Transformers, like other electromagnetic devices, produce a “hum” caused by the alternating flux in the transformer core. This “hum”, known as magnetostriction, is primarily produced at a fundamental frequency of twice the applied frequency. The relative loudness depends on the construction of the transformer, the manner of installation and the ambient sound level at the site. The sound produced by a transformer has a fundamental frequency of 120 Hz, accompanied by harmonics of 240, 360, 480, 600, etc.
Controlling Transformer Sound
Sound control becomes more important as power demands increase and transformers are placed closer to their loads. Planning of transformer placement and specification is especially important in designing high rise apartments, hospitals and office buildings.
Proper installation can significantly reduce transformer noise. For a quiet installation:
Consult your architect about the location of the transformer while the building is being designed.
Install the transformer as far as possible from areas where the sound could be objectionable.
Avoid placing near multiple reflective surfaces such as in a corner, near a ceiling or floor, or in a hallway.
Place sound-dampening pads between the transformer and the mounting surface. (Pads may be neoprene with sandwiched cork material or spring loaded with a rubber base.)
Use flexible conduit couplings between the transformer and the wiring system. Mount the transformer on walls or structural members sufficient to support its weight.
To avoid amplifying the sound, mount the transformer on a surface with as large a mass as possible.
Judge transformer sound only when the building is finished, occupied and functioning.
Sound Testing Standards
NEMA ST 1-4 (ANSI-C89.1) section 2.7 covers “Audible Sound Level Test.” For a thorough understanding of these tests it should be read in its entirety.
Briefly, the transformer is tested at its rated frequency and voltage under no-load conditions in a room which is 10 feet larger on all sides than the transformer. The ambient sound level of the room must be at least 5 db, and preferably 10 db, below the ambient level plus the transformer level. Five sound readings are taken with an approved sound meter one foot from each side of the transformer enclosure and one foot above the enclosure. The sound rating is the average of these five readings.
For three-phase transformers, the NEMA maximum allowable averages of the readings in decibels are shown in the chart below.