Transformer Hum

Power transformers are everywhere: in our equipment and in our homes. They transform the utility service voltage (120 or 240 VAC in Canada) to a higher or lower voltage needed by an electrical device. Those that transfer high power are large: dimensions, weight and cost. Copper isn't cheap. Although they can be remarkably efficient there is power loss.

Heat is the most noticable attribute of power loss. Another is sound. The intense magnetic field can vibrate loose components in the transformer that are subject to the field. Those are the core and the windings. All transformers vibrate but an improperly constructed transformer or one that is old and suffering from degradation of chemical binders and insulating layers can be objectionably loud. 

The greater the power the transformer is handling the stronger the vibration. If the equipment is in the shack you'll notice it! For example, the power transformer in a kilowatt amplifier when it is transmitting.

The sound has a fundamental of 120 Hz (counting both positive and negative swings at 60 Hz) and there can be substantial harmonic content. The latter is more noticable since our ears' sensitivity rolls off at low frequencies. The transformer's enclosure and how it is mounted can amplify the sound, and those harmonics, in the same fashion as a speaker and its enclosure. In severe cases it can be quite loud and annoying. 

Headphones are not very good at blocking low frequency sounds, and other noise such as rig and amplifier fans don't cover it up very well. It is better to solve the problem at the source. Unfortunately that isn't always possible, depending on the condition of the transformer and its placement inside the equipment. 

Let's look at a few cases I've dealt with, or tried to deal with, that may give you a few ideas should you have to deal with loud transformer hum. Don't confuse this with the other kind of audio hum that can be cured with a transformer.

The sheet metal enclosure for the Drake L7 amplifier's separate power supply is prone to vibration since it is large and thin with little structural bracing. It is close to the transformer on 3 sides of the large plate transformer core. When I first set it up it was loud. The sound was amplified when I placed it on a shelf under the operating desk since the desk and wall acted as an acoustic enclosure.

The enclosure is not easy to remove so I waited until I had to open it to replace the rectifier and filter boards. The first thing I tried was to tighten the bolts that press together the laminations comprising the core. They may loosen over the years from the 60 Hz vibrations. That helped a little but not nearly enough.

The next step was to add dampening to the enclosure. Fibreglass insulation was attached to the top and sides of transformer core. The layer was just thick enough to fill the gap between the core and enclosure. This material works well since it fire and heat resistance and provides excellent soundproofing. It is important to allow adequate airflow when you do this since the transformer heat has to escape or the transformer will prematurely fail -- fibreglass insulation slows heat transfer (obviously). Use it sparingly.

The sheet metal vibration is greatly dampened by the insulation it is in contact with. The noise was sufficiently suppressed that I considered the problem solved. The hum is still audible though quiet enough that it almost completely disappears when wearing headphones.

This spring I purchased a small Astron linear power supply to replace, perhaps permanently, another that was damaged in my most recent lightning strike. Transformer hum was objectionably loud. Simple measures such as its placement on and under the operating desk were ineffective. A friend loaned me a power supply so I forgot about for a few months. About a week ago I decided to try again.

I'll tell you up front that I was not successful. Failures are interesting so I thought it would be worthwhile to show what I did, what effect it had and my final assessment of the power supply. Apart from the transformer hum the power supply works perfectly well.

The transformer is attached to the chassis with rivets. Unlike bolts and nuts, which have adjustable torque, rivets are not adjustable once installed. Transformer vibrations can be amplified and chatter if they are even slightly loose. They appeared to be tight but I removed them anyway. A drill is used to split the outer plate from the shank with a bit about the same diameter as the rivet shank.

I experimented with a more absorptive mount with the objective of attenuating the coupling of transformer vibration to the chassis. Plate steel chassis are notorious for vibrating and amplifying sound from within it. 

Screws and nuts replace the rivets, with fibre washers under the bolt head and nut. There is a thin layer of fibreglass insulation between the chassis and transformer flanges. With the enclosure still open, the level of audible hum was reduced. More insulation was loosely wrapped around the transformer to see if that would help. It didn't.

Before doing that, I tightened the 4 long screws at the corners of the transformer core. Only a little additional torque could be applied before the soft metal of the screw heads began to distort. Vibration was unaffected.

A small C-vise was applied to the vertical plates on the sides of the core to see if additional pressure would have an effect. It didn't. This was safer than torquing the screws beyond their limit. 

Had it worked I would have considered replacing the screws. This is not a good idea most of the time since the lamination and their coatings can be damaged. 

Should you try it, avoid steel screws unless the allow is designed for this application. Otherwise they may be heated by the magnetic flux. Brass or a similar non-magnetic metal can be used. The metal determines the maximum torque that can be applied. Be careful.

At this point I was running out of ideas. The core does not appear to be the source of the vibration. I concluded that the windings are at fault. This can be due to age or poor manufacture. I hesitate to blame Astron since this hasn't been common with their power supply products in my experience. However, I've sampled only a small quantity of their products.

My final attempt was to fully stuff the enclosure with insulation. This is not a particularly wise move but I had nothing to lose. Heat isn't able to freely escape and bits of insulation can cause a mess. I don't recommend it as a permanent solution. In any case it didn't work.

Well, that's not quite true. It did work well with the top cover off. When the cover was slipped on, screwed on or not, the noise returned. It was barely attenuated at all by all that insulation. In the photo you can see what it looks like. (Note: the cover was accidentally reversed when I took the picture but that's not how I tested it!)

The enclosure is amplifying the noise and there is no easy cure. It's partly due to its design. Notice that there are only two screw holes on top where the cover attaches to tabs on the rear panel by the heat sink. The front panel is free to vibrate, as is the top of the cover. Perhaps fabricated tabs and screwing it together would help. I don't plan on doing that since it isn't worth the effort for such an inexpensive product. I've exhausted my patience.

I am left with several options.

• Use it with the cover off. While not the safest thing to do it will not be in reach of inquisitive fingers. A soft plastic cover can avoid the bulk of the electrocution risk.
• Place it where the noise doesn't disturb the operator. I can do that, except turning it on and off becomes a chore.
• Replace the transformer. That's an overkill repair and not worth the expense. Rewinding the transformer is a job for a ham braver than me, or one with far too much time on their hands.
  
• Repair the lightning damaged power supply and relegate this one to the workshop. I have tried repairing the malfunctioning Pyramid power supply but it involves replacing almost every semiconductor on the PCB. There could be one or many faulty devices. It, too, isn't worth the effort.
• Buy or build a new power supply.
  

I will use it for a while with the cover off and replace it at some point. I have two power supplies damaged by lightning and perhaps I'll get lucky getting one of them working again. The application is my station automation system which does not require a highly regulated 13.8 VDC. Relays aren't fussy. I have 5 VDC regulators to tame the power supply for the Arduino and relays.

Working on this problem was more about personal interest rather than necessity. I enjoy playing with broken things to see if I can get them working. I spent little time on it because my interest level isn't high. I've exhausted the learning experience dealing with vibrating power transformers. 

Is this report useful to readers? Perhaps it is, and that's why I took a few minutes to write it up.