Subject: Re: Boosting air compressor output?

From: [email-masked] (James Wilkins)
Date: Thu, 05 Dec 1996 08:56:32 -0400

> Using 50lbs does not cut it when bead blasting aluminum. The beads are
> round and are not intended to remove metal. I have toi use 90psi otherwise
> it takes forever to do large surfaces evenly. At 90psi my compressor runs
> out of puff very quickly.
> 
> I am not sure I like the idea of increasing the pully size until the
> overload kicks out. It sounds a little too heavy duty for me. 

The overload has never kicked out in 20 years of use. The idea was to find
the manufacturer's maximum allowable current rating and size the pulleys
to get near it but only for short intervals. 

A car starter motor is an extreme example of using the mass of metal to
absorb the heat from a brief overload. Those little things can put out
about 5HP for maybe 30 seconds, yet they are smaller than regular 1/4 HP
motors.

-- 
James Wilkins
The Mitre Corp.
Bedford, MA

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From: [email-masked] (Reckless)
Date: Sat, 07 Dec 1996 11:55:36 GMT

[email-masked] wrote:

>In article <[email-masked]>, [email-masked]
>(James Wilkins) writes:

>Yup, I understoof that from your post but my concern would be if an air
>hose breaks or develops a bad leak then the compressor runs full time
>until someone notices or in  this case the overload trips or *worse*
>doesn't trip! That could be days if we go away fro the weekend. It eats
>too far into the safety margin.

>The Motors on my Campbell Hausfields are rated for continuous use. The
>only thing that gets really hot is the air delivery tube from the
>compressor to the receiver. That would not be too much of a problem if it
>ran for three days with a big leak.

>Peter

With Mr. Wilkins' configuration a broken air line or other rapid leak
would not cause the motor to burn up.  Motor current rises as the
pressure rises, so a fast leak will drop the pressure and keep the
motor from burning up.

In such a configuration, rapid start/stop operation would seriously
overheat the start windings, but that is not the manner in which he
appears to use it.

BTW almost all compressor motors are rated at 1.15 service factor
which indicates they can all run continously at rated HP and withstand
fluctuations in input voltage and momentary overload (like TDC
condition of the pump).  

-Reckless

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From: [email-masked]
Date: 8 Dec 1996 17:10:29 GMT

>pressure rises, so a fast leak will drop the pressure and keep the

You are most likely right, hadn't thought about it in that light, thanks.

>fluctuations in input voltage and momentary overload (like TDC

I assumed the reverse to this. When a motor is advertised as "compressor
duty" I thought it meant intermittent and therefore not a good candidate
for continuous running. That's why I mentioned the C-H as being rated
specifically for continuous. I thought this was a bit unique for
compressors.

Are you sure all "compressor duty" motors mean 1.15 duty factor??

Peter

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From: [email-masked]
Date: 3 Dec 1996 07:24:45 GMT

In article <[email-masked]>, [email-masked]
(James Wilkins) writes:

>I can run an old Sears sandblasting gun at 50 PSI all day with this
setup:

I am not sure I like the idea of increasing the pully size until the
overload kicks out. It sounds a little too heavy duty for me. As it is the
compressor runs continuously. I can bead for about 10 minutes before the
pressure drops to about 50lbs then I wait until it cycles off again before
starting the process again.

My soultion has been to order another compressor.

Given some time I will try the bigger pullety idea and see what extra I
can eek out of the existing system. My problem in this instance was never
pressure but cfm.

Peter

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From: [email-masked]
Date: 10 Dec 1996 02:38:11 GMT

In article <[email-masked]>, [email-masked]
(Alex Sallwey) writes:

>I also went to Home Depo on Sat.  The C-H 6.5hp that someone here said 
>could be bought from C-H refurbished and delivered for $430(?) was priced

>at $389.  I wonder if it is the same one?  Probably.  Interestingly the
4.
>5 hp C-H motor draws the same amps as my measly 1 hp.  Although the 
>compressor is probably larger as it is rated around 6 cfm at 40 psi.  
>
>

Oooops my dyslexic fingers, that was $349 delivered. Being out of state we
save the tax too. Mine is on the way, hope it arrives this week.

Peter
Dyslexics of the world untie.

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From: [email-masked] (Fitch R. Williams)
Date: Tue, 10 Dec 1996 05:18:21 GMT

[email-masked] (Reckless) wrote:

<snip>

>I would not say that *all* compressor motors are 1.15 SF.  The vast
>majority of the units I have seen are, but not all of them.

