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  1. #1
    Weekend Mig is offline Plastic
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    Default Air Compressor Motor Size???

    So, I hope I posted this in the right place...

    What size motor (hp) do I need to run a 3-cylinder, single-stage compressor pump?

    There is no stamp on the motor, but from what I can find on the web, similar pumps are usually called "4 or 5 HP" pumps - rated to run around 1200rpm. Does this mean I need a 5 HP motor to run it?

    I bought this 60gal vertical compressor "used" of course. The existing motor is a Baldor 2hp running at 1725 rpm (nice motor). But in order to get around 1200 rpm on the pump (a 12.5" flywheel"), I installed a 9" pulley on the motor. This causes it to draw almost twice the rated amperage - which will obviously burn the motor eventually.

    So, I would like to get a different motor that runs faster. I see that most compressor motors run at around 3450rpm (twice as fast as mine). At this speed, I would need a 4.5" diameter drive pulley - much more standard.

    So again, do I need a 5hp motor (expensive), or can I use something smaller?

    Oh, I am using 220V / 30A power.

    Thanks for the help.

    Weekend Mig

  2. #2
    Mike C. is offline Diamond
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    To drive a real live 5hp pump, you'll need a 7.5hp motor. You really need to find out more about that pump. Exactly what make and model it is, before deciding to whiz it up to 1200rpm. Remember, wear on rotating machinery increases as the square of speed. Double the speed, 4 times the wear. Make sure that pump is rated for that speed.

    Compressor hp is properly measured by the delivery capacity of the compressor, not what size motor is on it. Rule of thumb is 5cfm at 90psi = 1hp... and that's a real horse, not one of those little chinese ones. If you go down to the local big box or HF and use that measure of the compressors, you will find they are typically rated at two to three times their actual output. They take a big 60gl tank, slap a 2hp motor on it and rate it as a 5hp-7hp "peak" motor. In other words... it can withstand an instantaneous 200% load for startup to get over top dead, so that's how much power they rate it at.

    I had a guy try to sell me one of these over rated ones a few months back. It was a 60 gallon "7hp" compressor. It drew 14amps at 110V. Anybody see a discrepancy here? lol. It might be 2hp on a real good day, more like 1 1/2. I have an old crappy BlackMax "3 1/2hp" airless that puts out more (a measured 8cfm at 90psi).

    Point here is that you cannot get more hp out of the pump than the motor delivers, in fact, you will get significantly less. That's why your motor is so overloaded trying to pull that pump. Neither the motor or pump is coming to full speed, as well. That's gonna cook your motor in very short order (like a few quick test runs).

    You will probably be very happy with a 5hp motor on that compressor, once you figure out the speed and specs on it. It'll put out a real world 3.5hp (15-18cfm at 90psi), which will run about anything you need, short of a 3" sandblast hose.

  3. #3
    Weekend Mig is offline Plastic
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    Default

    Thanks for the quick response and info. So here's some more...

    The pump has no name plate, but it looks like a "el-cheapo" pump sold by Princess Auto (the Canadian version of your Harbour Freight Tools). They typically sell these pumps as 4 HP, 18 CFM @ 1200 RPM.

    So here's the deal with my situation...

    I use my compressor a fair amount, but not for continuous use tools. So typically, I am using the air in the receiver only, the pump simply comes on to recharge the receiver. I do not need the compressor output to "keep up with the tools". I sure hope that made sense.

    What I want to do is to recharge the receiver as quickly as possible (to keep the shop quite if nothing else), so I thought to have the pump run at the 1200 RPM would be best - it takes just over a minute for the receiver to recover from 100 to 140 psi at that speed.

    With the motor/pulley setup I have, to reach the desired pump speed, the motor is WAY over-worked, and I do not want to continue to do this.

    So if all I'm after is pump speed, maybe HP is not my issue? Could I get away with a smaller, faster RPM motor - like I said most compressors I have seen have 3450 RPM motors? Would a 2HP, 3450rpm do the trick?

    Any thoughts?

  4. #4
    Garwood is offline Stainless
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    First off, if your compressor is single stage it likely should not be pumping to 140 PSI. Single stage units do not work very well past 125 or so.

    Most compressors are not 3450 RPM, just the junk ones. In recip compressor land you always want lower RPM, not higher. 1200 pump RPM sounds about 300 RPM too high to me, but it's your compressor.

    I would do one of two things- put a size pulley on your motor that keeps the motor amp draw below it's rated capacity at the highest tank pressure. Or stick a 1750 RPM motor on thier that's larger than needed and drive the pump at whatever RPM you deam safe (I would keep it around 900 myself).

  5. #5
    Mike C. is offline Diamond
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    Using the figure I quoted you of 5cfm at 90psi, you'll see that is actually a solid 3hp compressor at best. They are calling it a four because at full output, you'll need a 5hp motor to drive it.

    Let's get some basics down here. If you turn that compressor faster, it pumps more air. That's why the tank fills faster. Pumping more air requires more power, and power is virtually unlimited in the form of electricity out of the wires. Your motor converts electricity to power, but is limited on the amount of power it can continuously convert. That's why the motor draws twice its rated amperage tryig to pul the compressor that fast. You can drop the pressure shutoff point on the switch to help out a little, but not much. The compressor will then run almost continuously to maintain the lower pressure, because you are still using the same amount of air in your tools, just not holding anything in reserve.

