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Do I have the right motor here, or not ?

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  • Do I have the right motor here, or not ?

    My air compressor developed a bad tank, so I got a new tank. This is one of the big box store variety of compressor, it says its 6hp but it is the smallest 6hp I ever saw. I personnaly think it is a fake HP thing.
    Years ago I had a body shop with a huge slow turning compressor, best I can recall it was 3ph 3hp. It turned slow like a brummp-brummp-brummp, and with spray gun & sander it made more useable air than any I saw prior nor since. I want to duplicate this best I can.
    This thing I currently have makes more noise & heat than good air, so I decided to get new pump & motor to sit on top the new tank. it is vertical 60gallon.
    it is one of those high speed noisy 3450 motors the pump is really buzzing along. the motor doesnt even admit to its HP on nameplate (decal) but it does say 15amps.
    So I googled amps per HP and got a number of 6. (220v). 6 into 15 gives me estimated 2.5HP, and that is more believeable to me for air it puts out.
    I have now purchased a much larger pump, larger displacement and expect to turn it slower for similar HP, to lower heat and moisture at higher CFM.
    A talk with Quincy tech rep tells me of their large bore single stage makes 11CFM on 3HP @ 425rpm. Now thats more like it.
    A CFM test on the current compressor tells it is 10CFM @ 950RPM. now I am really thinking the fake 6hp high speed motor is really about 3hp, and it does amp clamp at 15amps.
    Fast track to now, I visited my nearest electric motor repair shop looking for a 1750rpm motor, an honest to god 3hp. This is the same guy I got my 5hp motor for my RPC that I built.
    We looked and looked at row after row of motors in all imagineable sizes & shapes. No 3hp to be found. He found a 1ph 5hp as big as a volkswagon, but that wouldn't fit on a vertical tank.
    So he comes upon a 7.5HP 3ph motor in good shape, and on a side comment says swap this out with your 5hp RPC and run compressor on that !!
    Hmmm, interesting. We struck a deal and I got a 7.5HP 213T frame motor.
    My 5hp idler is a 184 frame, a bit smaller.
    But then I can't understand this rub:
    I found a math formula that says amps x volts / 746 = HP
    that makes my original cheesy motor 15amps x 230v / 746 = 4.6hp. what ???
    what am I going to gain by swapping in all this extra motors & RPC?
    My gut tells me that original cheesy motor ain't even close to 5hp, but the math now tells me otherwise.
    why would I swap one 5hp motor for another?
    Did I buy the wrong stuff?

  • #2
    Something I've been railing about for years, is cheesey manufacturers using stall/burnout amperage instead of run amperage. ShopVac is notorious for claiming 6 horsepower, but a clamp on ampmeter will only show 5 to 6 amps running. Who the hell cares what it draws when you jam a screwdriver into the fan and it's drawing 6'000 watts the millisecond before it goes up in flames! I believe every on this forum is intelligent enough to realize a 20 amp, 120 volt circuit can only handle 2400 watts of draw.

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    • #3
      3 phase motors are inherently very efficient, cool- running and smooth. A 3- phase motor rated at 7.5 hp will probably make 7.5 hp for about 20 years before it dies of old age.

      And yes, the Shop- Vac at 3 hp will probably work for 30 seconds as a hair dryer, and then only as a way to get a VERY smelly fire started.

      I think your formula's missing something- RPM, for sure.
      As HorseyPowers are torques x distance...
      so a 3600 rpm '3 hp' motor is going to make half the torque of an 1800 rpm motor.
      The 1800 is going to need more magnet to make more torque... and thus, be 'heavier'.

      t
      has 3 '2 horse' compressors running in parallel.
      rusting in Seattle

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      • #4
        Your formula is correct except it is leaving out an efficiency factor. if the motor were 100% efficient it would produce 1 HP for every 746 watts (volts X amps) drawn. Most common motors are somewhere around 80% efficient so multiply your 4.6 HP by .8 = 3.68 Hp. Also, many cheap motors do not come close to 80%, some are as low as 40%. I just took a look at a code table for motor current and the code calculations for single phase motors are showing about 58% expected efficiency. With that, your 4.6Hp X .58 = 2.668 Hp... Sounds closer doesn't it? 😁
        Robin

        Happily working on my second million Gave up on the first

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        • #5
          yeah, its all slowly becoming clearer to me ,
          Since I have my RPC, I have noticed how much quieter/smoother a 3ph motor runs than 1ph, since I haven't been able to find a single phase motor for my re-doing air compressor, I looking into running 3ph with RPC. I wonder how much efficiency I lose by going that direction.

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          • #6
            Originally posted by Corbettprime View Post
            Something I've been railing about for years, is cheesey manufacturers using stall/burnout amperage instead of run amperage. ShopVac is notorious for claiming 6 horsepower, but a clamp on ampmeter will only show 5 to 6 amps running. Who the hell cares what it draws when you jam a screwdriver into the fan and it's drawing 6'000 watts the millisecond before it goes up in flames! I believe every on this forum is intelligent enough to realize a 20 amp, 120 volt circuit can only handle 2400 watts of draw.
            There is a lot of false advertising out there. I've seen a few compressor mfg. boast of their compressors having 5 HP motors with stickers plastered all over them stating that "Big Selling Point" and in small (microscopic print in an obscure and almost hidden area there is a sticker stating motor develops 5 HP at maximum RPM. I used to call them "Fake Fives"

            JL...................

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            • #7
              I have same motor on my Compressor,I believe the 5hp rating thing on these is listed on motor as SP which I assume means Special lol! I will be replacing it with Farm Duty 1ph motor when it fails,1ph motors vary a lot in quality have had numerous ones from 1/4 to 10hp on the Farm most run very smooth&quite but have seen the odd noisy one.

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              • #8
                For power, It's not just "efficiency" you also have to account for pf - power factor. pf varies from like 0.1 at "idle" to 0.9 for a good motor at full power. I doubt your motor is much more than 2 real hp, and rated for that intermittently. Look for the right-sized motor and a SF (service factor) of 1.15 or more. It will last...

                Assume you motor is a POS. Power =( V x A X eff. x pf ) / 764 = hp so 220 x 15 x 0.6 x 0.75 = 2hp. Lots of assumptions here even that your Meter is true rms, that that at full load (not over loaded), that the voltage at the motor terminals is 220, blah blah...
                Last edited by lakeside53; 02-18-2021, 11:18 AM.

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                • #9
                  I bought one of those importium compressor heads off the internet, in hopes of a larger slower turning unit. With the much larger flywheel, I can pulley down the speed from the crap motor 3450 to the pump 575. the original pump was going 925+ on a digital tach
                  and the pump arrived all broken up like it was in a wreck in transit

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                  • #10
                    Does a Delta wound motor idler effect your RPC? My (new to me) 7.5hp idler motor was connected 460v, and as I went to connect it down to 230v I realized it is a Delta motor.
                    How might this effect it as my RPC idler?

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