Aces High Bulletin Board
General Forums => Hardware and Software => Topic started by: bloom25 on October 27, 2002, 05:57:38 PM
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I spotted a great case of this today.
Take a look at the horsepower ratings of a Shop-Vac vacuum. I bought a small one today that has a 3 HP rating on it. Elsewhere on the box it notes the power consumption is 7 amps (it uses a standard 120V AC plug).
Here's some electrical fundamentals to get you guys up to speed:
Power (in watts)= Volts x Amps. 1 HP (horsepower) = 746 Watts.
If we multiply 120V * 7 Amps we get 840 Watts. Divide 840 by 746 and we have 1.12 Horsepower. That means the electrical power consumed by the motor is 1.12 Horsepower (maximum). No motor is perfectly efficient, so the mechanical power cannot possibly exceed 1.12 Horsepower. The BEST motors I've ever seen are around 90% efficient. Assuming that's true (and that's a BIG assumption) I get 1.01 Horsepower.
So here we have it, a 1 Horsepower motor (more likely 3/4 horsepower) somehow outputs 3 Horsepower.
I looked on Shop-Vacs site, and if you read really close you'll see that they claim 3 Peak Horsepower. I still don't buy that. If the motor was perfectly efficient it would have to consume:
746 x 3 = 2238 Watts
Meaning it would draw: 2238 / 120 Volts = 18.65 Amps of current.
Now we have a second problem. All 120 V AC outlets in the US are rated at no more than 15 Amps. Consider also that the cord on this vacuum is 20 feet long and uses 18 Gauge wire. There is NO POSSIBLE WAY anything close to 18.65 amps is getting through a 20 foot long cord off an outlet with a 15 Amp breaker. (Those of you who have ran power tools off a long extension cord know what I'm talking about here.)
Shop-Vac also advertises 6 HP + vacuums using 12' cords off 120V AC. If you think this example is bad, that would be even worse. It's completely impossible to run a 6 HP motor off a standard 120V electrical outlet!
I think what Shop-Vac is doing is taking the motor, connecting a very heavy weight to the output shaft and then bringing the motor to a sudden stop and measuring the impact force and using that to determine their "peak horsepower" rating. I can't see any other possible explaination. (This would most certainly damage the motor.)
I'll bet the motor used in their "3 HP" (it doesn't say peak on the package, only on the website) would actually rate somewhere around 1/2 to 3/4 horsepower. There's about a 300% to 600% difference between what's claimed and what's actually possible!
(I also have a 5 Horsepower motor on an air compressor. It's nameplate indicates a draw of 25 Amps at 220 Volts AC. That's 5500 Watts of power (or 7.37 HP). That motor would have to be(assuming the nameplate is correct) is 5/7.37=.678 or 67.8% efficient to output 5 HP (mechanical). This motor weighs close to 80 Pounds by the way. I was awefully easy on Shop-Vac on my example above by assuming their motor was 90% efficient!)
(I have a BS in Electrical Engineering, so I do know what I'm talking about here.)
What do you guys think about this?
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I think you just spent waay too much time thinking about this. If the shop-vac does the job you need it to do and you feel it was worth the price you paid, use it and don't worry about the marketing BS behind it.
SOB
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Maybe so SOB. I think I'm just angry about getting a jury duty letter in the mail Friday, considering I'm doing 3 seperate jobs right now. This must be because I haven't been able to play AH (or anything else) for weeks now.
I did actually figure out how they came up with the 3 HP rating though (and it's definately a scam):
I was actually right up above. Somehow vacuum cleaner motors (and ONLY vacuum cleaner motors) are rated in a totally BS manner by tying as much load as possible to them (without burning them up) in the form of a big weighted flywheel and then bringing them to a stop instantly. They get the "peak horsepower" rating from that by basing it on the assumption that a one horsepower motor can deliver 550 foot/pounds of torque for one second. (That formula is only valid if the motor can continue to provide that amount of torque.) Because of that, vacuum cleaner motors are rated in "peak instantaneous horsepower."
