Re: Air Compresssor Advice Needed


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Posted by D Sherman [72.47.153.24] on Thursday, April 12, 2012 at 03:04:06 :

In Reply to: Air Compresssor Advice Needed posted by dave horvath [24.131.120.209] on Wednesday, April 11, 2012 at 20:49:09 :

As others have mentioned, the decision may be made for you by your electrical service. It's very rare to find an honest 7.5 HP motor in single phase these days. Most of the time once you get above 5 real HP, it's 3 phase. Cheap air compressor motors tend to be rated ridiculously high in HP, though, so the "7.5 HP" unit might be equivalent to 2 real HP. Check the nameplate amps and see what it really draws. They lie about the HP but not about the amps.

You can use a "static phase converter", which is just a fancy name for a capacitor box, to make a 3-phase motor run on single phase, but you lose some power doing that. It sounds like the limiting factor is going to be the wire from the house to the garage. Even if it's rated for higher current circuit breakers (one on each end) than you have in there now, which is unlikely, the voltage drop on start-up might be hard on the compressor. Compressors draw a lot of current starting up and they don't like a "soft" power supply.

If you're stuck with a low-hp compressor due to electrical limitations, you can always add another receiver (air tank). Once it's charged up it might provide enough air to keep you working until you're ready to take a break anyway. Lots of things can be used as receivers, especially if you're not subject to safety agency inspections -- long lengths of large-diameter pipe are popular as are old water heater tanks. Of course they can also blow up and kill you.

It's too bad nobody's making screw compressors in the smaller sizes. They are really amazing -- much quieter and no pressure pulsations in the line. Picture two large tapered worm-type screws meshing together, a bit like the gears in an oil pump but extending out longer than their diameter and tapering. The one we have for the mine tour is one of the smaller ones and it's 40 HP. Cost about $5000 used, in good shape, and overhauled by the dealer. The real mines typically have compressors in the 400+ HP range.



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