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#1
02-19-2009, 09:05 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
Back EMF vs. Collapsing Magnetic Field Spike

Hi Everyone,

This does come up time to time and it isn't a matter of semantics. Back EMF simply is not the spike that you get from a coil when the field collapses.

You charge a coil - lenz's law describes the counter current or back emf that opposes the forward current and resists the forward current's ability to bring the coil's charge up...

Once the coil is charged and you disconnect power, the spike you get back is the "inductive spike" or "transient spike."

You can see Lenz's law here:
It is at the bottom.

Look at this nice simple answer:
WikiAnswers - What is the formula for transient spike computation in an inductive load

"E=I x R. The inductive spike occurs as the circuit is opened. The collapsing magnetic field causes the inductor to become the source of the circuit. For example consider a circuit consisting of a 10 volt battery, a 10 mh inductor, and a 10 ohm resistor all in series. With the switch closed, 1 amp will flow through the circuit (after 5 mS). The 5 mS is the time it takes the current to rise from 0 to 1 amp. This is given by the formual TC=L/R where TC is the time constant in seconds, L is the inductance in henries, and R is the resistance in ohms. It takes 5 time constants for the current to reach the maximum current which is determined by I=E/R (Ohm's Law). The delay is caused by the counter EMF generated in the coil as flux lines cut through adjacent turns of the inductor. After 5 time constants, the current is at 1 amp. When we open the switch, it will take 5 time constants for the current to drop to 0 amps. However, this will not be 5 mS because the resistance is now much larger do to the opening switch contacts. The voltage across the switch contacts will be whatever is necessary to maintain the current flow for the 5 time constants. After one time constant, the current will have dropped to 32% of the maximum current or in this case, 0.32 amps. If the resistance of the switch gap is 1 megohm, the the voltage will be 320,000 volts. More than enough to ionize the air and create a conductive path. If we assume an average resistance of 1 megohm, it will take 50 nS for the current to drop to 0. Of course during this time, the switch contact gets zapped. Placing a diode across the inductor such that the diode is reverse biased with the switch closed will give the current an alternate path as the polarity of the inductor reverses when the magnetic field collapses and the inductor becomes the source. This lowers the voltage from 1,000,000 volts to 0.7 volts. The downside is that the time it takes for the current decrease increases bo the ratio of 1,000,000/0.7. In a relay, this may cause the relay to "chatter" when opening. Adding a zener diode in series anode to anode with the spike suppressing diode will alleivate most chattering problems. A 34.3 volt zener will raise the voltage from 0,7v to 35v and shorten the time by a factor of 50 (35/.0.7). "

So you can see that it takes 5ms to charge the coil because the back emf opposes the forward current...that is the delay of charging the coil...the back emf.

You can see it takes 50ns to go back to 0. Why so fast? There is no more back emf opposing anything.

I don't agree that the calculation of the spike is as straight forward as this because other things come into play with sharp gradients.

But you can clearly see the back emf is NOT the spike that comes back. The spikes we are capturing and putting to use is the "inductive spike" or "transient spike" and I believe it does matter what it is called because there are very specific names for these very specific well-known events that have been established for a really long time.

People experimenting with the free energy stuff won't have much credibility in the general world of science calling the spike back emf.

They can believe what the want, that is fine but it is simply ample evidence for them to show that people in this "free energy" field don't even know what they're talking about and they would be correct. Let's not give them any ammunition. If they see that we do know the difference, it is just less resistance (back emf) that we have to work against in getting this stuff out there. It really is an inductive spike or transient spike and the back emf is already gone.
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Sincerely,
Aaron Murakami

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#2
02-19-2009, 10:02 AM
 Inquorate Gold Member Join Date: Oct 2008 Location: Sydney, Aus Posts: 2,104
Beating lenz?

@ Aaron - I am currently building a bedini motor with which I hope I can sidestep Lenz's law, and back EMF.

What would happen in an open coil while the core is magnetised? I am thinking not much.

Next step, the magnet activates a reed switch as it passes the core, which closes the coil to bridge rectifier, capturing inductive collapse.