I found a funner explaination that claimed they get the measurement by:
"Dropping the motor off the Empire State building onto the head of the engineer that came up with the peak horsepower rating and measuring the impact force."
If you guys are ever shopping for vacuums, realize the HP ratings are total BS and vary from manufacturer to manufacturer.
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OK Bloom.. whatever you say. Just don't get a shack and write a manifesto on us now.
Step away from the box. Turn on the shopvac and use it for what most college kids buy them for. You'll feel much more relaxed afterwards.
AKDejaVu
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Just cause it can't draw the requesite amperage from a standard socket does not mean the motor can't produce 3HP under a different load.
F.
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Bloom if I am reading this right you are saying they figure out the stall torque and then multiply that by a time and they get horsepower? LOL!
Only in America.
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LOL, maybe I overreacted a little bit. ;) Things like this just irritate me.
How do you figure that Furious? There is no way a 120V, 7 Amp, motor can possibly produce 3 HP continuously (as all other electric motors are rated). That would require it to be almost 300% efficient in converting electrical to mechanical power. If you can prove me wrong here I need to contract you to make me a perpetual motion machine. :)
As I posted above, vacuum cleaner motors are rated differently than all other electric motors. What they are rating is not the same as all other electric motors and isn't really horsepower at all.
BTW: The vacuum cleaner worked fine. It really sucks. DejaVu, do you know where I can get a good deal on a shack and publisher somewhere? :D
I'm actually not in college anymore. I was hired by a Eugene company back in July, but because of NDAs I can't talk about what I've been working on for them. I'd like to, because I know many of you would find it very interesting. (It's not related to vacuum cleaners in any way...)
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It looks like I'm not insane yet. (Don't worry about the shack and publisher anymore DejaVu. ;) ) http://www.ristenbatt.com/smpower.mv
It looks like that's exactly what they are doing Funked. It's pretty cheesy if you ask me.
I figured the rest of you would feel the same, but it looks like I was wrong. :(
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Originally posted by bloom25
I'm actually not in college anymore. I was hired by a Eugene company back in July, but because of NDAs I can't talk about what I've been working on for them. I'd like to, because I know many of you would find it very interesting. (It's not related to vacuum cleaners in any way...)
What company?
I've dealt with a couple from Eugene.
AKDejaVu
P.S. You wouldn't believe what I'm working on now;)
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"Peak" horsepower is probably a marketing term, but I'm not sure that continuous power would mean much for a vacuum cleaner, since it's operating load varies over a wide range.
Also, I don't think there would be any problem getting 20 (or 30) amps from a "15 amp" household circuit for a few seconds at least. Most fuses and circuit breakers will pass 200-300 percent of their rated value for a short time. (They do this so that motors and incandescent lamps can draw large surge currents when they turn on.) So, you could probably get 3 "peak" horsepower from a 15 amp circuit, as long as the duty cycle is very low.
http://www.faqs.org/faqs/woodworking/motors/
http://www.airpaxppp.com/pppsite/ialop.html#400
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Bloom,
It was meant to be read as tongue-in-cheek.
I was pointing out that if you put enough juice through it, it might output 3hp right before it catches fire.
F.
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Bet they put a MEG in it WOW 300% :rolleyes:
http://www.rexresearch.com/meg/meg.htm
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The lack of truth in advertising is rampant and disgusting....the fact that it's spilled over into politics us evidence of how well it actualy works.
I'm hoping that one day the Fed will levy huge fines for the worst cases and substantial fines for the lesser ones. Right now about the only protection we have is from consumer advocacy groups which have a hard time getting any legeslation passed because of the payola that goes on connected with lobbyists.
I wanna burger that looks like the one in the picture.
I wanna a car that has ALL the windows tinted.
Finaly...I want Red Bull Wings damnit!!! I'm tired of walkin'.
