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., 240, 360, etc). In addition, a lower magnitude vibration also occurs at 60 Hz and its odd multiples, i.e. 180, 300, etc.

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.


Lowering the Transmission of Structure-borne Noise

Proper vibration isolation of a transformer from its support structure lowers the transmission of its vibration to the structure and thus abates the structure-borne noise. Vibration isolation of a transformer involves the following steps:

Study the flexibility of the support structure/floor and see how it would affect the vibration isolation effectiveness of the isolators.
Note that even the most rigid of structures are only rigid up to their first resonant frequency.
Selection of vibration isolation scheme (one stage or two stage),
This decision is normally made depending on the spectrum of the structure-borne noise, rigidity of the support structure, etc.
Design of the brackets to allow the structure of transformer to accommodate the isolators, and
Design of the hard stops in case the resiliently mounted transformer moves more than it should (in the unlikely event of an earthquake, for example).

Lowering the Transmission of Air-borne Noise

In addition to addressing the structure-borne noise, one needs to abate the transmission of the air-borne noise. This is mainly done via the acoustic treatment of all surfaces (walls, ceiling, and floor) of the room housing the transformer by sound barriers and sound absorbing material. Moreover, soundproof doors should be used for that room.Structure-borne Transformer Noise Reduction by Air Isolation(PDF) Vibration Isolation | Sound and Vibration Control