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Author Topic: Electromagnet impedance vs transformer impedance  (Read 5921 times)

nix85

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Electromagnet impedance vs transformer impedance
« on: May 15, 2019, 08:30:18 PM »
I want to clear something up and i am sure there are people here who will instantly know what i am asking. So...

For a speaker coil impedance is said to be mostly due to dc resistance and in small part due to reactance. In other words we can measure speaker coil's dc resistance and we know it's impedance is little bit above that, like so. > https://www.youtube.com/watch?v=4Ocgo1p8Z3Y

On the other hand, a transformer, which is basically also an electromagnet is said to mostly consume energy due to reactive power and dc resistance being just a small part of it's overall impedance.

If secondary of a transformer is open, no load connected, it looks like near perfect inductor to the driving circuit. This means current lags the voltage by 90° and average power through the primary is 0. In reality there is little power consumed due to resistance and core losses.

If load is connected ti the circuit driving the primary it appears as if it got more resistive, thus voltage and current which are almost 90° off phase in no load condition become more and more in phase and more power is consumed.

This is explained in more detail here. Don't read the confused question, just the answer below. https://engineering.stackexchange.com/questions/7485/transformers-with-no-load

More about Transformer on No Load Condition > https://circuitglobe.com/transformer-on-no-load-condition.html

My question is how come speaker coil mostly consumes power due to dc resistance, why does it not behave like primary of a transfomer with no load - making current lag the voltage by 90° and overall power ~0?

Few related formulas:

XL= 2πfL
XC= 1/2πfC
Z = sqrt(R² + (Xc - Xl)²)
F = 1/6.28(LC)
F = 1/2π√LC
power per second: P = IV and P = V²/R and P = I²R
true power P=VIcosφ



nix85

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Re: Electromagnet impedance vs transformer impedance
« Reply #1 on: May 17, 2019, 11:45:29 PM »
BUMP

citfta

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Re: Electromagnet impedance vs transformer impedance
« Reply #2 on: May 18, 2019, 01:33:28 AM »

The simple answer is that the speaker is not a transformer.  It is in a sense a motor.  When signal is applied to the coil the magnetic field of the coil interacts with the permanent magnet of the speaker and causes the coil to move which then moves the cone of the speaker.  The cone has to move air when it moves and that is the load on the coil which causes it to draw current.

Carroll

nix85

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Re: Electromagnet impedance vs transformer impedance
« Reply #3 on: May 18, 2019, 11:33:57 AM »
The simple answer is that the speaker is not a transformer.  It is in a sense a motor.  When signal is applied to the coil the magnetic field of the coil interacts with the permanent magnet of the speaker and causes the coil to move which then moves the cone of the speaker.  The cone has to move air when it moves and that is the load on the coil which causes it to draw current.

Carroll

Simple but good answer, exactly what i assumed.

gyulasun

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Re: Electromagnet impedance vs transformer impedance
« Reply #4 on: May 18, 2019, 08:39:34 PM »

...
My question is how come speaker coil mostly consumes power due to dc resistance, why does it not behave like primary of a transfomer with no load - making current lag the voltage by 90° and overall power ~0?

...
Hi,
There would be one more thing to consider as an addition to the explanation:  a speaker coil has much smaller self inductance than that of a primary coil of a transformer.  You can consider speaker coil inductances in the order of some hundred microHenry to some milliHenry, while a primary coil in a transformer starts from at least several hundred milliHenry to over a few Henries, 
And the actual audio frequency a speaker coil receives does count of course: at the some kHz and over 10 kHz the impedance may increase much higher of course.  With an L meter, you can estimate this better for speaker coils.

Gyula

nix85

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Re: Electromagnet impedance vs transformer impedance
« Reply #5 on: May 18, 2019, 09:21:34 PM »
at the some kHz and over 10 kHz the impedance may increase much higher of course.

Do you think this is the main reason for amp power drop of 1.5-2 decibels at ~20KHz?

gyulasun

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Re: Electromagnet impedance vs transformer impedance
« Reply #6 on: May 18, 2019, 09:30:29 PM »
I am not an expert on loudspeakers,  so cannot give you a correct answer for your question. If you google for    loudspeaker coil inductance   or   voice coil inductance,   you will find some good papers on the behaviour of speaker coils at the higher audio frequency ranges.  It seems a complex problem.   
Consider the effect of cross over networks (if used such) too. 
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.


nix85

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Re: Electromagnet impedance vs transformer impedance
« Reply #7 on: May 18, 2019, 10:39:16 PM »
Consider the effect of cross over networks (if used such) too. 
Amplifier power drop : if you use a dummy load instead of loudspeaker(s),  then you can check the amplifier behaviour first, you surely know this.