What are your thoughts? I know there will still be magnetic drag from coil core, but there will be no counter emf?

Ta
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Atoms move for free. It's all about resonance and phase. Make the circuit open and build a generator.
#3
02-19-2009, 11:41 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
getting around Lenz's law

Inquorate,

These go hand in hand:

Lenz’s Law: Heinrich Emil Lenz (1804-1865) formulated Lenz's law in 1834, which states, "The EMF induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux."

Faraday’s Law of Induction: Michael Faraday (1791-1867) formulated Faraday’s Law of Induction in 1831, which states, “The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.”

How can a coil get charged with a magnetic field and defeat the above two "laws"?

The answer is right there in both of those Laws.
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Sincerely,
Aaron Murakami

#4
02-19-2009, 10:39 PM
 cody Senior Member Join Date: Jun 2008 Posts: 407
around lenz

Man i wish i could follow you with whatever it is in Lenz and Faradays statements that tells you how to get around it. I was thinking maybe by not allowing the closed circuit enough time for the emf to change the magnetic flux, but then there wouldnt be a strong magnetic field if you dont give it time to charge i think im missing something
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#5
02-19-2009, 11:04 PM
 redeagle Senior Member Join Date: Sep 2008 Posts: 172
Thank you for posting this Aaron. It might not be a bad idea to make this thread a sticky.

Matt
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#6
02-19-2009, 11:33 PM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
Ohm's Law

Might as well throw ohm's law in there too:

Ohm’s Law: Georg Ohm (1791-1867) formulated Ohm’s Law of Induction in 1827, which states, “The current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.”
__________________
Sincerely,
Aaron Murakami

#7
02-20-2009, 01:48 AM
 sucahyo Platinum Member Join Date: Dec 2008 Posts: 3,073
Would 5ms coil charging time apply to any coil including flat spiral coil?
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#8
02-20-2009, 02:57 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
time constant

Every coil has different inductance and resistance so would be different from coil to coil I believe.
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Sincerely,
Aaron Murakami

#9
02-20-2009, 02:58 AM
 cody Senior Member Join Date: Jun 2008 Posts: 407
sucahyo,
i believe that 5ms is only for the specific example that aaron used. It will be a different time depending on your coil inductance and the power supply you are using.

So when using ohms law with the others you posted, im thinking maybe im right about the time being the key? Im thinking that getting around lenz law would allow our circuits to draw less current while doing the same amount of work. So since current is inversely proportional to resistance(ohms law), we could possibly use this to our advantage, not by directly adding a resistor to the circuit but by the increasing impedance which happens on resonant points. Once the impedance goes up at resonance there is very small current draw, which is what we want right, so since the current went down, does that mean that we have reduced the lenz effect? Is that all it is, or am i just off completely?
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#10
02-20-2009, 05:50 AM
 sucahyo Platinum Member Join Date: Dec 2008 Posts: 3,073
Thank you Aaron and cody, do resistance here means AC resistance?

And do I assume correctly that flat spiral coil would have less BEMF than tubular coil at the same inductance? And if we can make them both have same resistance and inductance we would have same induction spike?
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#11
02-20-2009, 06:08 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
calculations

I don't have the background in these calculations. This should be simple for any EE's to explain. When I want to figure these #'s, I just use calculators.
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Sincerely,
Aaron Murakami

#12
02-21-2009, 03:32 AM
 sucahyo Platinum Member Join Date: Dec 2008 Posts: 3,073
@Aaron, thank you.

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#13
02-24-2009, 07:20 PM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
This is my (latest) conclusion about the different kinds of EMF:

* back EMF appears when a coil is building up its magnetic field and is opposite to the applied current
* forward EMF appears when the magnetic field in a coil collapses and generates a current in the same direction as the interrupted current
* counter EMF is generated in dc-motors as the motor is also a generator, but is usally called back EMF

Just as a capacitor can be used to smoothen out variations in voltage, a coil can be used to smoothen out variations in current.
If You shorten a capacitor You get a current rush, if You shorten a coil You get a voltage spike.
A Bedini motor cannot work as a generator and therefore do not generate any counter EMF, but the coil in it first generates back EMF when the transistor opens and then forward EMF (as a spike) when it closes.

