Central Vac overload

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Oh, yeah, you could also wire this up so that a relay cuts off the motor, and locks out, AND sounds an alarm.
--
EA



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<stuff snipped>

This is one of those things that I am going to have to check out for myself because it is so counter-intuitive because the motor whine gets so loud when it's blocked. But now that you mentioning, a laboring motor *shouldn't* speed up at all, so that fact alone means you're mostly likely correct that I'll see a drop in amperage.

There are a number of possible issues, from problems with the automation system that controls the vacuum, to the motor heat overload to an issue with the "smart outlets." My original concern was based on what I thought was a thermal overload kickout, but when I sealed up the unit I left plenty of room for airflow and even put in a small fan to make sure nothing overheated. I realize now I ran those tests during an average vacuuming session that didn't include the occasional serious clog, so I will have to see what happens when I deliberately block the hose and monitoring the motor temperature.
Most of this has been rendered moot by Art T's location of a ready-to-go valve system designed to detect such problems and remedy them.
http://www.thinkvacuums.com/beam/valve.html
Thanks again, Art. (And thanks to you, EA, too.)
-- Bobby G.
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It is very possible that the power unit uses the filtered air from the vacuum as cooling.
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Yep, when I tested the output flow with my hand, a clog caused the airflow to drop steeply and heat up quickly. Very likely hose air used for cooling.
-- Bobby G.
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Robert Green wrote:

Is that the output from the vacuum or the motor cooling?
With no air flow, the air in the blower will warm up as it sucks up the motors power, even without a motor being inside.
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I would be sure there would be a really high current even without any current flowing.
Huh?
With no air flow, the air in the blower will warm up as it sucks up the motors power, even without a motor being inside.
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wrote in message <stuff snipped>

Exactly correct. Very odd to see, but about 4A less when screaming like a rocket.
-- Bobby G.
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You may find the current doesn't drop significantly as the power consumprion does.
When load is removed from sysncronous motors the power factor decreses to the bottom, the current stays relatively in the same ball park, depending on motor design.
If you put a Kill-a-Watt monitor on it you may a huge difference in consumption.
wrote in message <stuff snipped>

Exactly correct. Very odd to see, but about 4A less when screaming like a rocket.
-- Bobby G.
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Is it possible that your "silensing" cabinet is restricting the cooling air to the top of the motor? Wasn't your central vac in the garage? Do you really need a silencing cabinet?
You're right about the overload safety switch. It is not designed to operate repeatedly.
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<stuff snipped>

<Is it possible that your "silensing" cabinet is restricting the cooling air to the top of the motor? Wasn't your central vac in the garage? Do you really need a silencing cabinet?>
I ran extensive tests, but not with the hose plugged! I will go do that road if Art T's solution fails for some reason. As someone pointed out, restricting air into the unit because of a clogged pipe could cause an overheat if the unit depends on "vacuumed in" air for cooling.
<You're right about the overload safety switch. It is not designed to operate repeatedly.>
Learned from experience. Once worked in a building with an electric furnace in the space above the suspended ceiling that clicked an awful lot when running. Turns out it was miswired so that a Klixon was operating as a limit switch, hence all the clicking. One day, the thermal overload stopped operating as a limit switch or a thermal protectory and the furnace burned up, along with a lot of other stuff. Turns out there was a defective sail switch, as well as some other issues, and a situation ensued where the unit called for heat, but not fan. The fire marshal commented on how deep the alligatoring was in the beams directly above the unit and wondered why the unit didn't drop down to the floor below. We even made the evening news!
So yes, don't use a thermal shutoff as a switch.
-- Bobby G.
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On 3/30/2010 7:23 AM, Robert Green wrote:

If you are using it and suddenly there is no or greatly diminished vacuum wouldn't that be a sufficient indicator that there is a problem?
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On 3/30/2010 10:45 AM, George wrote:

operated beater brush, you really can't hear the loss of suction over the motor noise. I know, both me and my wife have had it happen in our old house. What can alert you is that you have to go over the same spot several times to pick up the one little crumb.
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First the OP must see if the blocked hose speeds up or slows down the motor, and see if the current increases or decreases.
I like the suggestion of Mr. Todesco that puts a vacuum bypass on the inlet to the motor/vacuum pump. That guarantees you will not damage the motor unless the spring in the bypass suddenly gets stronger or the diaphram jams with dirt. That is unlikely to happen. If the current does increase, a loop of wire through a ferrite core, with a second coil of many turns on the same core can generate an increased voltage when the current thru the motor increases, if that is what is happening. That increased voltage could be used to do any number of things, but that is for a different group (sci.electronics.repairor misc).
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wrote:

