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Author Topic: The Back emf misnomer  (Read 16898 times)
Group: Guest
One of the most annoying things about participating in these free energy forums is the way it seems to be accepted as fact by almost everyone
that the discharge of energy from the magnetic field of a coil is (back emf).

My understanding is that it is regular emf because it is in the same direction as the applied emf, the back emf is only there to oppose the charging of the coil,
and the discharging of the coil if through another inductance.

So what is everyone else's opinions ? Is the emf produced from the release of energy from a magnetic field of a coil (emf) or is it (Back emf) ?

I'm interested in the reason it would be consistently considered back emf, and not emf as it should be in my opinion.

Cheers
   
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I think it is this misconception that fuels a lot of mistaken claims, people see the coil discharge referred to as back emf and think the coils magnetic field is comprised
of back emf. I think this leads some to believe that the coil discharge is free energy from the environment, And maybe that back emf consumes energy from the supply
or something. People also seem to think that reactive power is back emf. Same misconception i think.

I think this confuses newer experimenters to no end and ruins or retards their understanding right from the beginning of their learning.

Hardly anyone says anything to rectify the situation and it seems to me when no one says any different the newer experimenters assume it must be fact.

I'm kind of bemused by this and disappointed.

Cheers
   

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@Farmhand

The reason many don't respond to these things is because we have already done so too often. Not long after you help someone understand these things another piper comes along and erases the correct understanding.

'Back emf' is constantly misused. This term has more to do with motors than anything else. Counter emf is the term used for networks of inductance. Both are used interchangeably and incorrectly by almost everyone.

The emf created by the collapse of a magnetic field around a coil is a voltage (emf). emf doesn't even define energy.

It doesn't matter because there is no more free energy in B/Cemf than there is when you release a compressed spring.

The greatest mistake is to rely upon the Wiki's. In general, the information is good. The problem is that any jerk-off can edit the information. With the Wiki folks overloaded, it could be forever before they correct the edited information.



---------------------------
"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   
Group: Guest
Yes exactly emf doesn't define energy, I'm very much still a learner and will be always. But I don't understand the mindset of some people.

As soon as Carroll (Citfa) explained it to me it made complete sense (it was logically correct as far as I could tell ) and I did some quick research to confirm it.

And yes Counter emf is a better term when not talking of motors, that makes sense too, point taken.  O0

I don't want to try to get too technical because i may confuse myself. But I think this is an important part of the basics.

When an unqualified person like me tries to explain these simple things we often get swamped with all kinds of BS and are expected to then explain everything from
tuned circuits to black holes. hahahaha

People with the qualifications need to state these things every now and then clearly and simply, with a simple logical explanation of why it is so.

Thanks ! I need to check occasionally to make sure I'm not having a nightmare or going mad. :D


Cheers

   

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Quote from: Farmhand
But I don't understand the mindset of some people.

Aye, it is an ages old phenomenon.  The meanings of
certain words or expressions within any language
change over the course of time.

In many cases the changes come about unwittingly
caused by those who lack education or experience.
And once the new meanings are accepted by those
within the "following" it is already too late to stop
the corruption.  The change may not make it into
the dictionaries and reference works as a permanent
change but in the minds of those convinced it will
be handed down from generation to generation to
live on within the "following."

In the earliest stages of our technical educations we
pass through that embryonic phase where we
"know just enough to be dangerous."  It is while
we are there that we tend to use terms incorrectly.

As we progress on to higher education and understanding
most of us will "self-correct" our wrong usage.  But
always present is a certain "rebel" element who delight
in warped exposition and make it their goal to change the
way things are said...

While we're on the subject of misnomers, is anyone able
to make sense of this?


---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   
Group: Guest
While we're on the subject of misnomers, is anyone able
to make sense of this?

I can't make any sense of it, but i think what they are seeing is a combination of effects. One being the good batteries natural behavior of rising terminal voltage when loaded
along with the switched inductor (motor) causing some shock release of energy from the "dead" but still contains charge battery.

Several misconceptions going on there I think. The dead battery can only put out a negative voltage if the voltage is measured "upside down". Regardless it is still just a difference of potential.

I think a similar effect could be had from a switched inductor of similar inductance to the motor and a capacitor and resistor with similar capacitance and resistance as the battery.

