Eddy current losses in core of a electromagnetic motor

[sophiecentaur]

I agree. It is subjective and may be psychological. I mentioned DIY-ability as well.
Regarding SS amps, circuits I saw in past were frightening big, with hundreds of elements. May be today's IC schematics are more simple. What schematic would you recommend for a ribbon mic preamp with 80-90dB gain and low noise?

So we won't have a fight about that then! Good man.

It seems that a transformer is an accepted way to match a ribbon to a normal amplifier and, I guess they tend to be integral, meaning that, if you have a microphone, you will also have that transformer. But, if you do not have a transformer, it may cost you a bit to buy one so the electronic alternative would be attractive. The industry is very conservative (as are musicians, too) so just because you can't buy such a system doesn't necessarily mean it wouldn't be a better solution.
It's years since I did any circuit design but there is a lot of information about circuits out there. It's just a matter of searching all those constructor sites for suitable circuits to try.
I have been trawling around google and have come across agreement (on various enthusiast's forums) about using a common base configuration for low impedance sources. Also, I have seen op amp current to voltage converters - basically, virtual Earth amps with no series resistor in the negative input.
This gives V_out = I_inR_feedback

For a ribbon (and cable) impedance of around 1Ω and and emf of about 100μVpeak, you can expect 100μA into the virtual earth. A feedback resistor of 10kΩ would produce a first preamp stage output of 1V peak.
I am reluctant to talk in terms of dB because the impedances are different and dB are essentially a power ratio. There are some very good, low noise Op Amps around but you could establish the principle of amplification with a 741 even. Using a balanced pair of op amps and an opto coupler could eliminate hum.

 

[markmal]

...
I'm pretty sure if you consider the amp-turns per inch of airgap in the magnetic circuit of your microphone you'll find the Δflux from ribbon current is infinitesimal compared with the flux from your magnet.
In other words it's calculable; but I think it's so small as to be not measurable and an old hand would likely say 'don't worry about it'..
only way to be sure in measure it.
Carl Pugh addressed this in posts 9 and 11.

Yes, Carl, you and some other guys told me that the fluctuations of flux are very little comparing to flux from magnegnet and should be ignored.
But I have some feeling that something is not right in this comparison. Let me explain you why.
Example.
Atmospheric pressure is big, heavy - 14.7psi at sea level.
Pressure of mosquito wings is infinitesimal compared with the atmospheric pressure. It is probably 1mln times less.
However, we can hear mosquito quite well from distance 1-2m.
Huge atmospheric pressure does not hide infinitesimal fluctuations of itself.

This is what I mean by still asking "will flux in core fluctuate impacted by ribbon? how much (%) energy will be lost on these fluctuations of flux?"
It is like how much energy mosquito looses on making sound around itself instead of directing this energy on flight, counteracting with gravity and wind. :)
(Yes, making waves can take big portion of energy, that is why ship hull engineers try to elimitate them.)
So, the question is not in comparisson a mosquito and the Earth athmosfere, but rather getting understanding about efficiency of mosquito itself.

The little transformer has many more turns and no air gap. That's where the flux from your ribbon current will be significant - inside the transformer where you want it.
If it's a worry perhaps you could use nonconductive ceramic magnets for pole pieces?
Or place a few turns of tiny wire in the airgap and measure Δflux with an oscilloscope.

ceramic magnets will not produce that amount of flux that can be concentrated by Fe Co core.
I plan to use laminated core. By two reasons: 1 - it is more technological for me. 2 - it can be used to fight (possible) Eddy currents.

Is it moot anyway? Since eddy currents in iron oppose a change in flux it seems to me they're helping you cancel out what in a motor would be called "Armature Reaction".
"Armature reaction" is modulation of field flux by armature current.

Yes, this idea is used in loudspeakers. They put Faraday ring there exactly for this.
I am thinking about this too. It is not that complex, elongated copper ring around each pole.

Maybe that's why you don't see laminations in the pole pieces of old microphones. Something to ask on a microphone design forum.

:) This is exactly what I hoped to find an answer here, on physics forum. I hope people here know electromagnetics better. ;)

Also I point out - a motor has two large mmf's that interact to produce lots of torque.
You have one huge mmf(magnet) and one infinitesimal mmf(current in the ribbon) that you'd prefer don't interact.
How many amp-turns are required to push your 1 tesla through your air gap?
1/1000th as many amps through your 1 turn ribbon will affect your flux by a part in a thousand.
That puts a number on it for you.

I looked at TI's application guide and they recommend this opamp for a microphone transformer replacement, but I think they mean the one at preamp input:

http://www.ti.com/lit/ds/sbos003/sbos003.pdfA
microphone design forum.
Time for some experimenting.

It looks like that.

I haven't heard you mention your acoustic design. This ribbon has inertia and stiffness, and you want it to move with feeble variations in air pressure... F=MA ...
Look at its output when subjected to square wave sound at various frequencies and with a range of resistive loads..

