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OverUnity Research > General - Free Energy Related > Miscellaneous Discussions (Moderator: Allcanadian) > The Back emf misnomer
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img  EMdevices
Re: The Back emf misnomer « Reply #50 on: 2012-12-11, 03:31:07 »
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years ago I read of an interesting way to cancel the backEMF  or maybe just generate extra energy,  or whatever,  the point was that they were claiming it worked and they had photos of this special coil and the theory was quite interesting.   I'll try to describe what I remember:

The basic principle was to electrostaticaly charge a coil, or perhaps place it into a high E-field,  and the coil if tuned with a capacitor will begin to oscillate all by itself.

The principle was that it becomes a negative resistance, and the inventors were describing the charges on the wire and the currents and how all of this comes about etc...

Anybody have any recollection?  It was one of those "we give this to the world" type of disclosures.

EM
   
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Re: The Back emf misnomer « Reply #51 on: 2012-12-11, 14:08:09 »
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Quote from: WaveWatcher on 2012-12-11, 02:30:22
The flux density will never be perfectly uniform between attracting magnets. If the coil is the same diameter as the magnets you should find a distance between magnets where the density is almost uniform and therefore create little to no induction in the movements shown.

If you move them closer the density increases.

No matter what orientation you choose a change in flux density occurs and will induce an emf in the coil.

What is the goal of these experiments?

Thanks again,

I thought so.  I guess it's similar to the electric field between parallel plate volts/meter.  You can make a uniform field but if the magnet gets closer, flux density increase.  If density does not increase, then we would have free energy. 

I have a thought experiment that induction with PMM could have COP 2. 

Suppose we have two superconductor rings.  Inject two equal current into the rings.  Now let them attract to each other like PMM.  As they speed up toward each other, EMF induces in both rings and current energy convert into motion.  Overall, we converted all input from current into motion energy.   Now if one ring is a permanent magnet, we only need to inject current into one ring.  The same thing happens, but this time only 1 current used up with the same motion energy.
   
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Re: The Back emf misnomer « Reply #52 on: 2012-12-11, 14:42:28 »
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Interesting thoughts.

However, it is my understanding that a superconductor completely rejects external magnetic fields. It should be impossible to induce an emf into a superconducting ring.
Also, since superconductors reject all external magnetic fields there is no magnetic attraction like associated between two magnetic materials. There is 'field pinning' but that results in something more like station keeping than attraction or repelling.
   
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Re: The Back emf misnomer « Reply #53 on: 2018-08-16, 15:04:42 »

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Looking back at some threads tonight,i happened across this one.

I hope those here today have a better understanding of BEMF than those of times past.

Quote: When i say "Back emf" I mean (counter emf), "counter emf" or "back emf" being the force resisting the applied emf.

The collapsing of the magnetic field of a coil does not produce "counter emf" it produces emf, counter emf is in opposition to applied emf,
the emf produced when the magnetic field of a coil collapses does not oppose the applied emf, it emulates it or aids it.

Energy returning to the supply is not back emf either it is reactive power. That's how I see it.

Back emf or counter emf can only be the force that opposes emf while the emf is applied.

Reverse emf is not Back emf or Counter emf either.

If you wanted to apply a reverse emf to a circuit supplied by a battery as the source of emf then just reverse the polarity of the battery connections to the circuit and a reverse emf would be applied.
Reverse emf would be opposed by counter emf also just like the forward emf.

I hope this member now knows better,along with  everyone else here.
If not,then maybe the mistakes above should be corrected.


Brad


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Re: The Back emf misnomer « Reply #54 on: 2018-08-16, 18:38:41 »

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Just to clarify a little more,  CEMF happens on the rise of current and opposes the direction of flow.  BEMF occurs on the reduction of current and aids the direction of current flow, what is so hard about this, it is first year electronics.  They are distinctly two different things, not the same at all, and I think easy to see.

Room


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Re: The Back emf misnomer « Reply #55 on: 2018-08-16, 23:21:13 »

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Could it be that "Back EMF" is in reality a shortening of
KickBack EMF?

CEMF occurs during 'charging' of the inductor.

KickBack occurs during the 'discharging' of the inductor.

Yes, it really is all very clear and easy to see in both DC and
AC applications.

Except, of course, to 'Newbies" and "Embryonics,' but even
they shall see it in due course as they continue learning.


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Re: The Back emf misnomer « Reply #56 on: 2018-08-17, 03:32:40 »

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Quote from: Room3327 on 2018-08-16, 18:38:41
Just to clarify a little more,  CEMF happens on the rise of current and opposes the direction of flow.  BEMF occurs on the reduction of current and aids the direction of current flow, what is so hard about this, it is first year electronics.  They are distinctly two different things, not the same at all, and I think easy to see.

Room

Well no actually.
CEMF dose not appose the direction of current flow-it reduces current flow by decreasing the potential difference between the source EMF and the self induced EMF.
It could be seen as an impedance,but not an opposition.

The same applies with BEMF, which is a self generated EMF by motoring action,where the motor also acts as a generator.

Inductive kickback or flyback is a result of the magnetic field reversing direction through the coil,where the windings of the coil are the reference point.
This causes the current to continue to flow through the coil in the same direction it was before the source was disconnected,but where the voltage across the coil inverts.

The load impedance value across the coil during this flyback period determines as to how fast the field around the coil collapses.


