# Toroidal coil rotating magnet

there is a toroidal coil , a magnet rotates in the middle of the coil , I wonder what happens when first the magnet that rotates is a permanent magnet , second case if the magnet that rotates is a electromagnet supplied with a certain frequency AC current?

I wonder what kind of output we see on the toroid coil, in the case of the permanent magnet i'm not sure in the case of the electromagnet i think we should see the input ac frequency of the rotating magnet + some additional power from the rotation.?

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Hesch
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I wonder what kind of output we see on the toroid coil, in the case of the permanent magnet i'm not sure in the case of the electromagnet i think we should see the input ac frequency of the rotating magnet + some additional power from the rotation.?
It depends on the shape of the rotating magnet and how the toroid coil is wound.

well it's really hard for me to drawn anything that you could understand better than my words but for simplicity we can assume the toroid being a round tube , and the magnet a sphere rotating inside that tube.

well the same thing just from another perspective would be a linear coil with an endless stream of magnets seperated each by a distance running through that coil, in once case the magnets are all permanent magnets in other case assume they are electromagnets which are fed with a certain frequency and waveform AC. say 50hz.
in both cases what kind of waveform and frequency i would see if i attached an oscilloscope to the coils terminals?

jim hardy
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Have you looked into magnetic amplifiers ?
I wonder what kind of output we see on the toroid coil, in the case of the permanent magnet i'm not sure in the case of the electromagnet i think we should see the input ac frequency of the rotating magnet + some additional power from the rotation.?
i think you need to draw it then apply right hand rule
see hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

Does flux inside the toroid change ?

Hesch
Gold Member
i think you need to draw it then apply right hand rule
Also the width of the coil is important: Will the coil cover 30° or 360° of the toroid ?
the magnet a sphere rotating inside that tube.
How is that sphere magnetized? Something like this:

with the poles: N-S-N-S (two pole pairs), or what?

thank you Jim , yes i have looked into magnetic amplifiers and I have to say this thing im trying to work out is kinda different.
the flux changes along the path of the coil contantly.

well we could say that coil covers almoust all 360° of the toroid.basically there are two cases , one case (more a theoretical than practical one) were the magnets travels inside the very core of the toroid , not the window in the middle but the very core , which in normal transformers is filled with iron laminations.as the magnet travels in the core the flux engulfs the loop fully.

the physical idea of a generator was this, the same toroid with a coil on it , but in the middle window which is normally empty , there is rotor shaft, from the toroid core there come say two extensions attached to the toroid core at one end and an airgap wth the rotor at the other end.field coils are located on those extensions.the field coils are arranged such that one pole always points into the toroid core while the other into the rotor, the rotor has two blades at each side of the toroid those blades have an airgap facing the sides of the toroid. in other words the flux from the field coils goes into the rotor where via the blades it is directed back into the stator of the toroid were the other pole is. as the rotor moves this flux passing out the blades through the wire into the core is dragged around the toroid so the high density flux area travels around the coil.
since the flux goes into the core from both sides the direction of flux is opposite at each side of the toroid , co the induced curent doesnt cancel out.

i was talking about this elsewere but the poeple said that it wouldnt work because the flux isnt the same and doesnt go through the whole loop of the coil.

Hesch
Gold Member
we could say that coil covers almoust all 360° of the toroid.
If the coil covers 360°, and the flux is symmetric in the toroid, no emf will be induced in the coil.

But if the coil covers 180°, and the magnet has one pole pair, the shape of the emf will be . . . . . .

Well, sketch a curve of the flux passing through the windings ( the Ψn - value ) when the magnet rotates. Emf = dΨn/dt.

Be careful with the signs.

jim hardy
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all i see is symmetry cancelling out any voltage

well it's really hard for me to drawn anything that you could understand better than my words
there's a reason they say a picture is worth 103 words

Ok then I will try my best attempt at a drawing a bit later on.

Hesch
Gold Member
But if the coil covers 180°, and the magnet has one pole pair, the shape of the emf will be . . . . . .
all i see is symmetry cancelling out any voltage
Only if the coil covers 360° of the toroid, as shown in the attached image in #9.

