Classical transformer BEMF study

  • Last Post 29 June 2019
cd_sharp posted this 22 June 2019

Hey, guys

Let's examine what is going on in a classical transformer. I have a 1 : 3 ferrite transformer, the secondary over the primary winding, both CW.

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cd_sharp posted this 22 June 2019

Yellow: primary voltage

Blue: primary current

Pink: secondary current

The moment we are turning off the input, we have a change in primary magnetic field which causes almost instantly an opposite change in the secondary magnetic field. This change induces a voltage in the primary coil which comes back up almost to the value it had before we turned our input off.

Then, the secondary current starts ringing, this is where we see an LCR resonance effect. We have 2 coils carrying currents, we have some resistance but where is the capacitance? Why do we see LCR resonance when we have no capacitor?

Vidura posted this 22 June 2019

Very good idea to do tests on what most people are taking as granted. All components and conductive objects have a capacitance, although a small one. The ringing after switching off is likely a longitudinal oscillation, like in tesla coil secondary. In my opinion there is a huge potencial in this , as it is perpendicular to the transverse, magnetic wave in the conductor. you could do a simple test by adding a capacitance (a conductive object) to one end of the coil, the frequency should decrease notably. Earth ground should impact even more, as a huge capacitance. We can learn a lot from this simple experiments! I can not see exactly as yellow and purple trace are overlapped, but it seems that the currents are ar the beginning in phase, and then shift to 180ª ? Referred to the ringing.

thanks for sharing, Vidura


Chris posted this 22 June 2019

My Friends,

Agreed! Good work CD! Please feel free to use arrows as diagrams from the Lenz's Law Experiment Thread, also on the Lenz's Law Thread, we see supporting evidence.

Good work, spur on those that have not yet got themselves up to speed!


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Marathonman posted this 22 June 2019


It's called interwinding capacitance and it is from the general proximity of the coils from one layer to the next and from one winding to the next.  this electrostatic coupling is actually what they call noise but in reality it is the ringing you are experiencing at shut off of input.  this ringing is why medical and audio power supplies have isolated primaries and secondaries to mitigate or minimize this effect.

the capacitance even though small is still present  and distributed between each winding and between each layer.  this effect can actually be tuned  to give the highest resonant value if need be which is the opposite of the inductry standard.  tune it for maximum ringing effect might give a surprising result thus worth investigating. once you hit the right value of resonance you transformer should sing like a fat lady at an opera concert. (literally)

yes Vidura is correct as the secondary through this resonant effect is thrown 180 out of phase thus the signal is bounced back and fourth amplifying the effects. when in operation the secondary is kept in phase with the primary but the moment it is shut off it phase shifts thus the ringing.



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Dadrev posted this 22 June 2019

The moment we are turning off the input, we have a change in primary magnetic field which causes almost instantly an opposite change in the secondary magnetic field.

Digamos que la bobina primaria está "comprimiendo" el campo magnético de la secundaria. Al apagar la primaria, ese campo secundario se "descomprime" (cambia de polaridad) e induce una corriente similar, y opuesta, en la entrada. 

Es así?





Let's say that the primary coil is "compressing" the magnetic field of the secondary one. When the primary is turned off, that secondary field is "decompressed", changes its polarity and induces a similar, and opposite, current at the entrance.

Is it like that?


Marathonman posted this 23 June 2019


Let's say that the primary coil is "compressing" the magnetic field of the secondary one. When the primary is turned off, that secondary field is "decompressed", changes its polarity and induces a similar, and opposite, current at the entrance.

Is it like that?"

That would be correct.


Chris posted this 23 June 2019

My Friends,

Textbook theory on Transformers and Induction in General, is a birds nest of inaccuracy's! At the same time, it is just enough to make Transformers work as Predicted. So we understand enough to make Transformers work, to make "Generators" work, and Inductive devices in general work.

I wrote an article: Does Science truly understand Electromagnetic Induction

I have referenced this article here before.

Lenz's Law is not adequately described in Electromagnetic induction! Many things are vague to incorrect! I have outlined a few things in the PDF above, but I urge you to look at the Effects on the bench at face value!

I urge all readers to question everything, look at the basics, learn all you can!



Dadrev posted this 23 June 2019

Entonces, si es así, entiendo que sería hacer algo similar a esto:

Cómo empujar un columpio y que la gravedad (en este caso, el magnetismo) haga el resto. Como inflar un globo y soltar el aire por otro lado. Quizá no sea tan simple ...




