Eddy current losses in core of a electromagnetic motor

[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!