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do magnetic flux lines rotate or no

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Salvador

Fri Apr 24 2015, 09:57AM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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I have a question that I thought about.To make it simply I will try to expalin using a known device.
Imagine I have a toridal transformer with primary and secondary windings on it.The winding ration is 1:1
Now I put given amount of power in and say it's A ,then I add some rotational torque and spin the core of the transformer giving it power B.
In the secondary would I get out A from the primary + added B due to the torque from the spinning core which has flux through it , or just A?

Or the other example would be a generator with a solid steel rotor. there are two sets of coils , the stator coils and smaller induction coils at the ends of the rotor. I now put AC current through those smaller coils at the ends of the rotor so that the induced flux path cuts the stator coils through the rotor , In the stator coils would I get the amount of power that I originally put into the smaller rotor coils or would I also get some added power from the flux lines cutting the coils due to rotor rotation? If the lines rotate with the rotor at all ofcourse.Which I'm not sure.

BigBad

Fri Apr 24 2015, 01:55PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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The way to work this kind of stuff out is Faraday's law. If the *total* flux cutting a coil changes, then there's an EMF. If lines go in and out at the same rate all the time, so that the total doesn't change, then there's no EMF.

So that's the only question you have to ask, is the total flux constant or not?

That's pretty much all you need to know (there's corner cases where you have stuff like slipping contacts as in Faraday's disk, but then you use Faraday-Maxwell equation which is the fully general equation.)

Wastrel

Fri Apr 24 2015, 07:02PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
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I don't quite follow the question set up and I'm finding it difficult to see how you can arrange an experiment to test. The thing with the Faraday disc is that assuming the flux does not rotate provides exactly the same answer is assuming it does. An EMF results from a difference in both halves.

Salvador

Fri Apr 24 2015, 08:27PM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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I actually have to say thanks to Bigbad, when the total flux cutting the coil is the same along the length of the whole coil it behves just like a transformer it gets induction because the very flux is changing with time if I move a solid rotor around a stator which has coils on it then I guess I don't get any of the rotational torque converted to current via the flux lines cutting the coils correct?

So even if the flux lines passing through the rotor are AC even then I would only get transformer action but no generator action right?
That's why I talked about the toroidal transformer example , not that it's easy or even possible to set up as an experiment but just as a thought experiment what would happen.That's why I asked about the toridal transformer\s core roating, but I guess looking at what Bigbad said nothing would happen apart from the usual induction because either the core rotates or ot the flux lines at any given moment from the core through the coil does not change.

So what's the deal can a solid rotor induce something if it has AC flux through it , apart from the flux itself , or do only rotors with physical poles or " holes" in them work for this as to change the fklux srength repeatedly?

You see I'm aski9ng this because rotors that have poles are restricted to only induce an AC whose frequency is proportional to the speed at which the rotor rotates or rpm and pole and winding count, but If a rotor of what I hough could work then the frequency could be made independant of the rotor speed and said controlled via a varable capacitor in series with the rotor flux field coil.

BigBad

Fri Apr 24 2015, 11:31PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Yeah, I'm not getting the exact scenarios either, but if you have a solid rotor, then the poles will create eddy currents each cycle; the loops from the point of view of Faraday's law are then arbitrary loops through the solid metal; and so if you have an inhomogeneous field, then spinning the rotor will cause eddy currents also.

Salvador

Sat Apr 25 2015, 07:25PM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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Well I'm sorry i didnt quite understood what you said.
Well for the simplest possible example imagine just a AC induction motor similar rotor , a solid barrel or a barrel made from laminated soft steel for the example doesnt matter that much.
then the stator houses both the load and field coils. well you probably would ask maybe you just mena taking an AC induction motor and running it faser than the frequency of the field coils and it becomes a generator , no not exactly like that, the idea is to use a solid rotor as I said before and then run the field flux through that rotor and the rotor would bend the flux through the load coils which would output the same frequency AC that the field coils putted through the rotor just with a much stringer current and voltage which would come from the rotational energy supplied.

But the idea is that the frequency would be controol,ed only by the flux from the field coil, in ordinary generators the frequency is a property of the rotor pole count and winding sonnections etc.

Do you see where I;m going with this?

BigBad

Sat Apr 25 2015, 10:49PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Wouldn't the solid rotor short circuit the poles?

Salvador

Sun Apr 26 2015, 07:47AM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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yes but no more than a speaker magnets end plates short circuit the poles of the magnet, in other words the design is such (if it works ) that the idea being that the short circuited flux or lets just say just the flux flows through the windings at the right angle.the only difference being that instead of seperate poles rotating by the windings here the whole solid rotor moves past them and the changing flux through the winding should create induction much like in a transformer yet the lorentz forces from the rotor flux should transfer the rotational energy to electrical.

the idea is to have and AC flux which can induce currents with or without movememt but the rotation of the rotor would then move that flux lines so that the generator effect would kick in.
thas why I asked abut whether the field lines rotate or no if the medium through which they flow rotates , because if they don't then this doesnt work and it would only be an innefficient transformer.

as I said if you put poles on a rotor as in ordinary machines your outoput frequency is very dependant on the rotational speed of the rotor , here the frequency could be entirely determined only the by AC supplied to the field coil which then would make the flux through the rotor and sator.

Ash Small

Sun Apr 26 2015, 10:21AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Would this not be analagous to superimposing an AC signal onto an AC carrier wave (much the same as radio transmission, but at lower frequency, and then feeding the combined wave into the motor?

It would just result in the motor trying to accelerate and decelerate at the frequency of the superimposed signal, or, supposing the rotor had sufficient, or significant, rotational inertia, ....I'm not quite sure what happens in that case...wierd back EMF?

Or did you mean something else?

Salvador

Sun Apr 26 2015, 06:00PM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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Well remeber as this is a generator idea so two main components should work out , firstly you need a magnetic field to set a magnetic flux through the rotor and the output coils and then you need to use that field together with the rotational torque provided to generate current which then would be the input field current + supplied rotational energy.

Now controlling the frequency of the flux via the field coil is easy but would that flux then create the same frequency current but with much more power in the output coils due to the added rotational energy.

In other words would a solid rotor drag its flux with itself and produce curent in the stator coils or would the rotor's rotation be of no importance to the flux and the flux would just go through as in a transformer core because in that case the device totally fails as a generator.
that's why I asked at the beginning what would happen if one were to take a toroidal transformer supply some power to the pirmary and then spin the core would the secondary then have the power from the primary + some additional EMF due to the core spinning.

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