All energized transformers "hum" due to the alternating flux in the core, producing a phenomenon known as magnetostriction. Commonly referred to as "noise," it is primarily produced by the core at a fundamental frequency of twice the applied frequency. Noise is an inherent characteristic of the core and cannot be completely eliminated.
When selecting a transformer, make certain that the sound levels presented by the manufacturer have been measured in accordance with the American National Standards Institute, and certified by the manufacturer. NEMA Publication No. ST20 and ANSI Standard C89.2 establish the following maximum sound levels for dry type transformers:
- up to 9 kVA: 40 dB
- 10-50 kVA: 45 dB
- 51-150 kVA: 50 dB
- 151-300 kVA: 55 dB
- 301-500 kVA: 60 dB
A good quality transformer will have low sound levels due to the manner in which the core and coils are internally isolated from the enclosure. This is done by allowing the entire unit to "float" on vibration dampening pads.
Installation Affects Noise
One of the major reasons for transformer noise complaints is improper installation. Together with location, improper installation can increase transformer sound levels 10 dB or more. Considering that a 3 dB increase in sound level has the effect of approximately doubling the sound volume as detected by the human ear, a 10 dB increase cannot be tolerated.
The first step in low sound level transformer installation is specifying the proper location. With the increased popularity in cost saving advantages of high voltage distribution in modern buildings today, it is necessary to locate small dry type transformers relatively close to (or within) occupied areas. Transformers should be located in areas where the noise would be the least objectionable. The maximum sound limit of the transformer to be installed should be compared to the ambient sound level of the installation location. If the transformer is expected to be louder than the ambient of the site, it should be located elsewhere.
Don't place a transformer near multiple reflective surfaces, such as a corner near the ceiling or the floor. Either of these locations presents three reflecting surfaces, which will act as a megaphone for the transformer sound. Halls are undesirable too, because of the short distance between opposing reflecting surfaces.
Once the best possible location has been found, the next step is mounting. Transformers should be mounted on a floor, wall, or structure with as great a mass as possible. One guideline is that the mounting surface should weigh at least 10 times as much as the transformer. Take care not to mount a transformer on a thin wall (i.e. plywood or a curtain wall) as they amplify the noise, acting much as a drumhead does. The prime noise source in the transformer is in the core and coil. The noise from this source is amplified and reflected by any structure solidly connected to it, including incoming conduit and conductors.
