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Author Topic: Cooling fan (Patent 4757547)  (Read 10695 times)

John Roberts {JR}

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Re: Cooling fan (Patent 4757547)
« Reply #10 on: September 14, 2006, 01:34:12 am »

Grant Rider wrote on Wed, 13 September 2006 23:18



I think you are right there. The air must be cool.

The fan isn't needed because the air blasts from the cone are strong. Using the cone as the pump makes the fan not needed. There is no pressure differential offset problem that way either.



If you go back to the patent search website and look in the same category or patent class as the one referenced above you will probably find a long list of other related patents. This is not a new problem and smart folks have been working on it for years.

JR
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Al Limberg

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Re: Cooling fan (Patent 6837333)
« Reply #11 on: September 14, 2006, 09:27:15 am »

From my understanding (and I am NOT the resident expert!) only two complete Air Force Systems were sold.  The cabinets were quite large and very heavy and VERY loud and VERY expensive.  As I recall Community also went thru two different ways of powering the cooling fans, initially running them from the actual power amp signal and then running them from a seperate power supply running thru pins 7 & 8 of an NL8 connector.

Al
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Tom Danley

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Re: Cooling fan (Patent 4757547)
« Reply #12 on: September 15, 2006, 12:15:36 pm »

Hi

The power cooling trick in the patent you mention was used on all Servodrive BT-7’s and most SDL-4 and SDL-5’s and proved to be very effective.
It can also be applied to a normal loudspeaker if configured as shown in the patent.
Intersonics inc, the company I worked for at the time licensed the patent to Bond acoustics in Canada who was headed by Steve Hutt, now director of R&D at Harmon Motive.
While I was prohibited from using “conventional drivers” most of the time, Steve applied it to a range of products and got more than a factor of two in increased RMS power capacity.
As the power to the fan tracks the input power, it is not difficult to make the fans operation inaudible.

It is important to do the venting as show or risk cone offset proportional to air pressure.
On the other hand, if the air is withdrawn from the VC pocket, then the cone is not affected.
Also, it doesn’t take much flow past the coil to cause a large increase in PHC.
Keep in mind that the “un-glued” temp in a modern VC is around 300 – 350 deg C. that is darn hot and any net flow across that coil causes a nice temperature drop.

In the patent drawing, is shown the power cooling applied to a conventional VC driver, note where the air is drawn out.  
Attached is a photo from the “Olden days” back at Intersonics (around the time I applied for the patent).  
I was demonstrating a switching amplifier and control circuitry on a subwoofer at the time.  
In the foreground was the Eminance 12 inch driver I had modified to add power cooling.
This driver (like most) did not have access to the VC pocket, I drilled a pair of holes and pressed in he copper tubes sticking out the back.
I am on the far left side and John Halliburton (a lab poster) is on the far right.
Hope that helps

Tom Danley
index.php/fa/5880/0/
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Grant Rider

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Re: Cooling fan (Patent 4757547)
« Reply #13 on: September 15, 2006, 03:55:03 pm »

Thank you for responding.

I hooked up a fan and had problems just blowing air so I took it off. Maybe I will try this again. Maybe if I drill holes like you did and use different size holes or two different fans or something I can make it work. I will try with a cheap woofer first.

The Community patent talks about the problems from pressure difference in your patent. Your patent shows a pressure side and a return side but it does not talk about matching force using different pressures through different size holes. The places air goes through are different so the drag force will be different.

What can be done to balance pressure? The area in front is different than the area in back. It has to be adjusted somehow so the force from the vacuum pulling equals the force from the pressure pushing. I think the vacuum against the gap must be much stronger than the positive pressure against the inside of the cone. The vacuum air pulls through the voice coil gap which drops pressure and reduces force. This makes them unequal. Maybe blow through a tiny hole same area as the gap to match pressure? That way the back vacuum sucking through the gap will be the same as the front pressure blowing through this tiny hole?
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David Trotter

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Re: Cooling fan (Patent 4757547)
« Reply #14 on: September 15, 2006, 04:39:27 pm »



Seeing as the cone movement is sucking and blowing air in and out of the vent with every push pull, would you not benefit from using a fan blowing across the rear magnet to ensure that the hot air comming out was blown away and replaced with freash air?

has anyone tried cooling with water? what about valves which makes sure the air flow is unidirectional?