Brad


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Re: The Back emf misnomer « Reply #57 on: 2018-08-17, 05:24:18 »

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In Electronics Education of the '60s Counter Electromotive Force
included not only Inductive Action but also the Counter EMF
developed in a motor by generator action.  Counter EMF was
defined to be any opposition to Source EMF and Current.

When AC is applied to any Inductor we see opposition to
current flow while the AC current is increasing during each
half cycle.  We also see inductive discharge aiding source
current while AC current is decreasing during each half cycle.
'ELI the ICE man' was drilled into our heads to aid our
remembering that Inductive Current Lags the Applied
Voltage.

The term 'Back EMF' was not part of the Electrical/Electronics
technical vocabulary back then.  I wonder when it entered into
the jargon?

The meaning of 'counter emf' is intuitively quite clear.  The
meaning of 'back emf' is a bit ambiguous.  Is Back EMF intended
to mean 'giving back' or 'pushing back?'
« Last Edit: 2018-08-18, 20:00:51 by muDped »


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Re: The Back emf misnomer « Reply #58 on: 2018-08-17, 11:45:56 »

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author=muDped link=topic=1585.msg69249#msg69249 date=1534479858]





Quote
When AC is applied to any Inductor we see opposition to
current flow while the AC current is increasing during each
half cycle.

I do not agree with this muDped,as there is never any opposition to current flow.
There is (what could be called)an impedance to current flow,due to the low potential difference between the source EMF and the self induced EMF(CEMF)
This dose not only apply to AC,as the same will be seen when pulsed DC is applied.

  We also see inductive discharge aiding source
current while AC current is decreasing during each half cycle.

I have seen this with pulsed DC,but never with AC.
I do not believe that an AC frequency high enough could be produced that would exceed the speed of a collapsing magnetic field.

Quote
The term 'Back EMF' was not part of the Electrical/Electronics
technical vocabulary back then.  I wonder when it entered into
the jargon?

When they realised that there was a difference between a self induced EMF of an inductor connected to a source,and a magnetically generated EMF like that of a PM DC motor.

Quote
The
meaning of 'back emf' is a bit ambiguous.  Is Back EMF intended
to mean 'giving back' or 'pushing back?'

Pretty much. BEMF is power generated by the actions of the rotor windings of the motor cutting through the magnetic fields of the stator,thus producing a current flow that is in the same direction to that of the source.
The current value the source provides is source voltage divided by winding resistance minus the self generated current. So the faster the motor spins,the more self generated current it make's,and the less current the source has to supply.

With inductors,there is no change in speed,where the frequency remains the same,and so we have CEMF.

So as you can see,the two are different.


Brad


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Re: The Back emf misnomer « Reply #59 on: 2018-08-17, 18:53:00 »

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Quote from: TinMan on 2018-08-17, 11:45:56
author=muDped link=topic=1585.msg69249#msg69249 date=1534479858]





I do not agree with this muDped,as there is never any opposition to current flow.
There is (what could be called)an impedance to current flow,due to the low potential difference between the source EMF and the self induced EMF(CEMF)
This dose not only apply to AC,as the same will be seen when pulsed DC is applied.

  We also see inductive discharge aiding source
current while AC current is decreasing during each half cycle.

I have seen this with pulsed DC,but never with AC.
I do not believe that an AC frequency high enough could be produced that would exceed the speed of a collapsing magnetic field.

When they realised that there was a difference between a self induced EMF of an inductor connected to a source,and a magnetically generated EMF like that of a PM DC motor.

Pretty much. BEMF is power generated by the actions of the rotor windings of the motor cutting through the magnetic fields of the stator,thus producing a current flow that is in the same direction to that of the source.
The current value the source provides is source voltage divided by winding resistance minus the self generated current. So the faster the motor spins,the more self generated current it make's,and the less current the source has to supply.

With inductors,there is no change in speed,where the frequency remains the same,and so we have CEMF.

So as you can see,the two are different.


Brad

Yes let's confuse the issue again, we don't want anyone to understand this stuff, let's fight over symantics and how one person views something different over another.  Oppose subtract call it something else,  the fact is when current is rising in a coil it self induces a reverse or counter EMF in the same coil. That voltage is reducing the applied EMF by some factor, call it opposing or call it subtracting, from the applied emf or COUNTERING it either way it does the same thing. When the applied emf on a coil is removed or reduced the stored energy in the coil is released and induces an emf in the coil that aids or adds to the applied emf of that coil, this is called Back EMF because it is giving BACK some of the input energy.  Is that correct enough for you? Sorry if I didn't use the words you wanted me to.


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Re: The Back emf misnomer « Reply #60 on: 2018-08-17, 19:54:26 »

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For those who are mystified by BEMF manifesting itself as high voltage spikes this is what causes that phenomena.

A coil is charged and has manifested a magnetic field around itself, then power is cut off and that field collapses.  When it collapses it self induces into itself a pulse of energy.  In that pulse is all the energy that it took to manifest the magnetic field minus a small amount for losses.  If that pulse of energy is going into a load it will supply to that load the same amount of energy (VA) that made the field, it might be (example) 12 V and 3 A or 36 W. using some of the BEMF energy as amps reduces the voltage (example) you use 200 ma.  36 W /.2 you will have 180 Volts. Now if you take that same pulse of energy and apply it to no load or very high impedance load (meter) that energy has to go somewhere and with very little current flow it manifests as voltage (example) 36,000 V and 1ma. also equals 36 W.  That's how you can get 100's and even thousands of volts from a BEMF pulse, but that pulse will never return you more energy then went into it because there are losses, so you can never get more out then you put in with BEMF, yes you can get a lot more voltage but never more energy.

