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Noise & Vibration

Sound levels for transformers vary from 40 dbA for smaller distribution units to 68 dbA for 3000 kVA power transformers and higher for larger units. All transformers vibrate at 120 Hz because the EMF vibrates the unit due to 60 Hz oscillations. The audible noise is measured at no-load and tested in a low ambient noise environment with walls or reflecting surfaces at least 10’ away from the transformer per NEMA ST-20. When installed in more confining electrical rooms and connected to a load, transformers will exhibit higher sound levels than these standards. Transformers can be ordered that produce less noise that the NEMA ST-20 standard, generally at -3 and -5 dB but lower levels are also available. Rubber pads or springs can also be installed between the enclosure and floor to further reduce vibrations and noise. Harmonics generally don’t increase noise too much.

Check: Solution:
High Input Voltage Verify taps are correctly set for the input voltage.
High Frequency If power is coming from a local generator, adjust the generator. If the frequency can’t be adjusted, the transformer may have to be rebuilt.
Unbalanced Loads Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for each phase (1/3 of total kVA for three phase units).
Damaged Core. Check any core welds or clamped brackets when fully denergized to make sure the welds are intact and the brackets haven’t slipped. Loose brackets can be tightened on-site but broken welds or cores that have shifted will need to be sent back to the factory for repair.
Missing Core to Enclosure anti-vibration pads. All ventilated transformers should have rubber pads between the core and enclosure or mounting points. If these are missing they can either be field replaced or the unit can be sent back to the factory.
Loose Enclosure Tighten enclosure screws and bolts were necessary.
Transformer mounted on a suspended floor or wall may create and echo chamber and increase noise. Reinforce the floor/wall or move the transformer to a more solid mounting area.
Multiple transformers installedin one room. Multiple transformers in one area can sometimes resonate with each other and increase the overall noise. The transformers either have to be moved further apart or isolation pads or springs can be mounted under the transformer enclosure.
Transformer is running correctly but still producing too much ambient noise. A low audible dB transformer can be used to replace the existing unit and/or vibration isolation pads and springs can be installed between the transformer base and floor.

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