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Author Topic: Magnetic fields within a toroid inductor.  (Read 77723 times)

xee2

• Hero Member
• Posts: 1610
Re: Magnetic fields within a toroid inductor.
« Reply #60 on: September 15, 2013, 05:11:08 PM »
That's true. However, the "dl" factor tells us that the integral of B around the full  toroid loop should be the same within the inner and outer radius.

If the radius of the toroid is >> than the radius of the cross-section, then the B-field intensity inside the toroid core will, for practical purposes be constant from the inner to outer radius.

?? If you draw the line integral anywhere inside the toroid will it not always enclose the same amount of current?

Re: Magnetic fields within a toroid inductor.
« Reply #60 on: September 15, 2013, 05:11:08 PM »

poynt99

• TPU-Elite
• Hero Member
• Posts: 3582
Re: Magnetic fields within a toroid inductor.
« Reply #61 on: September 15, 2013, 05:15:44 PM »
Yes. That was my point.

However, in a toroid, if you did have a very small Hall sensor that you could insert inside the toroid core at any radius, we would see a stronger reading at the inside radius vs. the outside radius.

tinman

• Hero Member
• Posts: 5236
Re: Magnetic fields within a toroid inductor.
« Reply #62 on: September 15, 2013, 05:20:12 PM »
Ok,i was refering to the cross section center of the core itself-not the air hole in the middle of the toroid.
So if the magnetic field is strongest within the core itself,why dont we put the secondary coil in the center of the core?,rather than around the outside with the primary winding, like a standard transformer. Dose anyone know what would happen with the secondary placed inside the core itself?. Would this reduce or remove any BackEMF that would effect the primary?.

There is another thing about the toroid coil that may answer another debate that was raised in another thread(MH will know this one im talking about) It is in reguards to this bloch wall claim.If it exist within a PM or EM,then it must exist in all PM's and EM's. So when we pass current through a toriod inductor,where exactly would this bloch wall be? Is the toroid the perfect example of magnetic fields looping around a magnet,without this bloch wall?

Re: Magnetic fields within a toroid inductor.
« Reply #62 on: September 15, 2013, 05:20:12 PM »

MileHigh

• Hero Member
• Posts: 7600
Re: Magnetic fields within a toroid inductor.
« Reply #63 on: September 15, 2013, 05:24:04 PM »
Xee2:

Quote
If your point is that the field is uniform inside the toroid

My point is that the field is not uniform inside the toroid.  The field inside the toroid is stronger the closer you go towards the center of the toroid.

Quote
For an infinitely long solendoid, the field is uniform inside the solendoid.

I agree, and that's a nice little thought experiment that proves that the field is uniform inside an infinitely long solenoid.  You just keep making the radius of the toroid larger and larger until you get a straight line.

Poynt:

Quote
If the radius of the toroid is >> than the radius of the cross-section, then the B-field intensity inside the toroid core will, for practical purposes be constant from the inner to outer radius.

Yes but on the bench we normally work with toroids that are typically "doughnut sized" and the formula from the clip is relevant to bench work.

It also makes sense intuitively.  When you are hugging the inside wall of the toroidal cross section, the density of the wire loops per tangential centimeter is the highest.  So you would think that the magnetic field would be stronger where there is a higher wire loop density (as compared to the outside wall of the toroid where the wire loop density per tangential centimeter is lower.)

MileHigh

poynt99

• TPU-Elite
• Hero Member
• Posts: 3582
Re: Magnetic fields within a toroid inductor.
« Reply #64 on: September 15, 2013, 05:24:10 PM »
Mags and tinman,

Forget about toroid coils for a moment, and consider this thought experiment.

If you had a very long solenoid coil, say 1" in diameter and 1 mile long, wound evenly along the entire length of the solenoid, and you energized the solenoid. Would you be able to detect any magnetic field anywhere around the solenoid, other than at the ends or within its core?

In other words, if you were standing 2640 feet from one end of the solenoid, and you had a Hall sensor with you, would you detect any magnetic field anywhere near the solenoid (assuming you could not probe inside it)?

