AC voltage from single magnetic pole

[nix85]

This is strange and possibly the reason i got so little voltage from my all N rotor magnets and aircore toroid stator.

We all know when magnet approaches a normal coil you get voltage in one direction, 0 in the middle and opposite voltage as it crosses the other side of the coil.

All sweet. But look at this. I suggest you watch the whole vid but first 4 and half min are more important.

https://www.youtube.com/watch?v=KmENeg5YSCw

When he sweeped that coil exposing just one side to field, he got AC voltage, first small negative voltage from side N flux, then higher positive voltage as coil passed the center of the magnet and then suprisingly JUST AS HIGH or even higher negative voltage as coil crosses from strong central S flux to weak side N flux...

If you look at 3:23 he does it again, it really produces equal voltage in both directions using just one side of the coil and sweep in just one direction. Slow it down to 0.25x and you will see clearly a full AC spike from one sweep to the left.

But when he crosses the magnet across the coil horizontally using both sides of the coil, in usual manner, the second spike is lower.

What troubles me is does this mean we cannot use only N poles to induce DC voltage over aircore toroid?

Has anyone tried this? If not, i'd appreciate if you do, i tore my toroid apart so can't do it at the moment.

It is crucial that we know this. Does single pole really produce equal voltage in both directions due to sudden change of flux from dense central flux to much weaker opposite side flux.

When i gave it bit more thought it can't be any other way, significant change of flux must produce voltage, it's just that induction is usually not done this way so we tend to forget that just cause you are using one pole does not mean you will get DC.

[verpies]

When i gave it bit more thought it can't be any other way, significant change of flux must produce voltage, it's just that induction is usually not done this way so we tend to forget that just cause you are using one pole does not mean you will get DC.

You are confusing induced voltage with induced current.
Also, you are conflating the behavior of an unloaded (or open) coils with loaded (or shorted coils).

Finally, you are not accounting for the return flux of the permanent magnet.


To stir you mind, I will write that when an ideal coil is shorted and the return flux of the permanent magnet is kept away in such manner that it does not enter that coil as it is moved, then the current induced in that coil is unipolar. You may call it pulsating DC.
This video illustrates this unipolarity of the induced current.


However, when the coil is open, while all other things are kept the same, then no current flows in the coil but the voltage induced across its terminals is bipolar according to the Lenz law.
Namely, one polarity of voltage when the amount of flux encompassed by the coil increases and the opposite polarity of voltage when that flux decreases.

All of this has already been discussed in this thread started by TinMan.

[shylo]

Hi Verpies
In your unipolarity video, can we not collect twice? Positive one way ,negative the other?
I can't access the Tinman thread ,you need to be a member of OUR
I use half bridges on each coil leg to feed the caps, but loading the caps in a split manner eliminates lenz drag.
Split the caps  between the half bridges.
Thanks artv.

[nix85]

You are confusing induced voltage with induced current.

No, i'm not, i am talking about voltage here, current is irrelevant here.

As for current, it will be in-phase with voltage unless we go into very high inductance coils and high frequencies (ala Hanes). For normal alternators reactance will be minimal. This is denoted as power factor and is usually above 0.9 for most alternators meaning 10% or less power is consumed by inductive reactance.

Also, you are conflating the behavior of an unloaded (or open) coils with loaded (or shorted coils).

I am not conflating anything, i am talking about induced voltage in unloaded coil but there is no significant difference for loaded coil within usual parameters.

Finally, you are not accounting for the return flux of the permanent magnet.

Not true, i mentioned return flux or side flux multiple times.

To stir you mind, I will write that when an ideal coil is shorted and the return flux of the permanent magnet is kept away in such manner that it does not enter that coil as it is moved, then the current induced in that coil is unipolar. You may call it pulsating DC.
This video illustrates this unipolarity of the induced current.

Not true, with or without return flux, as coil passes the edge of a magnet and enters a zone where thick flux is absent opposite voltage will be induced according to dB/dT.

However, when the coil is open, while all other things are kept the same, then no current flows in the coil but the voltage induced across its terminals is bipolar according to the Lenz law.

No current in open circuit, who would say. With current or without, induced voltage polarity is the same.

Read my post again, watch the video again, there is a difference when he sweeps that coil with one side across the magnet vertically and when he sweeps magnet across both sides of the coil horizontally. Both produce AC voltage but in second one second spike is lower.

Namely, one polarity of voltage when the amount of flux encompassed by the coil increases and the opposite polarity of voltage when that flux decreases.

All of this has already been discussed in this thread started by TinMan.

AC voltage is produced in both scenarios, shorted or not, with side flux or not.

Take a single wire to keep it simple, sweep it across a magnet, as it enters the field you get voltage in one direction and as it leaves in opposite. Short the wire and you will get same voltage with insginificant difference of coil's backEMF when it's shorted.

I suggest you study lenz, here are few videos to start.

https://youtu.be/bkSsgTQOXVI < first part of this vid is misleading, if flux in same direction increases or decreases over both sides of the coil at the same time no voltage will be induced as shown in next vid
https://www.youtube.com/watch?v=6NDztGfWpe4

Few general rules to keep in mind..

First pic below, as north pole of a magnet sweeps across the wire, electrons will first go up and as magnet leaves the wire they will go down.

Two left hand rules..

If wire is placed perpendicular to a N-S field and electrons in the wire are going to the right, wire will experience a downward force.

If electrons coil into the screen in up part of the coil as shown north pole will be on the right.

[verpies]

...current is irrelevant here.

