actransformer
I have a large 2600W 230V -> 28V transformer, and it makes a constant quite loud bzzzzzz sound while it works. Is there any way to make it more silent (other then buying an equivalent SMPS which I don't have money for now)?
I am looking for some quick and cheap ways to make it less loud. Wrapping the whole thing in a large towel might not be the best idea as it will probably prevent the transformer from cooling properly.
Why does it make that annoying sound anyway?
A large air gap will give you a large leakage inductance, which appears as series inductance in your model.
Normally a transformer designer tries to minimize leakage inductance since it negatively affects regulation, but sometimes it's necessary (as in your case) or even an advantage (as in the case of discharge tube transformers).
Input to output voltage ratio is the same as the primary-secondary turns ratio.
For 42 see this.
So, if you want 220V out for 12V in the turns ratio is 18.3333.
Next you want to know how many turns for the primary (12V) winding and this is more complex. When driving the primary with a 12V RMS sine wave you don't want to be taking loads of current (unloaded output) because the core will saturate. This means you need to have a certain minimum inductance for the primary.
Most 230V AC transformers are in the realm of 1 henry inductance. For a 12V input running at 100mH for 50Hz will be about in the right range.
If you know what core size and laminations you want, the manufacturer of the core will likely give you a figure for a parameter called \$A_L\$. This is the microhenries inductance per turn squared and if you have two turns clearly the winding inductance quadruples.
Let's say they give a figure of 10uH and you need 100mH for the whole winding. This means that 100 turns will do the job because 10uH x 100 x 100 = 100mH. With 12V RMS at 50Hz the current is about 380mA. You'll need to double check to see if this current through 100 turns saturates the core.
Core saturation is due to flux density and all you know is RMS current and turns. Peak current is going to be about 540mA and therefore peak magneto motive force is this figure x turns = 54 ampere-turns. Divide this by the mean length that the magnetic field takes around the core.
Maybe this length is 500mm so, H (magnetic field strength measured in amp-turns per metre) is 108. Then use the BH curve of the core material to see what flux density is. Check to see that it is not high enough to cause too much saturation and you are in business.
Best Answer
The sound is a result of the system creating large magnetic fields as a function of how it operates. These fields are fluctuating, probably at 50Hz based on your input voltage but maybe 60Hz if your in the US.
These magnetic fields push and pull on components in the system, since perfect dampening isn't possible these components vibrate. Some of what you hear isn't at 50/60Hz though, it is also harmonics of that frequency and interactions with other components.
You definitely shouldn't do anything that will limit air flow or otherwise create a situation where the transformer could overheat.
You can poke around a bit (carefully) and see if you can find a particular components or panel that is vibrating and try to use something to dampen the vibration. High Temp Dampening material may be an option.
That being said, transformers of that power are basically impossible to silence. The transformer core or the windings are probably vibrating and causing most of the noise. At high enough power levels the iron core itself actually stretches and contracts as well causing vibration.
Some designs try to isolate the core to prevent vibrations transferring to other components but its not something you can likely fix in an after-market fashion.