The lighter ribbon is the more signal level it produces. Amasingly frequency response does not depend on ribbon mass. It is because mechanic inertia is square of movement and electric output is square of movement as well.
They compensate each other, but overall level decreases if mass increases.
Because of this ribbons are very thin - ~2 micron.
Highest frequency that ribbon can take depends on width and depth of baffles on each side of ribbon. These baffles are formed usually by poles, sometimes by magnets too. 0.5x0.5" baffles will allow max ~8KHz
(explanation and calculator is here http://www.diyaudiocomponents.com/ribboninfo.php?sub=3).
But, decreasing width of baffles lowers sensitivity because it decreases pressure gradient.

 

[sophiecentaur]

I do not see any parallel between this situation and acoustic power vs atmospheric pressure. Static pressure is of no consequence, whereas the emf induced on the ribbon will be proportional to the static field.
The linkage between the ribbon and the magnet is very slight so I cannot see how 'eddy currents' could be of any possible concern - what you are suggesting is that the 100μA in the ribbon would produce any significant current in the core. What effect on the performance could you possibly expect this to have? Could it be frequency response? If you are bothered by losses, the resistance of the ribbon (0.5Ω) plus leads and amplifier would be far more of a power loss mechanism (affecting SNR). With a transformer primary in series, the total primary circuit resistance will be even higher and this will all correspond to resistive loss. As I have already pointed out, it is Signal Power that is the basic concern and not Voltages - hence the use of a matching transformer.

You refer to the eddy current effects in a loudspeaker as being relevant but, in the case of a loudspeaker, the coupling is far higher (there are around 100 turns on a typical voice coil , compared with a fraction of a turn, which the ribbon represents). Also, one of the problems with speakers is over heating; not a problem with a microphone.

I think you need to split this project into its individual parts and deal with one at a time. The pre-amp is one that could be investigated more fruitfully, I think.

 

[jim hardy]

"will flux in core fluctuate impacted by ribbon? how much (%) energy will be lost on these fluctuations of flux?"

The aluminum ribbon being nonmagnetic has no effect on flux.
Current flowing in the ribbon does create magnetomotive force which affects flux.
To put a number on it, you must know both the current and the reluctance of the magnetic circuit.
That reluctance is mostly the substantial air gap in which the ribbon moves.
Because it's a substantial airgap it has substantial reluctance.

The current in the ribbon is the voltage generated in it divided by impedance of the external circuit connected to it.
Voltage is BlV,
B= flux
l=length of ribbon moving in the flux
V=velocity of ribbon

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir3.html#c1

Current is BlV/Z

Flux is μ₀NIA/(length of magnetic path)
u₀ is a physical constant 4∏X10^-7
N, number of turns is one,
I = current
A=area of airgap
just use airgap width for that length term because iron is almost a short circuit for flux


so the only unknown is velocity

The ribbon will follow the equations of motion , just sum forces.
Trouble is as you know this thing is a [STRIKE]motor[/STRIKE] dynamo delivering signal power to the preamp
so there's a force besides air pressure arising from the current that has to be included.

Force = power/velocity
and instantaneous power = Volts X Amps

and that's why the microphone is sensitive to preamp loading,
from simple [STRIKE]motor[/STRIKE] dynamo action.

Here's what I would have done in younger days:
write a Basic computer program that solves equations of motion for, say sinewave pressure input , get your equations of motion working. Then calculate Vout. Then run frequency response plots at various loadings.. note with high enough impedance load the [STRIKE]motor[/STRIKE] dynamo action term disappears. So do variations in flux.

I encourage you to have a go at some numerical estimates. It always increases one's confidence.
You can be sure the "old timers" did.

"When you can measure what you are speaking about and express it in numbers, you know something about it."
Lecture to the Institution of Civil Engineers, 3 May 1883
~ Lord Kelvin ~

 

[sophiecentaur]

and that's why the microphone is sensitive to preamp loading,
from simple motor action.

Why are we all going along with this terminology? It's acting as a generator and not a motor. There is no electrical power supply. You know that and I know that. It all seems to spring from one web page which is practical rather than academic and it just makes things confusing. Can anyone think of another example where the terms are used so blatantly back to front?

 

[jim hardy]

thanks sophie quite right

in my mind motor and generator are both dynamos

and I should have said 'generator action'

sloppy use of words on my part.
That's one of those bugaboos that plague me .

thanks for the correction.

Were my differential equations and my Latex better it'd be fun to tackle those equations of motion. That'd help a fellow understand damping.

......

I had no idea there was such an active DIY microphone community out there.
It appears the transformers are as you'd expect, made for ultra low loss.

http://www.samaraudiodesign.com/RibbonMicrophoneTransformers.html
http://www.diyrecordingequipment.com/projects/samar-ribbon-transformers/

Mumetal is 80% nickel with additions of iron and molybdenum as main elements in turn this alloy offers an extremely high initial permeability. It is only moderately resistant to moisture and atmospheric corrosion, it used mainly in in transformer cores but is well known for its magnetic properties which are used for shielding electrical components from magnetic fields. "HYMU 80" has a minimum DC permeability of 80,000 at a flux density of 40 gauss when heat treated by the recommended procedure and test in accordance with ASTM 596

http://aircraftmaterials.com/data/nickel/mumetal.html


and they call their ribbons "Motors"
http://www.diyrecordingequipment.com/projects/re-154-ribbon/

DIYAC RE154 Ribbon

Component Microphone Component Ribbon Microphone

The RE-154 is a ribbon motor intended for DIY ribbon microphone projects. The ribbon motor is available either fully assembled (hand made in Europe) or as a parts kit. It is manufactured and distributed by DIY Audio Components.

