When transformers are rigidly (not resiliently) mounted on the floor (or any other structure supporting them), their vibration will transmit to the support structure and find its way in neighboring spaces (rooms, offices, laboratories, etc.), causing an annoying, tiring, tonal noise. This structure-borne noise issue can be addressed by:
- Isolating the core and coils of the transformer from the ground/floor,
using proper vibration isolators. In air cooled dry transformers this means to
isolate the core and coil from its support structure. For an oil filled unit
it means to isolate the core and coil from its tank base and isolate its tank
base from the support structure.
- Making certain all the connections to the surrounding are flexible.
This includes incoming cables, busbars, stand-off insulators, etc. Note that any rigid
connection from the vibrating transformer to a solid structure will transmit vibration.
- Avoiding the use of a room, to house a transformer, with dimensions corresponding to half wavelength of the transformer vibration/noise frequencies. That is, stay clear of the room acoustic resonances being perturbed by the transformer.
Transformer vibration (and consequently noise) is caused by the magnetostriction of the core laminates (the extension and contraction of the core laminates when magnetized). Under alternating fluxes, this extension and contraction takes place twice during a normal voltage or current cycle, resulting in vibration to occur mainly at 120 Hz and its higher order harmonics (multiples of 120 Hz, i.e., 120, 240, 360, etc). In addition, a lower magnitude vibration also occurs at off multiples of 60 Hz, i.e. 60, 180, 300, etc.