Couldn't a bad battery be modeled with a capacitor and a resistor, maybe several capacitors in series with different resistors across only some. Like some bad cells worse than others and maybe one good cell.
Possibly with a low voltage battery cell in parallel with an unresistored capacitor/cell.

Cheers
   
Group: Guest
...
So what is everyone else's opinions ? Is the emf produced from the release of energy from a magnetic field of a coil (emf) or is it (Back emf) ?
...

Back emf is emf. What's the problem? It is well named. Emf is not specifically linked  to the magnetic field. When you power a circuit with a battery, the battery provides an electromotive force acting onto the electrons. When this circuit is a coil, a magnetic field is created by the electrons moved by the electromotive force from the battery. If you open the circuit, the magnetic field collapse becomes the source of a new electromotive force through the induction phenomenon, and so, we get "back" emf.

There is a complete parallel between coil and capacitor, magnetic field and electric field, current and voltage, open a circuit and short cut a circuit.

When you charge a capacitor from a battery, you store energy in the electric field thanks to the electromotive force from the battery that moves the electrons from one plate to the other one. When you short the capacitor, you collapse the electric field and recover the energy from the pulse.
When you power a coil from a battery, you store energy in the magnetic field thanks to the electromotive force from the battery that put the electrons in motion, creating a current. When you open the coil, you collapse the magnetic field and recover the energy from the pulse.

Although the question of a capacitor seems not problematic, "back emf" which is the coil equivalence of shorting a capacitor, seems to be a magic formula in the free energy field, while there is strictly no difference of principle.
The only difference is practical: our capacitors are almost perfect because the dielectric resistance is so high that it can be considered as infinite, while our coils should have no resistance, i.e be superconductors, to be equivalent to our near perfect capacitors. This is why we always need a generator to maintain a current in a coil after having established it while we don't need a generator for maintaining the voltage of a capacitor after it is charged, at least for a significant time. But once again, to open a coil or short a capacitor is the same thing: to recover "back" the emf that has been used to create respectively the magnetic or electric field.

   
Group: Guest
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.

Cheers
   

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Posts: 1852
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.


I would say that you already knew what Back emf was.

Now you can fight the good fight and educate the others  C.C

Good Luck  O0


---------------------------
"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   
Group: Guest
There cannot be EMF without BEMF.  It is an action reaction pair and cannot be separate under any circumstances.  I just take it from my view that back emf is taken from mechanic of a reaction force.  For the case of friction and static force, there is still a reaction force though not dynamic.  We'll just stick to inductor case.

In mechanic, we have F = ma, more specific, F = m dv/dt .  In electric we have V= L di/dt. 

When you push a heavy ball, you will feel a force exerted back with the magnitude of  m dv/dt .  This is the reaction force.  Now what if you push a ball of paper?  If you can match any speed, you will still experience the same magnitude because it has more acceleration now with less mass.   

What if you try to stop a ball that in motion?  You will exert a force V=L di/dt. 

You cannot separate V=L di/dt.   It's better to see V as EMF, L di/dt as a reaction force and not mix both of them together as one kind. 
   

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Posts: 1332
Frequency equals matter...
I can't make any sense of it, but i think what they are seeing is a combination of effects. One being the good batteries natural behavior of rising terminal voltage when loaded
along with the switched inductor (motor) causing some shock release of energy from the "dead" but still contains charge battery.

Several misconceptions going on there I think. The dead battery can only put out a negative voltage if the voltage is measured "upside down". Regardless it is still just a difference of potential.

I think a similar effect could be had from a switched inductor of similar inductance to the motor and a capacitor and resistor with similar capacitance and resistance as the battery.

Couldn't a bad battery be modeled with a capacitor and a resistor, maybe several capacitors in series with different resistors across only some. Like some bad cells worse than others and maybe one good cell.
Possibly with a low voltage battery cell in parallel with an unresistored capacitor/cell.

Cheers

Special meter, mysterious part, Creative commons license, patent. Such foolishness...


---------------------------
   
Group: Guest
...
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.

When the field is collapsing, the current in the coil tends to prevent the collapse. For this purpose the current "tries" to continue to circulate as before the collapse, thus opposing the field decrease. That's why there is no current reverse at the moment of the coil opening, and that's why the voltage increases under the effect of the current which tries to circulate and escape at the coil end. Back emf is a counter emf figthing the collapse of the field.
I believe that I now understand what you mean, you think "counter emf = emf against emf" while I think "emf against field variation", with paying a great attention to what is the cause and the effect to distinguish between direct and counter effect.

