Running compressor "too slowly"

It's likely that the manufacturer offered different models with the same pump on them, but with different HP motors (and outputs).
A pump that's used on a 5HP model may be the same one used on a 7.5HP model, with the 7.5HP model having a slighly larger motor pulley diameter.
A pump used on a 10HP model probably isn't the same one used on a 5HP model.
I will probably be more difficult to find a larger pump pulley, than to find a smaller motor pulley. A motor pulley that's too small could have a tendancy to slip at startup if the belt contact area with the pulley is too small.
The pump manufacturer may provide minimum and maximum pump speed recommendations. Proper lubrication is probably the only vital issue with reducing the pump speed.
Reducing the upper pressure limit cut-out is another option, in conjunction with a slower pump speed. The upper end of the cut-out limit is where the highest power demand is involved. Reducing an original upper limit of 175psi to 150psi, for example.
--
WB
.........
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One more issue is that if the pump does not get hot sometimes, then there could be condensation in oil (water in oil, milky oil).
i
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Ecnerwal wrote:

I certainly wouldn't want to run a splash lube compressor too slow. I pressure lube one shouldn't have a problem at any speed.
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I'm running a compressor underpowered but at specified speed.
I bought an old Atlas Copco 3-stage, 400PSI compressor with a 10 HP 3450 RPM motor. I put a 3 HP 3450 RPM motor (previously on a seawater pump) on it. Connected to an 80 gal. 600 PSI-rated Halon tank, I get 150 PSI of air okay. Sluggish starting.
Upgrade: Replaced the 3HP with a 5HP 3450 RPM motor from a large fan. Somewhat better starting.
PROBLEM: Starting torque is okay with either motor in warm weather.
For either motor, at winter temps (say, 20F to 0F or lower) I have to turn a propane salamander on the compressor for maybe 15 minutes in order to start it. Once started, and with medium air use, its own heat keeps it warm enough to restart in the cold.
Bonus question: Anybody have a manual, exploded parts diagram or parts list for an Atlas Copco model "KT 630 A1"? (The company doesn't admit to ever having made this old (50 yrs?) model.) Email me if you do.
--
Mike Spencer Nova Scotia, Canada
snipped-for-privacy@tallwhistlesships.ca
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Does it have unloaders? If not, perhaps electrically unloading it would help to start it. Then again, gearing it down might also help - particularly if it already has unloaders.
Sketch of an electric unloader:
Compressor out======+======checkvalve====tank===piping to shop=== | | | | Electrically operated valve to air
Set up where the valve is operated by a time delay relay such that the valve is opened for the first 10-15 seconds each time the motor starts.
--
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Yes. The unloader works fine. It's just a very clunky old compressor. Three pistons and crank with cold oil. Probably *should* have a special motor designed for prolonged starting load.

I use it so little in very cold weather that it's not worth the cost of the pulley or the trouble or the reduced pump-up time in warm weather.
The object of the post was to offer a possibly useful data point for the OP who was thinking of gearing down in a similar situation, i.e. that I get by without reducing speed except for extra bother in cold weather.
--
Mike Spencer Nova Scotia, Canada

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I am the OP, so that's fine and appreciated. Of course, you're also using a 400 psi compressor at considerably lower pressure, so that probably helps to keep your load down from the original design load. I won't really know until I get to inspect things exactly what I'm looking at, but I would guess they are typical shop air compressors aimed at 150-175 psi, which I probably can turn down to 110-120 for my purposes just fine and gain some that way, but I suspect I'll still need to slow whichever one I get (if any - never can tell with auctions, and I'm not going into a bidding frenzy) down some to run it on a motor I can actually run without ticking off my electric company (rural line, single phase, motor HP limited).
I used to have to throw a blanket over my lathe to keep the heat in and put a lightbulb under it to get it to start in the winter, since it was in an unheated shed. I wonder if there's synthetic compressor oil, and if so, if that would help any.
--
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On Feb 7, 6:01am, Ecnerwal I wonder if there's synthetic compressor oil, and

There is synthetic compressor oil. Should help. WW Grainger lists some in their catalog.
Dan
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On Feb 7, 6:01 am, Ecnerwal I wonder if there's synthetic compressor oil, and

There is synthetic compressor oil. Should help. WW Grainger lists some in their catalog.
Dan *******************
Very good stuff!
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