Subject: Re: How much heat is safe for motor and compressor

From: Robert Bickers <[email-masked]>
Date: 11 Dec 1996 23:49:57 GMT

[email-masked] (VAntonova) wrote:
>I was sandblasting with my 5HP 2 stage 80gallon compressor and noticed
>that the compressor and the motor (a 5 HP Marathon) were VERY hot. I
>couldn't touch the motor, and I have my gas generator is cooler than the
>compressor. Somebody recomended spraying the dark blue compressor with
>silver paint. What else should I do (I don't think painting will help
>much)? How much heat is safe? Thanks for any advice.

Do not paint it silver, flat black if you must which won't be a lot different
than what you have. What make of compressor is it? Maybe yours isn't a model meant for 
continuous duty. Check the temp rise on the 
motor label, it should say what the normal rise is, I have motors that run at 
90 degrees C. Cheers Bob

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From: [email-masked] (Robert Nichols)
Date: Thu, 12 Dec 1996 02:14:44 GMT

Note: E-mailed *and* posted.

In article <[email-masked]>,
VAntonova <[email-masked]> wrote:
:I was sandblasting with my 5HP 2 stage 80gallon compressor and noticed
:that the compressor and the motor (a 5 HP Marathon) were VERY hot. I
:couldn't touch the motor,
[...]

The motor's rated temperature rise should be on the nameplate.  A rated
rise of 50 C is common.  Added to a 25 C room temp, that would put the
temperature of the motor up around 170 F, which is a bit too hot to hold
your hand on.

Whether you trust the motor to run continuously near its maximum rated
temperature is another matter.

-- 
Bob Nichols         [email-masked]
Finger [email-masked] for PGP public key.
PGP public key 1024/9A9C7955
Key fingerprint = 2F E5 82 F8 5D 06 A2 59  20 65 44 68 87 EC A7 D7

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From: Brian & Angie Vaughn <[email-masked]>
Date: Fri, 13 Dec 1996 02:05:08 -0500

> much)? How much heat is safe? Thanks for any advice.

I don't think I would paint it at all.  Adding paint thickness also adds
an insulating blanket on it.  I just had the raditor of my Model A
redone and they painted it with a very thin coat of flat black. 
Radiators are not black just for looks.

Brian

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From: Paul Pierce <[email-masked]>
Date: Thu, 12 Dec 1996 19:59:26 -0700

VAntonova wrote:
> that the compressor and the motor (a 5 HP Marathon) were VERY hot. ...

Get a clampon ammeter on the line and see what amps the unit is pulling.
If below the nameplate it SHOULD be safe.  If at or above the nameplate
amps start looking for the problem.
 
-- 
Paul in AJ AZ  NRA Endowment Member

Checkout the Metal Web News at:
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From: [email-masked]
Date: 13 Dec 1996 17:26:40 GMT

In article <[email-masked]>,
[email-masked](Roger Loving) writes:

>a propeller to blow air across the compressor? Come to think of it...My
>old compressor has spokes that do this. Could it be that yours does too
>but someone has put the pulley on backwards??
> 

Good thought....but fans is fans no matter which way you put them they
push the air the same way.

Peter

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From: [email-masked](Roger Loving)
Date: 13 Dec 1996 03:27:45 GMT

In <[email-masked]> [email-masked]
writes: 
>[email-masked] (VAntonova) writes:
>
>> was sandblasting with my 5HP 2 stage 80gallon compressor and noticed

    Just a thought:  Most compressors have a fairly large pulley...is
there anyway you could rig that pulley up so that it also functions as
a propeller to blow air across the compressor? Come to think of it...My
old compressor has spokes that do this. Could it be that yours does too
but someone has put the pulley on backwards??
                                                    Roger L.

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From: [email-masked]
Date: 13 Dec 1996 00:06:53 GMT

>much)? How much heat is safe? Thanks for any advice.

 I don't know what will help other than adding a blower to help move more
air over the motor. One thing I do know is don't paint it silver. Unless
it is out in the sun where it can absorb heat from the light black paint
will let more heat radiate from the object. From the sounds of it I would
leave it a dark blue which is almost as good as black.

   Wayne Cook

Quality is like buying oats,
        if you want nice, clean, fresh oats you must pay a fair price.

However, 
        if you can be satisfied with oats that have already been through
the horse...
                          
That comes a little cheaper...

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From: [email-masked] (James Kirkpatrick)
Date: 13 Dec 96 13:46:23 MST

In article <[email-masked]>, Robert Bickers <[email-masked]> writes:
|>[email-masked] (VAntonova) wrote:
|>>I was sandblasting with my 5HP 2 stage 80gallon compressor and noticed
|>>that the compressor and the motor (a 5 HP Marathon) were VERY hot. I
|>>couldn't touch the motor, and I have my gas generator is cooler than the
|>>compressor. Somebody recomended spraying the dark blue compressor with
|>>silver paint. What else should I do (I don't think painting will help
|>>much)? How much heat is safe? Thanks for any advice.
|>
|>Do not paint it silver, flat black if you must ...

