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Why does motor HP affect CFM of a compressor?

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  • Why does motor HP affect CFM of a compressor?

    Why would a 5hp motor deliver more CFM that a 1.5HP motor?

    Assume that pulley's, RPM etc all the same...

    If the smaller motor is able to make the pump work at 1.5hp, it should be no different than a 5hp motor in my mind.

  • #2
    Only if you spin the the pump faster, but they all have a max rpm....

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    • #3
      A pump and an engine have a lot in common. They have a certain pumping efficiency. In race engines we work hard to increase the engines pumping efficiency to reduce the power required for the engine to breathe. A pump is basically the same, but it is far cheaper to just add more power than to port, polish and play with valve events. Most compressors use reed valves, and that adds a level of difficulty to making it more efficient.

      It takes a certain amount of power to do anything. A fan moves more air (within reason) by adding more HP to it. A fan does not necessarily move more air because it is turning faster. A pump does not necessarily move more air because it is turning faster. They both have an efficiency range, designed around a certain RPM range, but adding more power will move more air. There are calculations to figure it all out.

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      • #4
        I'm a bit unsure of what your asking here? Are you intending to remove a 1.5hp motor from a compressor and replace it with a 5hp? Then no increase in air will be observed.

        The amount of air delivered is a function of the pump displacement (piston pump) times the RPM's. If the 1.5 hp motor will sustain the RPM's then going to a bigger motor will gain you nothing unless something else changes.

        lg
        no neat sig line

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        • #5
          Air compressors are rated in CFM at some pressure. A compressor will deliver a given CFM of free air at a given RPM regardless of HP, but as pressure increases the work required increases and the HP requirements increase to maintain the same CFM.
          Jim H.

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          • #6
            Originally posted by cuemaker
            If the smaller motor is able to make the pump work at 1.5hp, it should be no different than a 5hp motor in my mind.
            doesn't take much to turn the pump over initially, but listen to how it starts having to work as the pressure builds. Presumably the designer balanced that heavy load and duty cycle and safety factor and the answer was 5hp......if 1.5 will fill the tank in time and not burn out and meet the duty cycle requirement, well then, the the 5 was over spec'd
            .

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            • #7
              Well, its getting time to replace my motor on the compressor as the current one is getting to hot and tripping itself off (its really old, had burnt windings when I cleaned it out after I purchase the compressor)...

              I was trying to figure just how big a motor I needed and in doing google stuff, lots of different things I found said to increase CFM, increase the HP...

              I am currently happy with the CFM my little 1.5hp is giving. I am able to run my sandblaster, the only problem I am having is after 5 or 6 cylces in an hour the motor gets to hot and trips the thermal protection.

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              • #8
                A 2 HP or 3 HP motor certainly will not harm anything. It may give you a longer life cycle. I don't think I would go to a 5 HP, as the mount and tank may not be designed for the additional load.

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                • #9
                  If the motor is overheating, it can be due to built up dust and dirt in the motor. It is worthwhile to clean the motor internally and see if the overheating persists.

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                  • #10
                    The old motor is rated at 1.5 hp and barely works. You probably need more. Two would be better, might even be optimum. Three would be all you could use near efficiently, anything larger would be a waste of energy.

                    Just be sure that the replacement motor is accurately rated. The old 1.5 hp motor was likely a true 1.5, where a new compressor rated 2 hp would likely be 3/4 hp or less.
                    North Central Arkansas

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                    • #11
                      You only get more SFM out of a given compressor with more HP if you incress the RPM as well, ie incress the load

                      a 5hp 'compressor' produces more air then a 2hp one because the compression cylinder(s) is usally designed to compress 2.5x as much air per revolution, hence consumes 2.5x as much horsepower.

                      You likey do not want to run your current compressor at any higher RPM's then its currently running or IT can overheat/wear out prematurely.

                      IE: more HP only makes more air when mated with the correctly sized compressor. a motors HP rating is just when it overloads/cooks. It will keep spining untill it you draw that much HP and make it overheat/stall.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        Umloader

                        Check your unloader valve. Its purpose is to see that motor starts under minimum load.

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                        • #13
                          Originally posted by oldtiffie
                          Check your unloader valve. Its purpose is to see that motor starts under minimum load.
                          Unloader valve works...but I do have some belt slippage and a small air leak at the junction of the tube and unloader valve. All of which means I will go through and tighten up the system to ensure I am getting the most of the effort being applied...

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                          • #14
                            and how are the valves in the compressor pump itself ? ? ?

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                            • #15
                              On a related note, you can reduce the HP used by a compressor by simpley lowering the RPM (smaller motor pully) but less CFM will result.

                              Also, while maybe not a big factor in efficency per say, if your compressor sounds 'clacky', it could benifit from a modren all teflon proppet style check valve. I installed one in my slow old clacky 2hp compressor and its so quite now you can't even hear it running inside (it lives outside in a small alcove)
                              I leave it setup to pump up whenever and the neighbors never complain about it kicking in at random hours during the night (especialy if I leave too much hose/tool pluged in with those leaky quickconnects)
                              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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