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Do I have the right motor here, or not ?

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  • #16
    The sketchy pumps from the big box store are not rated for continuous duty. Yes it can (briefly) make 6HP, but it'll self immolate after thirty seconds of continuous running. There may be a thermal fuse/cutout embedded in the motor windings that prevents the thing from running continuously. The same applies to the pump itself, and it probably doesn't have the metal to run continuously without warping into an egg shaped cylinder.
    -paul

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    • #17
      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.
      They all play that game. The motors may develop that HP at max RPM's but certainly not off the start or any where near starting or ramp up R's. It's a huge selling point for them.

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

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      • #18
        yeah, thats why I'm trying to get an 'honest' 3hp or 4hp compressor. A large bore pump turning slow.
        I'm upgrading the 5hp RPC up to 7.5 and run the 5hp (prior idler) to the pump hopefully at a 3hp amp load

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        • #19
          Originally posted by Ringo View Post
          yeah, thats why I'm trying to get an 'honest' 3hp or 4hp compressor. A large bore pump turning slow.
          I'm upgrading the 5hp RPC up to 7.5 and run the 5hp (prior idler) to the pump hopefully at a 3hp amp load
          I know exactly what you mean. I think to find honest ratings, you have to go back to the 1960's or something. BTW the most honest shop-vac I ever saw was when we had a wood stove, had to call the chimney sweep every summer. Dude had a regular 1 horse induction motor bolted on top of a 55-gal steel barrel.
          25 miles north of Buffalo NY, USA

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          • #20
            in a woodshop, that would be called a 'dust collector', for sukking sawdust an chips

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            • #21
              Yes, YES, YES! Motors are not 100% efficient. So there are TWO HP numbers associated with any particular motor/load configuration. The first is the electrical HP that is required to run the motor and that one can be accurately calculated from the Voltage and Amperage. But it means little or nothing on the output or shaft side of the motor. Mechanical HP that is actually produced AND delivered to the load is what counts there. And that can depend not only on the losses in the motor, but also in the characteristics of the load itself.

              Yes, you can calculate a general figure from the electrical numbers, but that is only a starting point.



              Originally posted by rdfeil View Post
              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? 😁
              Paul A.
              SE Texas

              And if you look REAL close at an analog signal,
              You will find that it has discrete steps.

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              • #22
                OK guys, tell me this one?
                The crappy fake 5hp 3450 motor takes 10amp to run at free idle.
                I twiddle with the caps on the 7.5rpc and add the 5hp motor my total is guess what?? ...10.2amps.
                How does the 7.5 idler plus the 5hp (idling) work out the same amps as the crappy 1ph fake?
                Yeah, I get it that everything is idling, but, that's 12hp idling vs 5hp idling. all for similar amp load.(give or take half amp)
                yes, reading all this from single phase side of things
                all 3ph motors are 17xx rpm

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                • #23
                  Originally posted by Ringo View Post
                  OK guys, tell me this one?
                  The crappy fake 5hp 3450 motor takes 10amp to run at free idle.
                  I twiddle with the caps on the 7.5rpc and add the 5hp motor my total is guess what?? ...10.2amps.
                  How does the 7.5 idler plus the 5hp (idling) work out the same amps as the crappy 1ph fake?
                  Yeah, I get it that everything is idling, but, that's 12hp idling vs 5hp idling. all for similar amp load.(give or take half amp)
                  yes, reading all this from single phase side of things
                  all 3ph motors are 17xx rpm
                  Wait, what??? thats weird, I would think the crappy motor has a short in it if it needs that much juice to idle. No load on it, right?
                  25 miles north of Buffalo NY, USA

