That's the path I was headed down but I think Art T. found the perfect answer to the problem - a "relief" valve designed for central vacuums for $10.
Thanks for your input. Say, how close are you to "the Old Man in the Mountains?" When I was a kid we took a trip to New England and New Hampshire in the summer was about the greenest place I have ever seen.
Hope it's still green.
-- Bobby G.
While cruising cable last week I saw a show called "Smash Cuts" where they were flinging small English sports cars and flaming barrels full of something flammable from a three story trebuchet. Very impressive. Can you imagine manning the castle walls when the first one to be invented rolled up on you?
-- Bobby G.
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.
-- Bobby G.
Which I may do despite ordering the device that Art T. pointed me to
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.
It's counter-intuitive, but I guess the motor RPM would decrease while overloaded, not obviously increase, which does make for a sort of "runaway Toyota" feel.
Mine is connected to an outlet pipe and muffler to the outside, so it's doable, but hopefully not necessary with Art's relief valve.
Otherwise,
Not sure about this issue. I suppose I could test it. For now, I am going to mount a temperature sensor in the unit and hook it up to my home automation controller to shut the unit down if the temperature exceeds a certain level. That will probably have to be a trial and error sort of thing.
Exactly. Which is why the temperature sensor might be a good thing to add in addition to the relief valve. In fact, what I should do is hook up a recording thermometer to a probe inside the silencing box to monitor a few sample runs to try to estimate the temperature rise and where it plateaus.
I've burned up enough gear accidentally (a few AMD CPUs, a few Fujitsu tablet PCs, a Sony receiver and many, many many more) that I am now hard over in the other direction, adding cooling fans to stereos and equipment racks to make sure everything gets good airflow. I believe heat really hastens the aging process and fans help reverse it. Now only if I could fan my bad knees and get them working again!
And those failures would be heat-related, as in winding insulation and bearing lube.
it > still "picks up" a piece of pocket litter.
The point that Art made applies here. An electrically driven motor brush head will appear to be picking up dust but in reality is just sucking it into the brush head. If there's not much dirt, it's very easy to think you're vacuuming when you're not. The motorized carpet sweeper head make enough noise that you really can't hear the change in the motor RPM or the lack of vacuum hiss.
-- Bobby G.
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.
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.
Just make sure you clean the heat sinks and fans occasionally, or eventually, dust will plug them and they will overheat. I just had a video board start to create visual artifacts, and then crash my computer. I blew out the board fan with compressed air, and all the problems went away.
Blocked and whining 8.83A, Unblocked free running inlet port - no house piping attached (cleanout, open actually) 12.96A
External air output pipe ran free when unblocked but dropped to very low flow when the hose was blocked. If there's an internal check valve, it's venting to someplace other than the vacuum's output pipe.
I used a compresible coil type cheap plastic hose in the basement, and when blocked at the nozzle it contracted considerably indicating that there was still a serious vacuum being maintained.
Another theory. It takes more work to create a vacuum than maintain it, so when the hose has contracted as much as it can, the motor has less load on it and thus the current drops. With an open vacuum, the motor is constantly sucking and never achieving even a partial vacuum within the tubing.
C'mon. There are smart people here.
Why would a plugged vacuum cleaner draw less current?
-- Bobby G.
Somebody else answered that once already. Basically it's not moving a bunch of air so it's not "working" any more. Now that you know the current you should able to design a circuit that alarms at the 8 amp range and is off at the 12 amp range. A 1/4ohm power resistor will give you about a 1 volt variation between the two.
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.
If you want an alarm (other than that the vacuum tool is not picking up as much as usual) you could add an air flow sensor in the line to the safety valve. If there is flow, the alarm is triggered.
It is very possible that the power unit uses the filtered air from the vacuum as cooling.
Actually, the motor increases in speed because there is less load on it. It is much easier to spin the fan in less than normal air without doing any work to move air. This is the reason that the noise level goes up.
No internal check valve on most vacuums. They expect that the increased noise will let you know. With a check valve, almost no change in sound.
A vacuum does it's work by pumping air. If it is not pumping air, it is not doing as much work.
The amount of vacuum goes up when it is clogged because air is not flowing into the system.
Read the rest of the comments. This is a moronic device for a self protected motor / fan system.
No air movement = higher motor speed = less work being done = less stress on motor = less current draw
-- Bobby G.
He's been told, many time here but doesn't seems to understand that.
Ya but .... In the central vac unit the air flow over or through the motor is what cools the motor. The "moronic device" would solve that problem by providing an inlet for air, when the lines are plugged. I'm not sure I agree with the vendor's picture of the house and garage burning down. The motor thermal protector should prevent that. That all said, I had a central vac in my previous house for 36 years and can count on one hand the number of times the thing actually plugged up. Robert, if it is plugging in the hose, there might be something wrong in the hose causing it, like a tear which would catch fuzz going by. If the plug is occurring in the wall piping, again there might be a reason. I remember my
2 year old dumping some small wooden blocks in the inlet near his room. They went through the pipe with no problem but couldn't make it through a Tee. It was about 1" x 1" x 3". This caused a place for new dirt to catch and eventually plug up.
Which of course then begs the question, so what's the problem? I agree that with many pumps the speed increases when the line is plugged, because the impeller is cavitating and just spinning freely. That means the power and current go DOWN. So, why the need for a system to prevent the motor from overheating?
The only logical conclusion would be that the motor depends on the air moving through the vacuum for cooling. Even given that, I can't see how the vacuum could stay plugged up and running long enough for that to happen frequently. Surely after a minute or two of vacuuming you'd notice the head is no longer picking up dirt, that the head moves freely over carpet instead of being sucked down, etc.
It would seem to me that if this were a problem that occured frequently enough to matter, all systems would have some built-in protection besides the thermal cuttoff and the thermal cuttoff is OK for something that happens once in a blue moon.
Regarding a blue moon, I ask again, how often does this occur? I've had a central vac system for 17 years and it hasn't clogged once. If it's happening frequently, sounds like whatever is causing it is the real problem that needs addressing.
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