Any way to lower transformer hum in a built system?

[TheUnnamedNewbie]

Hi guys,

I recenetly bought two older HP lab supplies. They are working great and I'm very happy with them (although I'm already thinking up ways to give them some extras like bringing the sense terminals to the front, etc).

However, the transformers hum quite loudly. I've already verified that this isn't because there is a defect inside, they don't draw any more current than I would expect. Is there anything I can do to lower this noise? Most transformers are potted to lower this hum a bit, but I think that is not viable here since it's already built and the thermal performance would be ruind. Any other tips?

[bktemp]

Is this one of those linear power supplies with a SCR/TRIAC preregulator?
They produce a louder hum than other transformers because of the leading edge phase cut control. In my case some load conditions caused vibrations on the lid of the case. Adding some soft material inside to dampen the resonance solved the problem.

[Ian.M]

If there's clearance, mod them to mount the transformer resiliently. E.g. 3M foam VHB tape under the transformer and  a thick rubber washer (+ a plain washer on top) under the head of each fixing screw.  However maintaining ground integrity to the core may be problematic

Case panel vibrations can be damped by gluing a small wooden batten diagonally across the inside of the panel using a rubbery glue.

[hammy]

Could you share a picture?
How are the transformers constructed? Is the core build with stacked layer of  metal-sheets or solid? Is there a tin cover over the transformer, or is it an exposed one? Are the windings covered?

It depends where the hum is created. Sometimes it is the core, sometimes the windings, sometimes the mount.

I had success in the past with cable ties, foam, rubber from a bicycle tube, superglue. Another time I was able to compress the metal cover a little bit with a pipe wrench. It depends ...

Cheers
hammy

[Ash]

Is this one of those linear power supplies with a SCR/TRIAC preregulator?
They produce a louder hum than other transformers because of the leading edge phase cut control. In my case some load conditions caused vibrations on the lid of the case. Adding some soft material inside to dampen the resonance solved the problem.

Oh good - It isn't just my unit then.. I have a HP E3640A that makes an annoying hum. I did roughly test it was related to input voltage by testing it on my variac.. I assumed it was something to do with the pre-regulator... Good to know it is "standard", even if it is annoying.   

[cdev]

I have the same problem on an older lab supply.

As I understand it, the varnish (between the transformer plates) wears out or becomes brittle and then they start to hum.

But it is fixable, if you're willing to set up your own jig to impregnate them (it doesn't sound super hard, but it requires having a vacuum pump and bell jar- and its potentially smelly if you use an oven to bake dry it afterward.) or pay a bit for it (sounds preferable if its available).

 I have not inquired as to how much this costs but I plan to, if the isolation approach doesn't quiet it enough.

Some people also seem to have had an experience where they performed some intervention (re-varnishing) to fix it, which seemed unsuccessful at first in curing it, but over time, afterwards, with use, the transformer became much less noisy.

[German_EE]

In the past I have had good results using a large syringe full of varnish. The heat of the transformer ensures that it sets quickly.

[cdev]

wet shellac while its running?

Wouldn't it short out?

[Ian.M]

Shellac is only slightly conductive when wet - the solvent for it is dry methanol, ethanol or isopropanol and even small proportions of water cause clouding problems.  Most varnishes will also be nonconductive as they are resins and drying oils diluted with an organic solvent.

However it wouldn't be a good idea to apply either to an operating high voltage or high frequency transformer or coil.   A safer procedure would be to run the equipment at the highest permissible ambient temperature as you can at full load, long enoough to reach its maximum steady state temperature, then switch off and immediately apply it. As long as you get a good initial coat on, as the transformer cools the varnish or shellac will be sucked into any voids.  After its reasonably dry it can be run again to warm it up and bake out remaining solvent.

[TheUnnamedNewbie]

Shellac is only slightly conductive when wet - the solvent for it is dry methanol, ethanol or isopropanol and even small proportions of water cause clouding problems.  Most varnishes will also be nonconductive as they are resins and drying oils diluted with an organic solvent.

However it wouldn't be a good idea to apply either to an operating high voltage or high frequency transformer or coil.   A safer procedure would be to run the equipment at the highest permissible ambient temperature as you can at full load, long enoough to reach its maximum steady state temperature, then switch off and immediately apply it. As long as you get a good initial coat on, as the transformer cools the varnish or shellac will be sucked into any voids.  After its reasonably dry it can be run again to warm it up and bake out remaining solvent.

Thanks for the responses, I'll look at giving that a shot. I also suspect there is some mounting component to this, as when I bend my bench every so slightly (by leaning on it, for instance) I can tell that the noise changes. Will have to take a look. But at least, now I have somewhere to start.

[edpalmer42]

Sometimes you can reduce hum by just tightening every nut and bolt in the unit.  Over time they loosen up and everything starts vibrating.  It isn't the actual mains frequency hum that you're hearing, it's the harmonics from the vibrating parts.  In any case, it's a free and easy thing to try.

Ed

[ebclr]

To get good advice you should provide more information about the transformer that has the noise problem. Such as:

- core material and grade

- flux density in yoke and legs

- the method of construction (wound, stacked, mitred, butt/lap)

- size and method of assembly

But here is some general advice that may assist. The noise usually emanates from the yoke. Reducing the flux density in the yoke (adding laminations) will help to reduce the problem. Eliminating air gaps in the core is also effective, so make sure laminations are cut precisely and edges mate without gaps. Laminations may have a slight bur on cut edges, removing this will also create a significant improvement. Finally, clamp the core tightly and evenly. Then impregnate (vacuum impregnate if possible).

[cdev]

Look at the bottom of this page..

http://www.antiqueradios.com/forums/viewtopic.php?f=3&t=12858&view=next

Vacuum is not essential.. varnish is.

[wraper]

Those transformers are not humming, they are very silent. Cover is what is vibrating and emitting noise under the impact of magnetic flux from transformer.

[darrellg]

However, the transformers hum quite loudly. I've already verified that this isn't because there is a defect inside, they don't draw any more current than I would expect. Is there anything I can do to lower this noise? Most transformers are potted to lower this hum a bit, but I think that is not viable here since it's already built and the thermal performance would be ruind. Any other tips?

They hum because they don't know the words. 

[Iwanushka]

Hi,

As others said varnish is your friend, other trics are compressing and welding edges for E cores. Also you can you nail polish instead of varnish works fine if you know what to do.