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Ever consider macramae or cross-stitch bloom? :D
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My Vac sucks :-)
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I dont see how a cheap vacuum cleaner can have the strenght of three horses
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Is this where I need to ask about the air speed velocity of an unladen shop vac?
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I'll sell anyone a slightly used shop vac for $50 bucks plus shipping if they want to buy it......Naaa I can't do that I got it for free and don't have a house yet to use it in. :D
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I know exactly how they get that "peak" HP rating...you are partially correct in your idea of how they do it. However, they do not put a big flywheel on them, but they do put a frictional load on them to the point they stop turning. They don't measure any amount of torque or the like.
Something funny happens when an electric motor is stalled...it draws huge amounts of current and creates massive torque, especially in induction motors when the capacitors kick in (just like when you first flip the switch on). In the labs, they will literally stall a motor to the point it burns up and they read the maximum amperage drawn. They then do the math, just like you did, and give the "peak" HP rating based on that amperage draw.
What they don't tell you though is that your regular old house wiring will not supply that maximum amperage draw...because if it did, one of two things will happen, you will pop a breaker (which you may have noticed if you've ever stalled a good sized electric motor), or you'll be calling the fire department to put your house out.
So yes, while it is cheesy advertising claims, technically speaking, the motor IS capable of outputing that power, albeit under extreme conditions, only for a split second, and the motor will be no good once it does it.
It's about the same thing as saying my 360 cubic inch V-8 truck motor has a "peak" of 10,000 RPMs. So you put it in neutral, slam the gas pedal down and it winds up and hits 10,000 RPM's just as it disentigrates into tiny fragments....yep, the engine is toast, but hey, it made it to 10,000 RPM, if even only for a split second....same thing.
Many people don't realize just how powerful an electric motor can be. I saw a show a couple years ago on TLC and some guy built an electric powered drag bike. It had a single electric motor about the size of a 1HP induction motor, and it used an aweful lot of car batteries. It was capable of 165 MPH in the quarter mile. That sucker drew 900 amps when the rider cranked the "throttle". It was amazingly quite too :D
Sears is probably the worst retailer that harks those "peak" HP ratings. They do it on nearly every electric tool or appliance they sell.
The best way to rate a vac is by it's suction power, or "static pressure", not by how powerful its motor is, or is claimed to be.
Static pressure is measured in "inches or millimeters (mmH2O, or InH2O) of water", meaning that's how many inches or millimeters of water it could suck up into a calibrated tube and hold it there.
To kind of keep things on topic here in the computer world...some of you overclockers that have tinkered with cooling solutions may have noticed that larger fans on your heat sink don't necessarily translate into better cooling, even though the bigger fans may provide double or more the CFM of air flow....the reason is: static pressure of the fan :D
The fan needs sufficient static pressure to force the air down through the heatsink fins (it has to overcome the resistence of the fins...the "load"). Smaller, high RPM fans will provide more static pressure than a larger, slower turning fan, so they are better able to handle the "load" and get air moving through the heatsink (unfortunately, they are also a lot louder). It's a tradeoff really and you have to find the happy medium.
As an example:
A YS-Tech 92x32mm fan turns at 4,000 RPM and moves 73.3 CFM of air, but it only has a static pressure of 10.7 mmH2O.
On the other hand, the screamin 60x38mm Delta fan turns at 6800 RPM, moves only 38CFM of air, but has a static pressure of a whopping 21.1 mmH2O
Almost half the CFM of air moving ability as the YS-Tech, but nearly double the static pressure. Guess which one will cool a heatsink better? :D
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Bloom,
Yes, I have noticed the same thing. It did not bother me as much as it has you though. Guess I've seen too many "Impossible" claims made through the years to be surprised.
I first noticed this in air compressor motors. The "5HP rated " air compressor that pulls 17 amps at 110Vac. Which of course, is impossible.
It could be a British standard of some sort. After all, we are talking about "Horse Power" here. More likely, just advertising hype.
Could be it's to ensure you go back to the manufacturer for spare parts. The motor HP is not rated that way by the motor vendor, only the appliance manufacturer.