Effects of Recirculating BEMF to Coil

[gotoluc]

Here's a question: Does the inductance of the magnet-lifting coil change, as the magnet lifts?

Humm, I don't think a magnet has inductance but I think you maybe referring to the coil. Like I said in the video, the magnet does not affect the effect. It is just used as a visual aid as to be able to calculate change when changes have been made. Anyways, I've tried it also without the magnet and it shows actually a very small efficiency.

It would seem on casual inspection that straight DC would lift the magnet higher than the reversing field from AC, at the same power levels. I think.

You may think that! but not according to my tests. I even have that test on video! you may have missed it. Here it is again: http://www.youtube.com/watch?v=HpaP__5Kd38

Got to build me one. Where did I put that mile of magnet wire....

Just make sure your 555 timer circuit is the one that gives you 10% or less duty cycles

Luc

[gyulasun]

Here's a question: Does the inductance of the magnet-lifting coil change, as the magnet lifts? I really don't know, and I think it would be interesting to try to find out. I can sort of argue both ways, at the present state of my knowledge.
I do know that bringing a powerful magnet close to the toroid of an operating Joule Thief causes interesting effects...it can quench or substantially brighten the LED...

It would seem on casual inspection that straight DC would lift the magnet higher than the reversing field from AC, at the same power levels. I think.

Got to build me one. Where did I put that mile of magnet wire....

@TinselKoala

On your first question: the inductance of an air core coil does not change but very little when moving a permanent magnet in it  because the permeability of permanent magnets approach pretty close to unity i.e. to that of the air.
If a coil has got a ferromagnetic core and you approach a permanent  magnet to it, then the flux changes inside the core (thus the permeability of the core) so the result is you move the magnetic working point of the core on the B-H curve, depending on the poles, and even you can saturate it.

rgds,  Gyula

EDIT  Permeability of Neo magnets ranges from 1.03 to 1.1 or so, ceramic magnets has 1.1-1.2 or so.

[gotoluc]

Ok just read some more of the thread and it does look like the bulb is in series with the supply cap and the device under test.
If this is so then the bulb is only telling half the story for the consumpion of power drained from the supply cap, you must also account for the voltage supplied by the power supply itself.

The bulb is lighting because of a voltage drop across itself, which is related to the current drawn through it and it's own resistance, but you then would need to multiply this current by the power supply voltage to get a true value of power consumed.
So keeping the bulb at a certain brightness but altering the supply voltage is indeed increasing the power consumed by the device under test.

Hope that helps

Peter

Hi Peter,

I'm sorry but that is not correct!  if you read this posts: http://www.overunity.com/index.php?topic=7713.msg190619#msg190619  you will find that user .99 has addressed that since I specifically asked the question before doing my last two videos.

Luc

[poynt99]

Luc,

Could I get a bit of info?

I'd like to know what that 22 Ohm resistor and the auto bulb each read on your inductance meter. Also, could you measure what the hot resistance is of the bulb? The "hot" resistance would be soon after disconnecting it from the circuit and measuring with your Ohm-meter.

One last thing, is the bulb the first thing in series with the + side of your storage capacitor for all your tests so far?

[Edit] I also assume that the 22 Ohm in series with the MOSFET Source lead is no longer in-circuit and not being used?

Thanks,
.99

[TinselKoala]

Humm, I don't think a magnet has inductance but I think you maybe referring to the coil. Like I said in the video, the magnet does not affect the effect. It is just used as a visual aid as to be able to calculate change when changes have been made. Anyways, I've tried it also without the magnet and it shows actually a very small efficiency.

You may think that! but not according to my tests. I even have that test on video! you may have missed it. Here it is again: http://www.youtube.com/watch?v=HpaP__5Kd38

Just make sure your 555 timer circuit is the one that gives you 10% or less duty cycles

Luc

Thanks, Luc and Gyula.
Hmm...I see.
But there is a secondary lifting effect that occurs when AC is applied to a coil and a (non-magnetic) conductive surface is placed above it--eddy current levitation, which can be quite strong. I wonder if the eddy current levitation due to the AC is adding to the normal electromagnet-permanent magnet repulsion.
As you turn the frequency down and down, approaching DC, does the height of the magnet vary, at the same input power? (Hmm--lock-in amplifiers, positive feedback loop, phase-locked loop, optical sensing, HPIB data logging and experimental control....might be interesting to explore....hmmm.......)

And I see that the low permeability of the magnet shouldn't affect the coil's inductance...but surely the magnetic field of the PM must be interacting with the coil's field in some manner. Besides simple repulsion or attraction I mean.  I mean, an increase in permeability means basically that more field lines are in there; sticking a ferromagnetic core increases the flux density (increases inductance) within the coil by providing a high permeability path for the field... the opposing fields of the coil and PM surely change the flux density around and perhaps in the coil...?

I think it's an interesting feature, regardless.

And I agree with an earlier point of Luc's : If the bulb brightness is kept quite low, eyeballing its relative brightness will be more accurate than it is at brighter light levels. I'm still thinking about instrumenting this measurement, though.

In fact it should be possible to construct a simple feedback loop that would vary input peak current to the bulb to keep the bulb at a constant repeatable brightness; then the bias current would be the dependent measure.

Hmm--looks like I've had too much coffee.

(adjourns for medication adjustment.)