Hot Coil Electromagnet

[Peterae]

We are all familiar with an electromagnet see picture below.

If i place a hall effect transducer at one end, as current increases the magnetic poles at the ends of the iron nail gets stronger.

but what happens if the current is increased until the electromagnet coil starts glowing.

Would you expect the temperature of the coil to affect the magnetism strength at the ends where measurement is being made as the temperature of the wire increases to red hot.

Does it make any difference when the wire glows as opposed to not glowing?

Offcourse we already know what happens when we heat a permanent magnet to a highish temperature.

[Grumpy]

I expect the magnetic field to decrease as the current decreases since the rod-hot wire will have very high resistance.

[Peterae]

Sounds reasonable.
I need to get some high temperature insulation to be sure, although i have spent an hour so far trying this, sometimes i do not see an increase in mag field after a certain point and sometimes i see a sudden jerk down in mag field, i was using stainless steel thin wire and will need to change this as it does not need to be many turns on an iron nail to have a high resistance and therefore the measured mag field is barley detectable using my UGN3503

[ion]

I have an infra red room  heater consisting of a  two foot long coiled wire of Nichrome (I guess) inside a quartz tube of approximately 0.5" I.D..

This has always emitted a loud buzzing when first turned on and operates silently after the wire reaches an orange glow at about 11.07 Amps. This has always puzzled me but I never thought to research it until this thread was started.

The Amps drawn from the line does not change much, going from 11.50 Amps cold to 10.86  Amps fully heated, around 5.9% change.

I will try to measure the change in the emitted AC magnetic field, which theoretically should only drop by about 5-6%, reflecting the change in Amps drawn.

[Peterae]

Thanks ION very interesting,i have also seen this in heating elements.
I will follow up this thread when i get some high temperature bits & pieces.

It was a bit of a crazy idea while thinking about the heater in the valve i am working on, and the other thing that made me think about this was the valve filament kick at power on, the difference here is that the kick is produced while cold while relaxation occurs when hot.

[exnihiloest]

...
Would you expect the temperature of the coil to affect the magnetism strength at the ends where measurement is being made as the temperature of the wire increases to red hot.
...

The magnetic field depends only on the current in the coil. It doesn't depend on the wire temperature. That's what we expect. If you suspect a different behavior, only a careful experiment will be able to show it. The nail temperature will have also to be measured. The temperature dependancy of the magnetic permeability is well known and so influences the magnetism strength of the nail.

[ion]

This morning I ran further tests on the infra-red heater.

My setup was scoped across a 0.01 Ohm shunt resistor to measure the current. Previously I used a Kill-A-Watt meter.

I measured the current and there are no spikes shown, the voltage and current waveforms are identical and in phase as you would expect from a resistive heater.

The strange thing is when I use a magnetic pickup coil into the scope, I get a very small 60 Hz. sine wave with a huge positive and negative going spike that I cannot account for. It dies out at around 11.07 amps leaving a small sine wave when the audible sound dies out.

The questions are:

Why does the extremely spiky waveform show up with the external magnetic pickup coil, but no such spike is shown when using the current shunt resistor.

Why does the spike diminish completely around 11.07 amps, right when the heater stops emitting the audible buzz and enters the glow phase?

I urge anyone with a heater to replicate this experiment.

[Peterae]

I'm getting the same noise from my rectifier valve but only when it's turned on from stone cold and probably if i am slightly over driving it, it disappears after a couple of seconds, a low Hz vibrating noise, and i am also familiar with the noise you mention with the heater.
It looks like i need to add a sniffer coil to the valve  

I think the audible noise is from where the heating element coil is supported, but then the question would be why does the coil quiver while cold and not hot, specially when the hot metal would move a lot easier, maybe it's the ac magnetic field being attracted and repulsed relative to any other metal or iron inside the heater.
Still does not explain the kicks.

[forcefield]

There might be a power control circuit that limits the current when you first turn the heater on.  That would help prevent tripping a circuit breaker.  That circuit might account for the noise and spikes that you are picking up with your magnetic probe.  There might be a line filter that would prevent detection with your 0.01 ohm line resistor.

I don't have one of these heaters, so this is just a guess.

[Peterae]

No definitely not a power control circuit, i was brought up with large panel electric heaters  
As a kid we had one 4 foot long in the early 70's and it had a grill along the top front, when you first switched it on you could see the heating element resonate and clunk against the supports down it's length, quiet a loud noise, once warmed it stops after a few seconds, the elements were nearly the 4 foot length and were air coiled.

