DIY Overhead crown guard with dust extraction.

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DBT85

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If a loaded motor uses less energy then set it to drive a generator and get free energy!
I'm not sure I get you Jacob?

If you cover the inlet to an impeller the load goes down, not up. The impeller is at that point spinning in a vacuum (as any air in the impeller housing is pretty much blown out of the outlet) with no pesky air getting in the way of the blades creating resistance.
 

Jacob

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I'm not sure I get you Jacob?

If you cover the inlet to an impeller the load goes down, not up. The impeller is at that point spinning in a vacuum (as any air in the impeller housing is pretty much blown out of the outlet) with no pesky air getting in the way of the blades creating resistance.
If there was a vacuum in the housing (there wouldn't be but there might be lower pressure) it would be working against the flow of air being impelled out and the impeller would encounter more resistance than if the thing was open both ends. If you put your hand over the inlet or outlet of a vacuum cleaner you can hear the motor using more power and speeding up. So much so that if you left it blocked the motor might overheat and/or the current blow the fuse.
It's very basic physics. If it's Bill Pentz's theory then he's probably wrong about everything else as well! Which is a relief as I wasn't looking forward to wading through his dense and long-winded output and I'll forego the pleasure!
 
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DBT85

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If there was a vacuum in the housing (there wouldn't be but there might be lower pressure) it would be working against the flow of air being impelled out and the impeller would encounter more resistance than if the thing was open both ends. If you put your hand over the inlet or outlet of a vacuum cleaner you can hear the motor using more power and speeding up. So much so that if you left it blocked the motor might overheat and/or the current blow the fuse.
It's very basic physics. If it's Bill Pentz's theory then he's probably wrong about everything else as well! Which is a relief as I wasn't looking forward to wading through his dense and long-winded output and I'll forego the pleasure!
I do not believe you are correct Jacob. Once covered, the housing has expelled any air inside and the pressure is lower, if the pressure is lower there is less resistance to the blades and it is that resistance that increases the load. The more air getting to the impeller that harder it has to work to move that air out of the way.

I am happy to be proved wrong, however. I'll provide my own numbers by the weekend for anyone curious.

Here is Mattias Wandel showing you the current draw on a centrifugal blower (as used in these extractors) dropping when the inlet is covered, from 2:30 on the video. The fan also speeds up which is more noticeable if you wind back a bit. Only in that case becase the impellor was so large that the motor couldn't spin it at the correct speed I believe.



I do not know if the same applies to ordinary vacuums, though the issue with those is that with blocked airflow the motor can no longer be cooled and so will heat up. Not the case on extractors as the motor is not cooled by the air it is moving.

I'll try and take my own readings of both for the purposes of science though maybe what appears basic physics isn;t quite that straightforward after all.
 
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Doug71

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As I understand when you block the end of a vac hose with your hand it sounds like it's working harder but it's not, it's just spinning faster and freely because it isn't having to move any air. This can cause them to burn out because they are not designed to spin that fast plus there is no air cooling the motor. Some of the better vacs like Festool have a bypass built in so the motor is still cooled even when the hose is blocked.
 

Jacob

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As I understand when you block the end of a vac hose with your hand it sounds like it's working harder but it's not, it's just spinning faster and freely because it isn't having to move any air. This can cause them to burn out because they are not designed to spin that fast plus there is no air cooling the motor. Some of the better vacs like Festool have a bypass built in so the motor is still cooled even when the hose is blocked.
Just checked - I am right and wrong! :mad:
A brushed motor speeds up under no load and vice versa.
An induction motor doesn't - stays close to design speed 1500 or 3000 rpm most common. More load takes more power.
Most dust extractors are induction motors, well mine is anyway (bigger motor but cheaper and quieter more reliable). Most vacuum cleaners (and hand electric tools) are brushed motors (smaller and lighter) so I was wrong about that. Oh well thats what being on a forum is all about!
 
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Sideways

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Just to confirm, here's the current draw on my vacuum running with nothing attached
20210324_104828.jpg

And despite the increase in speed / noise, the current goes DOWN when you block the hose. (at least until it overheats from lack of cooling).
20210324_104856.jpg
 

DBT85

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Just checked - I am right and wrong! :mad:
A brushed motor speeds up under no load and vice versa.
An induction motor doesn't - stays close to design speed 1500 or 3000 rpm most common. More load takes more power.
Most dust extractors are induction motors, well mine is anyway (bigger motor but cheaper and quieter more reliable). Most vacuum cleaners (and hand electric tools) are brushed motors (smaller and lighter) so I was wrong about that. Oh well thats what being on a forum is all about!

More load on an induction does take more power. Never in question. Just a matter of what "load" is.

This is what I like about a forum, a spirited discussion about something interesting (to us).
 

Inspector

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I had a look at Bill Pentz's site but he appears to be another over enthusiastic enthusiast, a bit like a sharpening nutter - masses of dubious experiments and observations generating a huge fog of vague mis-information.

Jacob you are one that will never understand dust and it's dangers because you don't want to make the effort to learn and that's okay for you. Telling everyone far and wide that Bill's information is wrong is a danger to newer woodworkers. You're "knowledge" of dust collection is rooted in the distant past and because you have always been doing things a certain way you are not open to learn that it may be wrong and can be improved. What Bill said on his site about fine dust a couple decades ago is being realized in todays regulations. The EU standards for dust are among the most stringent in the world now and are magnitudes lower than allowed when you got your start. When Bill came out with his information he got dumped on by lots like yourself including duct collection manufactures. Funny thing is that many have copied his cyclone because it is that good. The only company that makes his cyclone and is decent enough to pay him a royalty is Clear Vue. The rest just poached his work. His work has been verified and confirmed by others and it isn't, as you believe, "masses of dubious experiments and observations generating a huge fog of vague mis-information."

