What causes transformer noise, and how can it be eliminated?

Q: I am planning a new job which requires a 300 KVA, 3Ø transformer, several 25 KVA, 1Ø transformers and a 30 KVA, 3Ø transformer. This is essentially the same type of installation which our firm completed a year ago. In this installation we received numerous complaints about transformer noise. What causes transformer noise, and how can it be eliminated on the new job?

A: Transformer noise cannot be eliminated. It can, however, be reduced through proper design and assembly, and/or masked through proper consideration of the installation.

The basic cause of transformer noise is magnetostriction: the expansion and contraction of the iron core (laminations) due to the magnetic effect of alternation current flowing through the transformer coils. This produces an audible hum. Magnetostriction may be partially controlled by the transformer design, but it cannot be totally eliminated.

The fundamental sound frequency is twice the power line operating frequency of the transformer (i.e., a 50 Hz transformer produces sound at 100 Hz and a 60 Hz transformer produces sound at 120 Hz). In addition to the fundamental frequency, harmonics are also produced.

Since the average office environment is about 50 decibels, depending upon the equipment within the office, the location of a 300 KVA transformer in such an area would produce complaints. The 5 decibels (50db vs. 55db) difference between the normal office ambient sound level and the maximum sound level of the transformer may not seem significant; however, it is recognized that a 3 decibel increase in sound level measurement represents a doubling of the actual sound level, so the 5 decibel increase becomes quite large.

As this example shows, even a well designed transformer can be a problem if it is located without consideration of its surroundings. Transformer noise can be brought under control through proper installation. The following guidelines will help assure you of an acceptable installation.

  1. The maximum sound level of the transformer to be used should be compared with the estimated ambient of its location. If it is higher than its estimated ambient, the unit should be relocated.
  2. When installing transformers in “people areas”, such as office buildings or motels, plan to have at least one “non-people” room between the transformer and the “people area”.
  3. The mounting surface for a transformer should not amplify the sound level. A general rule of thumb is the mounting base (such as a concrete floor) should weigh at least 10 times as much as the transformer.
  4. A corner location should be avoided since the sound will be reflected out into the room.
  5. Don’t mount unit on thin walls, such as plywood or curtain walls. They amplify the noise.
  6. The manufacturer’s installation instructions should be followed so that any vibration suppression devices incorporated in the transformer design are utilized.
All Acme transformers operate at sounds levels below NEMA standards and each unit, 30 kVA and larger, is shipped from the factory with vibration isolation pads to help assure you of the quietest installation possible. Encapsulation greatly suppresses noise in units under 30 kVA, and vibration isolation pads are not required.