Transformers installed indoors transmit vibrations to the surrounding areas, resulting in an unpleasant tonal noise commonly known as ‘transformer hum’. In addition to structure-borne noise, transformers generate airborne noise (noise emitted through air), albeit to a lesser degree. Given that structure-borne noise is the primary mechanism of noise transmission in a transformer, it is essential to isolate the core and coils from the support structure (floor system). This isolation would reduce the transmission of vibrations from the core and coils to the support structure, thereby mitigating structure-borne noise in the surrounding areas.
The airborne part of the transformer noise is normally abated by enclosing the transformer in a soundproof enclosure (room). The structure-borne part of the noise should be treated by properly isolating the transformer from the structure, abating the transmission of transformer vibration to the structure.
DEICON implemented a vibration isolation scheme using air springs for a 3300 KVA transformer weighing 12,000 pounds. This transformer was installed on the top floor of a 16-story corporate office building, directly above the offices of the CEO and other corporate officers as well as the conference rooms. Figure 1 shows one side of the core & coil before and after the air isolation.
Air springs a) provide the highest degree of low-frequency isolation of any type vibration isolator, b) have large load-bearing without, excessive static deflection and c) has low damping enhancing high-frequency isolation.
The air isolation, lowered the transmission of vibration from the transformer to the structure drastically abating the structure-borne noise. Figure 2 shows the power spectra and time responses of acceleration, measured across one of air isolators.
The blue traces show the measurement on the core & coil side and the red traces show the same measurement on the support floor, over the frequency range of 0-1200 Hz. Comparison of measured vibration across an air spring, points to the high effectiveness of air isolation.
Figure 2 Power spectra and time traces before across one of mounting feet of the core & coil
The sound measurements before and after changing the mounts were done in the the floor space below the transformer (the office area); not shown for the sake of brevity. Improvement in line with that of vibration abatement was observed.