Re: Magnetic fields within a toroid inductor.
« Reply #64 on: September 15, 2013, 05:24:10 PM »

MileHigh

• Hero Member
• Posts: 7600
Re: Magnetic fields within a toroid inductor.
« Reply #65 on: September 15, 2013, 05:28:25 PM »
Xee2:

Quote
?? If you draw the line integral anywhere inside the toroid will it not always enclose the same amount of current?

Yes but if the summation of all of the dl's is larger then the B has to be smaller because there is a constant amount of current inside the dl loop.

So that's telling you the larger the dl loop the smaller B has to be.  (no pun intended lol)

MileHigh

poynt99

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• Posts: 3582
Re: Magnetic fields within a toroid inductor.
« Reply #66 on: September 15, 2013, 05:34:54 PM »
Ok,i was refering to the cross section center of the core itself-not the air hole in the middle of the toroid.
So if the magnetic field is strongest within the core itself,why dont we put the secondary coil in the center of the core?,rather than around the outside with the primary winding, like a standard transformer. Dose anyone know what would happen with the secondary placed inside the core itself?. Would this reduce or remove any BackEMF that would effect the primary?.
Brad, in theory it does not matter how loosely the secondary is wound on the core, as long as it loops completely around it. Remember, it is not the magnetic field that induces the emf in the secondary, it is the resulting electric field which does. And this electric E-field emanates from the center of the core outward to OUTSIDE the core itself.

Quote
There is another thing about the toroid coil that may answer another debate that was raised in another thread(MH will know this one im talking about) It is in reguards to this bloch wall claim.If it exist within a PM or EM,then it must exist in all PM's and EM's. So when we pass current through a toriod inductor,where exactly would this bloch wall be? Is the toroid the perfect example of magnetic fields looping around a magnet,without this bloch wall?
There is no Bloch wall in a regular PM or EM. And this includes a toroid coil; there is no Bloch wall there either.

Re: Magnetic fields within a toroid inductor.
« Reply #66 on: September 15, 2013, 05:34:54 PM »

hanon

• Hero Member
• Posts: 614
Re: Magnetic fields within a toroid inductor.
« Reply #67 on: September 15, 2013, 05:50:44 PM »
So what dose the magnetic field look like within a toroid coil?. Is the magnetic field contained within the toroid core?,and wich field is it-north or south?.
My first test looking at a field being expelled outward from the toroid coil.But listen carefuly to the video near the end.

Hi,

Very interesting!!

Does anyone know why the induced voltage into the external coil has an exact  90º phase shift ? Could you test what happens at lower frequencies as 60 Hz or so?

Is is possible that you are getting in that video another type of induction?

xee2

• Hero Member
• Posts: 1610
Re: Magnetic fields within a toroid inductor.
« Reply #68 on: September 15, 2013, 06:01:00 PM »
Xee2:

Yes but if the summation of all of the dl's is larger then the B has to be smaller because there is a constant amount of current inside the dl loop.

So that's telling you the larger the dl loop the smaller B has to be.  (no pun intended lol)

MileHigh

MileHigh,
I think I see your point. Since the toroid walls are not parallel like a solenoid, the magnetic field will be stronger near the inner edge of the toroid than near the outer edged of toroid. Thanks for the corrections. I hope I have it correct now.

Re: Magnetic fields within a toroid inductor.
« Reply #68 on: September 15, 2013, 06:01:00 PM »

tinman

• Hero Member
• Posts: 5236
Re: Magnetic fields within a toroid inductor.
« Reply #69 on: September 15, 2013, 06:05:06 PM »
Mags and tinman,

Forget about toroid coils for a moment, and consider this thought experiment.

If you had a very long solenoid coil, say 1" in diameter and 1 mile long, wound evenly along the entire length of the solenoid, and you energized the solenoid. Would you be able to detect any magnetic field anywhere around the solenoid, other than at the ends or within its core?

In other words, if you were standing 2640 feet from one end of the solenoid, and you had a Hall sensor with you, would you detect any magnetic field anywhere near the solenoid (assuming you could not probe inside it)?
Well i believe that the fields must always loop from one end of the magnet(inductor) to the other,so a magnetic field should be detectable along the outside. The same go's for any length of straight wire that has current passing through it,there will be a magnetic field along that wire.If we look at the power line's that deliver our power to our home's,we know we can light a CFL from the magnetic radiation from those wires. The wires are also twisted to form loops,like that of an inductor-only the twists arnt as tight as they are in an inductor.

tinman

• Hero Member
• Posts: 5236
Re: Magnetic fields within a toroid inductor.
« Reply #70 on: September 15, 2013, 06:08:28 PM »
There is no Bloch wall in a regular PM or EM. And this includes a toroid coil; there is no Bloch wall there either.
Thank you point,and i was with you on this one.MH tried for some time to show this to other's,but-well lets just say,they love there bloch walls.