I do believe the benefits stated above. Just loading the driver backwards in the BPH horn i designed has meant we can put double the power in and the magnet only gets luke warm.

i for one look forward to day when you can use high-temp superconductors in your voice coil. Very Happy

-dave

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Tom Danley

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Re: Cooling fan (Patent 4757547)
« Reply #15 on: September 16, 2006, 09:06:36 pm »

Hi Grant, all

The issue of cone offset is a hydraulic one which can be solved as this approach was used on conventional drivers under license.

The cone off set due to pressure is proportional to the pressure per area times the area the pressure is acting on.  If one simply tries to blow or suck air from the center pole vent, then one has some pressure acting on a piston the diameter of the VC.  
By using an air rectifier to “self pump” a net flow,  this tends to be highly asymmetric and “not good” so far as distortion or Qm. One can pump air both ways and that helps.

By drawing air from the rear cavity as I had done in that old old photo, one leaves the vent open and air flows to the VC gap thru the spider and through the center vent.  
Here, the total piston area  is only the VC od area minus the VC ID area (small).
I would suggest you put the fan inside the speaker rear volume, this way you will have no differential pressures on the cone etc.

Other thoughts.
Air is a decent heat insulator and is used in homes (trapped in fiberglass and in foam) but at the same time, once heated, has decent heat energy per volume.
The voice coil, is something like the filament in a light bulb, it is depending on radiant heat which only becomes significant at high temperatures and some conduction through the air gap.
While the magnet can get “hot”, keep in mind the part that matters, can get to 500 –  600 degrees at failure.
Cooling the magnet is somewhat like trying to increase the power dissipation in the light bulb filament by immersing the glass in water.  Yes you can keep it cool and increase the power some, but you are not cooling the part that maters.
Moving air past the “hot part” is very effective and routinely used on many gasoline engines now and in the past.   High performance DC servomotors have been air cooled for the same reason, they have a wire armature suspended in air, within a magnetic field.
In that application, where the armature is limited to 150 deg C, one can nearly quadruple the continuous power dissipation with air cooling.   This, done by only passing air over one side of the armature, not both sides (inside and outside) as in the VC cooling patent drawing.

Try the air path as shown in the drawing, set up a water tube manometer so you can monitor pressure at the VC rear cavity.
Use a shop vac (normally has a universal AC/DC motor) with a Variac to adjust he speed /pressure.
Once you have a few inches of water pressure (the net flow that induces) on the rear pocket, you should have increased the heat capacity significantly.
After that, it is a matter of getting the heat outside of the box in a way that doesn’t interfere acoustically.
Hope that helps,

Tom Danley

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Wayne Parham

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Re: Cooling fan (Patent 4757547)
« Reply #16 on: September 17, 2006, 01:04:47 pm »


There is no doubt that forced air cooling will do some good.  There is also no doubt that other methods of cooling will do some good too.  Heat can be transferred by any of the three methods, conduction, convection and radiation.

Forced air is employed in the cooling vent, using the speaker motor and cone as the pump.  A fan can be employed, although it is difficult to do it without offset.  Whether positive or negative pressure is used, an offset is possible (probable).  One has to find a mechanism to ensure the forces are equal while at the same time introducing unidirectional flow.  It is possible, but it isn't automatic.  Pulling a vacuum on the gap rather than pressurizing the area behind the cap intuitively seems like it would produce less offset, but there still would be a pressure differential.  My solution was to allow airflow through the gap to move back and forth, but then to force the air passing through into a heat exchanger.  This was done by a device that produced unidirectional flow in the cooling path after the voice coil so no pressure differential existed across the cone.

As for the temperature of the pole piece and magnet, one definitely wants to keep it cool.  Radiated heat transfer is just as legitimate a method of cooling as any other.  It is the one that has been most overlooked by speaker manufacturers and so arguably where the most improvements can be found.  Cooling vents already do a lot to help where forced air is concerned, but when the magnet is 200
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Michael Hedden Jr.

