Considerations about motor noise level
This post provides few considerations about motor noise level with respect to inverter supply.
Noise might be considered as one performance criteria of the motor. Noise level of electric equipment is regulated by standards and regulations, often involving local law. Depending on the maximum level, the personnel working in the vicinity of the source of noise shall wear adequate personal protective equipment. There are also hygienic limits applying in the neighbourhood (particularly critical in residential areas). Thus, the requirements on motor noise level are getting stricter.
Sources of motor noise
There are multiple sources of motor noise:
- Aerodynamic noise
- Bearing friction
- Cooling system
- Magnetic circuit
- Structural vibration
Aerodynamic noise
Rotor friction in gaseous atmosphere is often the most dominant source of noise in medium-speed and high-speed machines. It is linked with the circumferential velocity of the rotor, i.e. a product of the angular velocity and radius of rotor.
Bearing noise
Large machines typically use sleeve bearings that do not create any considerable noise. Bearing noise is often caused by rotor unbalance. The frequency of bearing noise is thus linked to the fundamental mechanical frequency of the rotor.
Small machines with ball bearings may cause more significant bearing noise.
Cooling system
Noise generated by the cooling system strongly depends on the cooling design. Water-cooled motors are more silent than air-cooled ones. Air cooling can be realized as direct air cooling (open type) or utilizing air-to-air heat exchanger.
Mechanical noise (vibration)
Vibration of bearings or parts inside the machine may cause oscillations of larger surfaces or air ducts. This kind of noise is transported through the mechanical parts and on the surface converts into noise transmitted through air. if the root cause is an unbalanced machine, then mechanical noise is a low-frequency noise.
As there is a large variety of motor designs (aspects such as rated speed, cooling type, physical dimensions etc), the motor acoustic noise level can be quite different from one motor to other. Table in [1] states some typical sound pressure level for several categories of motor design.
Factory testing
Routine and type tests in the factory follow national and international standards. Most common standards are IEC 60034 and NEMA MG-1.
Motor sound level is tested at no-load and with sinusoidal supply. That is because the infrastructure in the test field often does not allow to load large machines with rated power. And the project-specific VFD is not available in the test field either. Naturally, the motor manufacturer can only guarantee what can be measured. Thus, sound pressure and sound power level is measured at no-load and sine wave.
Who guarantees the motor noise level in real operation?
Who is then responsible to guarantee the noise level in real operation? That is a bit tricky question with no general answer. Instead, it depends on the agreement. In some cases the system integrator may take up this responsibility. Alternatively, the final noise level may be just an indicative value without a contractual guarantee. Or the real noise level can be measured during a system test (full load combined test, string test etc) before supplying the equipment to site.
In any case, to provide a qualified and realistic estimation or a guarantee of motor noise level when supplied from a VFD, it is crucial to understand the effect of the modulated inverter voltage (or current) on final motor noise.
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Tolerance
When talking about motor noise level, it is essential to distinguish typical values (qualified estimation) and guaranteed values. Latter can be provided as absolute value (with any positive tolerance) or with certain tolerance.
A common way to express the sound level is with certain tolerance. This is typically 3 dB.
Motor noise under inverter supply
The noise level under inverter operation can be significantly higher than the sound level with a pure sinusoidal supply. Exact impact depends on the type of inverter, way of modulation and inverter parameterization (e.g. switching frequency):
- 2-level / 3-level voltage source inverter without a sine filter → high impact
- 5-level voltage source inverter → medium impact
- multi-level voltage source inverter → medium/low impact
- voltage source inverter with an output sine filter → low impact/negligible
- load-commutated inverter → medium impact
- PWM current source inverter → medium/low impact
High impact: 6-10 dB(A) increase ¹
Medium impact: 3-6 dB(A) increase ¹
Low impact: 1-3 dB(A) increase ¹
Negligible impact: < 1 dB(A) increase ¹
¹ increase of sound pressure level compared to sinusoidal supply
The above ranges are typical and represent kind of thumb rules. An acoustic study of inverter-fed motor is a complex study that requires a specialized software.
Measures to reduce motor noise
When the “standard” design does not fulfil the criteria, additional measures need to apply. They can include changes in the electrical design of the motor (e.g. reduction of flux density), changing type of cooling etc. Certain “tuning” of the VFD may help as well – for example adjusting the inverter switching frequency. Another way is to install an acoustic noise enclosure on the motor. If the main source of motor noise is the VFD then one (theoretical) option is to change the VFD model. However, that is not always feasible as alternative, more motor friendly VFD models may not fit the application requirements.
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Further considerations about motor noise level
There are couple of points to consider on top with respect to motor noise level. While the inverter output voltage usually has the dominant impact, it is always the combination of VFD-motor design that determines the noise level.
The additional noise due to inverter supply is not a constant value (offset). Instead, it changes with motor speed and the corresponding switching frequency. Moreover, the human ear is more sensitive to some frequencies than others. This is reflected in so-called A-weighting [2]. Thus, at specific speeds the motor appears to have higher acoustic noise level than at other speeds. A-weighted sound pressure level is indicated as dB(A).
Extra decibels caused by the VFD do not depend on the inverter topology only, but also other factors such as already mentioned switching frequency. The switching frequency is variable across the operation range. It is also a parameter that can be tuned meaning same type of inverter can generate slightly different additional noise in the motor depending on the parameterization.
In voltage source inverter type of drives the motor leakage reactance is decisive to smooth the stator current. Higher design leakage reactance leads to lower distortion of the motor current (assuming identical switching frequency) which has some positive effect on the additional noise. An external series reactance, e.g. as an output choke, helps to reduce the noise level in the same way.
Summary
Motor noise level can be an important design quantity. One reason are the hygienic limits for personnel working in the vicinity of the machine, other reason are the noise limits in the surroundings of the installation (such as site located close to a residential area).
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References
[1] Acoustic noise level, MB Drive Services, February 2020, available online, https://mb-drive-services.com/acoustic-noise-level/
[2] A-weighting, Wikipedia – The Free Encyclopedia, https://en.wikipedia.org/wiki/A-weighting