   

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Posts: 1172
The following is a good example of how the meanings
of technical terms are changed over time.

When both Counter e.m.f. and Inductive Kickback were
first discovered and named the meanings were very
clear.

Today, however, "back" as in Back e.m.f. can mean
either "pushing back in opposition during charge" or
it can mean "getting back by discharge."

As long as we take it upon ourselves to assign new
meanings to technical terms the confusion will persist.



Back emf is emf. What's the problem? It is well named. Emf is not specifically linked  to the magnetic field. When you power a circuit with a battery, the battery provides an electromotive force acting onto the electrons. When this circuit is a coil, a magnetic field is created by the electrons moved by the electromotive force from the battery. If you open the circuit, the magnetic field collapse becomes the source of a new electromotive force through the induction phenomenon, and so, we get "back" emf.

There is a complete parallel between coil and capacitor, magnetic field and electric field, current and voltage, open a circuit and short cut a circuit.

When you charge a capacitor from a battery, you store energy in the electric field thanks to the electromotive force from the battery that moves the electrons from one plate to the other one. When you short the capacitor, you collapse the electric field and recover the energy from the pulse.
When you power a coil from a battery, you store energy in the magnetic field thanks to the electromotive force from the battery that put the electrons in motion, creating a current. When you open the coil, you collapse the magnetic field and recover the energy from the pulse.

Although the question of a capacitor seems not problematic, "back emf" which is the coil equivalence of shorting a capacitor, seems to be a magic formula in the free energy field, while there is strictly no difference of principle.
The only difference is practical: our capacitors are almost perfect because the dielectric resistance is so high that it can be considered as infinite, while our coils should have no resistance, i.e be superconductors, to be equivalent to our near perfect capacitors. This is why we always need a generator to maintain a current in a coil after having established it while we don't need a generator for maintaining the voltage of a capacitor after it is charged, at least for a significant time. But once again, to open a coil or short a capacitor is the same thing: to recover "back" the emf that has been used to create respectively the magnetic or electric field.




---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   
Group: Guest
There cannot be EMF without BEMF.  It is an action reaction pair and cannot be separate under any circumstances.  I just take it from my view that back emf is taken from mechanic of a reaction force.  For the case of friction and static force, there is still a reaction force though not dynamic.  We'll just stick to inductor case.

In mechanic, we have F = ma, more specific, F = m dv/dt .  In electric we have V= L di/dt. 

When you push a heavy ball, you will feel a force exerted back with the magnitude of  m dv/dt .  This is the reaction force.  Now what if you push a ball of paper?  If you can match any speed, you will still experience the same magnitude because it has more acceleration now with less mass.   

What if you try to stop a ball that in motion?  You will exert a force V=L di/dt. 

You cannot separate V=L di/dt.   It's better to see V as EMF, L di/dt as a reaction force and not mix both of them together as one kind. 

I agree. Nevertheless there is a cause and an effect. When you increase a current, the field increases with a delay that can't be less than d/c, d being the distance from the flowing electrons, where the field is observed. The current is the cause, the field the effect. Inversely when you move a magnet near a coil, the change of the magnetic field can't be felt by the coil before a d/c delay and so the current is the effect of the moving field which is the cause. So the reaction force is a bit delayed but it doesn't change anything because the energy that is not "fighted" by the reaction force during the time d/c, is used to build the field, the whole phenomenon still obeying the energy conservation.

   
Group: Guest
...
Today, however, "back" as in Back e.m.f. can mean
either "pushing back in opposition during charge" or
it can mean "getting back by discharge."
...

That's why math is so important in physics. The need of measurements first initiated by Galileo who studied the fall of bodies along inclined planes and the need to avoid the amguities of the natural language have convinced the scholars after the Renaissance to adopt math as the best choice language.

   
Group: Guest
I've been here before, so here's my two-penneth just to clear up a minor point.

Relating to Bemf/Cemf. Some people seem to think that Cemf and Bemf are totally different things, referring to different phenomenons: They are not, they are the same thing. Back emf = Counter emf.