You will also notice that the higher the voltage spike the shorter the pulse length and the higher the current the longer the pulse length.

Room


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"Whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material"  Nicola Tesla

"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."  Edmund Burke
   
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Re: The Back emf misnomer « Reply #61 on: 2018-08-17, 21:27:57 »

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Believing in something false doesn't make it true.
 O0 O0 O0 O0

I agree completely with everything you just posted.  And since the time it takes to discharge the coil is dependent on the circuit impedance the discharge is going through, then it should be obvious that you can indeed have a frequency that is faster than the discharge of the coil.  Ham Radio operators are well aware of this because they have to tune their circuits to match the frequency being used.  Or more commonly called tuning to resonance.

Respectfully,
Carroll


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Re: The Back emf misnomer « Reply #62 on: 2018-08-17, 22:02:53 »

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The earliest experiments with electricity that I was able to do
as a young lad were with 6 Volt lantern batteries and transformers
salvaged from old discarded radios.  The flaming arcs one is able
to make when momentarily connecting the primary of the
transformer to the battery was quite amazing.  The arcs made a
slight 'poofing' sound as they developed.  Naturally, at some
point in playing with them one gets both fingers across the
wires where the arc develops and one receives a potent shock.
Truly amazing and magical stuff.  Even small audio transformers
of the day were able to be used as high voltage spark generators
at the secondary when the primary was driven with an auto radio
vibrator.


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Re: The Back emf misnomer « Reply #63 on: 2018-08-17, 23:45:06 »

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Frequency equals matter...


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Ah yes, semantics. The breakfast of idiots.Sorry to waste server space.  The reason why specific words are so important to some is because all they have is the tongue and no mind. This is the larger congregation of souls. This is also why idealists are crucified. One can find more bullets in the adventurer's back than arrows in the front.And we must not forget: blather, spew and repeat. But only in the codified language of the coddled masses...

And how does one get line breaks correct in the thread server?


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Re: The Back emf misnomer « Reply #64 on: 2018-08-18, 01:53:35 »

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 author=Room3327 link=topic=1585.msg69256#msg69256 date=1534528380]
     


Quote
Yes let's confuse the issue again, we don't want anyone to understand this stuff, let's fight over symantics and how one person views something different over another.

I am not trying to confuse the issue,i am trying to clarify it.
If we are to get anywhere,then clarity is needed,and it appears that clarity is just not here in this simple subject.

Quote
the fact is when current is rising in a coil it self induces a reverse or counter EMF in the same coil. That voltage is reducing the applied EMF by some factor

And this is what i am talking about.
To say
Quote
induces a reverse or counter EMF
to me sounds like a voltage of the opposite polarity is being produced by the coil. When you reverse something,that means opposite direction,or opposite polarity--as in-reverse the polarity of a circuit.
And to say that the CEMF reduces the applied EMF is just not right. The applied EMF will remain the same across the coil at all times,regardless of the CEMF value,because the CEMFs polarity is the same as,but always lower than the applied EMF.

Quote
When the applied emf on a coil is removed or reduced the stored energy in the coil is released and induces an emf in the coil that aids or adds to the applied emf of that coil,

As EMF denotes voltage(not current),then when the applied EMF is removed,the voltage across the coil is of the opposite polarity to that of the applied EMF. I fail to see how this adds to or aids the applied voltage.

Quote
this is called Back EMF because it is giving BACK some of the input energy.  Is that correct enough for you?

If i am to be honest,then no,it's not correct enough for me.
When an applied EMF is removed from a coil,you get what is well known as inductive kickback-or flyback. This is not BackEMF. Inductive kickback/flyback produces an EMF across the coil that is of the opposite polarity to that of what the applied EMF was. BackEMF produces a voltage that is the same polarity to that of the applied EMF.

Even wikipedia has it wrong

https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Counter-electromotive_force.html

Quote: For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.

The voltage's polarity is not the reverse of the input voltage at all.
The CEMFs polarity across the coil is the same as the applied EMF.
The only time the voltage polarity is reverse.opposite to that of the applied EMF,is when the source EMF is interrupted/broken/disconnected-->inductive kickback.


No wonder so many get confused with this subject.

Quote Giantkiller
Ah yes, semantics. The breakfast of idiots.Sorry to waste server space.  The reason why specific words are so important to some is because all they have is the tongue and no mind.

Mock me if you will,but in this game,things like this should be well defined.


Brad


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Re: The Back emf misnomer « Reply #65 on: 2018-08-18, 02:26:54 »

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Quote from: TinMan on 2018-08-18, 01:53:35
author=Room3327 link=topic=1585.msg69256#msg69256 date=1534528380]
     


I am not trying to confuse the issue,i am trying to clarify it.
If we are to get anywhere,then clarity is needed,and it appears that clarity is just not here in this simple subject.