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                  • #24
                    yes, seems weird, the crappy motor I checked before I started all this compressor hoopla. Crappy motor is 10amps with no belt on it and 15a when compressor fills the tank.
                    No shorts, no extra heat, no sparks, runs fine, just fake 5hp it seems.
                    ........been talking to service reps at the major compressor folks, my whole idea of running a pump 2x as big at 1/2 the speed to get more quiet and less heat.
                    the crappy fake was indeed a fake 5hp. A true 3hp should get about 11CFM and the crappy fake-5 was 10.
                    Come to find out an average compressor can make about 3.5 CFM/HP. And 4 CFM/HP would be high performance.
                    Now my next trick is to convert CFM to amps, and keep this new machine on the same old 20a circuit (my breaker panel is full)

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                    • #25
                      That sounds like very low PF. It may be helped by adding three capacitors phase-to-phase on the crappy motor. As for values, try some that will draw about the same, or maybe half, the current as the motor is drawing. 220 VAC and 10 amps works out to 22 ohms reactance. C = 1 / (2 * pi * f * X). I get 120 uF for 60 Hz.
                      http://pauleschoen.com/pix/PM08_P76_P54.png
                      Paul , P S Technology, Inc. and MrTibbs
                      USA Maryland 21030

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                      • #26
                        Well, I got a failure,,,, the sketchy fake 5hp motor draws 20amp on the new compressor pump head, it makes more air than the old pump, but more amps than my estimated 3hp. sketchy motor only rated at 15 so its 5amp over.
                        Setup the 7.5hp RPC turning the 5hp threephase motor hoping for a really true 3hp out of it. Nope. drawing way more amp than the sketchy motor. it was tripping the single phase breaker.
                        Moved it over to a 50amp breaker and it takes 28amps for the same pump RPM as the sketchy little motor.
                        Funny though, the crappy motor rated at 15 and running 20 gets real hot real fast.
                        The big 3ph motor rated at 14amp and running 20 don't get hot, it doesnt seem to care.
                        But at the breaker box, the 3ph motor takes lots more amps (thru RPC)
                        I got pulleys sized within 10rpm of either motor driving the pump.

                        Something not right here, and I can't figure out what.
                        Supposedly 11CFM is a 3hp air compressor. I'm getting my 11cfm, but drawing 5hp worth of amps to do it single phase.
                        On the RPC it looks like 10hp worth of amps for same 11cfm.

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                        • #27
                          Remember that VA is not power. The sketchy fake 5 HP motor drawing 20 amps at 240 VAC is 4800 volt-amps (VA). 5 HP with 0.9 power factor and 80% efficiency would be 5 * 748 * 1.1 * 1.2 = 4937 V-A. I assume you are reading single phase voltage and current into the RPC so there are power factors and efficiencies in the converter as well as the compressor. There may also be issues with phase angles and waveform distortion.
                          Last edited by PStechPaul; 03-24-2021, 01:29 AM.
                          http://pauleschoen.com/pix/PM08_P76_P54.png
                          Paul , P S Technology, Inc. and MrTibbs
                          USA Maryland 21030

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                          • #28
                            I read amp clamp at both ends of the deal. looking at amp at 1ph breaker, and motor, on both setups.
                            the fake 5hp idle 10a and ramped up to 20 with full tank, both ends of cord was same reading
                            RPC idle 10amp, and ramped up to 28a at the breaker, motor was 20 by itself
                            The 7.5hp idler AND the 5hp load motor was both exceeding amps

                            I tweeked around with capacitors in the rpc, and all I could was trade volts for amps. If I ran idle volts up high, I lower amps a little bit, and vice versa.
                            Put a cap across incoming L1-L2 lower idle amp a bit,,,, increase that to where idle amps was high, running amp went bit lower.
                            This is supposed to be 3hp pump but, it overdriving 2 different 5hp motors.

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                            • #29
                              Some of this discussion is a bit over my head, but I will say this: it's the amps that the power company is billing you for, since the voltage in their lines stays constant.
                              25 miles north of Buffalo NY, USA

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                              • #30
                                Are you turning the pump at the rpm at which the pump is rated at 3hp? In other words, do you have the speed reduction from the motor to the pump right?

                                Over speeding the pump will require more hp.

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