Compressor watts at PSI output

This certainly makes sense with a positive-displacement compressor and an induction motor -- as the pressure goes up the compressor shaft would require more torque to turn to move the air, for an induction motor higher torque = higher current.

This isn't the case with the blower in a vacuum cleaner. I'm not entirely straight on all the details, but put your hand over the vacuum cleaner hose and you can hear the motor run faster -- I think it's because the turbine is suddenly just spinning the air inside it's housing, instead of moving it through.

yup- it's doing less work (moving air, or really not moving work when blocked) and just speeds up as it sees less load.

squirrel cage blowers act the same, and you can see in data sheets that running some without any sort of choke will overheat/burn them out.

I've thought this would make a good way to do a home-brew dynamometer -- hook the motor up to a turbine pump or blower, and choke it off more or less to tune the load.

I wonder if something a brain-dead as a propeller would work for the turbine, if you had a way to shut off the air to it (my interest is in model engines, so a prop would be a natural thing to hang on the end of one of them).

I've wondered if this wouldn't be a good way to make a dynamometer for small motors -- some sort of blower or turbine pump, with a variable restriction on inlet and/or outlet.

In fact, it just now occurs to me that this may even work if the 'turbine' is a model airplane propeller; if you shrouded it completely the resistance should go way down, and you'd certainly have an easy selection of different turbines for different motors...

Why limit it to an air pump?

I see the same thing on an irrigation pump when the intake screen gets plugged.

Paul

...

It's interesting that at max load your "3 HP" compressor is only using

Bob

Well, in the first paragraph I wasn't, even though I didn't explicitly state that 'some sort of' included 'any fluid, gas or liquid'.

Clearly the second paragraph limits things severely, but as I'd be mostly interested in seeing what a model airplane motor could do, a model airplane propeller is a natural.

Trying something new- see

After reading my post, it's clear I keyed in a couple things wrong. 3 HP should read 3 gal, and the watts at 90 PSI is 244. I should spend more time proo- freeding. I do apologize.

Can't remember if it was this list. The question was raised, does compressor amperage decrease as pressure increases. I set up an informal test, using equipment on hand. Plugged in a Kill A Watt meter, and set it to read watts consumption. Plug in my

I turned on the compressor, and noted the reading as the pressure went from 0 PSI to 100 PSI. The readings are as follows:

PSI -WATTS

Hope the information is of some use to someone. If nothing else, it used a few minutes of my afternoon. And demonstrates that I am lousy at transcribing information from sheet to keyboard.

I am trying a couple things new.

• Correct the terms. 3 gal not 3 HP. 12 disciples, not 10.
• Transcribe the numbers accurately off the page. 90 PSI is

Sorry, folks, it makes a lot more sense when I copy accurately.

I think there's a detail that is still missing; a piston displacement compressor has a 'dead volume'; when at max pressure, the motor compresses the dead volume, opens the valve to the tank, and no gas flows. It then closes the valve to the tank and does another cycle. If the vent-to-atmosphere valve opens when the dead volume is still at high compression, you lose that compressed air and the FORWARD torque it applies to the motor. So, the timing of the big-intake-valve opening is crucial here. The action on the piston downstroke is ... unspecified, so we can't be sure if there's a useful recovery of compression energy or not.

Just out of curiosity, how about charting psi vs amps. Then calculate VA.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

When I have a lot to write I type it up and edit it in Wordpad and then copy and paste into a posting, leaving the top line in the Reply field intact to preserve the header.

jsw

You should also try to leave the "References: " header intact to make threading newsreaders work properly.

And my newsreader allows me to select my own choice of editor, instead of using one built in which I may not like. I opt to use "jove" as my editor with both mutt (e-mail client) and slrn (newsreader), both on unix of course, so this may not be as easy on Windows.

And, with jove I can always read in an existing file to add to the article with a "Control-X Control-i", so no cut and paste needed.

Enjoy, DoN.

Too many keystrokes for me. nano does it with "Ctrl-r"

That one is already used for "reverse search", which I do more often than insert from a file, so I prefer the less frequently used functions to have the longer sequences.

And no -- there is no shift needed on the "Control-X", just habit typing it as uppercase. (And actually, if I want to use an uppercase 'X' with the Control -- it works too. (Though I could re-assign it to serve some other function.)

Enjoy, DoN.

Normally I would have assumed that, but you gave one in lowercase and one in uppercase, so the natural inference was that it was significant.

I hadn't installed jove for quite awhile, and I find, much to my amazement, that it's actually now even