And this is what i am talking about.
To say  to me sounds like a voltage of the opposite polarity is being produced by the coil. When you reverse something,that means opposite direction,or opposite polarity--as in-reverse the polarity of a circuit.
And to say that the CEMF reduces the applied EMF is just not right. The applied EMF will remain the same across the coil at all times,regardless of the CEMF value,because the CEMFs polarity is the same as,but always lower than the applied EMF.

As EMF denotes voltage(not current),then when the applied EMF is removed,the voltage across the coil is of the opposite polarity to that of the applied EMF. I fail to see how this adds to or aids the applied voltage.

If i am to be honest,then no,it's not correct enough for me.
When an applied EMF is removed from a coil,you get what is well known as inductive kickback-or flyback. This is not BackEMF. Inductive kickback/flyback produces an EMF across the coil that is of the opposite polarity to that of what the applied EMF was. BackEMF produces a voltage that is the same polarity to that of the applied EMF.

Even wikipedia has it wrong

https://ipfs.io/ipfs/QmXoypizjW3WknFiJnKLwHCnL72vedxjQkDDP1mXWo6uco/wiki/Counter-electromotive_force.html

Quote: For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.

The voltage's polarity is not the reverse of the input voltage at all.
The CEMFs polarity across the coil is the same as the applied EMF.
The only time the voltage polarity is reverse.opposite to that of the applied EMF,is when the source EMF is interrupted/broken/disconnected-->inductive kickback.


No wonder so many get confused with this subject.

Quote Giantkiller
Ah yes, semantics. The breakfast of idiots.Sorry to waste server space.  The reason why specific words are so important to some is because all they have is the tongue and no mind.

Mock me if you will,but in this game,things like this should be well defined.


Brad

I'm not mocking you Brad, I am merely trying to educate some of those that do not understand this stuff without confusing them even more, for the most part you are right, I wasn't disagreeing with you, I was trying to simplify it for others.  Yes the emf across a coil reverses with BEMF and the current continues in the same direction it was going.  The counter EMF is in reverse to the applied EMF, the applied EMF is delayed from reaching it's maximum value across the inductor until the CEMF is over or the rise in current has stopped as far as I know, and the CEMF is the reverse polarity of the applied EMF, I think your wrong there.
Room


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"Whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material"  Nicola Tesla

"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."  Edmund Burke
   
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Re: The Back emf misnomer « Reply #66 on: 2018-08-18, 15:09:29 »
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Hi Folks,

My take on this topic is this (let's say we DC pulse a coil): when current starts flowing in a coil, a magnetic field starts also building up. Here is the "problem": the increasing magnetic field (in fact it is an electromagnetic field) induces an EMF (a potential, a voltage) in the same coil (as per Faraday induction) just because the coil surely embedded in its own changing magnetic field. This induced EMF always works against the input voltage that started to drive the input current into the coil (normal Lenz law).

I imagine this as if there are small variable voltage sources created in each turn of the coil: they immediately appear when input current starts flowing and as this current increases so does the amplitude of these internally induced voltages too. When the current reaches its maximum value possible (the 5L/R time just lapsed) and becomes steady state, then these internally induced voltages reduce to zero volt because the increasing magnetic field stops increasing any further, hence no more induction can happen in the coil. 

To model this internal voltage source, I imagine its effect as if you connect many variable 'batteries' in series with each turn of the coil, see the drawing, the switch is ON).
Now the question is the polarity direction of these internal voltage sources. We know the induced voltage opposes input current and we know the induced voltage in the coil cannot be higher than the applied voltage that started the coil current because the input current is still in the increasing state in the closed circuit. If we sum up each such voltage source, then the summed amplitude should approach that of the outside voltage source that started the input current flow but never reach it, the difference is always the I2R loss coming from coil wire resistance (+some other losses like radiation and core loss etc).

Maybe this approach helps better understand the process during the 5L/R time duration that happens in a coil when current starts flowing in it.

Gyula
   
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Re: The Back emf misnomer « Reply #67 on: 2018-08-18, 18:28:54 »

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Quote from: gyula on 2018-08-18, 15:09:29
Hi Folks,

My take on this topic is this (let's say we DC pulse a coil): when current starts flowing in a coil, a magnetic field starts also building up. Here is the "problem": the increasing magnetic field (in fact it is an electromagnetic field) induces an EMF (a potential, a voltage) in the same coil (as per Faraday induction) just because the coil surely embedded in its own changing magnetic field. This induced EMF always works against the input voltage that started to drive the input current into the coil (normal Lenz law).

I imagine this as if there are small variable voltage sources created in each turn of the coil: they immediately appear when input current starts flowing and as this current increases so does the amplitude of these internally induced voltages too. When the current reaches its maximum value possible (the 5L/R time just lapsed) and becomes steady state, then these internally induced voltages reduce to zero volt because the increasing magnetic field stops increasing any further, hence no more induction can happen in the coil. 

To model this internal voltage source, I imagine its effect as if you connect many variable 'batteries' in series with each turn of the coil, see the drawing, the switch is ON).
Now the question is the polarity direction of these internal voltage sources. We know the induced voltage opposes input current and we know the induced voltage in the coil cannot be higher than the applied voltage that started the coil current because the input current is still in the increasing state in the closed circuit. If we sum up each such voltage source, then the summed amplitude should approach that of the outside voltage source that started the input current flow but never reach it, the difference is always the I2R loss coming from coil wire resistance (+some other losses like radiation and core loss etc).

