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Re: Compressor-related motor question

To: <>
Subject: Re: Compressor-related motor question
From: "Karl Vacek" <>
Date: Thu, 15 Jul 2004 12:35:17 -0500
OK, thanks.  As far as all the assumptions about pulley sizes, I really
can't say, as I loaned that particular compressor out 20+ years ago and that
guy's still got it (my dentist pal - long story).  However, the compressor
pump itself was very similar in model to one I still have, and the pulley
ratio I have on that unit is less than 2:1 with a 1725 RPM motor, so maybe I
would have gone to somewhere near 4:1 - not a big deal, no real concerns
about wrap length, etc.  Or I could have added a jackshaft if it was a

As far as starting torque, before a decent-sized compressor is up to speed,
you'd need a doggone large accumulator or else need to run unloaded a few
revolutions (which is what lots of better compressors in this range in fact
do - centrifugal unloader) to alleviate most of the starting load.  No
realistic amount of longer pipe, etc. between the compressor and tank would
do much there.  Maybe a huge aftercooler...

I was an ME, not an EE, and don't really pay much attention to all the RMS
and Power Factor stuff -- Purdue made you guys learn all that - not us next
door in the ME Building.  I just figured there was something inherent in the
number of poles where more poles added efficiency, torque, etc.  Sounds good
to me anyway  ;-)


> However, my old electrician pal warned
> me off  -- he said the 3450 RPM motor had fewer poles and thus
> less start-up
> torque even when "geared" back to the same RPM by a pulley
> change.

IMO it's a myth, at least as stated.  AFAIK, all other things being equal,
the 3450 rpm motor should develop half the starting torque of the 1725,
which would make it the same once it was geared back to the same compressor
rpm.  But there might be some other factor at work, perhaps the 1725 you
were looking at was not designed for as much starting torque or whatever.
Some motors will overheat when used for cyclic loads like air compressors,
since they do have a higher peak-to-average power ratio than other services.
"Compressor duty" is one step beyond "continuous duty".

It's easy enough to reduce the starting torque requirement for an air
compressor anyway, all you do is increase the volume between the pump and
the check valve into the tank.  This volume is bled down to zero pressure
after each run, so the pump starts with no load.  A bigger volume gives the
motor more time to start.

Of course, achieving that 2:1 reduction in drive ratio might be a problem
too ... most compressors already have a 3:1 or 4:1 ratio.  Don't have my
design handbooks handy, but 8:1 sounds like an awfully big reduction for a
single belt.  There's also a minimum radius below which belts aren't as
efficient, so you might need to increase the pulley on the compressor
instead, which would be another PITA.


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