Back EMF vs. Collapsing Magnetic Field Spike

[Aaron]

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:
Faraday's Law
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.

[Inquorate]

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

[Aaron]

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.

[cody]

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

[redeagle]

Thank you for posting this Aaron. It might not be a bad idea to make this thread a sticky.

Matt

[Aaron]

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.”

[sucahyo]

Would 5ms coil charging time apply to any coil including flat spiral coil?

[Aaron]

time constant

Every coil has different inductance and resistance so would be different from coil to coil I believe.

[cody]

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?

[sucahyo]

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?

[Aaron]

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.

[sucahyo]

@Aaron, thank you.

I think anyone want to know more about spike should really read this thread bellow too:
http://www.energeticforum.com/renewa...nt-energy.html

[nilrehob]

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...

[Aaron]

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.

[nilrehob]

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.