Maybe this approach helps better understand the process during the 5L/R time duration that happens in a coil when current starts flowing in it.

Gyula
Yes, I think this does help people to see how CEMF works, thanks Gyula.
and thank you CITFTA for the thumbs up.
Room


---------------------------
"Whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material"  Nicola Tesla

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Re: The Back emf misnomer « Reply #68 on: 2018-08-19, 01:42:30 »

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 author=Room3327 link=topic=1585.msg69263#msg69263 date=1534555614]
     


Quote
I'm not mocking you Brad,

I was not referring to you room.

Quote
for the most part you are right,

Which part is wrong?.

Quote
I was trying to simplify it for others.  Yes the emf across a coil reverses with BEMF and the current continues in the same direction it was going.

No,no it dose not.
BEMF is of the same polarity to that of the applied EMF
BEMF is the self generated EMF within an electric motor.
Once again,BEMF is being mixed up with inductive kickback.

Quote
The counter EMF is in reverse to the applied EMF,

Once again,that is wrong.
The CEMF is of the same polarity to that of the applied EMF--just like BEMF is of the same polarity to that of the applied EMF in an electric motor.

Quote
, and the CEMF is the reverse polarity of the applied EMF, I think your wrong there.

I am not wrong.
Both BEMF and CEMF are of the same polarity to that of the applied EMF.

When you drop a voltage across an inductor,what happens?
Why is there very little current flow through that inductor at T=0 to the first time constant?.
If the polarity produced by the CEMF or BEMF was opposite to that of the applied EMF,what would be the value of current flow at T=0 to the first time constant?
Get your self two 12 volt batteries,and hook them up reverse polarity to each other,and see what happens-how much current flow is there at the connection points?.
Then get 1 battery with say 12.6volts across it(the EMF),and one with say 12.2 volts across it(the CEMF),hook them up so as the polarity is the same(parallel),and then measure what the current flow is between them?.
My guess is you will get smoke in one test,and very little current flow in the other test.

Quote
the applied EMF is delayed from reaching it's maximum value across the inductor until the CEMF is over or the rise in current has stopped as far as I know

EMF denotes voltage,where voltage is the value of that EMF.
When you drop a voltage across an inductor,there is no delay to that voltage reaching it's maximum value,as you just dropped that voltage across the inductor.
The voltage across the inductor will always be the value you dropped across it,providing the source and connecting wires are able to handle the maximum current flow through that inductor without voltage drop.
The value of the CEMF has nothing to do with the voltage across the inductor,only the current flowing through the inductor.

If the connecting wires or source cannot maintain the voltage across the coil as the current rises,then the opposite to what you stated above is true,where the maximum voltage will appear across the coil at T=0,and then drop off as the CEMF reduces-->current rises. But this is only due to either the source or connecting wires being unable to handle the maximum current flow through the coil.

 
Quote
I am merely trying to educate some of those that do not understand this stuff without confusing them even more,

It would seem that the teachers need to go back to school them self,as those teachers seem to be the cause of the confusion.

Quote
I wasn't disagreeing with you,

It would seem that you are.


Brad


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Re: The Back emf misnomer « Reply #69 on: 2018-08-19, 01:54:10 »

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Quote from: gyula on 2018-08-18, 15:09:29
Hi Folks,

My take on this topic is this (let's say we DC pulse a coil): when current starts flowing in a coil, a magnetic field starts also building up. Here is the "problem": the increasing magnetic field (in fact it is an electromagnetic field) induces an EMF (a potential, a voltage) in the same coil (as per Faraday induction) just because the coil surely embedded in its own changing magnetic field. This induced EMF always works against the input voltage that started to drive the input current into the coil (normal Lenz law).

I imagine this as if there are small variable voltage sources created in each turn of the coil: they immediately appear when input current starts flowing and as this current increases so does the amplitude of these internally induced voltages too. When the current reaches its maximum value possible (the 5L/R time just lapsed) and becomes steady state, then these internally induced voltages reduce to zero volt because the increasing magnetic field stops increasing any further, hence no more induction can happen in the coil. 

To model this internal voltage source, I imagine its effect as if you connect many variable 'batteries' in series with each turn of the coil, see the drawing, the switch is ON).
Now the question is the polarity direction of these internal voltage sources. We know the induced voltage opposes input current and we know the induced voltage in the coil cannot be higher than the applied voltage that started the coil current because the input current is still in the increasing state in the closed circuit. If we sum up each such voltage source, then the summed amplitude should approach that of the outside voltage source that started the input current flow but never reach it, the difference is always the I2R loss coming from coil wire resistance (+some other losses like radiation and core loss etc).

Maybe this approach helps better understand the process during the 5L/R time duration that happens in a coil when current starts flowing in it.

Gyula

Thank you Gyula.

As you can see,the CEMF is of the same polarity to that of the applied EMF.

We also now can work out why very little current flows at from T=0 to the first time constant-->the potential difference between the EMF and CEMF is very low. As time go's by,the rate of change of the magnetic field reduces,and in turn the value of the CEMF reduces,and the potential difference between the applied EMF and CEMF increases,causing a greater current flow.
This is why i asked Room to carry out the two tests in my last reply.


Brad


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Re: The Back emf misnomer « Reply #70 on: 2018-08-19, 17:49:01 »

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Quote from: TinMan on 2018-08-19, 01:42:30
author=Room3327 link=topic=1585.msg69263#msg69263 date=1534555614]
     


I was not referring to you room.

