Why do transformers make noise?

[Hal]

Is it just me, or have you also heard a large transformer "buzz" ?

I think its the transofrmer itself.  I'm pretty sure.

What is that noise?

[toneman]

laminations of the xfrmr cause the "buzzing".
torriods buzz less.
the "buzz"  is at the AC freq....60HZ in USA
staybuzzed
tone

[zachary vex]

if the laminations aren't all stuck together nicely with that gooey lacquer stuff they're dipped in, they can vibrate and cause that buzz.  it happens at high frequencies too... my television squeals (flyback transformer) for a few minutes every time i turn it on, until the flyback warms up and expands enough to stop.  

it happens in marshall amps too.  i had a head that i used to run in the control room (with the cab in another room at the end of a long speaker cable) and when the monitors were turned down, i could hear the audio output transformer making noises when i played.  my little iMP hi-fi amp does this sometimes too when i'm doing frequency generator tests into a dummy load.  most transformers can be made into somewhat of a transducer (speaker) at certain frequencies and under load or at higher levels.

[R.G.]

Let's think a moment.

We know that magnetic fields can move things around ( think of permanent magnets, solenoids, speakers, etc.)
We know that electrical fields can move things around (think of static electricity attracting dust).
So both electric and magnetic fields can exert a pull on other objects, given the right conditions. In a transformer, you have both electrical and magnetic fields as well as the correct materials (ferromagnetic for M-field and insulators for E-field) to be moved around.

If something is loose, especially the iron laminations, the magnetic field will move it a bit. If the loose something happens to be resonant at near the field frequency it moves even more. The iron laminations and the individual wires in the coils can move under pressure from the E-field. This is one reason that transformers are bolted together and then impregnated with varnish or other potting-gook; it holds things together and keeps them from rattling under the 120Hz shaking from the M and E fields, as well as helping heat get conducted out.

In low voltage (i.e. under 1kv) transformers, the E field is usually inconsequential compared to the M field. However, in high voltage transformers like CRT flyback transformers, the E-field is substantial and there is no loose ferromagnetic material to rattle. Flybacks have composite, sintered two piece bolted cores. When flybacks whine, it's because the E-field forces are actually making the pie-shaped high voltage winding expand and contract from the E-field forces on the distributed capacitances in the coil.

Iron transformers can do a similar thing. Iron not only is attracted by a magnetic field, it actually changes shape a  bit when a field is holding it. This is called "magnetostriction". In higher performance transformers where the M-field is particularly intense, the iron can be heard "singing" at three times the line frequency from shape changes due to magnetostriction.

And that's why they hum: movements due to E and M fields, and magnetostriction.

[R.G.]

Let's think a moment.

We know that magnetic fields can move things around ( think of permanent magnets, solenoids, speakers, etc.)
We know that electrical fields can move things around (think of static electricity attracting dust).
So both electric and magnetic fields can exert a pull on other objects, given the right conditions. In a transformer, you have both electrical and magnetic fields as well as the correct materials (ferromagnetic for M-field and insulators for E-field) to be moved around.

If something is loose, especially the iron laminations, the magnetic field will move it a bit. If the loose something happens to be resonant at near the field frequency it moves even more. The iron laminations and the individual wires in the coils can move under pressure from the E-field. This is one reason that transformers are bolted together and then impregnated with varnish or other potting-gook; it holds things together and keeps them from rattling under the 120Hz shaking from the M and E fields, as well as helping heat get conducted out.

In low voltage (i.e. under 1kv) transformers, the E field is usually inconsequential compared to the M field. However, in high voltage transformers like CRT flyback transformers, the E-field is substantial and there is no loose ferromagnetic material to rattle. Flybacks have composite, sintered two piece bolted cores. When flybacks whine, it's because the E-field forces are actually making the pie-shaped high voltage winding expand and contract from the E-field forces on the distributed capacitances in the coil.

Iron transformers can do a similar thing. Iron not only is attracted by a magnetic field, it actually changes shape a  bit when a field is holding it. This is called "magnetostriction". In higher performance transformers where the M-field is particularly intense, the iron can be heard "singing" at three times the line frequency from shape changes due to magnetostriction.

And that's why they hum: movements due to E and M fields, and magnetostriction.

[ESPguitar]

Thanks for the explanation R.G

RB

[Hal]

sounds good, RG, thanks :-D.  

I think I was thinking idealy - "there are no moving parts in there...how can current make noise?"  But there _are_ moving parts :-D

[ESPguitar]

Iron transformers can do a similar thing. Iron not only is attracted by a magnetic field, it actually changes shape a  bit when a field is holding it. This is called "magnetostriction". In higher performance transformers where the M-field is particularly intense, the iron can be heard "singing" at three times the line frequency from shape changes due to magnetostriction.

So here in europe with a line voltage at 230volts the buzzing from a tranny will be 150Hz??

RB

[Paul Perry (Frostwave)]

[So here in europe with a line voltage at 230volts the buzzing from a tranny will be 150Hz??RB

Yes, but I personally think it would be 100Hz... any bass players with perfect pitch?

[niftydog]

I don't have perfect pitch, but I sure can pick 50hz and 100hz, let me tell you! All bass players are well familiar with the mains cycle hum in their country!

So here in europe with a line voltage at 230volts the buzzing from a tranny will be 150Hz??

in the scenario R.G. described, and given the mains cycle is 50Hz, yes! But, I beleive they sometimes hum at the fundamental or 2x­Æ'.