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Old 27th September 2007, 09:23 AM   #11
el`Ol is offline el`Ol  Germany
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http://home5.inet.tele.dk/f-hammer/index.htm
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Old 27th September 2007, 09:45 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
it looks like that site has not been updated for at least a couple of years.

I had a look at copper and found that he is predicting about 2.4A for 2T. He has 1000turns and DCR of 3r1.
That is about 17W to be dissipated. This coil is going to get pretty warm. A big heatsink bolted to the back end will help keep the iron cooler, but the coils are pretty big and internal temperatures are going to be a lot higher than surface temperatures. =max volume of copper.

It is also obvious that the critical part of the iron circuit is right next to the gap. Elsewhere he appears to be using at least twice the area of iron in the magnetic circuit to keep iron flux below 1T.
It's a bit like a resistive circuit passing current. keep the resistance in the cables low and allow the main resistance where the power is to be used. here the flux is replaced by current, the area of iron is area of copper cables and flux density is current density. The gap is equivalent to the resistor dissipating the power. So concentrate the flux into the gap and make the rest of the circuit low flux density (= low resistance).

If we take that a little further, it may be that the central pole piece and the top plate use the best flux carrying iron and that the backplate and outer annular ring can be any lower quality steel that can easily pass the flux density in those areas. You can see that these two components can be made any thickness without impinging on the volume where the copper coil is. I think that copper volume is the critical design parameter that determines the achievable flux density in the gap.

Thanks to el`Ol for the link.
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Old 27th September 2007, 10:12 AM   #13
Geoff H is offline Geoff H  Australia
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I could get shot down for this. I think the more flux the better. Recent experiments I have done indicate the magnetic circuit of many speakers is not saturated, ie it will carry more flux.

Adding an additional magnet on some full rangers provided an extra 3db in sensitivity across its range. I can show curves to back that up.
Transient response is also improved. according to my ears.

Transformer laminations and cores in old 3000 type relays were silicon iron. From memory, the core in those relays is only about 5/8" dia. OK for high freq drivers, not much use for bass use.

Other sources could be starter solenoids from cars, and the rotor from an AC synchronous motor.

To get maximum flux, we need to fill the gap with the right no of turns of the right gauge. More turns of smaller gauge will increase resistance needing a higher voltage, and increasing losses.

Using less than optimum material for the poles/circuit could be offset using thicker plates. The centre pole piece will be the limiting factor.

Melting down existing materials could be a challenge. Back in the factory, the alnico mix was brewed in a high frequency furnace. No flame to add carbon. Also the varnish on the laminations would need to be disposed of.

Modifying existing drivers could be interesting. I don't think I could carve up a Goodmans or Wharfedale, but maybe a Richard Allen.

I might hide for a while. LOL

Edit: "It is also obvious that the critical part of the iron circuit is right next to the gap."

Yep, that's where it's needed.
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Old 27th September 2007, 10:23 AM   #14
Geoff H is offline Geoff H  Australia
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The power dissipation is probably not a big issue. In the old console radios I dragged home as a kid, the large openback cabinet also had a chassis with 5 or 6 octal tubes. The field coil doubled up as the B+ filter choke. The magnets ran warm, but not hot.

Did I have fun at show and tell in 6th grade!
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Old 27th September 2007, 12:47 PM   #15
OzMikeH is offline OzMikeH  Australia
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Here is what prompted the original question.

http://www.fullrange-speakers.com/eng/treiber-e.htm

According to that page: Variable field = variable Qts

I don't understand the comment "More turns of smaller gauge will increase resistance needing a higher voltage, and increasing losses"

Does it matter?
Assuming the magnetic circuit is sufficient the amount of power (Watts) applied should be approximately proportional to feild strength whether through many turns high voltage or few turns high current.

I'd love to see a cross sectional drawing of that feild coil, is it almost completely full of copper wound on the pole piece?
Is there 2 windings inside, one on the pole piece and the other wound the opposite way on the inside of the outside cylinder?

This thing is huge, What is the limiting factor for the size, heat dissipation or the amount of iron needed for the magnetic circuit around the outside?

Sorry for the novice questions, I just can't see why an electro magnet has to be 5 times the size and 10 times the price of a permanent magnet.
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Old 27th September 2007, 01:21 PM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by OzMikeH
I don't understand the comment "More turns of smaller gauge will increase resistance needing a higher voltage, and increasing losses"
more turns requires less current but has more resistance so the heat stays roughly the same. If you do the full calculation I think you'll find that the Watts stays the same if the volume of copper stays the same.
Quote:
Originally posted by OzMikeH

Does it matter?
Assuming the magnetic circuit is sufficient the amount of power (Watts) applied should be approximately proportional to field strength whether through many turns high voltage or few turns high current.
But push in more current and you get more AT and thus more flux in return for more heat.

Quote:
Originally posted by OzMikeH

I'd love to see a cross sectional drawing of that feild coil, is it almost completely full of copper wound on the pole piece?
Is there 2 windings inside, one on the pole piece and the other wound the opposite way on the inside of the outside cylinder?
It will probably be 10layers with 100turns per layer or something similar that gives 1000turns in total. Basically the space is crammed full of neatly wound copper to minimise the power dissipation and maximise the AT.
Quote:
Originally posted by OzMikeH

This thing is huge, What is the limiting factor for the size, heat dissipation or the amount of iron needed for the magnetic circuit around the outside?
The limiting factor are the losses around the magnetic circuit. This depends on how close each material in the circuit gets to it's inherent flux density limit. There comes a point for each material that the flux will not increase any further. Even reaching that point where the flux density vs AT curve has flattened out the copper will have probably reached the insulation failure temperature. That's the second limiting factor.
Quote:
Originally posted by OzMikeH
I just can't see why an electro magnet has to be 5 times the size and 10 times the price of a permanent magnet.
the cost of labour in accurately machining all those parts. The labour in assembling it. The capital investment in turning and winding machines. Buying in the specialised/expensive materials. The expertise in designing and developing the product for tiny sales volume. Compare that to moulding a ferrite block and zapping it with a high flux magnetic field. No contest.

But then ceramic cannot approach 2T
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Old 28th September 2007, 07:09 PM   #17
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Quote:
more turns requires less current but has more resistance so the heat stays roughly the same. If you do the full calculation I think you'll find that the Watts stays the same if the volume of copper stays the same
Are you sure?

The dissipation in the coil P = I^2 x R. For the same AT product, if you double the amount of turns this more or less doubles the resistance and cuts the current in half. Thus the dissipation will be reduced by a factor of four.

I also came across Finn Hammer's site and dug up the original drawing from Voigt's patent:
Click the image to open in full size.

This construction is supposed to minimize flux leakage and maximize the saturation at the airgap

The elements of this construction can be found in the
Lowther PM4A

If you take a look at the PM4A you'll see that they use permendure (2.4T saturation) only for the top plate and the pole piece.
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Old 4th October 2007, 08:45 PM   #18
Geoff H is offline Geoff H  Australia
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Some info on armco and steel:
www.key-to-steel.com/Articles/Art3.htm

and a source of armco, including 80mm rod:

http://www.goodfellow.com/csp/active...Condition.HTML

One could machine the assembly in only 2 pieces, keeping losses down.

Geoff.
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Old 16th November 2010, 01:56 PM   #19
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The goodfellow link appears dead. Any one know of another source?
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