Dry Type Transformer Sound Level

[samwgamgee]

I have a 75kVA dry type transformer. 480v:120/208v, wye-delta configuration. The transformer is one of two installed as part of a recent project, and is appreciably louder than the other transformer on the project.

When I measure the sound pressure level of the room with the 'noisy' transformer I obtain a 70dBA reading. When I measure the sound level of the room with the other transformer, I obtain a reading of 70dBA also. Based on how I read NEMA ST-20 it appears that both transformer installations are compliant with this standard for overall sound pressure levels.

Yet, when I do a spectral analysis of the sound, at the 125Hz range the sound level is 68dB for the 'noisy' transformer and 42dB for the 'quiet' transformer.

Both transformers are feeding the same types of loads (A mix of coffee pots, computers, refrigerators, copiers, incandescent lights, video monitors, etc. for a med/surg area in a hospital.) Both transformers are 30% loaded.

My acoustical engineer indicates that I should expect to see the most noise in the 125Hz range. Why is that?

Any thoughts as to why one is 'louder' than the other?


Thanks!

 

[jraef]

Mechanical isolation? Something about the noisier one is mechanically resonnating with your building's structure and making the transformer noise radiate from more areas than just the transformer itself.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[davidbeach]

I'd think that you'd get more noise at 120Hz than at 125Hz as the laminations get hit by the magnetic field 120 times per second (assuming 60Hz power given the voltages you list). Were the shipping anchors released on the noisy transformer?

 

[dpc]

Google: Magnetostriction

Iron expands and contracts (slightly) in varying magnetic field.

If transformers are identical and one is louder than the other, the location and what it is sitting on may be big factor.

If it's an issue, there are various rubber gizmos available for isolating the transformer from what it's sitting on.

 

[Zogzog]

Why do transformers hum? They forgot the words!, Really, its the domains aligning and resonating as posted above, you may want a IR scan to ensure that those loose connections causing the excessive vibrations are not current carrying parts, probally just loose panels.

 

[unclebob]

See

http://www.federalpacific.com

under training : transformer noise

 

[EEJaime]

I don't know if it is possible at this point, but we use an isolation pad at the mounting points to isolate the transformer vibrations from the building structure. This often reduces substantially the sound levels. One very good pad we use utilizes a sandwich of cork, steel and neoprene to absorb the vibrations. Look at CalDyn, (California Dynamics) at

http://www.caldyn.com

It also helps to assure that there are no rigid conduit connections between the transformer and the structure by use of flexible conduit connections. All transformers hum to a certain degree, some manufacturer's more than others. Square D for example, in their Sorgel energy efficient, "Watchdog" series, for transformers from 51 to 150 kVa guarantees a maximum of 50dB; 151 to 300 kVa a max of 55 dB and 301 to 500 kVa at 60 dB, (and these are at rated load). At only 30% load, one wouldn't expect the transformer you described to be generating anywhere near the sound levels you mentioned, so as the other gentlemen have stated, it may be a resonance problem with the mounting or other sources.

Especially in hospital applications, we specify as quiet an installation as we possibly can. This being a 'recent' project, I would look at the specifications and see if a sound level was specified, because it sounds as if the installation is not quite up to par. Is there installation details/specifications where isolation may have been covered?

It would be interesting to know what you discover. Good luck and keep us posted.

 

[DaveScott]

Apart from the installation differences, there may be a difference in the laminations of the core:
Loose/tight
Different chemical content & mechanical properties.

We have a different sound from three Toshiba Transformers (130MVA, 230/33kV), caused by the material for the core coming from different sources.

 

[samwgamgee]

Thank you everyone for your insight and comments.

To date we have determined that the shipping anchors were released and the conductors were properly terminated within the transformer housings. Since these measures were verified and the transformer appears to be operating within NEMA standards, I am tending to conclude the difference in noise is due to differences in manufacturing.

Regards!