Current is very relevant for coils. It is a major phenomenon of inductive energy storage and without it a coil does not store energy nor oppose external flux not attracts not repels anything.  In other words, a coil without current  is a Nothing Burger.
Also, current is directly proportional to the magnetic flux generated by the coil^* since the inductance of a coil is the ratio of its flux to the current flowing through it, in mathspeak: L=Φ/i   or  i=Φ/L.


* _{or an external flux attempting to change the total flux penetrating a closed coil}

Not true, with or without return flux...

Let's talk about it

I am not conflating anything

But you are.
Take a look at the piece of our discussion below.  I wrote about current but you countered with an argument about voltage.

To stir you mind, I will write that when an ideal coil is shorted and the return flux of the permanent magnet is kept away in such manner that it does not enter that coil as it is moved, then the current induced in that coil is unipolar. You may call it pulsating DC. This video illustrates this unipolarity of the induced current.

...as coil passes the edge of a magnet and enters a zone where thick flux is absent opposite voltage will be induced according to dB/dT.

Note, that in that exchange above you made a tacit assumption, that the direction of the voltage induced in an open coil unequivocally determines the direction of the current induced in that coil when it is shorted.

Also, you made a mistake writing that the voltage induced across an open coil depends on dB/dt.
This is wrong because the induced voltage across an open coil does not depend on the rate of change of magnetic flux density at all.  It depends only on the rate of change of magnetic flux, in mathspeak: dΦ/dt.

As for current, it will be in-phase with voltage...

Here you probably assumed that it works the same way as with a resistor when the direction of voltage applied to a resistor unequivocally determines the direction of current flowing through that resistor.
However that voltage vs. current relationship is not true for an inductor.

i am talking about induced voltage in unloaded coil but there is no significant difference for loaded coil within usual parameters.

I beg to differ.
Also, the direction of the induced voltage in an open ideal coil does not determine the direction of the induced current in that coil when it is shorted.  This is not a resistor!
That's why analyzing coils only with induced voltage leads you down the garden path.
Last, but not least it is impossible to even measure the induced voltage in an ideal shorted coil.

This is denoted as power factor and is usually above 0.9 for most alternators meaning 10% or less power is consumed by inductive reactance.

First of all now you are conflating power with energy. Power is the rate of change of energy so it cannot be consumed by definition.
Energy cannot be consumed either but it can be converted to other forms of energy.

However, my most important objection to the quote above is that inductive reactance does not consume energy permanently, because pure inductive reactance stores the energy as magnetic flux and then converts all of it back to electric current.
Notice that this cannot even be properly analysed with pure induced voltage without the consideration of the current flowing in the inductor.

Not true, i mentioned return flux or side flux multiple times.

OK, I grant you that you did but you do not seem to consider it in your analysis of an open coil being waved in front of a naked permanent magnet.

With current or without, induced voltage polarity is the same.

This statement is not even wrong until you notice that it is impossible to measure the induced voltage in an ideal shorted coil.

...watch the video again, there is a difference when he sweeps that coil with one side across the magnet vertically and when he sweeps magnet across both sides of the coil horizontally. Both produce AC voltage but in second one second spike is lower.

I do not see anything unusual in this video but I see a lot of misunderstanding what is happening and the constant assumption that the direction of the induced voltage somehow unequivocally determines the direction of the induced current like in a resistor according to Ohm's law.
Did you watch the video that I have linked from prof. Belcher and noticed that the direction of the induced current does not change?

AC voltage is produced in both scenarios, shorted or not, with side flux or not.

Of course, the total flux* penetrating the coil varies up and down and that induces voltage across an open coil in both direction.  There is nothing unusual about it according to Faraday's law.
Notice, that the video shows only an open coil being measured so you remark "shorted or not" is Ad Hoc.


* _{that also includes return flux.}

Take a single wire to keep it simple, sweep it across a magnet, as it enters the field

Notice that as it is being swept across a naked magnet the first thing it encounters is the return flux.

you get voltage in one direction and as it leaves in opposite.

Yes, that's the Faraday's law, but it has nothing to do with magnetic flux density dB/dt.
Also, with small coil and large magnet you get two double voltage pulses because it:
1) starts encompassing the return flux.
2) stops encompassing the return flux.
3) starts encompassing the flux at the magnet's surface.
4) stops encompassing the flux at the magnet's surface.

Short the wire and you will get same voltage

First of all, it is impossible to measure the voltage across an ideal wire.
Also, an ideal short forms an ideal 1-turn loop/coil with the ideal wire.


The direction of the induced current flowing in that loop/coil will be the same in cases 1 & 2 and the same but opposite in cases 3 & 4.  The direction of the current in cases 2 & 4 will be opposite, because the direction of the magnet's return flux is opposite to its surface flux.
The magnitude and direction of the induced current will generate magnetic flux that will oppose any attempt to change the total flux encompassed by this shorted coil. In consequence the total flux encompassed by the coil will remain constant.
Notice that the direction of the current induced in a shorted coil DOES NOT follow the direction of the voltage induced in the same coil when it is open.  You do not appear to know that and that is why I wrote that you are conflating the behavior of an unloaded (or open) coils with loaded (or shorted coils).

I suggest you study lenz, here are few videos to start.

Don't assume that this a new topic for me. I have been through it many times on this and other fora.

Notice, that the Lenz's law is a qualitative law that specifies the direction of induced current, but states nothing about its magnitude.
Also, sometimes it is said that the Lenz's law is manifested as the minus sign in the Faraday's law ε= - dΦ/dt.
That minus sign refers to your voltage induced across an open coil by a changing flux. Note the concept of this voltage fails the Ohm's law in a shorted ideal coil (just look at the magnitude of the result!) and that is one of the many reasons why it is better to analyze coils in the current domain than voltage domain....and the capacitors - just the opposite.