It simply comes down to this. In the UK we use the term Back emf, in the USA they use the term Counter emf. Just as we say 'anti-clockwise' and they say 'counter clockwise'. Simply two different ways of saying exactly the same thing.  :)
   

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It may also depend upon the industry specific terminology used.

Since rotating machinery is a fairly large part of my business I follow the terminology used in my industry.

For inductors, CEMF is analogous to the counter force in a spring when it is being compressed and BEMF is analogous to that same stored energy from the compression of the spring being released by the spring when it is released.

For motors, CEMF is not commonly used but clearly understood because the windings of a motors are inductors and BEMF relates to the motor slip angle or the rotational delay between driving energy and the actual following angular position of the rotor. BEMF is the term used when slip angle and angular position/velocity are needed.

Either term is a force - be it energy being stored or released. One can't exist without the other otherwise tank circuits wouldn't have any ring.

I never understood the UK usage of 'anti-' in such cases. In the U.S. (and every other country I work with except the U.K. and India) 'anti' would be applicable to comparing sine to cosine. Example: a resonant tank circuit ..... a series L/C tank will have a maximum of current when anti-resonant(resonant for a series L/C) and a parallel tank will have a minimum of current when resonant.

Because I deal with these things every day I must speak the same lingo as the rest of the folks. Should I proclaim that BEMF and CEMF were the same action/reaction .... well, it wouldn't be a good thing. They might cancel my IEEE subscription  :'(

I have come to realize that modern learning institutions are globbing concepts together, perhaps to streamline since most folks won't see the differences. Sad, IMHO.
 


---------------------------
"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   
Group: Guest
To me counter emf and back emf are one and the same thing, both only exist to oppose emf and neither has anything to do with regained energy or the emf produced when a magnetic field collapses.

Both are the same to me. And both can only exist to oppose emf and are always present when emf is applied either directly by us or indirectly applied by the coil.

For me the emf produced by the collapse of the magnetic field of a coil will always be just emf, and the emf produced by the coil is in fact opposed by counter emf while the emf is applied.

It can only be one or the other, either the emf produced by the collapsing magnetic field is emf or it is counter emf. Counter emf opposes the applied emf so it can only be regular emf.

That's logic. If the coil was to produce counter emf as a result of the magnetic field collapse then how could it be opposed by more counter emf in the discharge current path ?

Because when the coil discharges and current flows in the discharge path counter emf is present to oppose the force that created the current. Is that not so ?

Cheers

P.S. There is a similar misuse of the term "reactive power", people think they can power loads with reactive power. Which is impossible. Loads can only be powered by "real" power that dissipates energy in the load,
as soon as that happens the power is real power, reactive power can only be the unused part of the apparent power which is returning to the supply or being stored, if reactive power released from the coil of a motor
is stored in a capacitor then the power supplied by the capacitor back to the load must be considered as apparent the power, if returned to the capacitor it is reactive again the power dissipated in the load is real power.



..

   

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Posts: 1172
Quote from: Collins Dictionary
back emf

Definitions

noun

(electrical engineering) an electromagnetic force appearing
in an inductive circuit in such a direction as to oppose any
change of current in the circuit.

The above concise definition, while quite good, covers every
possible scenario.  In order to add precision to any application
which focuses on the concept it would be necessary to define
the circumstance which produces the emf.

Many experimenter discussions are characterized by a paucity
of explanatory verbiage and numerous abbreviations.

With a little more effort any potential ambiguities could be
prevented or minimized.  In this era of "text speak" and
weird spellings that is probably too much to hope for...

The key word is "change."


---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   
Group: Guest
That is a massive simplification in my opinion. In my opinion the "key" word there is "appearing".
Going by that definition the emf produced from the collapsing field is back emf and that back emf
is opposed by back emf because it is causing a change in the current in the discharge path, particularly
if the discharge path for the current from the coil discharge is a different path to the current path for charging the coil.
In the latter case it would involve back emf opposed by back emf. The coils field collapsing is in my opinion a direct result of the lack of emf to maintain it
and so it cannot remain and must collapse, I don't think it collapses just because there is a rule that says it must resist a change in current. I think it collapses because
of the lack of emf to maintain it, the emf it produces is a result of that, the result is it "appears" to resist a change in the flow of current. But In reality it doesn't collapse
for the purpose of resisting a change in current, it collapses because the emf that built and maintains it is removed. Just like a balloon the balloon doesn't deflate to maintain
air current it deflates because the force that inflated it and keeps it inflated is removed. If there was truly momentum involved, then when the applied emf was removed the magnetic field would continue to "build"
or increase for some time after the emf was removed.Is that the case ? I think it is more like pressure that is released when the force that created it is removed.
Truth is I think that the energy will go whichever way it can go easiest. If it is easier for the energy from the field collapse to escape by reversing the flow of current I think it will do that, but I couldn't be sure unless i tested it.

