Buying a used air compressor: tips?

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N8N wrote:

Not entirely correct, but you are confused enough already, so we can just call that good enough for now. The important point is that the motor is not rated as a 3HP motor, but instead, as I have already stated a half-dozen times in this thread, a 3HP SPL motor.

And it is, of course.
I think the main issue here is not so much the way that motors are rated, but at the reluctance of a typical end user educate himself beyond his ignorance.
Jon
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On Feb 7, 5:18 pm, "Jon Danniken"

Which is clearly marketing BS. I don't know or care what "SPL" means nor is a web search much help (other than some suggestions by forum posters that it stands for "Special" or "Split Phase" the latter of which is clearly false.) What is important to me is what the full load but running power of a motor is. Or in this context, really, what the compressor as a whole's max CFM is at what PSI.

I know enough to know that I take any electric motor's "HP" ratings with a grain of salt and look at amp draw and in the context of an air compressor, CFM@PSI instead when trying to compare.
nate
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I know enough to know that I take any electric motor's "HP" ratings with a grain of salt and look at amp draw and in the context of an air compressor, CFM@PSI instead when trying to compare.
nate
And I believe none of what I hear and half of what I see when it comes to hp and cfm ratings on compressors. I tend to overbuy on everything, figuring I'll take it to burnout mode at least once. That's when you see how it really runs with the tool you want to use. And sometimes, you have to compromise and let it get a full tank to work that half inch impact to get those lug nuts off. And it won't run some other heavy demand air tools. But unless you are making money with it, you can live with it. I got a stand up 33 gallon tank 2 hp (IIRC) Husky, and it does all I want it to, although sometimes I have to wait for it to catch up. If it was costing me money, I'd go get one of those honking tall as I am two stage brand name ones.
Steve
Heart surgery pending? Read up and prepare. Learn how to care for a friend. Download the book. http://cabgbypasssurgery.com
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?

Once apon a time, sleazy tool makers used a horsepower rating taken just as the motor stalls from load rather than actual running power like they should. Craftsman saws were all 3 or more HP and compressors used similar tactics. The SPL probably stands for something like "Stall Power Load" or similar fairy tale rating.
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On 2/7/2011 5:18 PM, Jon Danniken wrote:

apparently accept whatever nonsense the big box marketing department invents instead of calling them on their sleezy tactics?
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N8N wrote:

Yea g, what do you know about motors...

In addition to efficiency there is also power factor.
The NEC has motor tables that are used for sizing wire and some other purposes. A 120V 3HP motor has a table value of 34A. A real 3HP motor is likely somewhat less. --------------- IMHO if you connect the motor, the NEC requires the conductors be rated 125% of 34A = 42A. The NEC says nothing about SPL.

There is also the problem of starting the motor when the starting current is about 6x the run current. ------------- A lot of people here work with electric motors - no one seems to know what SPL is. Jon might but he isn't saying much. Does not appear to be a standard motor term. I agree with everyone but Jon that it is an excuse to lie.
--
bud--


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bud-- wrote:

The two local motor rewinders that I called yesterday told me it stood for "special", as in it is a particular type of rating used in motors for specific applications, namely compressors.
I'm sure you could contact Magnetek/Century (the manufacturer of the motor on my compressor) for a more specific answer.

The only "lie" would be calling a 3HP SPL (15A/115V) motor a 3HP motor, which may have been the case in the past with less than reputable merchants such as Sears/Craftsman, among others. To some of the participants on this newsgroup, that is an excuse to consider legitimate motor manufacturers guilty of the same type of deception, even when they properly label their products.
I believe the phrase "when all you've got is a hammer, everything looks like a nail" is appropriate here.
Jon
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Jon Danniken wrote:

Much more appropriately I saw someone call it "Someone's Probably Lying".

Now apparently owned by AO Smith. IMHO their website sucks. I looked earlier for info on SPL and couldn't find anything, including no phone number.

There was a class action lawsuit about 5 years ago about fraudulent HP ratings on compressors/motors. The manufacturers lost and were supposed to use real HP ratings. You have the old fraudulent rating hidden behind "SPL". Your 3HP rating is a lie intended to deceive. The same as in class action lawsuit (with a minor cover your ass).
gfretwell wrote "if it is 120v it will not be more than 1HP." He is right. Yours works because it is not a 3HP motor. Because of past, and apparently current, fraud about everyone here has said ignore HP ratings.

Is there any reason why the "real" HP is not used? How is a 3HP SPL motor different from a motor with a real HP rating? What possible reason is there to use "SPL"?
I can think of one reason. It is the same fraud as in the class action lawsuit about 5 years ago, with a minor cover-your-ass.
--
bud--

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bud-- wrote:

Yes, the motor on my compressor is not a 3HP motor, it is a 3HP SPL 15A/115V motor. That's about 2.3HP Continuous, which is more than the 1HP you and "gfretwell" claim is the absolute limit for air compressors.
Jon
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On Tue, 8 Feb 2011 16:55:13 -0800, "Jon Danniken"

A 100% efficient motor. I want one of them!
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snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

And 100% power factor too. Excellent engineering using unobtanium.
For real motors the NEC gives a table value of 16A 115V for a 1HP motor.
--
bud--

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On Tue, 8 Feb 2011 16:55:13 -0800, "Jon Danniken"