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Re: Cooling fan (Patent 4757547)
« Reply #17 on: September 17, 2006, 07:53:55 pm »

Didn't I see this in a Jason or Freddy movie or maybe it was that classic "Race with the Devil" where everyone screams run, run for your lives, it's back, the attack of the cooling plug thread!  

Mike Hedden
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Phil Pope

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Re: Cooling fan (Patent 4757547)
« Reply #18 on: September 18, 2006, 11:12:19 am »

i have tried this cooling fan on a woofer I have

http://www.eighteensound.it/index.aspx?mainMenu=view_product &pid=203

which already has a ring of vent holes behind the voice coil.

I got a cheap rectifier and some capacitors to make the circuit and tried using a cheap 12V computer fan. cut a tube from some 1 3/4" aquarium pipe that fits snug into the pole vent and made a little wooden box to house the fan, components and duct to the pipe.

I have as yet been unable to demonstrate a detectable reduction in power compression.

this is either because

I am not running at a high enough level for fear of damaging my fan as I don't have proper means to power it. Danley's patent shows a resistor to limit the power taken by the fan circuit. this would need to be something like a 50W lightbulb. I am thinking of using a transformer but am having difficulty finding a suitable one locally.

the driver I am using is designed to pump air through itself so it is hard to achieve additional air cooling

the fan I am using is too small 56 cubic feet per min. where do I get high powered fans? all the vacuum cleaners I have looked in are AC motors. I tried looking in my girlfriends hair-dryer and it LOOKS like an AC motor but I think if i take it to bits to check I might get hurt Rolling Eyes

I have tried sticking my shop vac onto the backs of the 18 sound driver which gave relatively little diaphragm movement and a Ciare 12.00SW which moved to about half of xmax. will try on a Lab12 when I open one of the boxes up.

will let you know of any success.

cheers
Phil
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Tom Danley

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Re: Cooling fan (Patent 4757547)
« Reply #19 on: September 18, 2006, 02:18:52 pm »

Hi Phil

Nice driver, I don’t have one of those but you might check to see “IF” that driver also has vents on the top plate (under the spider).  These holes (that usually accompany the ones on the rear)  allow air to by pass the critical flow area, the gap.
Remember the key to this is making the air path involve passing between the magnet and Voice coil, as in the drawing, it does so on the inside and outside.

If you have an “old” or sacrificial driver available, you can drill into that rear cavity area like I did using a vacuum cleaner running, next to the bit while drilling.
If you do this to a LAB-12, I would find out the magnet ID and drill through as far from the center pole as possible (as this area is likely very High B already).
I would not use hose smaller that say 1 /2 inch ID as the pressures are low and there is net flow.

I would set up a test like I described before.
With air flow, like voltage or sound radiation, there is an “impedance” issue. With DC it is the ratio of voltage to current, with air or “what is the right fan?”, it is Volume to Pressure ratio.
Since you don’t know what you actually need in the beginning, you also need some simple “air” tools, an air meter and variable air power supply.

Make up a water tube manometer so you can keep an eye on what pressure is / does.
This can be as  simple as a  “U” clear plastic tube with some water in it, taped to a yard stick to read from, the measure is in “inches of water” pressure.    
One of the tube is open (or connected somewhere if you want to measure a differential pressure) and the other end goes to your pressure measurement point (at the duct coming out of the speaker).
To start with, use a shop Vac or vacuum cleaner, these nearly always have a universal motor and are perfectly happy running on a variac or variable DC supply.  
This (a variac on a shop vac) gives you an easy to control source of vacuum from none up to about 40 – 60 inches of water depending on the vacuum motor.
You are not dealing with high pressures here, 26.9 inches of water pressure is only 1 psi, you will get good results at less than 1 / 10 of that.
At least in the speakers that were in production using this approach, none had problems with offset.
If I am not mistaken, they used something like a hand held vacuum motor (small DC) and with pink noise had about a 3X increase in capacity over “un cooled”.
Have fun, keep me posted on your results.
Best,

Tom
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