Central Vac overload

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.

Yup. I learned this many years ago in working on pipe organs. Normally, when the blower is running, it is basically plugged. The pressure is at its normal operating pressure and no air is being moved, except to any leaks. And in pipe organs there are usually many tiny leaks which eventually do add up. But, if a wind line is opened, the blower current goes up because it is now moving air and thus, doing more work. In an organ blower, the motor is usually an induction motor. It doesn't speed up as in a vacuum cleaner. In a vacuum cleaner, the motor is usually a series (sometimes called universal) motor. These motors will actually generate a back EMF. When the motor is not doing an work, i.e. sealed off, this back EMF will actually "self power" the motor a little, making it turn faster. Back in high school shop, they said that a series motor with no load, will keep going faster and faster until it literally breaks apart. I've run a small sewing machine motor, no load, and indeed it kept going faster and faster. I was afraid to keep going, but I suspect it would eventually find a top speed as even the bearings and cooling fan blade do some loading, albeit small.

I like the cuirrent sensing solution tied to a triac to turn the thing off when it clogs. That way you know right away and you don't waste you time vacuuming an area with the system clogged. Sometimes when you vacuum a relatively clean area with a beater head you do not easily notice that the suction has stopped. The circuit would be fairly straighforward and could probably be done with an opamp, an optoisolator and a 20amp triac. An alarm would have to be pretty loud to be heard over the beater anywhere in the house.

jamesgangnc wrote in news:b4f81cd1-dad7-4e35-85a3- snipped-for-privacy@g10g2000yqh.googlegroups.com:

And don't forget a delay or other override to handle the situation on startup and shutdown as you pass through the range getting to 12 amps.

Most appliance motors do have a thermal cutout designed into them. Many people repairing their own stuff will just cut it out of the circuit once it goes.

Get the Vac unit into a repair place and get the thermal cutout replaced.

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.

eventually, > dust will plug them and they will overheat. I just had a video board start to

fan > with compressed air, and all the problems went away.

You're lucky you caught it before it fried. A full "dust cap" is a fine heat retainer and could have easily cooked your VPU. Smart PC'ers use programs like Motherboard Monitor that will tell them when their fan speeds have dropped enough to indicate that they are getting clogged.

I do a lot of PC repair. I have a rough rule of thumb. In a normal environment, it's probably OK to let fans go unchecked for 24 months. Subtract 1 year if the equipment's on the floor. Subtract 2 months for each shorthair dog or car in the house, 4 months for every longhair. Subtract 1 month for fans under 80 centimeters and another month for those under 40.

One neighbor with 5 longhairs and a floor tower with a teeny video card fan needed monthly cleaning so we added some more fans and covered the front intake with air conditioner filter material held in place by a little magnetic frame. Now she just vacuums the front and the machine can go almost a year without a blow-out. (For anyone considering this, the clips on the cheap case face were so weak they broke during the procedure but we replaced them with some neo mags and hot melt glue so snapping the face off to vacuum the filter was even easier. Most case faces can't withstand frequent removal without those damn little tab clips breaking.)

I *would* post a picture of a super small video card sleeve bearing cooling fan and finned heat sink mount I removed from the 5 cat machine because I had never seen anything so completely caked in dust. The fan spun, but moved no air. I replaced it with a much larger ball bearing fan simply because it was less prone to clogging and the space permitted it. However, I see the picture police are on patrol, enforcing "laws" created when bits were moved around via acoustic modem. Like so many rules of that era it has been completely outmoded by technical advances. Anyone who says that posting a link is as easy as posting a picture with a message isn't being honest. Personally I'd rather see a relevant picture *with* the message rather than clicking on a link to who knows where.

One reason we got the central vac is that we like to air cool in the summer with a big attic fan and that brings in an awful lot of dust and pollen. Switching from A/C to "free air" cooling cured my wife's allergy. Turns out that being cloistered inside superclean, highly filtered office and A/C house hyper-sensitized her to pollen. She used to sneeze up to nine times when she left an A/C'ed building and stepped out into the late spring air, which *is* mostly pollen. Our standing joke is "we don't go around trying to mate with trees, so why are they trying to have sex with our noses?"

-- Bobby G.

Certainly not in my mind and I'm the OP.

Yes, the fire was a little much, but it almost never hurts to have an extra layer of protection, especially against catastrophic outcomes.

You've hit on the problem. A dog that loves to tear pieces of rag and nylabone just large enough to catch on a crimped section of the hose.

When I tested the output of the unit's exhaust port with the hose blocked, I noticed that although the exhaust flow cfm's dropped dramatically, the feel of heat on my hand increased appreciably.

That was clear evidence that the motor depends on good hose airflow for cooling and to let it run clogged was risking heating the bearing lube until it evaporated and possibly burning the insulation on the motor windings. Only a moron would expose an expensive motor system to unnecessary stress if there was a ten dollar way around it.

I would rather notice lack of suction from a bypass opening than from a clog that was burning up the motor. One wastes maybe 1 minute of extra electricity and the other subjects a motor to 1 minute of unnecessary stress, resulting in who knows how much shorter a life. That's a no brainer.

-- Bobby G.