[Amps] Buzzing transformer

[Manfred Mornhinweg]

Hi,

some general rules about buzzing transformers:

- Tighten it up. Torque all the bolts, and don't forget to tighten up 
the center leg too. That's best done by driving insulating wedges of 
suitable thickness into the space between the core's center leg and the 
bobbin. Don't overdo that, or you might damage the bobbin or first layer 
of windings. The idea is to stop any loose laminations from vibrating, 
not to put a tremendous pressure on the thing.

- Impregnate it. Vacuum pressure imprgenation is best, but most of us 
can't do that at home. So, what you can do at home is a basic 
impregnation: Loosen up the bolts, put the transformer in a pan, slowly 
pour varnish over it. You can do that with a paintbrush, working the 
varnish on all surfaces and trying to push it in. Let it soak in while 
adding more. Collect the run-off from the pan and re-use it. When it 
looks like neither the core nor the windings accept any more varnish, 
tighten the bolts, insert wedges along the center leg if possible, stand 
the transformer on some spacers to keep it from sticking to the pan, and 
then heat-dry the varnish by applying a well calculated amount of DC to 
the biggest winding, in order to warm up the transformer to working 
temperature.
Transformer varnish exists in many types and grades. Use one with the 
thermal rating your transformer needs, class G or H would be typical 
for a modern transformer, while an old transformer is OK with a lower 
thermal grade varnish. "Air-drying" varnish dries faster than the 
"oven-drying" variety, but without warming the transformer, even 
air-drying varnish takes forever and a day to dry deep in the 
transformer, so do warm the transformer to dry it.

- Make sure the transformer isn't working at an overvoltage. Over the 
years much of Europe went from 220 to 230V, the US slowly went from 110 
to 127V and higher, so it's easy that an old transformer now is simply 
getting so much primary voltage that the core saturates. Both bolt 
tightening and impregnation can only cure noise coming from loose parts 
vibrating, but they cannot cure noise coming from magnetostriction. And 
magnetostrictions gets much stronger when the iron gets deeper into its 
saturation curve.

- And of course, make sure the stray field of the transformer isn't 
making steel cabinet parts vibrate, but that's something most people are 
well aware of.

One cause of a previously silent transformer starting to make noise 
could be that the waveform of your power source has changed, so that 
there are stronger harmonics in it. This often happens when some large 
non-linear load is added (electronic equipment), which could be in your 
own home, in some neighbor's, or by the power company. In most cases 
these harmonics cause much stronger audible noise from transformers, 
than the 50 or 60Hz fundamental.

And yet another way to make a transformer hum very loudly is having a DC 
component (even a very small one) in the applied net voltage. This can 
happen when the load on the transformer is asymmetric, such as when an 
open diode turns a bridge rectifier into a half wave rectifier, and it 
can also happen when there is an asymmetric load on the house on in a 
neighbor's house. Such asymmetric loads can be diode-fed heaters, for 
example my Black&Decker toaster oven places the heater element in series 
with a big diode to half the power! That makes EVERY transformer in my 
home hum loudly! Light dimmers using TRIACs also often don't trigger the 
TRIAC symmetrically under all conditions, causing a DC component in the 
line current. The hum happens because the DC component de-centers the 
flux in the transformer core, causing unilateral saturation.

So a humming transformer is not always to be blamed!

Manfred

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