Anyway i am hearing the same thing with my valve, it's as if the coil couples to something when first switched on, this maybe just because the current is much higher at this point and hence the mag field is much stronger allowing coupling.

but the interesting point ION brings are the spikes, the audible noise is nothing new to me.

We had a fan heater a few years ago that made the noise as well.

[Peterae]

OK i have listened to my valve each time i power it up and i can hear all sorts of oscillations and vibrations from it fairly easily as it warms up.

Here's a recording of it warming up, sounds a bit like those double pulsing coil noises i was getting.

[ion]

There might be a power control circuit that limits the current when you first turn the heater on.  That would help prevent tripping a circuit breaker.  That circuit might account for the noise and spikes that you are picking up with your magnetic probe.  There might be a line filter that would prevent detection with your 0.01 ohm line resistor.

I don't have one of these heaters, so this is just a guess.

There is no power control circuit or line filter in this heater, just an on-off bi-metal thermostat. Anyway, if you read my first post you will note that there is no large inrush current, just around 5.9% higher current at first  turn on, not enough to cause concern.

Tungsten wound lamps have a huge inrush by comparison. I've worked in this field, designing soft start SCR circuits for such lamps to eliminate nuisance fuse blowing upon turn on. The effect I see here is something different.

I will attack this anomaly from a few different angles and report back as time permits.

This is a vertical infra-red coiled heater with a quartz or glass tube over the element. It is not sealed at the ends.

Coiled Nichrome is the standard for this type of heater.

The chart attached is in line with what I measure. Good data on this site:

http://www.wiretron.com/nicrdat.html

[Grumpy]

can you add time measurements to the table above?

how long after the initial turn-on does the noise stop?

[Chef]

This morning I ran further tests on the infra-red heater.

My setup was scoped across a 0.01 Ohm shunt resistor to measure the current. Previously I used a Kill-A-Watt meter.

I measured the current and there are no spikes shown, the voltage and current waveforms are identical and in phase as you would expect from a resistive heater.

The strange thing is when I use a magnetic pickup coil into the scope, I get a very small 60 Hz. sine wave with a huge positive and negative going spike that I cannot account for. It dies out at around 11.07 amps leaving a small sine wave when the audible sound dies out.

The questions are:

Why does the extremely spiky waveform show up with the external magnetic pickup coil, but no such spike is shown when using the current shunt resistor.

Why does the spike diminish completely around 11.07 amps, right when the heater stops emitting the audible buzz and enters the glow phase?

I urge anyone with a heater to replicate this experiment.

I assume the heater wire in which the current flows is coiled. I guess, before the current flows the heater wires are in a room temperature, have a definite length to this temperature. When switched on, the current rises, and the wires expend because of rising temperature, and the opposing magnetic fields. As the sine wave goes on, current flow changing directions, the wires vibrate back and force with less and less amplitude, until they  reach the glowing temperature, where there will be no more movements. I guess the back vibration when the current falling, because of the fast heat dissipation, so related to the material temperature related length change.

I guess it's all happening because the wires need several AC cycle to reach the temperature, where the heat generated by the previous cycles, doesn't dissipate fast enough until the next current rise coming. When the glowing temperature reached, the will be no temperature related movements.

The pick up coil may pick up the field spikes generated by the current carrying vibrating wires. You cannot measure with a shunt resistor those spikes, because your measuring point should have in an outside reference frame, from where the movement of the wires (which are now moving magnetic field sources) could adding to the flowing current generated waveform.

I could be totally wrong.

[EMdevices]

The strange thing is when I use a magnetic pickup coil into the scope, I get a very small 60 Hz. sine wave with a huge positive and negative going spike that I cannot account for. It dies out at around 11.07 amps leaving a small sine wave when the audible sound dies out.

Ion,  i've seen this same type of waveform from one of my toroid coils, and it was due to magnetic saturation.  As the material saturates magneticaly, the inductance drops faster and faster, and so the current increases rapidly, so the dI/dt is fast and creates a large dB/dt response which induces the voltage spike.

EM

[Peterae]

Theres a problem with that EMdevices. Here's a quote from ION

I measured the current and there are no spikes shown, the voltage and current waveforms are identical and in phase as you would expect from a resistive heater.