So, if so, I understand that I would do something similar to this:


How to push a swing and that gravity (in this case, magnetism) do the rest. How to inflate a balloon and release the air on the other side. Maybe it's not so simple ...


solarlab posted this 23 June 2019

The Off Shore Wind Farm folks had some major problems a while back (2014?) with transformer interwinding arcs, etc.. Search: "Transients in Transformers" - some excellent, detailed, information is available. Examples here:


Chris, Good article - thanks. (btw, the looped-back coil drawing is a "Hooper Winding" scheme (I think). Not Bifilar in a sense but reduces the "inductance" - quite helpful in non-symetrical air core transformers - "grenade coil" for example.


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cd_sharp posted this 23 June 2019

Let's say that the primary coil is "compressing" the magnetic field of the secondary one. When the primary is turned off, that secondary field is "decompressed", changes its polarity and induces a similar, and opposite, current at the entrance.

We have a way to make a visible analogy:

(jump directly to 01:41, for some reason it doesn't do it automatically, although the link looks good)

It's compression and decompression alright. Notice how the magnet bounces back.

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Marathonman posted this 23 June 2019

It is actually quite amazing what you can learn when you discard main stream Science thinking outside the box.

Good thread thank you. exploring alternative possibilities to the uncomplete path of science and Physics.



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Chris posted this 23 June 2019

My Friends,

Please do not be confused, when Science deems the Term "Eddy Currents", these Currents are no different than the Current in an Electrical "Generator". The only difference is, in an Electrical "Generator" Insulation guides the "Eddy Currents" down an Insulated Copper Wire out to the Load.

We see "Eddy Currents" build in the Copper, this is the reason the Magnet stops dead. Equal and Opposite, less losses. However, the Currents are the same! Thus the very principal of How Electrical "Generators" work!

The definition of Eddy is as follows:

a circular movement of water causing a small whirlpool.


All we do in an Electrical "Generator" is guide the Currents in our own Eddy, around the Turns in the Coil and out to the Load.

So, do not be confused by this!


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Marathonman posted this 23 June 2019

Eddies are created the same way yes, an incoming flux produces emf but in this case a solid core lets eddies circulate in excess causing current to flow which according to the Lenz Law the field produced will oppose the original flux. when the magnet comes at high velocity a very high opposing flux is created in the core of the copper. this is the reason for Walter Russel putting the iron core on the outside which supports the inside circulating flux.

Sorry cd-sharp off topic kinda.



Chris posted this 24 June 2019

My Friends,

I agree with Marathonman, however, MM has pointed out one major Inaccuracy, a total misunderstanding that Science has upheld until present day.

Lenz's Law is the Negative Sign in Faradays Law of Electromagnetic Induction, so before Lenz:

E.M.F = N ⋅ dΦ / dt

Adding Heinrich Lenz's contribution gives us:


E.M.F = -N ⋅ dΦ / dt


All Lens says if the E.M.F will be Negative! Remember, it does not state anything about Current, none of the Equations deal in any way with Current, except when you move to Ohms Laws. E.M.F is a Voltage ONLY Component, Faradays Laws and Lenz's Laws do not predict in any way Current!

It is important to note: Lenz's Law was one of the reasons the Conservation of Energy Laws were conceived!

Science is flawed in this area, there is no prediction about Current, it is ASSUMED Current will flow from Negative to Positive, but this as we know is not always the case! The Mr Preva Experiment proves this fact!

Note: Science used to Assume Current flowing from Positive to Negative, this is considered Conventional Current. The correct Flow is Negative to Positive, as is understood of today.

I hope this helps some!


cd_sharp posted this 24 June 2019

Guys, what we see here

is exactly this:

Two equal and opposite currents, practically simultaneous. Also, as time moves on from the moment of the input is switched off (yellow), the input current (blue) starts looking more and more like a triangle. We know this is the sign of current pumping:

The only difference is the periods are about equal.

Chris posted this 24 June 2019

My Friends,

CD is correct.

Each MMF is approximately equal but Opposite. Thus the Opposing Red Arrows.

Note: The average Textbook does not ever consider any Opposing Magnetic Field from the Secondary Coil. You can see:





Did you notice the error in the second image, above? Try applying the Right Hand Grip Rule:




Oddly, and only partially correct, the above image does show two Fluxes, Φσ1 and Φσ2 however, only marked as Leakage Flux. This is still not correct! It is inaccurate!

