example) wastes budget.
• Ignoring the VFD carrier. Leaving PWM at 4 kHz guarantees an audible whine that no mechanical fix removes.
• Comparing bare dB numbers. A supplier’s “50 dB” at 0.5 m in an anechoic chamber is not your “50 dB” at 1 m on a resonant panel.
• Over-greasing bearings. Too much grease churns, heats and raises both noise and failure rate; follow the specified fill.
• Sealing a TEFC motor. Blanket enclosures that block the cooling path trade noise for overheating and a shorter life.
• Specifying only the motor. A motor that passes IEC 60034-9 can still be loud in a product if the mounting, panel or gearbox radiate. Design the system, not the part.

Troubleshooting — Problem, Cause, Solution

FAQ

Why Choose Greensky Power?

Greensky Power designs and manufactures custom electric motors and complete OEM actuator assemblies, so we treat the motor, the reduction gear and the controller as one matched, low-noise system rather than three parts bought separately. For your program we supply:

• Standard and custom 24 V BLDC, BLDC, brushed DC and servo motors with IEC B5/B14 flanges, low-noise bearing options and verified shaft dimensions.
• Planetary, worm, cycloidal and micro gearboxes with documented rated-torque, efficiency, backlash and acoustic data for noise budgeting.
• Engineering support for industrial motor selection, torque calculation and converter-supply acoustic verification.

Related Resources

• Electric motor basics 
• Gearbox overview 
• Speed-reducer motor selection guide
• How to select a reduction gearbox for an electric motor
• Different types of speed reducers
• Harmonic drive vs. planetary gear
• What is the use of a DC geared motor?
• BLDC vs. servo motor comparison
• Motor efficiency classes
• DC motors category

References

1. IEC 60034-9:2021 — Rotating electrical machines, Part 9: Noise limits. webstore.iec.ch/publication/66184
2. IEC 60034-1:2022 — Rotating electrical machines, general requirements (insulation, mounting, converter supply context). webstore.iec.ch/en/publication/65446
3. ISO 1680:2013 — Acoustics, test code for airborne noise emitted by rotating electrical machines. iso.org/standard/63318.html
4. ANSI/NEMA MG 1-2021 — Motors and Generators (Part 30: sound of converter-fed motors). webstore.ansi.org/standards/nema/ansinemamg2021
5. NEMA — Motor and generator product guidance. nema.org/products/pages/motor-and-generator.aspx
6. U.S. DOE — Motor systems efficiency and load guidance. energy.gov — 10097517.pdf
7. IEA — Electric motors and industrial efficiency. iea.org/energy-system/industry/electric-motors
8. SKF — Bearing failures and their causes. skf.com — bearing failures
9. Siemens — SIMOTICS electric motors product range. siemens.com — electric motors
10. IEEE Transactions on Industry Applications — Zhou & Shen, “Rotor Notching for Electromagnetic Noise Reduction of Induction Motors” (5 dB(A) on 5.5 kW), DOI 10.1109/TIA.2017.2681969. doi.org/10.1109/TIA.2017.2681969
11. Journal of Sound and Vibration — Lin, Zuo, Deng & Wu, “Reduction of vibration and acoustic noise in PMSM by optimizing magnetic forces”. sciencedirect.com — S0022460X1830302X
12. maxon — EC technology and gearhead selection. maxongroup.com — ec-technology
13. FAULHABER — Brushless DC motors and micro-drive know-how. faulhaber.com — brushless DC motors
14. Yaskawa — Motion and servo system technical downloads. yaskawa.com/downloads/search-index
15. Tsubaki — Gearmotor selection technical reference (service-factor method). tsubakimoto.co.jp — selection reference

Technical content reviewed by the Greensky Power engineering team. Standards citations reflect editions current as of August 2026; confirm the latest edition with the issuing body before specification.