If this is correct it means that I'm wrong in my videos, I'm not trying to capture the back EMF but rather the forward EMF.

...I think...
__________________
Hob Nilre
#14
02-24-2009, 09:17 PM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
forward emf

I think forward emf was used in Bedini's patent...possibly to get through the patent examiners.

The inductive spike is for the most part, pure potential. Literally, I don't believe there is much FORCE in an inductive spike so it may not really be forward emf...otherwise, that spike could produce a magnetic field in the coil as it discharges but it doesn't...the field is collapsing from the original applied power that caused real EMF.

The collapsing spike is just that, an inductive spike or transient spike.

Forward EMF literally would be a current made that assists the applied field accelerating the coil's charging speed...of course in the same direction but we know this doesn't happen when we charge a coil with voltage and current.

Just my 2 cents.
__________________
Sincerely,
Aaron Murakami

#15
02-25-2009, 08:22 AM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
Aaron, I'm not convinced.

I don't have much personal experience, yet, but reading in the (in my opinion very good) 2'nd edition (which has much more theory than the 1'st edition) of "Practical Electronics for Inventors" (how could You resist such a title),
I stay with my conclusion in my previous post.
In the book, Paul Scherz uses the terms "back" and "forward" Emf to separate the two events increasing and decreasing the current.
The average voltage V over an ideal coil when the current drops I amps in t seconds is V=L*I/t and would be a spike with infinite voltage if the decrease in current is instant.
But infinite spikes never happens, of course, as a real coil has both resistance and capacitance,
but they can be quite big, as we all have observed.

The reaction in capacitors and batteries when receiving a forward Emf spike is quite fortunate as the are able to collect the energy in it.
__________________
Hob Nilre
#16
02-25-2009, 08:31 AM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
Quote:
 Originally Posted by Aaron "The EMF induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux." “The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.”
__________________
Hob Nilre
#17
02-25-2009, 08:35 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
inductive spike

Back EMF is the same as Counter EMF and in either case, including Forward EMF, none of these are the inductive spike from a coil.

If he is saying in the book that forward emf is the inductive spike then he is misusing the entire concept of force as in electromotive force. Is there force with zero work or pure potential moving through a coil and out of it? It might sound good but is is like an oxymoron...kind of like "military intelligence."

I have the same book from a friend it is a great reference nevertheless.
__________________
Sincerely,
Aaron Murakami

#18
02-25-2009, 09:49 AM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
Quote:
 Originally Posted by Aaron Back EMF is the same as Counter EMF and in either case, including Forward EMF, none of these are the inductive spike from a coil. If he is saying in the book that forward emf is the inductive spike then he is misusing the entire concept of force as in electromotive force. Is there force with zero work or pure potential moving through a coil and out of it? It might sound good but is is like an oxymoron...kind of like "military intelligence." I have the same book from a friend it is a great reference nevertheless.
Please forgive me for being a skeptic enthusiast trying to keep both feet on the ground.
I have built two Bedini SSG's and I'm currently doing a few experiments on the Tesla switch and read and watch as much as I can about this.
I'm not saying that You or anyone else is right or wrong, as I actually don't know.
But could You, when You have the time, point out where in the book (2nd ed) You think Paul Scherz is wrong, and also how he is wrong? (maybe around pages 124-126, 140, 142-147) I would really appreciate it.
__________________
Hob Nilre
#19
02-25-2009, 10:56 AM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
spike not current

Quote:
 Originally Posted by nilrehob * forward EMF appears when the magnetic field in a coil collapses and generates a current in the same direction as the interrupted current
EMF isn't really a force according to the experts. I say there isn't much force to the spike and there isn't.

Do you realize you're claiming (or the man in the book) there is a net gain in forward current.

Here is the chronology of what happens according to that claim.

1. There is applied power and this would technically be a Forward EMF.
2. Back EMF or back current opposes the forward current.
3. The current was interrupted (the applied/forward current is interrupted)
4. now there is a generation of current that goes in the same direction as the original forward current that was interrupted according to what you've posted.