Sure, there are other factors too why amp power drops, some mentioned here. https://answers.yahoo.com/question/index?qid=20190510203041AAZbp3b

Coil Inductance Calculator
https://www.allaboutcircuits.com/tools/coil-inductance-calculator/

nix85

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Re: Electromagnet impedance vs transformer impedance
« Reply #8 on: March 20, 2023, 11:35:11 PM »
What citfta replied above is simplistic and does not answer the question. Of course it draws more current cause it is doing work, as it is in a sense a linear motor. But that is not the answer to why most of speaker coil's resistance is ohmic resistance.

As i wrote in both cases, transformer and the speaker, just like in all kinds of motors, reason for increase of current when doing work is due to demagnetization aka lenz, backemf across the working inductor falls and more current rushes in. Formula for this is Ia=Ve-Eb/Zs where Ia is the current, Ve is the input voltage, Eb is the backEMF and Zs is the impedance. Reason why most of speaker coil impedance is DC resistance aka heating is due to the fact speakers do not need much power to produce relatively loud sound, so relatively high resistance wire is used to limit the current, otherwise it would be like a short at lower frequencies. Not to get into the fact that at lower frequencies power must be increased for human ear to hear the bass at same level as higher frequencies (higher energy) considering higher frequencies are more energetic and also our hearing is logarithmic.

Similar example for light would be LED where big resistor is often used to limit the current and 1W LED may have a 10W resistor limiting the current to acceptable value, so 90% of energy is wasted into heat. Of course there are more efficient ways like inductive current limiter, so inductive reactance is used instead of ohmic resistance.

floodrod

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Re: Electromagnet impedance vs transformer impedance
« Reply #9 on: March 21, 2023, 02:47:30 AM »
Deleted.   I should have known better. LOL
« Last Edit: March 21, 2023, 11:32:26 AM by floodrod »

kolbacict

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Re: Electromagnet impedance vs transformer impedance
« Reply #10 on: March 21, 2023, 07:25:00 AM »
Therefore, telephone capsules with a perfect magnetic system give out a much greater sound output for the same electrical power.
But the sound there is such that no one wants to listen. :(

C.O.P.1000

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Re: Electromagnet impedance vs transformer impedance
« Reply #11 on: March 21, 2023, 08:51:42 AM »
Your views are welcome. I watched your video and i appreciate your views but you have not told me anything new, and the way you try to explain things is not exactly correct either, as most people, you lack laser precision, let me elaborate on it.

Firstly, if you look at my first post you will see the formula for inductive reactance.

XL= 2πfL

So obviously and i have spoken about this many times, smaller the inductance smaller the reactance aka self-induction. It is also clearly implied in my last post that voice coil has low inductance and low reactance and thus needs ohmic resistance to limit the current, especially at low frequencies, all written clearly.

And obviously air core coil has lower inductance than one with iron core, i don't think you had to make a video to show such obvious and widely known basic thing but on the other hand why not, i like the video.

In any case, you said in the video air coil has no self-inductance, well it does, just less. I think you know that and you just expressed yourself like that. This is related with the common formula E = 444 f NAB volts rms. Also, you repeat multiple times how without core coil supposedly "cannot complete the induction path". This is wrong. Flux always closes on itself, with or without a core. Key reason is iron core amplifies the magnetic field, not just providing the path of lesser reluctance, so

B = μH

aka flux density is permeability of the core times magnetizing force

or longer formula

flux density = amper x turns x core permeability x core area / m² (T)

and magnetizing force H is closely related to popular term ampere turns, namely

H (magnetizing force) = ampere x turns / length of a coil

And to further elaborate on self-induction....