>After looking through my Grainger catalog, I see that GE makes motors
>used on C-H, Sanborn and other compressors.  These are rated at 1.0SF
>"compressor duty" which seems somewhat of an oxymoron.

<snip>

>-Reckless

Near as I can tell "compressor-anything" is a euphemism for
mysterious violation of usual rules.  After my investigation
of compressor horsepower, compressor duty seems highly
suspect.  I think it is, at best ,a way of saying if you
want a *real* 2 hp motor, don't use this one, it will fry on
you.  That is not necessarily bad, just like Ketterings
application of duty cycle to electric starting on cars as
was mentioned before.  It does take the margin out of the
rating though since it is rated by counting on it only
delivering its rated hp for the last few seconds of the pump
up cycle.

The motor pulley exercise described earlier was actually
quite clever since what was done was to take the motor right
to its max nameplate when it finally pumped up to full
pressure.  Note that the use of the compressor in the
authors application reached equilibrium at a much lower
pressure than cutoff - i.e. when he was using air the supply
equaled demand at well under the set pressure of the switch,
so the motor just sat there happily spinning the compressor
at under rated load for hours with out overheating.  The
author also indicated that a gage was provided to permit the
pressure to be monitored.  What he did was to optimize his
air system for a particular purpose as opposed to the normal
pump it up and shut it off general purpose scenario.  In his
case the pressure switch was more of a safety device than
enything else.

This can work just fine as long as no one uses the system
who doesn't understand it.  It is considerably less fool
proof.

Fitch

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From: [email-masked] (Alex Sallwey)
Date: 10 Dec 1996 15:17:51 GMT

Ok, ok, I got the cfm reversed, and the NON dropped out when I talked 
about the oil, it should have been non-detergent.  Sorry about that.
Alex

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From: [email-masked] (Reckless)
Date: Mon, 09 Dec 1996 08:23:53 GMT

As far as continous duty, that depends upon the type of insulation
used and the temperature rise.  Some motors will say "continous duty",
and others may not state it but can be under the right circumstances,
like in a cool environment, but not when in a 140-degree attic area.

Compressors would rather be running continously rather than rapidly
cycling.  The start windings are much smaller wire than the run
windings and get switched out once the motor is at speed.  In a rapid
cycle situation the start windings are in use much more often and are
likely to burn up.  The little "contractor" compressors with 1-6
gallon tanks are infamous for killing motors due to rapid cycling.

-Reckless

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From: [email-masked] (Alex Sallwey)
Date: 9 Dec 1996 18:55:05 GMT

This has been an interesting thread.  FWIW, on Friday I remembered to 
brush the dust off the nameplace of my 1971 twin cylinder Sears with 1hp 
motor.  My motor is rated at 17.5 amps at 120v, and the compressor is 
rated at 4.5 cfm at 45 psi and I think around 6 cfm at 90 psi.  The belt 
is a flat belt and the motor pulley is about 1 3/4".  I was thinking of 
trying a larger pulley, but being a flat belt this may be difficult.  I 
have not checked with Sears to see if they still use this belt, but most 
belts I have seen are v-shaped.

at $389.  I wonder if it is the same one?  Probably.  Interestingly the 4.

I can understand the desire for more air, I too want to run a sand 
blaster and could use more.  Just thought I would toss out my findings.

One more thing, my compressor instruction sheet said to use 10w-30 
detergent oil.  I guess this was before the days of specific oil for 
compressors.  I'll have to check at Sears the next time I'm there to see 
if they recommend it for their new compressors.

Alex

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From: [email-masked] (Jens)
Date: Sun, 08 Dec 1996 23:48:05 -0800

[email-masked] (Alex Sallwey) wrote:

>the compressor is 
>rated at 4.5 cfm at 45 psi and I think around 6 cfm at 90 psi.  

> Interestingly the 4.

>detergent oil. 

I am very confused here ..... more cfm at higher pressure ????  At
that rate if I raise the pressure on my compressor to maybe 200 PSI I
should get a real breeze going .....

Also I presume you meant to say NON DETERGENT oil ?????

Jens
(confused as always)

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From: [email-masked]
Date: 6 Dec 1996 15:55:50 GMT

Peter

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From: [email-masked]
Date: 10 Dec 1996 17:17:41 GMT

In article <[email-masked]>, [email-masked]

>about the oil, it should have been non-detergent.  Sorry about that.

Hey Alex, got a small tube so I can syphon this Mobil-1 out of my new
compressor before I do some damage? Grrrr

Peter
Just kidding. <g>