|The manufacturer must first insure that he achieves the noise level as specified by the appropriate specification. If something unusual is required by way of a very low noise level then there should be discussions and agreement between the manufacturer and the user, as to what steps to take. Remember the only course left to the manufacturer is usually to lower the flux density and this means increased cost. There have to be trade-offs between cost and noise annoyance or treatments.
If the manufacturer is only supplying core and coils, then what happens next is in the hands of the user, assuming all noise level requirements have been met. If the core and coil is mounted in a containing cabinet then the manufacturer has some precautions to take.
He must insure that the core and coils are correctly resiliently mounted, for it they are not, the noise level will increase. The stiffness of the mounts must be such that they do not weaken the installation by being too soft or spoil their attenuation properties by being too hard.
The choice of the resilient must be carefully considered. It has to absorb transformer frequencies which, by most commercial shock absorber systems are very low. Shore hardness (resilience), ability to withstand the environment and stiffness sufficient to carry the unit, are all important design parameters.
Bus bar or other connections to the core must not transmit vibrations. Flexibility is the key. Ventilators must be carefully positioned. The core must be designed to avoid transformer frequencies of half wave-length dimensions, or multiples of these dimensions. If this cannot be achieved, then consideration of damping material applied to the case, is required. This is an added cost and must be part of the arrangement with the user.
Now comes the interface with the user. For shipment purposes, it is often necessary to block out the core and coils against the case, to avoid shipping damage. This can include holding down bolts which if left in a fastened condition, can short circuit the anti-vibration effects of the resilient mounts. The manufacturer can draw attention to these bolts by painting them with a florescent paint, and advising his customer to remove all such marked bolts before use. All other blocking and wedging, not part of the design, should be carefully removed since these might interfere with the vibration isolation. The user should be made aware of any of these requirements. After this, it is up to the USER!
|The user thinking should start at the conceptual stage. If he can, he must consider if he has a noise problem before he specifies his transformer. If he does, a noise survey including frequency analysis, would be advisable. If for instance, a building is only in the conceptual stage, then a little thought beforehand will make sure that transformers are not placed in small reverberant rooms next to a proposed board room, sleeping areas, study areas or other occupied areas where the normal sound level is low. Closets under stairs seem, very popular for dry type transformers but are usually acoustically bad. Some discussion with the manufacturer is useful at this point.
A word of warning here. The noise level as measured and given by the transformer manufacturer is usually for the core and coils inside a cubicle. There is no way that the manufacturer can assess the effect on the transformer noise level by the location in which the unit will be place. It is advisable that if a user wants to maintain a particular noise level in a particular environment they should work backwards.
First of all assess what level is tolerable (say 65dBA). Allow for the effect of the room (say 3dB). Allow for the efficiency of all the connections (say 2dB) and as a result ask for a transformer to meet 60dBA! This will ensure that the required noise level is met. Advice on how to assess these corrections is available within Federal Pacific.
The design of the room to house the transformer is the next consideration. Avoiding half wave-lengths of transformer noise, or multiples thereof, is advisable. This includes dimensions in all directions, including the ceiling. If these dimensions concurrences are involved. This is a caution against using acoustical treatments which are only effective for speech frequencies. Choose damping materials for the noise frequencies to be damped.
Isolation of the transformer from the ground is vital.
Installation instructions must ensure that nobody tightens down shipping bolts but removes them. Connecting cables must be as flexible as possible. Ventilation ducts must be placed in positions where these are effective thermally without affecting the acoustic performance.
After taking all the precautions, a noise survey after installation, with the transformer excited might be useful.
|It might be useful to review the salient points and give some extra pointers:
1. Transformer noise is difficult to change at the source. Flux density reduction is the main thrust, but this means increased cost.
2. Transformer core constructions help to a degree. Reputable manufacturers will use good joints, flat steel, consistent thickness, good core supports, few bolts, etc.
3 Transformer current loading has little or no effect on the noise level.
4. Placing transformers in liquid (oil) does not help since oils are incompressible.
5. Vibration isolating core and coils within a tank does assist vibration isolation although isolation of the whole tank is still needed.
6. Noise reduction by distance is the simplest form of attenuation. If it can be achieved without cost excellent. Usually it cannot.
7. Noise reduction by screens, bushes, etc., is the next simplest but use should be made of the topography of the site. Remember the shadow effect means the noise could be heard outside the shadow of that screen.
8. Full enclosure is usually the only option left to a troublesome transformer.
9. Full enclosure can be made of any material with a high mass/weight ratio. Brick, concrete, steel have been used. Expect 25.30dBA reductions.
10. Full enclosures using masonry products are not easily demountable. Prefabricated concrete block is the best for this application.
11. Steel, mass or limp panel techniques make good demountable enclosures. A 15 20dBA reduction is possible with properly designed enclosures.
12. External cooling to the enclosure requires flexible treatments to the connecting pipe work.
13. Enclosure mounts should be separate from the transformer base or at least, isolated somehow.
14. All connections - cables, etc. to enclosed transformers should be flexible.
15. Remember bushings vibrate and losses (acoustic) are experienced through them. Flexible acoustic protection between enclosure and bushings are needed.
16. Bushings used in an enclosure might have to have a longer ground sleeve to accommodate the enclosure roof distances.
17. Pay close attention to access doors and removable covers on enclosures. Tight fits are essential.
18. Watch the dimensions of rooms in which units are mounted. Damp them if necessary, suitable for transformer frequencies.
19. Damping materials are needed if standing waves or reverberations are possible.
20. Choose damping materials compatible with trans former frequencies.
21. If steel plates are used for enclosures ensure that they are gasketed. Isolate the fastening down bolts.
22. Carry out sound surveys before and after installations. Remember to do a frequency analysis so that transformer noise can be differentiated.
23. Anticipate transformer noise problems when accommodating them inside a building - especially for dry types.
24. Pay careful attention to removing unnecessary bolting or stiffening used originally for shipping. Make sure the manufacturer identifies what can and cannot be used or removed.
25. Remember transformers need cooling air in rooms. Be careful (acoustically) when you position air ducts, ventilators and grilles, etc.
26. Pay attention to flexible connections inside rooms containing transformers.
27. Make sure the vibration isolators are correctly mounted and will accommodate transformer frequencies.
28. Select rooms which are not near potential complaint areas.
29. Check the voltage on the system. Increased flux density by having a higher than normal system voltage will raise the noise level.
30. When assessing the required noise level of a transformer work backwards from the required noise level at a location. Consider the inefficiencies of the site.
31. Consider very carefully where transformers will be mounted. Resilient structures such as wooden mezzanines might be harmful.
It has not been possible to give all the points and suggestions that might assist a user in producing a trouble free (noise) site. However, we are always available for assistance.
The purpose of this leaflet is to make people aware of the important points. If we have encouraged users to plan ahead with their noise problems then we have succeeded.
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