Acoustic noise level
This post discusses the topic of noise generated by variable frequency drive system. We will talk about acoustic noise in applications utilizing VFD technology. To be precise we shall use terms of sound pressure level and sound power level as explained further.
Acoustic noise and health
Why is the acoustic noise level important? Noise affects human body and mind. Strictly speaking noise is a pollution. Same as for air pollution or water pollution there are hygienic standards defining the noise limits.
Sensitivity of human ear
Healthy human ear can hear frequencies (pressure waves) in the range of approx. 20 Hz to 20 kHz. Frequencies below and above that are outside of audible range and people cannot hear them (however, some animals do). The ear is capable of registering pressure variation in wide range. The threshold of hearing is 0 dB which corresponds to less than one billionth of atmospheric pressure. Sound pressure of 130 dB is called threshold of pain.
Sound pressure versus sound power
Two quantities are frequently used to characterize the noise – sound pressure and sound power. People sometimes confuse them when talking about ‘sound level’… [1], [2].
Sound power level (SWL, Lw) is the acoustical energy emitted by the source of sound. The measurement unit is Watt (W). It is an absolute value, i.e. not dependent on distance or environment –> sound power is an absolute value.
Sound pressure level (SPL, Lp) is a pressure disturbance in the atmosphere. Measurement unit is Pascal (Pa). The intensity is influenced not only by the strength of the source, but also by the distance between source and receiver –> sound pressure level shall always be indicated at specific distance (e.g. 70 dB(A) at 1 m). Sound pressure is what our ears hear.
Sound pressure can be converted into sound power and vice versa. Be careful to use correct quantity. Both sound power and sound pressure have decibel (dB) as common unit. However, decibel is a logarithmic ratio between the measured value and reference number. As the decibel scale is logarithmic, the difference of few decibel might be very significant (and technically challenging when it comes to reduction).
Third quantity often used to characterize sound is sound intensity in W/m2.
Limits for noise level
Companies (end users) usually specify certain maximum level of noise that shall not be exceeded. The values may differ quite a lot as they have different background and motivation.
Reason 1 – Health / Ear protection
When working in environment with increased noise the workers shall be equipped with adequate ear protection. The threshold for ear protection is around 80-85 dB(A) of sound pressure.
Reason 2 – Noise limits in residential areas
When the equipment is installed in or nearby a residential area, noise limits usually apply. These limits are often very tough and require lot of specific measures to minimize emitted noise. The required level for equipment located outdoor can be as low as 60 dB(A) or lower and is very challenging to meet.
Source of noise in drive system components
1. Input isolation transformer
Transformers are relatively silent devices compared to other equipment. However, there might be several sources of audible noise. The first source is the characteristic “humming”. Its root cause is the magnetostriction effect in the magnetic core. This noise from the core can be minimized by using premium quality steel sheets and proper clamping and tightening of magnetic circuit. Another source might be the cooling system. While it is negligible for naturally cooled transformers (AN, ONAN, KNAN) it becomes more significant for forced cooled units (medium noise for ONAF, OFAF and OFWF; medium/high for AF, AFAF, AFWF). Transformers feeding non-linear loads such as variable frequency drives also emit certain noise due to harmonic currents and voltages. The harmonic currents do not impact the noise level too much. Voltage harmonics, on the other hand, have significant influence on noise as they directly impact the waveform of the flux.
2. Variable frequency drive
The main source of noise in a variable frequency drive is the cooling system. Liquid cooled VFDs are generally more silent (70-75 dB(A)) than air cooled VFDs (80-85 dB(A)). In current source inverters the reactor in dc link might also be a source of noise audible at specific ranges of output frequency. The reactor might be “singing” during ramp up of motor speed. In case the VFD is equipped with an integrated transformer the items from previous paragraph apply as well. For more details on VFD cooling please also refer to [3].
Liquid cooled VFDs usually operate with constant internal flow rate regardless of the load. Air cooled VFDs nowadays mostly use speed controlled fans. Therefore, the noise level is reduced at partial load due to decreased speed of cooling fans.
3. Electric motor
The electric motors are available in a wide range of different designs. Accordingly the noise level differs significantly from one design to another one. Rotor friction is one source of noise. The higher the surface velocity the higher the friction and noise. Therefore, this portion is considerable for motors with high speed and larger diameter (v = ω⋅r). Another source, as for previous components, is the cooling. Air cooled machines normally generate higher noise than water cooled motors. It is because air is much less efficient coolant and larger volumetric flow is required. The supply waveform affects the motor noise as well. VFDs with an output sine filter or generally motor friendly waveform have small impact on motor noise while VFDs with distorted waveform influence the motor noise considerably. Simplified inverter voltage waveforms are shown in figure 3. Increased number of voltage levels (steps) or output sine filter are the two common ways to improve the motor voltage. Motor-VFD compatibility is also discussed in [4].