Which part is wrong?.

No,no it dose not.
BEMF is of the same polarity to that of the applied EMF
BEMF is the self generated EMF within an electric motor.
Once again,BEMF is being mixed up with inductive kickback.

Once again,that is wrong.
The CEMF is of the same polarity to that of the applied EMF--just like BEMF is of the same polarity to that of the applied EMF in an electric motor.

I am not wrong.
Both BEMF and CEMF are of the same polarity to that of the applied EMF.

When you drop a voltage across an inductor,what happens?
Why is there very little current flow through that inductor at T=0 to the first time constant?.
If the polarity produced by the CEMF or BEMF was opposite to that of the applied EMF,what would be the value of current flow at T=0 to the first time constant?
Get your self two 12 volt batteries,and hook them up reverse polarity to each other,and see what happens-how much current flow is there at the connection points?.
Then get 1 battery with say 12.6volts across it(the EMF),and one with say 12.2 volts across it(the CEMF),hook them up so as the polarity is the same(parallel),and then measure what the current flow is between them?.
My guess is you will get smoke in one test,and very little current flow in the other test.

EMF denotes voltage,where voltage is the value of that EMF.
When you drop a voltage across an inductor,there is no delay to that voltage reaching it's maximum value,as you just dropped that voltage across the inductor.
The voltage across the inductor will always be the value you dropped across it,providing the source and connecting wires are able to handle the maximum current flow through that inductor without voltage drop.
The value of the CEMF has nothing to do with the voltage across the inductor,only the current flowing through the inductor.

If the connecting wires or source cannot maintain the voltage across the coil as the current rises,then the opposite to what you stated above is true,where the maximum voltage will appear across the coil at T=0,and then drop off as the CEMF reduces-->current rises. But this is only due to either the source or connecting wires being unable to handle the maximum current flow through the coil.

 
It would seem that the teachers need to go back to school them self,as those teachers seem to be the cause of the confusion.

It would seem that you are.


Brad

Brad I am not going to continue the argument, and it would seem that one of us needs to go back to school.  As no one has taken your side in this, I would reconsider WHO needs to go back to school.  I don't know where you got your erroneous information, but I like you, so I tried to be diplomatic with you on correcting your erroneous ideas but that time has passed. Get your act together and quit confusing everyone on this subject.
Room


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"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."  Edmund Burke
   
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Re: The Back emf misnomer « Reply #71 on: 2018-08-19, 18:08:07 »

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I guess I'm not done I just spotted this. In Blue my response.

Quote from: TinMan on 2018-08-19, 01:42:30
author=Room3327 link=topic=1585.msg69263#msg69263 date=1534555614]
     


I was not referring to you room.

Which part is wrong?.

No,no it dose not.
BEMF is of the same polarity to that of the applied EMF
BEMF is the self generated EMF within an electric motor.
Once again,BEMF is being mixed up with inductive kickback.

Once again,that is wrong.
The CEMF is of the same polarity to that of the applied EMF--just like BEMF is of the same polarity to that of the applied EMF in an electric motor.

I am not wrong.
Both BEMF and CEMF are of the same polarity to that of the applied EMF.

When you drop a voltage across an inductor,what happens?
Why is there very little current flow through that inductor at T=0 to the first time constant?.
If the polarity produced by the CEMF or BEMF was opposite to that of the applied EMF,what would be the value of current flow at T=0 to the first time constant?
Get your self two 12 volt batteries,and hook them up reverse polarity to each other,and see what happens-how much current flow is there at the connection points?.
Then get 1 battery with say 12.6volts across it(the EMF),and one with say 12.2 volts across it(the CEMF),hook them up so as the polarity is the same(parallel),and then measure what the current flow is between them?.
My guess is you will get smoke in one test,and very little current flow in the other test.

EMF denotes voltage,where voltage is the value of that EMF.
When you drop a voltage across an inductor,there is no delay to that voltage reaching it's maximum value,as you just dropped that voltage across the inductor.

That is if you look at it as being in parallel with the CEMF, but in reality the CEMF is in series with the source not parallel, could this be your problem in seeing this correctly?

The voltage across the inductor will always be the value you dropped across it,providing the source and connecting wires are able to handle the maximum current flow through that inductor without voltage drop.
The value of the CEMF has nothing to do with the voltage across the inductor,only the current flowing through the inductor.

If the connecting wires or source cannot maintain the voltage across the coil as the current rises,then the opposite to what you stated above is true,where the maximum voltage will appear across the coil at T=0,and then drop off as the CEMF reduces-->current rises. But this is only due to either the source or connecting wires being unable to handle the maximum current flow through the coil.

 
It would seem that the teachers need to go back to school them self,as those teachers seem to be the cause of the confusion.

It would seem that you are.


Brad

Room


---------------------------
"Whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material"  Nicola Tesla

"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."  Edmund Burke
   
img  gyula
Re: The Back emf misnomer « Reply #72 on: 2018-08-19, 23:09:48 »
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Dear Room,

I chime in if you do not mind with a question on one of your sentences you wrote in Reply #65 to Brad, I quote:

"Yes the emf across a coil reverses with BEMF and the current continues in the same direction it was going. The counter EMF is in reverse to the applied EMF, the applied EMF is delayed from reaching it's maximum value across the inductor until the CEMF is over or the rise in current has stopped as far as I know, and the CEMF is the reverse polarity of the applied EMF, I think your wrong there."