Thanks to all who've posted for the eye openers.

What an age we live in - so many projects so little time !
I have ten antique outboards and three antique pickup trucks in the queue.

i'll bow out of this one - hope to have helped with magnetics basics a little bit. It's all I can do anymore...

old jim

 

[sophiecentaur]

I'm not having a go at you, Jim. I'm just amazed that no one but me has really picked up on some bad terminology. It makes me wonder about the veracity of the rest of those websites. But it may just be a language thing.
Ten outboards*? I have just one inboard and that is quite enough responsibility, thank you. Mind you, I do rely on it when actually out there on the 'open sea' - to help when the wind fails to do what I want it to do.

* Would you, in a million years, ever refer to them as ' generators'? lol

 

[technician]

I'm just amazed that no one but me has really picked up on some bad terminology. It makes me wonder about the veracity of the rest of those websites. But it may just be a language thing.

In my humble opinion this is a problem with 'websites'...this is one...use of correct terminology is sloppy.
text books are the best reference for terminology (the best have stood the test of time) and are not often quoted.
The dreaded WIKI is the worst, but usually the first reference.

 

[jim hardy]

I'm not having a go at you,

Wasn't taken as such - just I smacked myself in forehead.

I try to be precise in wording but as you see I too often backslide.

the sciences have made progress, because philosophers have applied themselves with more attention to observe, and have communicated to their language that precision and accuracy which they have employed in their observations: In correcting their language they reason better." lavoisier

and it's me who so often quotes him...

I really am happy to have stumbled across this thread. Learned a lot.

 

[markmal]

The aluminum ribbon being nonmagnetic has no effect on flux.
Current flowing in the ribbon does create magnetomotive force which affects flux.
To put a number on it, you must know both the current and the reluctance of the magnetic circuit.
That reluctance is mostly the substantial air gap in which the ribbon moves.
Because it's a substantial airgap it has substantial reluctance.
...

Thank you Jim, I'll try to program your formulas in spreadsheet.

 

[markmal]

I do not see any parallel between this situation and acoustic power vs atmospheric pressure. Static pressure is of no consequence, whereas the emf induced on the ribbon will be proportional to the static field.

My example just shows a principle where tiny fluctations of some parameter of a media are not hidden by an absolute value of the parameter. Pressure can be very high but even tiny fluctuations of it are detectable.
Same with magnetic field. It can be weak or strong, but tiny fluctuations of it can be created and detected.

The linkage between the ribbon and the magnet is very slight so I cannot see how 'eddy currents' could be of any possible concern - what you are suggesting is that the 100μA in the ribbon would produce any significant current in the core. What effect on the performance could you possibly expect this to have? Could it be frequency response?

Answers on these questions I am trying to find or figure out here with your help. :)

If you are bothered by losses, the resistance of the ribbon (0.5Ω) plus leads and amplifier would be far more of a power loss mechanism (affecting SNR). With a transformer primary in series, the total primary circuit resistance will be even higher and this will all correspond to resistive loss.

I agree, but I can do nothing here. Aluminum is currently best material for ribbon, from mass and resistance perspective. I read about superconducting graphene nano-films/tubes. But today it is in theory only.
For wires copper is very good. It is possible to use silver for leads and primary. But silver wound trannies are too expensive. Supercondactive wires would be ideal :), but not invented yet for temperature 20C.

As I have already pointed out, it is Signal Power that is the basic concern and not Voltages - hence the use of a matching transformer.

Sorry, I am not getting this. Everywhere is recommnded to have low impedance source and high impedance load to have good frequency response.
Ribbon is low impedance, preapm (tube) is high impedance.
How impedance matching can be applied here?

You refer to the eddy current effects in a loudspeaker as being relevant but, in the case of a loudspeaker, the coupling is far higher (there are around 100 turns on a typical voice coil , compared with a fraction of a turn, which the ribbon represents).

Yes, but are principles same?
The difference I see is - a loudspeaker works as generator when voice coil goes back. It produces power that is transformed to heat partly in core and partly in power amp output cascade.
Ribbon mic works as generator all the time. It produces power when ribbon goes back or forth.
So, I assume, same principles are applicable, aren't?

Also, one of the problems with speakers is over heating; not a problem with a microphone.
I think you need to split this project into its individual parts and deal with one at a time. The pre-amp is one that could be investigated more fruitfully, I think.

I agree. And they are separate. But I need to know parameters of one to account them in another and vice versa.

 

[markmal]

Thank you Jim, I'll try to program your formulas in spreadsheet.

Placed all in spreadsheet.

It shows "reaction flux" is 2.5E-10 % from the original magnet flux.
Eddy current losses of power is 5.8E-12% from power of ribbon current. using solid core.
With using laminated core it will be 5.8E-16%

So, yes, you guys are right! These losses can be ignored.

Thank you very much!