Typical sound pressure values of drive system components
Table below provides some indicative sound pressure levels of drive system components. Since variable frequency drive systems are used across wide range of applications, exceptions to below values of acoustic noise are possible. It is not feasible to cover all different design options and variants in one table.
For transformers the sound pressure level depends on the size (kVA rating) of the unit as well. It is much easier to make a small transformer with silent design rather than unit with large capacity. For typical sound pressure > 60-65 dB(A) the additional noise at load compared to no-load is negligible. However, in case if very silent design each and every effect has an impact. When the transformer is installed outdoors an additional firewall with noise attenuation might be the most economic solution.
For multiple VFDs installed in an electric room the responsible company shall check possible noise reflections that can increase the total acoustic noise level.
Table above does not consider cases where the VFD harmonics excite a structural resonance of the transformer or motor. It is assumed that such cases are rectified e.g. by changing the switching pattern.
Measures for reduction of acoustic noise
What means can be used to reduce the acoustic sound / acoustic noise level?
Transformers
If the main source of noise is the core then the most effective measures are reduced nominal flux density along with high quality steel sheets and proper tightening and core clamping.
Natural cooling inherently has very low noise level. Water cooling is also rather silent. Noise from air cooling depends on the cooling fans (fan speed, shape of blades, quality of bearings).
Harmonics generated by VFD also contribute to vibrations. Current harmonics do not have too significant impact. Luckily, since they are usually generated by (multi-pulse) diode rectifiers and cannot be actively controlled. Voltage harmonics influence the acoustic noise level much more. They are generated by active front end voltage source inverters (AFE VSI). Some optimization can be achieved by changing the pulse pattern of the modulation, resp. the carrier switching frequency.
Variable frequency drives
VFDs are usually not of concern when it comes to acoustic noise. Liquid cooled VFDs typically reach 75 dB(A) @ 1 m of sound pressure or less. Air cooled VFDs are somewhat more noisy with sound pressure up to 85 dB(A) @ 1m. One measure to reduce the noise are speed controlled (EC) fans. Another measure might be air ducting.
In rare cases the VFD internal components might create acoustic noise linked with the switching frequency. Changing of switching pattern might help in such situation.
Electric motors
For motors with higher circumferential speed the friction of rotor in the gas (usually air) is the main source of noise. This is hard to change, but an acoustic enclosure or a silencer can be used for noise dampening. Reduced stator flux can help for noise reduction, especially is the noise is generated by the magnetic yoke. Forced cooling is also a vibration source. Water cooled machines are generally more silent than air cooled motors. One possible source can also be structural vibration. In such case the foundations shall be reviewed. Finally the harmonic distortion caused by the VFD contributes to the noise level, too. Motor manufacturer (almost) always guarantees sound pressure level for sinusoidal supply. Most manufacturers even do not have the possibility to test the motor with the project specific VFD in their test field. ‘Motor friendly’ VFDs, such as those with output sine filter or with multi-level inverter topology, have small to negligible impact on motor noise. Less motor friendly VFDs impact the motor noise much more. The additional noise can easily add 3-6 dB. Attention shall also be paid to the selection of switching frequency. Excitation of structural resonance inside the motor shall be avoided.
Example – Sound level of a railway substation
In below video you can hear the noise of an outdoor substation feeding the railway grid. The transformer humming due to magnetostriction is what you can hear as a characteristic sound. In addition, there is a time period when the noise level is higher. The reason seems to be some structural vibration (maybe enclosure?).
Example – sound level of an industrial re-cooler
Bear in mind that also an external re-cooler might be a significant contributor to the overall acoustic noise level of the installation. In case of strict noise requirements pay attention to the specification and design of re-cooler.
References
[1] Sound power calculator (online), http://www.sengpielaudio.com/calculator-soundpower.htm
[2] Sound power level (wikipedia), https://en.wikipedia.org/wiki/Sound_power
[3] How to choose a medium voltage VFD: Cooling type, https://mb-drive-services.com/choose-mv-vfd-cooling/
[4] How to choose a medium voltage VFD: Motor compatibility, https://mb-drive-services.com/choosing-mv-vfd-motor-compatibility/