Say it covers only 180° ( the right half of the toroid ) the magnetic field will still be symmetric, but the left half of the magnetic field will not pass through any windings.

jim hardy
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But if the coil covers 180°, and the magnet has one pole pair, the shape of the emf will be . . . . . .
aha i missed that, thanks

so here is my crude drawing , well first of all if i made the coil just half the toroid and rotated a bar shaped magnet inside the toroid window I would have just an oridnary generator and in fact not the best efficiency.

so the idea was like this there is a toroid with a coil wrapped on it spanning the whole length of the toroid. then there are field coils placed on the pink attachments which are fixed or the same material as the toroid core on which the coils sits, as you feed ac current into the field windings they are arranged such that both alike poles will point into the core of the toroid and the other two alike poles will face the airgap with the rotor , the rotor has blades or whatever you call them on it and is a low reluctance path for a amgnetic field so the opposite poles seek to find each other to establish flux and the flux should go from the field coil into the core through the windings into the blade back into the rotor and into the other end of the field coil, in other words i need the rotor to have a flux at the right angle with the toroid core so that once the rotor rotates the flux from the blades would cut the coil wire on the toroid core and induce current.
I don't know what do you think.

jim hardy
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you need to lead people's thoughts
not flood them with a run-on sentence that's a disjointed heap of thoughts with no flow from one to the next
so the idea was like this there is a toroid with a coil wrapped on it spanning the whole length of the toroid. then there are field coils placed on the pink attachments which are fixed or the same material as the toroid core on which the coils sits, as you feed ac current into the field windings they are arranged such that both alike poles will point into the core of the toroid and the other two alike poles will face the airgap with the rotor , the rotor has blades or whatever you call them on it and is a low reluctance path for a amgnetic field so the opposite poles seek to find each other to establish flux and the flux should go from the field coil into the core through the windings into the blade back into the rotor and into the other end of the field coil, in other words i need the rotor to have a flux at the right angle with the toroid core so that once the rotor rotates the flux from the blades would cut the coil wire on the toroid core and induce current.
Might it be easier to read like this ? my questions in blue
so the idea was like this ...

there is a toroid with a coil wrapped on it spanning the whole length of the toroid.

then there are field coils placed on the pink attachments (which are fixed or the same material as the toroid core on which the coils sits),

as you feed ac current into the field windings
they are arranged (What are arranged ?) such that

both alike poles (What are the poles? Where are they on your diagram ? )
will point into the core of the toroid and the other two alike poles (where are these on your diagram ?)
will face the airgap with the rotor , (airgap with rotor? I'm lost now, dont know what you mean)
the rotor has blades or whatever you call them on it (?? )

and is a low reluctance path for a amgnetic field

so the opposite poles seek to find each other to establish flux(are you suggesting motion? how do the blue ones get past the pink ones ?)
and the flux should go from the field coil into the core through the windings into the blade back into the rotor and into the other end of the field coil, (yes, both clockwise and counterclockwise around toroid)

in other words
i need the rotor to have a flux at the right angle with the toroid core so that once the rotor rotates the flux from the blades would cut the coil wire on the toroid core and induce current.
Voltage is induced, not current .
Amount of voltage that's induced is equal to number of turns of wire ,
multiplied by,
rate at which the magnetic flux encircled by those turns of wire is changing.
Draw the flux on your diagram, then show how that flux changes as the rotor turns.

the two field windings are wound such that both N poles for example face into the torus core and both S poles point into the rotor.

the airgap with the rotor is because the field coils are stationary together with the stator torus. the only thing that turns is the rotor which is made up of a shaft and two blades at each end so that the blades move past the sides of the torus as the rotor turns.

the field flux from the field coils is such that it travels through the core which is stationary as the field coils but then to complete the magnetic loop the flux travels through the blades and through the rotor back to the field coils. since the rotor is a low reluctance path for the field lines.
but since the rotor with the blades rotates the blades change their position contantly as they are gragged around the torus the high flux area between the core and the blades cuts the wire sitting on the torus.which is the output coil.
the field current in the field windings is AC.

jim hardy
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Ahhhh much better !

Hmmm will think on it

meantime

what if

you tapped your coil at three places
and energized the rotor with ac
would there be voltage between the taps ?

What would happen if you made two such apparati
connected their stators in parallel
and their rotors in parallel
?

I honstly don't know
but you might look up "selsyn"

.

as to the image you posted I think there will be no induced current nor between the taps not in overall, because the whole coil is one big coil and the magnet has two sides of flux direction both of them cutting the same coil just at different parts.so the total current will cancel out.well thats my take on it.

my picture with the blades has only one way flux direction at any part opf the coil so there shouldn't be cancelling currents.the flux itself changes direction as the AC field current does but at any given moment both blades or whatever the blade count all have the sam flux direction either from the rotor to stator or vice versa.

jim hardy
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as to the image you posted I think there will be no induced current nor between the taps not in overall,
i re-drew my diagram so the col is divided into three equal sections of six turns.

Magnetic flux that passes through loop of wire is said to link it.

Do you understand that induction in a loop of wire causes voltage in proportion to the rate at which magnetic flux linking those loops of wire i changes?

Does it look as if the flux through the individual loops of wire might change ?

As Hesch pointed out above, induction around the whole toroid will sum to zero
but around just a segment it will not.