<First the OP must see if the blocked hose speeds up or slows down the motor, and see if the current increases or decreases.>
Which I may do despite ordering the device that Art T. pointed me to
<I like the suggestion of Mr. Todesco that puts a vacuum bypass on the inlet to the motor/vacuum pump. That guarantees you will not damage the motor unless the spring in the bypass suddenly gets stronger or the diaphram jams with dirt. That is unlikely to happen. If the current does increase, a loop of wire through a ferrite core, with a second coil of many turns on the same core can generate an increased voltage when the current thru the motor increases, if that is what is happening. That increased voltage could be used to do any number of things, but that is for a different group (sci.electronics.repairor misc).>
I have a home automation controller (Ocelot) that's able to sense varied voltage conditions and take an action. That part's pretty easy. Well, at least for me. Someone without such a controller would have more difficulty.
Determining what conditions variable current levels represent will be a little trickier. A small "sail switch" in the output pipe that could detect the lack of strong air flow might work well, too. But Art's relief valve seems to be the most cost-efficient solution so far. If that fails (or succeeds) I'll come back and make a report. I'm hoping that I'll be able to report "problem solved" with.
Thanks for your input!
-- Bobby G.
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<stuff snipped>

Thanks, Art, for summing it up so well. I wouldn't have asked the question if it wasn't a problem. I was downstairs, right near the vac cabinet so I could hear the motor noise change, but my wife, vacuuming upstairs, didn't.
Based on some other comments, I need to do some testing, especially current draw, to see what's really happening when the vacuum becomes clogged.
-- Bobby G.
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Current sensing (for slightly low current, fans spinning air take less power than ones moving air) is one way but is likely to be more expensive than a vane on a switch to watch for no air flow on the exhaust. Another possibility is to look for a lower pressure (higher vacuum) just ahead of the main unit.
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He's been told, many time here but doesn't seems to understand that.
Current sensing (for slightly low current, fans spinning air take less power than ones moving air) is one way but is likely to be more expensive than a vane on a switch to watch for no air flow on the exhaust. Another possibility is to look for a lower pressure (higher vacuum) just ahead of the main unit.

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Well, you see, the house is infested with bats and though we're trained the werewolf puppies to use the vacuum cleaner to suck up the dead bats and not eat them, the bats are just the right size to plug the house. The little werewolf pups are still learning, so they can't tell when the damn thing gets clogged. Then the forget their training and eat the bats when they don't go down the tube, get rabies and begin to drool all over the carpet from hydrophobia so the that agitates Cujo, who comes and fights with the werewolves and all Hell breaks out. So you can see why it's a problem.
The serious answer is that with a motorized head, there's sufficient sweeping action to appear as if the vacuum is sucking things in. Couple that to the "silencer" cabinet I built around it, it actually IS hard to tell when vacuum is lost and the motor is making a high pitched noise - which could indicate that I already have the relief valve that I am thinking of buying and should test to see if the outlet air is still flowing when the hose is clogged. That would indicate that the unit itself has opened an internal bypass valve. Good work, George. You may be on to something . . .
Anyone still reading: Why DOES the motor RPM increase quite noticeably when the hose is clogged. Does an internal vacuum relief valve sense the blockage and open a relief valve? Since it would be close to the vacuum motor, probably in the head, it would not have to draw air from the entire piping system and thus the motor RPM would soar because of the lightened load. Sound right?
-- Bobby G.
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Robert Green wrote:

Block off the air flow, and air just goes around and around with the fan blades. Without the block, it is constantly accelerating air from the input to the output, which uses more power.
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Believe it or not, the "motor load" is usually at a minimum when the vac is completely blocked off.
If you want to monotor for a blocked inlet you might want to check flow in the exhaust (after the bag). Most central vacs exhaust through a tube which can be connected outside if your don't want to "re-cycle" the dirt the filter bag misses.

In many/most central vacs (and even some "shop vacs") the cooling air for the motor is separate from the air used to suck up dirt. If your's isn't separate then a blockage would definitely cause it to overheat. Otherwise, you may find that it actually runs slightly cooler when the inlet is blocked.
Frankly, sensing overheat isn't a bad way of seeing if something has gone wrong.
Modern motors "run hot" so don't worry about overheating shortening the life of the motor.
If anything, the life limiting is as likely to be the motor bearings as the windings.
If you like to worry about blockage, you can quickly check to see whether it still "picks up" a piece of pocket litter.

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