Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.
The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbor, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level.
Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear.
Definitely not the worst I have ever heard.
1. Transformer noise increases with age as the laminated steel in the core starts to get loose and the thin film of insulation on it wears off.
2. Quality conscientious mfrs use high grade Grain-Oriented Electrical Steel (GOES) and orient the grain of the steel core laminations to help prevent this for as long as possible by reducing the eddy currents in the steel. GOES is available in about 6 different grades of quality with the cost increasing as the grades get better. Go figure...
3. Challenger; cheap crap to start with, likely made with the lowest grade possible or maybe not even with GOES at all. Wore out faster than most.
I can't really tell from the pic, but if there are isolation mounts the trans is shipped with them tightened all the way down and you need to loosen them a bit. It appears to be an older install but I have seen alot done that way.
Here's a list of NEMA decibel ranges for dry transformers. Yours would be 50 decibels. That puts it at about the noise level of average conversation or a refrigerator or electric razor. How loud is that thing in person? I find a lot of the times, especially in offices, folks think any transformer noise is excessive, when it's usually completely normal.
If you can put a PQ meter on there, take a look at harmonics, I've done load tests on transformers that ran nice and quiet even at a 100% resistive load, but once installed they were running at 20% THD and hummed pretty significantly.
I know we've even swapped transformers for customers who were insistent even after we made it clear it was not likely to solve the problem, and it didn't.
I know our engineers kicked around the idea of putting in filters to try and get rid of the harmonic distortion causing the additional noise, but that seemed like a really complicated, expensive idea to me, and I'm not sure that's the best solution.
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