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how to vary magnetic reluctance

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Salvador

Tue Feb 17 2015, 11:43AM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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Hello, I was thinking is there any material out there which has good magnetic properties like those that are used in transformer cores, but whose reluctance could be varied by some external means , it would be in analogy like a transistor switch where you vary the resistance and so the current either increases or decreases, here I want to vary the reluctance of the material so that magnetic flux would vary.

I know this can be done mechanically like varying the air gap in a transformer core or in inductors with variable cores but I'm looking for a solid state way to do this.
Does anyone have any ideas , I have though much about it but cant think of any material who would suit these properties.

thanks.

Wolfram

Tue Feb 17 2015, 03:05PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
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A common method is to bias the core with a DC field. Reluctance is proportional to permeability, which is the slope of the magnetization curve. Typical magnetization curves get less steep as you move away from the center (zero bias). The easiest way to do this in practice is to have a winding on the core that you excite with a DC current. This effect is utilized in magnetic amplifiers, saturable reactors and in tunable ferrite loaded resonant cavities.

Note that the magnetisation required is often significant if you want to change the permeability over a large range.

Salvador

Tue Feb 17 2015, 09:54PM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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Oh I guess I forgot to add that my primary winding is excited by DC rather than AC so the biasing DC on the core will not help as the core will satured by the very DC power winding itself and the control winding as in magnetic amplifiers will become useless , it only works for AC I guess due to AC having impedance as a current limiter.

So with DC on the primary I can still make AC on the secondary if i physically move the secondary with respect to primary or change the reluctance periodically , something like a core with memory when it goes into saturation then it drops and lets itself into saturation once again , this could utilize the primary DC into usable secondary AC.
Hope now it's clear what I'm thinking here.

Eleccentric

Wed Feb 18 2015, 12:17AM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
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I think what you are asking for is impossible, barring some counter-intuitive metamaterial with properties allowing the magnetic characteristics to be altered by some means other than a magnetic field.

If you've got enough DC current on your primary to saturate the core, there is no electronic way to bring the core out of saturation other than reducing the current on the primary.

Physically altering the core or the positioning of the coils could "work," but why not just use a mechanical interrupter on the primary coil instead?

I wonder what the losses would be like with a device that, rather than mechanically interrupting primary current, mechanically opens and closes the core, or slides the position of a magnetic shunt back and forth, in order to induce AC on the secondary.

Would such a thing work at all? Would it just be the mechanical energy of the moving core bits that gets transformed into electrical energy on the secondary coil? I am curious what better minds than mine have to say..

BigBad

Wed Feb 18 2015, 03:56AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Not sure I really understand what you're trying to do.

Is this like an electropermanent magnet, or a logic gate or a transformer or what?

Salvador

Wed Feb 18 2015, 10:04AM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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no the very idea would work , you got a magnetic field which is set up by the primary winding and the DC current that goes through it that ios energy deposited in the field , the thing is you cannot induce this energy into a nearby winding because it is static once you make the nearby winding or the secondary winding to see the pirmary field but with interruption it can now induce current, all you need is something to change , but in order for the secondary to utilize the force of the primary you cannot move it physically with respect to the primary or that would be like a generator which has a field DC winding then the energy would come from the rotational energy , but if both windings are in place and just the magnetic field varies or is varied all the energy then comes from whatever supplies the field in our case the primary winding DC.

remember that once you switch the DC on any transformer you get a first induction spike because as you add the current firstly the magnetic flux rises as it rises it induces current into the secondary only when the core becomes satured it stops inducing , so i though that if we could make the core drop from satuiration repeatedly with a given frequency it would work as a DC transformer.

hen918

Wed Feb 18 2015, 07:01PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Salvador wrote ...

no the very idea would work , you got a magnetic field which is set up by the primary winding and the DC current that goes through it that ios energy deposited in the field , the thing is you cannot induce this energy into a nearby winding because it is static once you make the nearby winding or the secondary winding to see the pirmary field but with interruption it can now induce current, all you need is something to change , but in order for the secondary to utilize the force of the primary you cannot move it physically with respect to the primary or that would be like a generator which has a field DC winding then the energy would come from the rotational energy , but if both windings are in place and just the magnetic field varies or is varied all the energy then comes from whatever supplies the field in our case the primary winding DC.

remember that once you switch the DC on any transformer you get a first induction spike because as you add the current firstly the magnetic flux rises as it rises it induces current into the secondary only when the core becomes satured it stops inducing , so i though that if we could make the core drop from satuiration repeatedly with a given frequency it would work as a DC transformer.

What you describe would end up as a flyback transformer. These are fed with DC, however the "induction spike" would be negative, so your output is AC with a weird waveform. see

2Spoons

Wed Feb 18 2015, 09:38PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Salvador wrote ...

if both windings are in place and just the magnetic field varies or is varied all the energy then comes from whatever supplies the field in our case the primary winding DC.

I think you will find the energy actually comes from whatever is varying the magnetic field - because doing that requires energy transfer.

Salvador

Wed Feb 18 2015, 10:27PM

Registered Member #54402 Joined: Mon Feb 02 2015, 11:09PM
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Yes I know a flyback transformer , yet it is driven by DC pulses rather than pure DC so it is just another form of SMPS.

Well I guess you are right that the energy to the secondary would come from whatever varies the field but that is only fully true for mechanical systems because if we look at this mechanically it becomes a generator where you have a stationary DC field into which a pair of windings rotate and the current induced in them is a result of the rotational torque that goes into driving the shaft.

But here i'm looking at it differently , i have the primary DC winding and what if I could have a material which when driven into saturation by a given magnetic field cannot sustain this level of flux and atleast partly drops from the point of saturation and then the DC current pushes it into saturation again and again the core drops from taht point being unable to hold it for longer than a cerain period of time , is such a situation possible ? In this situation the secondary winding would be exposed to a time varying field even though the pirmary is a static field.

A side question but related to this , in an inductor when we shut off the current that flows through it rapidly enough the voltage rises as it tries to recover the current flow, the level to which it rises is dictated also by the inductor core ability to store the upbuilding magnetic field correct?what happens whne we shut off the current rapidly and after a very very short amount of time also drop out the core from the inductor , would the magnetic field and volatge still build up ?

2Spoons

Thu Feb 19 2015, 02:17AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Saturation does not mean that the flux decreases. Saturation is a reduction in Âµr, the relationship between H and B.

I suspect what you'd need is a material with a region of negative incremental Âµr in its B-H curve, if you want to create a "magnetic oscillator". Much like "negative resistance" oscillators (look up tunnel diodes). I'm not aware of any material with this property.

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