[ion]

Ion,  i've seen this same type of waveform from one of my toroid coils, and it was due to magnetic saturation.  As the material saturates magneticaly, the inductance drops faster and faster, and so the current increases rapidly, so the dI/dt is fast and creates a large dB/dt response which induces the voltage spike.

EM

EM:

There is no magnetic core to create any saturation effect.  Nichrome is very weakly magnetic and there is no closed magnetic structure that can saturate.

I know the effect you are talking about and have studied it carefully on my bench. This is not magnetic saturation effect.

I wish people would try the experiment with Nichrome wire, and carefully observe the effect on an external magnetic probe.

I'll try to post the some waveforms as time permits.

To All: please read my prior posts carefully before offering a "theory". Voltage and current waveforms are completely in phase showing a near pure resistive heater. Granted it is coiled but inductance will be very small at 60 Hz.

[Peterae]

ION did you hear the sound recording i did of the valve rectifier heater warming up in post 10.
Not sure if you missed it or not sorry if not.

[ion]

Peterae:

Yes, I listened and it sounds very interesting.

Attached is the sound of the Nichrome heater. Note that just before the pulse decays into the sine wave (around the middle of the file) there is an additional burst of sound like the beginning.

So there is a quick buildup to the first maximum amplitude, a slow decay, then two large buildups again before fading into silence.  The large amplitude waveforms occur as the spike is decaying back into the sine wave.

If you view this file you can see the changes. I did not fade the file, the sound dies out to a very low level.

[Grumpy]

Ion,


This magnetic kick is "continuous" until the wire heats up and the buzzing stops?

How far is the pickup from the wire?

[ion]

Ion,


This magnetic kick is "continuous" until the wire heats up and the buzzing stops?

How far is the pickup from the wire?

Yes, the spike picked up with the mag probe is continuous until the sound dies out, but the sound goes through three major amplitude changes during the heatup period. View the Nichrome Heater.mp3 audio file with an wave viewer to get a clearer view.

The Magnatek magnetic probe is oriented vertically near one end of the coil, about 1.5" away.

The microphone was placed about the same distance and location. Attached is a capture of the wave file. Do not confuse the audio file with the spike I am referring to. This will be posted later.

[ion]

Here are two camera shots of the scope traces. Forgive the fuzziness. Smooth sine curve is the heater current. Spike waveform is superimposed.


First shot (001) is spikes as picked up with mag probe full amplitude at initial time up to 5 seconds vs. current.

Second shot (002) shows spikes decaying after 5 seconds versus current.

[Grumpy]

Can you move the probe in-out to see if there are other peak points or is it anywhee within a zone arong the wire?  i.e. map the field perp to the wire.

[exnihiloest]

@ION

Here is a possible explanation. Under its own magnetic field, a coil tends to collapse on itself, because each turn of wire acts as a magnet and they attract one another.
With AC current, this effect produces a mechanical vibration that is hearable. It also changes the inductance which becomes time dependent at the rythm of the signal, and can disturb it (for example by producing the peaks, depending on the mechanical displacements).
When the wire heats, its mechanical characteristics change, it becomes softer and consequently the mechanical resonance is decreasing.
It's just an idea. Further tests are needed.

[ion]

@ION

Here is a possible explanation. Under its own magnetic field, a coil tends to collapse on itself, because each turn of wire acts as a magnet and they attract one another.
With AC current, this effect produces a mechanical vibration that is hearable. It also changes the inductance which becomes time dependent at the rythm of the signal, and can disturb it (for example by producing the peaks, depending on the mechanical displacements).
When the wire heats, its mechanical characteristics change, it becomes softer and consequently the mechanical resonance is decreasing.
It's just an idea. Further tests are needed.

Possibly, but the inductance is way too small to account for this, a few uH at best. I will measure it to satisfy.

I will continue testing. Here are two additional DSO shots, showing the large spikes (from the Magnatek probe) at the zero crossing, and how they nearly completely die out  as the wire heats up.

They are superimposed on the current waveform, which is quite clean. There are smaller amplitude spikes besides the large ones at the zero crossing.

Additional note: Although I state that the spikes are occurring at the zero crossing, this is only with reference to the 60 Hz current waveform. In actuality the spikes appear to be at the peaks of the sine wave as picked up by the Magnatek, the phase of which changes with location.

Do note that the current and voltage waveforms of the input power are exactly in phase. I have not posted this shot yet.