Please specify the circuit you had thought of when you wrote your 1st sentence. Is it a closed circuit that include a voltage source like a battery and a coil and a switch that was on? I need to know in which circuit you meant the "emf across a coil reverses with BEMF and the current continues in the same direction it was going".

If you had thought of an already open circuit where the current in a coil is just switched off, then I agree: the polarity of the appearing voltage spike across the coil (due to the sudden collapse of the field) will have a flipped polarity with respect to the input supply voltage that charged up the coil. 


You accepted my post #66 how CEMF works (at least that is how I think it works). In the drawing I had attached there, the polarity of the small CEMF 'batteries' for each turn of the coil is identical to that of the input voltage source. Of course this is a closed circuit and we need to consider the 5L/R time duration during which we examine the CEMF.

Why does Brad say this in his Reply #69:

"We also now can work out why very little current flows at from T=0 to the first time constant-->the potential difference between the EMF and CEMF is very low. As time go's by,the rate of change of the magnetic field reduces,and in turn the value of the CEMF reduces,and the potential difference between the applied EMF and CEMF increases,causing a greater current flow."

My answer is: because the amplitude of the CEMF is the highest at the t=0 and this highest value means an equal voltage value to the input supply voltage and as time starts passing from the t=0 moment, the value of CEMF will exponantially decrease. But we cannot measure CEMF during these moments (it is impossible I think), we can only measure a manifestation of it in the coil current as an exponential current flow.

I found a simulation for this here:
http://www.learnabout-electronics.org/ac_theory/images/Fig-4-5-1.swf  (Java applet needs to be allowed to run in your computer).  The simulation is embedded into this link:
http://www.learnabout-electronics.org/ac_theory/dc_ccts45.php 

Observe the exponential curve: it has the steepest rate of change when it just starts from zero.  Mathematically, the tangent line to the curve is quasi vertical near t=0 and becomes quasi horizontal as the curve flats out i.e. the rate of change (of the current hence magnetic flux) diminishes towards a minimum at t=5L/R .
 
In this link they call it back EMF (bemf, BEMF) which is created during the 5L/R time duration (I would prefer counter EMF, CEMF but English is my 2nd language) so no problem for me which of them is used as long as everybody knows what circuit and under what conditions they are used in.

You may ponder why the amplitude of the CEMF or BEMF is the highest at the moment the supply voltage is switched across a coil?
What is more: why the CEMF inside a coil has the same polarity the input supply has (as shown in my drawing)?   

Because this is the only explanation which describes reality at t=0 moment: the coil current is zero and starts building up from that value. This zero current is possible if you connect two quasi identical batteries in parallel, positive to positive and negative to negative: between them there is very little, quasi zero current (there are no identical batteries to the last fraction of mVs in practice, that is).

To put this CEMF "phenomena" otherwise: the moment a voltage source appears across a coil, the coil immediately "creates" the same voltage amplitude internally and first blocks current just because the created voltage polarity is the same as that of the supply voltage.

I am not for or against anybody here, I am for correct science whenever it is possible. 

Gyula
   
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Re: The Back emf misnomer « Reply #73 on: 2018-08-20, 00:11:51 »

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Hi gyula, I will try and answer your questions.

Quote from: gyula on 2018-08-19, 23:09:48
Dear Room,

I chime in if you do not mind with a question on one of your sentences you wrote in Reply #65 to Brad, I quote:

"Yes the emf across a coil reverses with BEMF and the current continues in the same direction it was going. The counter EMF is in reverse to the applied EMF, the applied EMF is delayed from reaching it's maximum value across the inductor until the CEMF is over or the rise in current has stopped as far as I know, and the CEMF is the reverse polarity of the applied EMF, I think your wrong there."

Please specify the circuit you had thought of when you wrote your 1st sentence. Is it a closed circuit that include a voltage source like a battery and a coil and a switch that was on? I need to know in which circuit you meant the "emf across a coil reverses with BEMF and the current continues in the same direction it was going".

If you had thought of an already open circuit where the current in a coil is just switched off, then I agree: the polarity of the appearing voltage spike across the coil (due to the sudden collapse of the field) will have a flipped polarity with respect to the input supply voltage that charged up the coil. 

In BEMF before the EMF is removed (example) current is flowing left to right through a coil from negative to positive.  So the left end of the coil is neg. and the right end is pos. ( its connected to source Positive current flow is neg. to pos.) so now the source is removed from the coil and we start the bemf cycle, in order for the BEMF to aid or give back its energy the coil must release it in the same direction it was flowing. Which means the coil now becomes neg. on the right and pos. on the left --It reverses In order to keep current flowing in the same direction neg. to pos.

You accepted my post #66 how CEMF works (at least that is how I think it works). In the drawing I had attached there, the polarity of the small CEMF 'batteries' for each turn of the coil is identical to that of the input voltage source. Of course this is a closed circuit and we need to consider the 5L/R time duration during which we examine the CEMF.

Why does Brad say this in his Reply #69:

"We also now can work out why very little current flows at from T=0 to the first time constant-->the potential difference between the EMF and CEMF is very low. As time go's by,the rate of change of the magnetic field reduces,and in turn the value of the CEMF reduces,and the potential difference between the applied EMF and CEMF increases,causing a greater current flow."

