I sometimes walk my dog near a local substation. As soon as I get within 50 feet of the substation I start to hear a characteristic hum. I’ve always taken it for granted that high voltage causes the wires to vibrate creating the hum, but what exactly is vibrating and why the characteristic sound? I’ve heard of 60 cycle hum, but that doesn’t really help me.
Can someone dumb this down and explain why high voltage running through wires causes a constant humming sound?
According to Wiki:
BTW- it isn’t the voltage, but rather the current that causes the hum.
A transformer is just a couple of coils of wire wrapped around an iron core. Transformers typically use laminated cores, which are basically stacks of thin pieces of metal instead of one big solid piece.
This image may help you picture that:
http://www.electronics-for-beginners.com/pictures/transformer.PNG
Coils of wire are basically electromagnets, so there’s a lot of strong magnetic fields involved in a transformer (which is how they work). As Chefguy posted, it’s these magnetic fields which cause things to vibrate, not only the transformer coils and the laminated core itself, but also the transformer case and other metal things nearby. A laminated core reduces eddy currents (basically current loops through the iron itself which waste energy) but a laminated core also hums more than a solid core.
With high voltages you can also get some hum from corona discharge. Basically, the voltage is high enough that it starts to break down the air and you get some electrical conduction through the air. If the air breaks down enough you’ll end up with actual arcing, which in a power system is generally a very bad thing.
For what it’s worth, the arcs hum as well.
The answer the part about 60Hz, the standard frequency for alternating current in North America is 60 cycles per second. That means that AC electric current reverses direction 60 times each second, which means the magnetic fields in transformers reverse their polarity 60 times per second, which means all the metal stuff in the transformers vibrates back and forth at 60 times per second, leading to the audible 60Hz hum.
I’ve also heard that a lot of the hum is due to the metal cases on the transformers vibrating as well, not just the cores.
Is it possible theoretically to make a transformer of that capacity which doesn’t audibly hum?
Here’s a fun vid with good audio of an arc humming. https://www.youtube.com/watch?v=hIkNY5xjy5k
it’s possible to quiet an inductor or transformer by potting it in wax or epoxy, but to your question I don’t think it is for a large power transformer. they’re typically oil-filled which cools it via circulation, often times through heat exchanger plates on the side (like so.)
Related to this, I have a ham radio rig I bought second-hand, part of which is this big boxy linear power supply which occasionally goes thump when I power it on. Confused and slightly concerned (but not enough to, you know, not use the radio) I asked around online and was informed that it was due to electromagnetic forces caused by the current, similar to the power transformer hum you observed.
The big difference is that the power supply converts AC to DC power, so instead of getting the hum of Geordi LaForge reversing the polarity 60 times a second, I just got a thump of the polarity getting set once, causing every magnetized bit of metal to move once, in sync. Specifically, most folks theorized it was the metal case doing the Snapple lid pop as it flexed with the electromagnetism.
As noted by Chefguy, the hum you hear is actually twice the line frequency, i.e. 120 Hz for North America.
And while the hum can be caused by a number of things, I am guessing most of it is due to vibration in the laminate plates that comprise the transformer’s iron core. In an ideal transformer the plates are perfectly flat and perfectly adhered to one another. In a real transformer, they’re not.
What fun! 
I’ve got a big ol’ linear Astron (35 amps, I think), also for ham radio, that thumps every time it turns on, too. I know it’s normal and harmless but agree that it isn’t desireable. It just sounds bad.
Thanks everyone. Ignorance fought.
thump
The thump is caused by* inrush current*. I assume the supply has a big transformer; when you switch it on there can be a big gulp of current taken to get the magnetism in the core into its working state. Its size depends on what state the transformer core was left in at the instant you switched it off last time, and the exact phase of the mains when you switch it on again.
That is why it only thumps sometimes.
Should not be a problem unless it is large enough to pop your circuit-breaker.
Modern transformers, big and small, hum much less than older ones. I can remember street transformers humming: not any more. And the last time I was in a substation, the physically-much-smaller-than-they-used-to-be substation transformers weren’t audibly humming at all.
Of course, part of that is my deafness. But I’m deafe at high frequencies: I haven’t lost much at 100Hz.
Also, wrt the wiki quote: high current substation transformers are specifically designed not to leak electric field as much as possible… If it causess external equipment to hum, it’s a piss-poor device. Humming in a switching yard might be caused by exteranal electric field, but I don’t think that’s coming from the transformers.
I think the last thing I heard up on a utility pole actually humming was the ballast for an older mercury-vapor street light. Even that’s rare now in my area as LEDs take over.
I’ve always heard corona discharge as a hiss or mild crackle when standing under high-tension wires. Generally speaking, there’s not much metal in close proximity to the high-tension wires themselves that can be influenced by the magnetic fields, and not much metal in the lines themselves to be subjected to magnetostriction. You can certainly hear the classic 120-Hz frequency, but it’s fairly subdued compared to what you hear standing near a substation.