You're saying that there are two emfs to begin with...forward and backward in opposition....coil is turned off and you wind up with a net gain of a forward current going in the same direction as the original interrupted forward current.

That means when the power on a coil is turned off, you wind up with a forward current or amperage. If that is true, then there is a magnetic field being maintained preventing a collapse. As there would be current in the coil going in the direction necessary to make the magnetic field to begin with.

The reality of what we actually do wind up with when the coil collapses is a high voltage spike with virtually no current at all - that is the whole point to calling it a spike..a spike means there is no pulse width to it and with no pulse width there isn't a time variable to speak of meaning if there is no time to speak of per spike, there isn't current flowing. That is why it is called voltage potential.

When the coil collapses, voltage goes through the roof while current goes through the floor.
__________________
Sincerely,
Aaron Murakami

#20
02-25-2009, 12:25 PM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
I just did a test using components that I had lying around: coil ~166mH, cap 47uF, diode 1N4007, bat 12V and a switch.
The amp draw of the coil was ~500mA.
Battery, coil and switch in series.
Coil, diode and cap in series.
Switching on/off a few times the cap easily reached 80V.
Doing this repeatedly and emptying the cap each time it reached around 80V I took the average of the changes in voltage for each on/off-cycle squared and got ~550V^2. (Edit: max change squared = 688V^2)
The energy in each forward EMF spike captured by the cap would then be about (47*10^-6*550)/2=0.013 J. (Edit: max energy = 0.016 J)
The energy in the coil before switched off would be near (166*10^-3*0.5^2)/2=0.021 J
The difference in energy I blame resistance and other losses and me calculating the inductance wrong (using supposed length of the wire and the physical dimension of the coil).

What is wrong with my assumptions in this exercise?
__________________
Hob Nilre

Last edited by nilrehob; 02-25-2009 at 12:44 PM.
#21
02-25-2009, 07:11 PM
 Sephiroth Silver Member Join Date: Sep 2007 Posts: 971
I'm going to step in to confuse matters a bit more

I personally think both Aaron and nilrehob are correct

but the output of bedini's circuits contains two componants and neither of them is back-emf as aaron has already mentioned.

but the transient spike is only part of the output.

when current has been cut off from the coil, then immeadiatly upon the opening of the switch we get the transient spike that can shoot off the top of our oscilloscopes. But following this transient event is the flyback current which I think could be described as forward emf as it is generated by the collapsing field in a similar way as current is generated when you pull a magnet off the top of the core.

The flyback current is recycling the energy we put into the system but the transient spike is the part that allows us to gain more energy in our systems.

My two cents
__________________
"Theory guides. Experiment decides."

“I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything.”
Nikola Tesla

Last edited by Sephiroth; 02-25-2009 at 07:42 PM.
#22
02-25-2009, 07:31 PM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
I have just found the "Radiant Energy"-thread so I have to chew on this for a while.
But I'm still *very* skeptic!
__________________
Hob Nilre
#23
02-26-2009, 02:59 PM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
Please forgive me, but I still can't see the forward EMF spike as anything else than the energy in the coil desperately trying to find another form as quick as possible, and as there are high resistance in the way You get high voltage and low current.

The spike has a width, and a current, albeit very small, just enough to make the spike carry the energy that was previously in the coil.

If the capacitor in the circuit in my previous post has a voltage higher than the battery, the energy in the coil compresses itself even more to an even higher (but thinner) spike to make its way into the capacitor. The only other alternative is to convert itself to heat.

I just tested a Bedini SSG charging a capacitor instead of a battery, and it shows very clearly that the spike grows in height as the capacitor collects more and more energy.

This again leads me to the conclusion that the driving battery connected to a coil should be connected as short as possible, just to "fill up" the coil, as any current flowing in the coil after the coil is "full" cannot be recollected.

I'm only looking at a single spike here, as a single event. A train of spikes may add to the story, but I'm not there yet.

Please correct me anyone if You think I'm wrong about the spikes, and a simple easy to replicate setup to show it would be really helpful.