My answer is: because the amplitude of the CEMF is the highest at the t=0 and this highest value means an equal voltage value to the input supply voltage and as time starts passing from the t=0 moment, the value of CEMF will exponantially decrease. But we cannot measure CEMF during these moments (it is impossible I think), we can only measure a manifestation of it in the coil current as an exponential current flow.

To be honest I am not certain what the question here is, at T=0 the current is almost a vertical line meaning it is increasing very rapidly as it increases more and more the CEMF is continually growing and by T=2 it is slowing down the rise in current till T=5. does this help?


I found a simulation for this here:
http://www.learnabout-electronics.org/ac_theory/images/Fig-4-5-1.swf  (Java applet needs to be allowed to run in your computer).  The simulation is embedded into this link:
http://www.learnabout-electronics.org/ac_theory/dc_ccts45.php 

Observe the exponential curve: it has the steepest rate of change when it just starts from zero.  Mathematically, the tangent line to the curve is quasi vertical near t=0 and becomes quasi horizontal as the curve flats out i.e. the rate of change (of the current hence magnetic flux) diminishes towards a minimum at t=5L/R .
 
In this link they call it back EMF (bemf, BEMF) which is created during the 5L/R time duration (I would prefer counter EMF, CEMF but English is my 2nd language) so no problem for me which of them is used as long as everybody knows what circuit and under what conditions they are used in.

Many people do not know the difference between CEMF and BEMF and think they can interchange them whenever they want.

You may ponder why the amplitude of the CEMF or BEMF is the highest at the moment the supply voltage is switched across a coil?
What is more: why the CEMF inside a coil has the same polarity the input supply has (as shown in my drawing)?   

It does not have the same polarity as I just explained to Brad Because they are not in parallel they are in series with the source. The CEMF is not at it's highest at the moment the switch is closed It is at it's lowest.

this is the only explanation which describes reality at t=0 moment: the coil current is zero and starts building up from that value. This zero current is possible if you connect two quasi identical batteries in parallel, positive to positive and negative to negative: between them there is very little, quasi zero current (there are no identical batteries to the last fraction of mVs in practice, that is).

To put this CEMF "phenomena" otherwise: the moment a voltage source appears across a coil, the coil immediately "creates" the same voltage amplitude internally and first blocks current just because the created voltage polarity is the same as that of the supply voltage.

It doesn't first block current it first allows the current and then as the CEMF develops it slows it down untill the coil is fully charged, any more energy added at that point is basically wasted depending on what you want to do.

I am not for or against anybody here, I am for correct science whenever it is possible. 

I agree I am not for or against anyone here, I too am for correct science.  Room

Gyula


---------------------------
"Whatever our resources of primary energy may be in the future, we must, to be rational, obtain it without consumption of any material"  Nicola Tesla

"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."  Edmund Burke
   
img  gyula
Re: The Back emf misnomer « Reply #74 on: 2018-08-20, 10:59:38 »
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Hi Room,

From what you answered last night: "In BEMF before the EMF is removed (example) current is flowing left to right through a coil from negative to positive.  So the left end of the coil is neg. and the right end is pos. ( its connected to source Positive current flow is neg. to pos.) so now the source is removed from the coil and we start the bemf cycle, in order for the BEMF to aid or give back its energy the coil must release it in the same direction it was flowing. Which means the coil now becomes neg. on the right and pos. on the left --It reverses In order to keep current flowing in the same direction neg. to pos."

it comes that you considered first the charge up of a coil and then you continued with switching the current off (so now the source is removed from the coil) and that the coil polarity reverses.  So this is the whole current on and off process for a coil.

But so far in my earlier two posts I have discussed only the charge up process of a coil from the moment a voltage source is switched across it till the moment the 5L/R time duration lapses. It is important to separate this initial switch-on transient event from the switch-off event and it is the latter event you mainly include in your answers to Brad and to me (at least it sounds to me like that).

Regarding your answer: "To be honest I am not certain what the question here is, at T=0 the current is almost a vertical line meaning it is increasing very rapidly as it increases more and more the CEMF is continually growing and by T=2 it is slowing down the rise in current till T=5. does this help?"

The question was from Brad to me, in fact it was not a question but an explaining sentence, I quote the revelant part he wrote to me: "We also now can work out why very little current flows at from T=0 to the first time constant..."

Well, notice in the simulation (I linked in my earlier post) the coil current starts from a zero value and then increases rapidly, almost vertically indeed. But it still has low values after the t=0 moments just after a voltage source appears across the coil. What you wrote in the second half of your answer: "as it increases more and more the CEMF is continually growing" is what we need to discuss.

In the simulation it is the coil current which is included on the vertical axis of the graph. 
I would like to understand why do you say the CEMF is continually growing?
The CEMF is a potential, a voltage appearing within the 5L/R time duration inside the coil and it cannot be measured directly, only a manifestation of it can be measured if we measure coil current. 

So why do you say the CEMF is increasing in this situation? (i.e. when there is a closed circuit for a coil and a voltage source)

I will continue later on tonight with commenting the rest of your answers (I will be busy till then) but I think as a first step this question needs to be clarified and we can go on from that point.

Thanks, 
Gyula
   
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