If the average Transformer is around 90% efficient, most of them are around this figure, and we are dealing with a step down Transformer, lets say 24 : 1, 240 Volts Input, 10 Volts Output. 240 Volts / 400 Turns on the Input, that's 1.66 Volts per turn. 17 Turns gives 10 Volts. Our Input is pulling One Ampere with a Load attached to the Secondary. Some figures:

  • Impedance ( Z ): 1+j239.998 in Ohms. ( Ω )
  • Inductive Reactance ( XL ): 239.998 in Ohms. ( Ω )
  • Capacitive Reactance ( XC ): 239.997916693793 in Ohms. ( Ω )
  • Phase Angle ( θ ): 89.7612689669011 degrees. ( θ )


Our power on the input calculates as follows:

Power ( P ) = Volts ( V ) ⋅ Amperes ( I ) ⋅  cos( θ ) = 240 ⋅ 1 ⋅ cos( 89.76 ) = 1.0 watts.

At 1 Watt on the Input, a 90% efficient Transformer will give you 0.9 Watts on the Output.

Magnetomotive force ( MMF ) = Fm = N ⋅ I = Ampere-Turns ( At ).

For illustration purposes I am going to consider DC Power. We are going to put aside Phase Angle for the moment. Assuming Unity Coupling:

  1. 400 Turns ⋅ 1 Amperes = 400 Ampere-Turns.
  2. 17 Turns ⋅ 24 Amperes = 408 Ampere-Turns.


We have a big problem here! Yes we have not considered Losses! Also, 17 turns is rounded up from Volts per Turn: 400 / 240 = 1.6666666666666666666666666666667 ( Turns Per Volt ) ⋅ 10 ( Volts ) = 16.666666666666666666666666666667‬ ( Turns ).

On the Output at 10 Volts we should get 24 Amperes without Losses and all Transformers have losses! So we have a 90% efficient Transformer! What is 10% off 400? 40 right? So more accurate estimation would be:

  1. 400 Turns ⋅ 1 Ampere = 400 Ampere-Turns.
  2. 17 Turns ⋅ 21.6 Amperes = 367.2 Ampere-Turns.


But, is this the correct way to look at this?

What is work? The Current or MMF is the work Component, and we have a Phase angle to think about. But Watts is not the Magnetising Force, MMF is! MMF does not consider where the Voltage is, it is only Turns multiplied by Amperes.

You can see, it is easy for confusion and therefore inaccuracy's to creep in!

367.2 Ampere-Turns / 400 Ampere-Turns = 0.918‬ or 91.8% of the work is producing Output, nearly 10% is lost to Hysteresis and Heat losses.‭‬

PrimaryMMF = Primary Turns ( N ) Primary Current ( I ) and SecondaryMMF = Secondary Turns ( N ) Secondary Current ( I ) so together we must conclude:


MMFPrimary ≅ -MMFSecondary



As you can see, there are Two Fluxes in a Transformer Core at any time the Secondary is Loaded! It is assumed the Secondary Current Opposes the Primary Current and most of the Time this is correct. Importantly, none of the equations predict the Current Direction, again, they only predict E.M.F.


Atti posted this 24 June 2019

I would add to the following.

Let us also differentiate the following.
Empty district flux
Primary flux
Secondary flux
Scattering Flux
Short-circuited (or heavily loaded! And here's not all about how much time we load!) The transformer's spreading flux during external and internal feeding.

Experiments on various transformer rods show that after short-circuit (or heavy load) in the column of the iron core, the flux may behave very differently depending on whether the outer or inner coil is fed.

In addition, we need to distinguish between serial and parallel flux paths.
In the column of the iron core, the flux may decrease to 3-15% of the idle value when the external coil is fed.
In the case of internal feeding, on the other hand, the flux may increase to 103-115% of its idle value.

We can legitimately ask: will a higher flux rate change our induced tension?
So in short-circuiting, if we ignore the resistance of the coils, there will be no flux in the column (or at least a little), and the scattered flux closes through the groove and the barrier column.
Take an example. Also, take informative measurements.
Read the description. The data is for information only.

So there is a coil of flux and we have iron flux (iron or ferrite no matter what, but no further question)
We can't influence the flux of the roll, but what can we do with the flux in the iron?
How does potential resonance work? (Reaching magnetic resonance suggested by Chris)
How does flux change and load capacity change by adding an external magnetic field?

Ask yourself.

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cd_sharp posted this 29 June 2019

Hey, guys

Let's examine what happens at the beginning of the cycle. I changed the core and the coils to a loose coupling transformer, ignore the third coil, we have only primary and secondary:

We have the traces:

-yellow : input voltage

-dark blue: secondary current

The secondary current builds up slowly, we see the storage and decay phases.

But let's zoom in the beginning of the cycle:

We see ringing, there is current fast changing in time through the secondary coil.

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