Re: Magnetic fields within a toroid inductor.
« Reply #70 on: September 15, 2013, 06:08:28 PM »

tinman

• Hero Member
• Posts: 5236
Re: Magnetic fields within a toroid inductor.
« Reply #71 on: September 15, 2013, 06:13:05 PM »
Hi,

Very interesting!!

Does anyone know why the induced voltage into the external coil has an exact  90º phase shift ? Could you test what happens at lower frequencies as 60 Hz or so?

Is is possible that you are getting in that video another type of induction?
Hi hanon
I can shift the phase angle from 0 to 180* out,by varing the frequency.At low frequencies,there is no phase shift.Only from about 5KHz dose the phase start to shift. I dont think there is anything to it,but once i hit a 90* phase shift,is when i get maximum amplitude from the secondary.

MileHigh

• Hero Member
• Posts: 7600
Re: Magnetic fields within a toroid inductor.
« Reply #72 on: September 15, 2013, 06:20:55 PM »
Xee2:

Quoting you:

Quote
Since the toroid walls are not parallel like a solenoid, the magnetic field will be stronger near the inner edge of the toroid than near the outer edged of toroid.

Yes indeed, and for posterity I am attaching the screen cap of the clip that shows the formula.

Some people may not be comfortable with doing derivations.  But for what it's worth the teacher that made all those wonderful clips I reference from time to time is excellent at doing and explaining derivations.  The essence of a derivation is taking what looks like a very complex problem and reducing it all down to a simple formula.

Some people may also not be comfortable with interpreting formulas and shy away from them.  But if you followed this thread and have a look at the formula in the attached screen capture and contemplate it, it may all come together.

If you look at the formula and the associated diagram in the screen cap and contemplate it for a while, you should be able to see that just the formula itself is telling you that the magnetic field strength is highest along the inside wall of the toroid.

MileHigh

MileHigh

• Hero Member
• Posts: 7600
Re: Magnetic fields within a toroid inductor.
« Reply #73 on: September 15, 2013, 06:31:14 PM »
Tinman:

Quote
I can shift the phase angle from 0 to 180* out,by varing the frequency.At low frequencies,there is no phase shift.Only from about 5KHz dose the phase start to shift. I dont think there is anything to it,but once i hit a 90* phase shift,is when i get maximum amplitude from the secondary.

You might recall that I stated that there should be no phase shift at lower frequencies.  Below 5 KHz your toroidal coil was acting like a single-turn primary to the N-turn secondary of your pickup coil.  This is due to the generation of a separate magnetic field that is not associated with the magnetic field that is generated inside the core.

You cannot ignore frequency effects when it comes to coils and capacitors but that is a whole other can of worms that's not for this thread.

MileHigh

poynt99

• TPU-Elite
• Hero Member
• Posts: 3582
Re: Magnetic fields within a toroid inductor.
« Reply #74 on: September 15, 2013, 09:38:46 PM »
Well i believe that the fields must always loop from one end of the magnet(inductor) to the other,so a magnetic field should be detectable along the outside. The same go's for any length of straight wire that has current passing through it,there will be a magnetic field along that wire.If we look at the power line's that deliver our power to our home's,we know we can light a CFL from the magnetic radiation from those wires. The wires are also twisted to form loops,like that of an inductor-only the twists arnt as tight as they are in an inductor.
When the solenoid length is >> than the solenoid diameter, no there will not be a detectable B-field outside the solenoid when you are half way from one end. In fact this will be the case for 99.999% of the length of that 1 mile long solenoid. You WILL detect a B-field outside the solenoid when you are very near the ends.

And so it goes with a toroid. There are no open ends for the flux to "escape" and return, hence the B-field outside the toroid is practically zero. The return path is through the center itself because the solenoid makes a complete circle.