Someday you will be in a bed and you can proudly say "I never.... cough.... bothered with.... cough hack.... fine dust collection.... because it gasp hack cough cough.... isn't problem....harrk cough.... and isn't needed. Cough hack gob spit gasp.... where's my oxygen?"

Pete, a nutter.
 

Jacob

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Jacob you are one that will never understand dust and it's dangers because you don't want to make the effort to learn and that's okay for you. Telling everyone far and wide that Bill's information is wrong is a danger to newer woodworkers. You're "knowledge" of dust collection is rooted in the distant past and because you have always been doing things a certain way you are not open to learn that it may be wrong and can be improved. What Bill said on his site about fine dust a couple decades ago is being realized in todays regulations. The EU standards for dust are among the most stringent in the world now and are magnitudes lower than allowed when you got your start. When Bill came out with his information he got dumped on by lots like yourself including duct collection manufactures. Funny thing is that many have copied his cyclone because it is that good. The only company that makes his cyclone and is decent enough to pay him a royalty is Clear Vue. The rest just poached his work. His work has been verified and confirmed by others and it isn't, as you believe, "masses of dubious experiments and observations generating a huge fog of vague mis-information."

Someday you will be in a bed and you can proudly say "I never.... cough.... bothered with.... cough hack.... fine dust collection.... because it gasp hack cough cough.... isn't problem....harrk cough.... and isn't needed. Cough hack gob spit gasp.... where's my oxygen?"

Pete, a nutter.
OK could be wrong! It's just all those pages of rambling stuff - he needs an editor.
Heart and lungs OK - I do take dust seriously. But dust generation for me is very intermittent - even more so now I'm retired. I've got a big extractor and a Triton vax/extractor with all the filters
Bigger problem has been hearing - I also wear ear muffs, but both my parents went deafer than me but neither of them worked in a noisy environment.
I also have all my fingers, thanks to two push sticks mainly!
As far as I know Pentz's cyclone is not a new idea - it's been in use with variations for a long time, particularly in grinding mills for flour etc. So I guess he couldn't patent it - he could only copyright his particular designs to some extent.
 
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Inspector

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Bill took the agricultural cyclones and added the inlet guide ramp and optimized the size and lengths for smaller woodworking shops. He may have made enough changes to qualify for a patent but you would have to take out worldwide patents and be prepared to defend them in court. Not practical for a pensioner to do for a product with a limited market.

Yes it is a long winded site but it was the norm 20 years ago when people had an attention span. It is repetitive because many, possibly like yourself, hop in for a short read of a specific subject and the important repeated information may get taken in so the info is in context. There is a cyclone design spreadsheet free for anyone to use that wants to make their own cyclone that prints out the cut patterns. It sizes it to match the impeller and motor power, the 3hp being a bigger diameter than the 5hp for instance. I'm sure Bill would not object to someone helping him upgrade/modernize/edit his site but I doubt anyone is lining up to do it so it will remain for the most part as is.

Don't get me wrong I respect your knowledge and abilities but you are known to spout off your opinions as fact when maybe you should be asking questions and doing a little research.

I'm done now as we have got off the topic that started this thread.

Pete
 

Jacob

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Bill took the agricultural cyclones and added the inlet guide ramp and optimized the size and lengths for smaller woodworking shops. He may have made enough changes to qualify for a patent but you would have to take out worldwide patents and be prepared to defend them in court. Not practical for a pensioner to do for a product with a limited market.

Yes it is a long winded site but it was the norm 20 years ago when people had an attention span. It is repetitive because many, possibly like yourself, hop in for a short read of a specific subject and the important repeated information may get taken in so the info is in context. There is a cyclone design spreadsheet free for anyone to use that wants to make their own cyclone that prints out the cut patterns. It sizes it to match the impeller and motor power, the 3hp being a bigger diameter than the 5hp for instance. I'm sure Bill would not object to someone helping him upgrade/modernize/edit his site but I doubt anyone is lining up to do it so it will remain for the most part as is.

Don't get me wrong I respect your knowledge and abilities but you are known to spout off your opinions as fact when maybe you should be asking questions and doing a little research.

I'm done now as we have got off the topic that started this thread.

Pete
All right all right I'm reading Mr Pentz!
Just hit on why a reduced pipe is not good branching into a larger pipe diameter - the reduced pipe reduces air flow and you get pile ups in the larger pipe. Thank you mr Pentz; its obvious when you think about it! That's why the small crown guard port needs it's own HPLV extraction. That'll do me for one session! Took a few thousand words to get that far
 
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Woodmatt

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I do, well kind of.

I have a Record DX 4000 on the bottom of my saw (think it's actually classed as hplv but it takes a 4" hose). When I split it to go to crown guard the extraction really seemed to suffer, I had a spare shop vac kicking around so put that to the crown guard instead. They both turn on together with remote switches, works really well.
Thats exactly the set up I have and it works pretty well but I'm never sure if each extractor is working against the other
 

Sideways

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Good to know :)

I have HVLP under the table on mine but I'm convinced by the theory and I'm (slowly) building a custom crown guard that will use my shop vac to create a concentrated suction at the front edge of the blade where it normally spits right out at you !
 

Jacob

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..... I'm never sure if each extractor is working against the other
I doubt the top extractor would have any effect on the bulk extraction going on below the table. Too far away, with a gap and the saw insert pate between them. The top extractor just catches the finer dust which the bottom has failed to pick up.
 
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