I'm sure there are things in "Tesla tech" (and others) than cannot be described in current theories and formulas, but I'm sure a spike in this context is easy to explain using common textbooks.
__________________
Hob Nilre

Last edited by nilrehob; 02-26-2009 at 03:34 PM. Reason: changed "no resistor" to "high resistance", works better in that way
#24
02-26-2009, 04:42 PM
 nilrehob Silver Member Join Date: Nov 2008 Location: Sweden Posts: 689
Quote:
 Originally Posted by Sephiroth I'm going to step in to confuse matters a bit more

The Bedini SSG has more components in it than my simple test above and thus I think it works somewhat differently. The passing magnet, for example, I'm sure adds to the equation.

I think I have heard Bedini say something like "the magnet sucks out the field" as it passes away from the coil. Maybe that is how energy is added in the mechanical SSG.
The SS SSG doesnt have the magnet but may work differently as it has a higher frequency (just a guess as I haven't built a SS SSG yet).
Would a SSG with no wheel, magnets or transistor work with a relay (instead of the transistor) switching in low freq? My guess is it won't, not even if You trim the width of the puls to match the size of the coil.

Wouldn't it be appropriate to try to separate the different mechanisms in the Bedini SSG so that we *really* understand what is happening?
__________________
Hob Nilre

Last edited by nilrehob; 02-26-2009 at 04:51 PM.
#25
02-26-2009, 05:43 PM
 boguslaw Gold Member Join Date: Aug 2007 Posts: 2,490

There is a magic in iron core. Magnetic field (or rather current) inside iron propagates quite slowly, there is a delay from first time when magnetic field touch core and to the point of "filling" all iron core. The same is for diminishing magnetic field.

Now if you have a coil with iron core and magnet attached to it so magnetic field is strengthen by permanent magnet field , nothing changes for coil operations because MAGNETIC FLUX CHANGE is source of current flow, not simply static magnetic field.

Thing are going to start interesting IF we have a nice way to QUICKLY remove permanent magnetic field exactly at the point when coil magnetic field is starting to collapse because of turn off of power source.Iron core is still magnetized but there is no external source of static magnetic field - so magnetic field of coil + magnetic field of iron core COLLAPSE.

You see how SSG is using permanent magnet and iron core to extract energy from static magnetic field.

How do you think about it ?
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#26
02-26-2009, 05:47 PM
 Jetijs Gold Member Join Date: Aug 2007 Posts: 2,134
If you want inductive spikes without any magnets involved, look at Bedini self oscillator circuits or Peters rotary attraction motor.
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#27
02-26-2009, 06:53 PM
 Sephiroth Silver Member Join Date: Sep 2007 Posts: 971
nilrehob - I am almost 100% confident that the passing magnets actually take energy away energy from electrical output and does not add to it.

Also, what you are saying in your previous post is correct that as the capactior charges the flyback increases in voltage and reduces in duration BUT remember that what you are calling the spike is actually the flyback current I mentioned and is not the transient spike that aaron is refering to. The transient spike preceeds the flyback.
__________________
"Theory guides. Experiment decides."

“I do not think there is any thrill that can go through the human heart like that felt by the inventor as he sees some creation of the brain unfolding to success... Such emotions make a man forget food, sleep, friends, love, everything.”
Nikola Tesla
#28
02-26-2009, 09:07 PM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
spike

Quote:
 Originally Posted by nilrehob The spike has a width, and a current, albeit very small, just enough to make the spike carry the energy that was previously in the coil.
Right, if you zoom into the spike close enough, you will see it is a triangle with some width getting wider towards the bottom. However, there are spikes like this but also true impulses without the current...this is firmly established.
__________________
Sincerely,
Aaron Murakami

#29
02-26-2009, 09:13 PM
 Aaron Co-Founder & Moderator Join Date: Feb 2007 Location: Washington State Posts: 10,955
current after disconnect

There is a way to have current in the coil AFTER
you remove the input power.
__________________
Sincerely,
Aaron Murakami

#30
02-26-2009, 10:09 PM
 boguslaw Gold Member Join Date: Aug 2007 Posts: 2,490
Aaron,

Great ! Now teach us how to circulate such current in closed path and extract energy from such generator !
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