Transformer hum

[AxleD]

Hi,

I just noticed that my new 15-0-15 center tap transformer has a very low hum when power is connected.  It is a relatively cheap transformer ($15) and i was wondering if this was normal?

Thanks

[electr_peter]

How big is it?
Transformers may hum because laminations are not glued fully or laminations are loose with respect to other parts. Nothing strange or unexpected. High temp glue/epoxy should fix that.

Bigger mains transformers hum (or force hum on metal cases) no matter what you do, magnetic forces are very strong.

[Zero999]

Loose laminations will make it worse but all transformers hum due to magnetostriction.

[AxleD]

Thanks I though it was normal but was a bit concerned because it wasn't there initially.

FYI, It is a relatively small transformer - maybe 7.5 cm (W) x 6.0 cm (H). Will post a pic if anyone is interested.

[Shock]

Does it have the holes to bolt the laminations together or existing bolts you can tighten? Do a test, you can use temporary clamps or make up a wooden one, just observe safety precautions and don't get anywhere near the clamps while it's powered on. If it has excessive hum the windings may eventually short. Perhaps make a video of it a couple of feet away, it's easier to tell if it has an issue. 

Protect the laminations from being marked in case you have to return it.

[dom0]

Loose laminations will make it worse but all transformers hum due to magnetostriction.

Transformers will also hum much louder if the mains voltage has a DC offset. This is especially an issue with toroidal transformers, where a seemingly small DC offset can cause the core to go into saturation (which blows the fuse due to the rapidly dropping inductance of the primary winding, which in turn leads to a rapid increase in primary current).


Transformers in metal cases (e.g. sheet metal) will also hum louder than ones in plastic cases since they essentially use the metal case as a loudspeaker membrane (bending it marginally with their magnetic field). Increasing the distance to the case will reduce the hum in that case.


Another cause of hum can be filter capacitors, this hum typically sounds much "brighter" (more higher frequency content). This is caused by electrostriction in the caps. Sometimes it's also caused by magnetostriction in the transformer due to the rapid current increase when the filter caps are charged (100 times a second).

And at very high currents even the rectifier diodes can start to make noises...

[AxleD]

Thanks for all the information.

I have attached a pic of the transformer and i don't see any easy way to tighten the lamination's. Following suggestions earlier, i clamped the outside of the transformer and it does make a noticeable difference. It therefore seems to be an issue with a loose outer casing.

An incorrect wire placement on my breadboard earlier resulted in some magic smoke, the end of the road for some op-amps and probably some momentary heavy current draw from the transformer (some bright lights on the breadboard as well and another few hours to rebuilding the circuit). Not sure if this was the cause but the transformer is now getting a little warm. I do have the transformer connected to a 2A fuse so it shouldn't have been a problem.

Final question, under heavy load i presume it is normal for the transformer to get warm/hot and how hot is acceptable?

Thanks

[c4757p]

I'd probably accept the bit of hum. That looks like a fairly hefty bugger, they do that. Some of my power supplies sing quite beautiful songs

Yes, it's expected to get hot under load. Not horribly so; it'll be somewhat uncomfortable to hold your hand on for a long time. 60C?

[dom0]

Cheaper transformers will get hotter than quality ones (they save on the cooper wire), but every transformer is at least rated to full load at 40 °C ambient. [Internal] temperature rise can exceed 100 °C.