Obviously self-induced voltage prevents current to flow instantly, i and others spoke of this many times here, and another thing i have figured myself and another person here also figured it independently - altho it is commonly said current cannot flow instantly through an inductor for back EMF which prevents instant current to flow to appear there first must be some small current cause without it voltage could not appear - so technically current is leading voltage, well, for a tiny instant. Or maybe it does not, this is only an assumption. In that case it is even stranger as it means voltage across an inductor jumps without change of current through it, something that should not happen. In any case, voltage jumps up to source voltage and then decreases gradually proportionally to 5 time constants of which i have also spoken before.

t = L/R inductor time constant, after 5t (transient time) current reaches 99.5%

Now the reason for the smaller inductance of a voice coil vs transformer coil. Well, firstly voice coils are usually physically smaller than transformers which usually means smaller inductance.

As for the gap, voice coil indeed has an airgap which lowers inductance but if you look how speaker is made you will notice that gap is actually small. It surely does reduce inductance but i would not agree it is the main reason.

https://youtu.be/RxdFP31QYAg?t=86

Then, you mention the permanent magnet, of which i was perfectly aware, and you say "also gets complicated by the permanent magnet's field". The thing is voice coil is working against the magnet half of the time so magnet is reducing it's inductance half of the time and other half of the time it is increasing it's flux but it would be incorrect to say it is increasing it's inductance since inductance of the coil is only the part of flux it creates by itself, additional flux can lead to saturation but does not increase inductance, it is but a bias. So it does complicate things but i would say magnet is overall reducing the inductance which is already quite low due to small core size and the airgap.

So, obviously, inductance of the voice coil is much smaller - usually in range from less than 1mH to few dozen mH compared to transformer primaries which are usually like hundreds of times more, for example i measured one of microwave oven transformers pri ind 351mH and sec ind 34H. I also measured inductance of my ~50W speakers and i don't remember exactly but it was tiny. And obviously, as i said before, smaller inductance means smaller self induction. These are simple basic things.

So, all that was clear and implied. But important correction, again, it is not that core allows for "south induct back into the top" flux always closes on itself, with or without a core, it is, as i said above and before, the fact that iron AMPLIFIES the field - altho mainstream still sleazily forces the term CONCENTRATES the flux instead of AMPLIFIES. Altho they are forced to admit flux is larger, up to a million times, with the core.

Obviously, larger L means longer time constant, so it takes longer for current to rise to max value and if you wanna force it you need more voltage, that is, more power, unless you are smart and you use the nature given parallel RLC i spoke of in the thread of same name.

As for your comment on "Speaker doing work", it seems you are confusing the reactive and active power. If you removed the coil from the speaker inductance would be even smaller and since the ability of a coil to do work (power) is flux times frequency (just like motor is torque times rpm), you would have to increase number of turns significantly to get the same inductance and same ability to do work but more turns means more ohmic resistance so your losses would go up. If you would not add more turns, current would be larger, obviously, you would again waste more power as heat in accord with P = I²R copper losses and would do less work. So of course air core coil is not optimal to do work.

So, one with iron core is of course more efficient, but not overunity. Conventional relation here stands, unless special configurations are made. So no magic there just by adding a core. Magic happens when you go into resonance or when you play with flyback aka collapsing field backEMF etc. Using the coil in classical manner, just like electric motors is surely not overunity, no matter how much iron or exotic core material you put.

Or to sum all this up in few words, of course inductance of a voice coil is less, this is obvious and clearly implied. Iron amplifies flux but this does not automatically mean overunity. It can and does lead to overunity in specific cases.

Nix

kolbacict

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Re: Electromagnet impedance vs transformer impedance
« Reply #12 on: March 21, 2023, 12:10:36 PM »
I have an interesting table.
Just do think, with an input electrical power of 10-13 watts, the radio operator can already received speach !
Sorry for russian.

C.O.P.1000

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Re: Electromagnet impedance vs transformer impedance
« Reply #13 on: March 21, 2023, 03:35:17 PM »
Something interesting, apparent overunity input output values on NSTs are usually said to be for momentary peak value, before the ionization, but this post says different

Just got off the phone with a Ventex Engineer, the power supplies are constant current constant voltage 35 khz symetrical waveform to each anode. So if a NST specification says it will supply 2500v at .028 mA in order to drive 30" of tube once the tube is lit it will sustain the 2500v at .028mA at 35khz to keep the tube lit.

From the Ventex engineers we have OU and wikipedia states the same requirements for the lighting function.

Which is why Don had the rectification diodes in place because when one anode is -2500 the other is 0.

https://www.overunityresearch.com/index.php?topic=25.msg7511#msg7511