1HP is not the absolute limit on 115v but if it has a NEMA 5-15 plug on it (15a breaker max) that is about all the motor you can reliably start. There are some 1.5 HP motors that run on 120v but they will require a 20a breaker and a NEMA 5-20 plug if they want to get a U/L listing.
If you want to believe you have a 3HP compressor, go for it but I would look at the air output myself.
There are about 100 articles on the net about "SPL" rated motors and they all have the word "lie" or "inflated" in them
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snipped-for-privacy@aol.com wrote:

I never stated that I had a 3HP compressor, I stated that the motor on my compressor is a 3HP SPL motor, with a rating of 15A at 115V.
You stated that " If it is 120v it will not be more than 1HP," which is complete and utter bullshit, as is most of the rest of the horseshit you have tried to pile on to cover your ass.
I should know better than to attempt to have an honest discussion with people on usenet.
Jon
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On Feb 9, 2:15 pm, "Jon Danniken"

please read the table of estimated current draws at the following link
http://www.cerusind.com/catalog/documents/calculators/MotorEngineeringFormulas.pdf
and then apologize to all the people you've abused and insulted in this thread.
The motor you have may or may not be greater than 1 HP, but if it *is* it is more efficient than the motor used for the NEC tables, and in any case, it is likely not significantly greater than 1 HP.
nate
PS - is your self worth really tied to the power output of your electric compressor motor?
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On Wed, 9 Feb 2011 11:15:34 -0800, "Jon Danniken"

motor. It is a special application motor that has the starting torque(roughly) of a regular 3 HP split phase motor. This is accomplished by the use of 2 capacitors (usually) which affect power factor and the phase shift between current and voltage in the motor. Once the motor comes up to speed it looses that advantage and it is simply a 2HP motor (maximum, under ideal conditions) with a little more "slip" built in (if it is a dual cap - or cap-start cap-run motor), which allows it to produce slightly higher torque at slighly lower RPM for short periods without stalling and going over-current.
1 HP is defined as either 745 watts or 550 ft lbs/second.
Since an electric motor will ALWAYS be less than 100% efficient, to get 745 watts (or 550 ft-lbs/second) of power out, slightly more will be required to be put in. If the motor was 90% efficient (and precious few are) that would require 828 watts in. At a line voltage of 123 volts (high side of average) that requires 6.75 amps at 100% power factor (or unity power factor, or Power Factor of 1, depending at how you want to quote it).At 117 volts (more realistic line voltage) that takes 7 amps.
So, at the very MOST you have a 2HP motor. And that is being extremely optimistic on both efficiency and power factor.(real world efficiency is closer to 80%) More realistically, a motor that will start and run on a 15 amp circuit in a 120 volt system is a 1.5 HP motor.
You can slice it and dice it and spin it any way you want - the laws of physics prevent anything more than 2 HP from being developed on a 120 volt 15 amp circuit, absolute best case.
That said, the 3HP SPL motor will start a bigger load without tripping the breaker or blowing the fuse than a regular 2HP motor will start. It might even, very optimistically, start the same load as a 3HP regular duty AC motor would start (under the right combination of conditions), and installing a 2HP utility motor on the compressor in place of the 3HP SPL would LIKELY reqwuire a smaller pulley to start the compressor reliably. I'd love to see a compressor with a variable ratio(variable speed) drive that would start the compressor at a high numerical ratio, then speed the compressor up under light load, and slow it down under heavier load, to get the maximum air power out of the least electrical power input possible - but that would not be your typical low-cost compressor any more.

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snipped-for-privacy@snyder.on.ca wrote:

No shit, sherlock, what you think I have been trying to explain for the last two days?
Jon
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On 02/09/2011 06:56 PM, Jon Danniken wrote:

We're still not sure.
nate
--
replace "roosters" with "cox" to reply.
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wrote:

I've understood what he's been saying since the beginning of the thread. He's never said it's anything other then what it's labeled as, a 3hp SPL and he's acknowledged that he's not claiming it's a 3hp continuous duty motor. He's simply said, several times, essentially the same thing clare just posted. Maybe there is some history here I'm not aware of but it seems like a lot of people went out of their way to attack him for pointing out there is a difference between a 'compressor' motor and a 'regular' motor.
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He said,
"Yes, the motor on my compressor is not a 3HP motor, it is a 3HP SPL 15A/115V motor. That's about 2.3HP Continuous, which is more than the 1HP you and gfretwell" claim is the absolute limit for air compressors."
Implying that it was a 2.3HP motor and that gfretwell's 1HP "limit" was wrong by a factor of 2.3. At *best* he's a troll.
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Jon Danniken wrote:

Is there any reason to believe that these motors have different characteristics than other cap start cap run motor (which is what this motor is)? Is there a reliable source that says compressor motors have a different characteristic? That is, is there any reason an honest company wouldn't use the real HP?
Perusing the Grainger catalog (real paper), many of the smaller compressors (maybe 6HP max) have a "peak HP" and a "run HP". Peak HP was what was used by companies that were sued in the class action. It is likely what "SPL" is. "Run HP" is the mechanical HP used at the rated air output (the real HP). "Run HP" generally ranges from about 30 to 65 % of "peak HP". In general the higher the HP the lower the percentage.
The companies in the class action presumably agreed to not mislabel their compressors. That puts them at a disadvantage to companies like Jon's.

It started when gfretwell said 1HP was about the max you could run on a 15A circuit. You said "nonsense" you run your 3HP motor on a 15A circuit. In the quote above clare told you it is not a 3HP motor. That is what everyone else has been telling you.
Then you said it was a 2.3HP motor - also wrong.
Nobody seems to know what you are saying, including you.
--
bud--

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