    A 2hp 3450 motor will be turning twice as fast, but have exactly half the torque of a 1700rpm 2hp motor. To make up that torque, you'll have to double the reduction to get twice the torque multiplication. Motor spins twice as fast, compressor spins the same speed as with the 1750 motor. It'll turn the compressor at the exact same speed in the end because it only puts out 2hp.

    hp is a measurement of work. 2hp will only do so much work. Turning that compressor at 600rpm is work. Turning it faster is more work. You can increase the speed until you max out the amp rating of the motor, and then that's it. If you use a larger motor, it can handle the larger amp draw, thus converting more electricity to power, and can run the compressor faster.

    The only way to do more work with that compressor (just like an engine, which is a compressor) is to make more air go through it in a given period of time. To do that, it has to spin faster, which is more work. If you want to do the work faster (fill the tank or recover faster), you need to get a bigger motor. No free lunch.

  6. #6
    Mark Rand is online now Titanium
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    Put the original pulley back on the motor. Go to the local muffler shop and get a middle box for a small car. Weld up flanges as appropriate and attach the muffler to the compressor inlet. Fit filter to the end of the pipe. 1 minute from 100 to 140 psi is not unreasonable.

  7. #7
    Weekend Mig is offline Plastic
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    Quote Originally Posted by Mark Rand View Post
    ... Go to the local muffler shop and get a middle box for a small car. Weld up flanges as appropriate and attach the muffler to the compressor inlet. Fit filter to the end of the pipe...
    That's an interesting set-up you describe, and since I have a 3-cylinder pump, it would be quite the little header I would be fabbing - a project for a "rainy day". I can just imagine what my friends (who already think I have WAY too many tools) would think...


    Quote Originally Posted by Mark Rand View Post
    Put the original pulley back on the motor... 1 minute from 100 to 140 psi is not unreasonable.
    This is with the pump running at around 1200 RPM. With the "original" 4.5" pulley, the pump ran around 600 RPM, and took quite a bit longer - I'd guess it was around 4 min.

    I've actually noticed that the pump runs smoother at higher speeds. At 600 RPM it seemed to bounce around a little. Closer to 1000 RPM, things are way smoother.

    I think I'll be looking for a bigger/faster motor.

    Thanks again.

  8. #8
    14tony is offline Hot Rolled
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    "Most compressors are not 3450 RPM, just the junk ones."

    Wow, I wonder if IR knows so many of their compressors are junk.

  9. #9
    Garwood is offline Stainless
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    Quote Originally Posted by 14tony View Post
    "Most compressors are not 3450 RPM, just the junk ones."

    Wow, I wonder if IR knows so many of their compressors are junk.
    Yes, I'm sure they do. IR is not a good name in compressors.

    There's no need to get in a pissing match over who's compressor is better. Compressors are very simple machines, cheap junk ones can still last years troublefree. Good ones run continuosly loaded, in the weather for decades with minor routine maintenance. Good ones are overhauled when they wear out. Cheap ones are tossed in the scrap pile and replaced. Quincy, Gardner-Denver, Worthington, Curtis- Those are great names in recip compressors. You will not find a compressor made by one of those with a 3450 RPM motor on it. Many companies today are still riding the coattails of the great name their products built for them 50+ years ago. Most compressors are junk today. IR is a prime example of a shit product under a once decent name.

  10. #10
    14tony is offline Hot Rolled
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    I think you are overly biased. I have two IR's. A 10 HP and a 7.5 HP both with 3400 RPM motors. The 10 is a 60's vintage and had it's first overhaul seven yerars ago. The 7.5 is a 70's vintage and had it's first overhaul last year. Both are run hard. The answer to long life with any compressor is proper maintenance. The last two Quincy's I dealt with
    (15 & 10 HP) lasted about three years each. The 15 was a constant headache. The Quincy tech couldn't even fix it. The 10 wasn't much better. The best I can say is, your experiance does not match mine.

  11. #11
    Weekend Mig is offline Plastic
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    Quote Originally Posted by Garwood View Post
    ...There's no need to get in a pissing match over who's compressor is better...
    I think it just happened...

    If IR makes cheapos - I'd hate to have someone comment on mine...


  12. #12
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    Mickey_D is offline Stainless
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    Ingersoll used to make very good compressors but a lot of their smaller stuff is made in India now. Indian quality makes you wish you had upgraded to the Chinese version... I replaced my Indian Ingersoll with an American Champion and could not be happier.

  13. #13
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    Metalcutter is offline Titanium
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    The first thing is to decide how much air you will be using. If the pump & motor will be making air for 5 minutes and off cycle for 20 minutes, then you can run the pump much slower and 5hp motor will be acting as a much lessor hp motor and not see so much wear.

    I have a two stage pump 5hp motor and I'm using a smaller motor sheive so the pump turns more slowly. I can usually lay my hand on the cylinders after pumping for an instant.

    50% time on and 50% time off is usually the turning point for continuous running.

    Regards,

    Stan-

  14. #14
    Mike C. is offline Diamond
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    "A 10 HP and a 7.5 HP both with 3400 RPM motors."

    Check the CFM ratings and compare with my numbers above. You'll see why Garwood says what he does. I'd take a 1750rpm motor compressor over a 3400rpm, anyday, and an 1100 is even better. Torque is king.

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