This would depend on whether the dominant heat-dumping mechanism is
conduction or radiation.  At compressor temperatures I would expect
conduction/convection to dominate, with radiation a very small part.
This suggests an unpainted surface would be superior to any paint, as
the paint itself no doubt would act as a thin insulator.

Make sure the compressor is not limited in its air flow (e.g. that other
stuff isn't blocking free air flow), that it's oriented as designed, and
you might consider a couple of cheap "muffin" fans for forced-air cooling.
Wire them in with the motor so they don't run all the time, for example.

Not sure how hot is "too hot" however.

Jim

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From: [email-masked] (Dave Corgill)
Date: Fri, 13 Dec 1996 21:06:30 GMT

>|>Do not paint it silver, flat black if you must ...

If this is an "Import" motor, they run very hot & burn up quick.
That's why you see the extra price for a "UL" listed American
motor in the machine tool cats.

A big fan would help.

.
[email-masked] & The Doberman gang in Dallas (Taxes)
Dave's Doberman page http://www.geocities.com/Athens/1877/
Doberworld home page http://www.geocities.com/Athens/1878/
El-Camino/Chevelle page http://www.geocities.com/Athens/1876/

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From: [email-masked] (Reckless)
Date: Sun, 15 Dec 1996 06:23:27 GMT

>Peter

Very true that they will move air in the same direction regardless of
which way the prop is facing, but if the blades have a curve to them,
they will be more efficient facing one way than the other.

Though the compressor pump flywheel/fans I have seen don't have curved
blades so that wouldn't make any difference here.

Is it running in the correct direction? CCW vs. CW?  From the factory
it should be correct, but if the motor was replaced it might not be
running the correct direction.

-Reckless

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From: [email-masked] (Reckless)
Date: Sun, 15 Dec 1996 06:28:58 GMT

>amps start looking for the problem.

Good advise, here's a few more thoughts:

Check the incoming voltage as well.  A 230V motor on 208V doesn't put
out full HP and does run extra warm.  Undersized and/or long supply
lines or dirty connectors can cause low voltage at the motor. 

BTW, too hot to touch is normal for a 50C rise motor.  Did you smell
burning insulation or paint?  If you did, it was definitely too hot.

-Reckless

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From: [email-masked]
Date: 15 Dec 1996 17:35:15 GMT

>blades so that wouldn't make any difference here.

I think you need to check that one again, Reckless. If you turn the blade
around, any curve will turn with it! If the curve is on the output edge of
the blade, it stays there no matter what. The only time the curve, if any
would be likely to affect the outcome is if the fan is run backwards. Even
then it is doubtful.

The curve is generally there to reduce noice rather than improve output.
It provides less shearing of the air at the tips. Therefore such a fan
running in reverse may well work more efficiently rather than less.

Peter

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From: [email-masked](Roger Loving)
Date: 16 Dec 1996 04:26:17 GMT

In <[email-masked]> [email-masked]
writes: 

    Peter, I think I inadvertantly started this debate with my ill-
considered comment that the compressor pulley might act as a fan and
might have been reversed on it's shaft. After spinning a prop on a 
shaft and looking at it, I see what you mean. Reversing the prop
changes nothing. I should have thought it through....of course
reversing the shaft direction - as in a CW to CCW rotation motor 
used as a replacement motor - is still a possibility. My old compressor
has large vanes on the pulley clearly designed for cooling.... 
    Thanks for straightening me out on this.
                                                Roger L.

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From: [email-masked]|-NOSPAM-| (James Vahn)
Date: 15 Dec 1996 14:13:38 -0800

VAntonova ([email-masked]) wrote:
> couldn't touch the motor, .........

Electric motors these days run too-hot-to-touch on the norm.
There is a duty-cycle rule... maybe someone will remember?
10 minutes on, 5 minutes off?

-eof-

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From: [email-masked]
Date: 16 Dec 1996 18:31:40 GMT

>    Thanks for straightening me out on this.

I think you are being a little hard on yourself here Roger, not
ill-considered just a little hasty. I had to make a propellor with a
twisted piece of paper to be sure before I responded. It didn't sound
right but I wasn't sure until the paper proved it.

The CW versus CCW is very much a possibility though. I am not sure but I
suspect a compressor would work in either direction as they pump on every
stroke much the way a two stroke motorcycle engine can run backwards if
the timing is advanced too far.

I had a 400cc Yamaha trail bike that would do that at the drop of a hat.
If you rtried to ride up over a log and faltered at the top and dropped
back the motor would be running backwards. Really exciting when you made
the next attempt at the log. The next attempt ususally began with much
more throttle than the last.

Actually, thinking about it, it is unlikely the compressor motor would run
backwards as it is more likely to be a single direction motor rather than
reversable.

Peter