Charge a coil, then completely open the negative end and see if it causes a current back to the supply so as to discharge the field energy. I think that could be considered as reactive power.



Cheers

   

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Posts: 1172
Inductors and their electrical characteristics can be
difficult to comprehend.  For some reason capacitors
are a whole lot easier for most of us.

T=RC is easily intuitive.

T=L/R requires a lot of thought.

I  can still fondly remember my own early experiences
with inductors.  I got my hands on a power transformer
from an old radio and used it for many experiments with
a 6 Volt lantern battery.  I was fascinated by the large
flamelike sparks that would be produced when breaking
contact with the battery energizing the transformer low
voltage output winding. 

And the occasional nearly half inch long sparks that could
be created from the high voltage primary winding when
making and breaking the connections to the battery and
the low voltage winding.

Not to mention the power of the shocks I'd get when
careless!

I was hooked on electricity and couldn't rest until I
knew how that stuff worked...

Later on I was able to obtain a Model T ignition coil
which seemed to me worth its weight in gold.  Used it
to make my first Jacob's Ladder and to ignite pieces of
paper in the very hot sparks.  And then a very rudimentary
spark gap radio transmitter.

Ah yes, those were the days!  The smell of Ozone brings
back many memories.



---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   
Group: Guest
...The coils field collapsing is in my opinion a direct result of the lack of emf to maintain it and so it cannot remain and must collapse
...

If there was a lack of emf, the field would collapse instantly.
The field must collapse but don't collapse instantly because each attempt of collapsing is partially compensated by an emf that prolongs the field. An emf always opposes its source, because the energy must be taken somewhere, and the question becomes a simple question of viewpoint about which source is that one to be preserved.
If you are using a generator to establish a field in a coil, the emf appears when the field is increasing (self-induction) and opposes the source, so you will say "it's back emf or it's counter emf, it is depleting my source". If you open the coil circuit, the field collapses and the coil itself is the source. You may say it's emf, not counter emf: it's an emf generated by the coil which is the source, a real magnetic generator as a dynamo providing current from a changing field. But you may also say that it's a counter emf: it opposes the field collapse and thus it opposes the current variation which should lead to stop it.
Question of viewpoint while the phenomenon at work is always the same, emf from field variation or field variation from emf, depending on which is the cause and which is the effect. "Back emf", "counter emf" are just words from professional jargon of electricians to qualify this emf relatively to the useful generator.

   

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Posts: 1172
Quote from: Farmhand
In my opinion the "key" word there is "appearing".

Could it be that "change" and "appearance" are
simultaneous?  Whenever current through an
inductor is non-changing there is no induced
emf.  Or, whenever the magnetic field strength
within an inductor is non-changing there is no
induced emf.

In both cases, absent change, the appearance
of the induced emf does not manifest.


---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   
Group: Guest
It may also depend upon the industry specific terminology used.


Maybe, but I'm of the opinion that you won't see 'counter emf' mentioned in a uk produced text book, only 'back emf'. Hence I'm also rather inclined to think the opposite would be true of an American text book.  I certainly never came across the term Cemf on the course I did, but the term Bemf I remember from inductor theory.

Happy to be proved wrong though!
   
Group: Guest


P.S. There is a similar misuse of the term "reactive power", people think they can power loads with reactive power. Which is impossible. Loads can only be powered by "real" power that dissipates energy in the load,
as soon as that happens the power is real power, reactive power can only be the unused part of the apparent power which is returning to the supply or being stored, if reactive power released from the coil of a motor
is stored in a capacitor then the power supplied by the capacitor back to the load must be considered as apparent the power, if returned to the capacitor it is reactive again the power dissipated in the load is real power.



..



If you have a primary and secondary.  Secondary has a resistive load.  The power giving out by the resistive load is real power.  The resistive  load is powered by the primary. 
   
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