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  1. #1
    PackardV8 is offline Cast Iron
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    Default 1140 vs 1725 vs 3450 RPM motor for air compressor

    The best motor minds on the net weigh in here, so help me with an answer in small words. I'm helping a friend re-power an air compressor. It has a HP/RPM/CFM chart for 3/4 to 1-1/2 horsepower motors. We have two 6.1 amp TFEC motors. One is a very heavy duty all-cast-iron 1140 RPM 3/4hp rating and the other is a medium duty 3450 1hp rating. The 1hp 3450 is a drop-in mount, using the existing 3.3" pulley. The physically larger and heavier 3/4hp will need an 8"-9" pulley with a 7/8" bore and a new belt, which are expensive additional costs.

    Naturally, the heavier motor would be our preference, but is there any real reason to spend the money? Would we notice any real performance or longevity difference? Any advantage to one RPM over the other?

    thnx, jack vines

  2. #2
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    The quickest/easiest/cheapest option is the same (or similar) rpm. The lower rpm motor does have the advantage of at least twice as long a bearing life, but the bearings in a 3450 rpm motor last years when used on an intermittent load - like a compressor. No matter which motor you use, the pump still needs to turn the same rpm as before, so it will still make as much noise as before.

    If noise reduction is important, use the 1140 rpm motor, realizing that cfm output will be only 33% of "normal". But you'll hardly know the pump is running.

    ---------
    Barry Milton

  3. #3
    Torque1st is offline Aluminum
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    Usually the pump needs to run within a specific RPM range for the oiling system to work. Otherwise the pump bearings may fail. Check with the pump manufacturer.

  4. #4
    rons is offline Stainless
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    Quote Originally Posted by precisionworks View Post
    The quickest/easiest/cheapest option is the same (or similar) rpm. The lower rpm motor does have the advantage of at least twice as long a bearing life, but the bearings in a 3450 rpm motor last years when used on an intermittent load - like a compressor. No matter which motor you use, the pump still needs to turn the same rpm as before, so it will still make as much noise as before.

    The tank will charge faster resulting in less motor time. So not really a valid
    statement unless continuous operation is the concern.

    If noise reduction is important, use the 1140 rpm motor, realizing that cfm output will be only 33% of "normal". But you'll hardly know the pump is running.

    Any reduction in cfm might occur when the tank is re-charging.
    ---------
    Barry Milton
    -------------------

  5. #5
    rons is offline Stainless
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    Quote Originally Posted by PackardV8 View Post
    The best motor minds on the net weigh in here, so help me with an answer in small words. I'm helping a friend re-power an air compressor. It has a HP/RPM/CFM chart for 3/4 to 1-1/2 horsepower motors. We have two 6.1 amp TFEC motors. One is a very heavy duty all-cast-iron 1140 RPM 3/4hp rating and the other is a medium duty 3450 1hp rating. The 1hp 3450 is a drop-in mount, using the existing 3.3" pulley. The physically larger and heavier 3/4hp will need an 8"-9" pulley with a 7/8" bore and a new belt, which are expensive additional costs.

    Naturally, the heavier motor would be our preference, but is there any real reason to spend the money? Would we notice any real performance or longevity difference? Any advantage to one RPM over the other?

    thnx, jack vines
    The title "1140 vs 1725 vs 3450 RPM motor"

    Sounds like the compressor had a 1725 motor. And you want to
    work with what you have.

    1140 motor - 34% speed decrease
    3450 motor - 100% speed increase.

    You did not mention the capacity of the tank or the Hp rating of the
    previous motor unit. That would matter if you were thinking about slowing
    the charge time by using a slower motor.

    What was previous motor rating (and speed).
    What is tank size.
    Going to use for spray painting where cfm is important or just going
    to inflate tires or run a nail gun?

  6. #6
    Mud's Avatar
    Mud
    Mud is offline Diamond
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    Quote Originally Posted by PackardV8 View Post
    The best motor minds on the net weigh in here, so help me with an answer in small words. I'm helping a friend re-power an air compressor. It has a HP/RPM/CFM chart for 3/4 to 1-1/2 horsepower motors. We have two 6.1 amp TFEC motors. One is a very heavy duty all-cast-iron 1140 RPM 3/4hp rating and the other is a medium duty 3450 1hp rating. The 1hp 3450 is a drop-in mount, using the existing 3.3" pulley. The physically larger and heavier 3/4hp will need an 8"-9" pulley with a 7/8" bore and a new belt, which are expensive additional costs.

    Naturally, the heavier motor would be our preference, but is there any real reason to spend the money? Would we notice any real performance or longevity difference? Any advantage to one RPM over the other?

    thnx, jack vines
    I'm not a motor expert, I don't even play one on TV.

    However I did pick up your mention of pulley sizes, and I deduce from your comments that your original motor is 3450 RPM, and that you have already worked out what you need to run the pump at the correct rpm. I'd stick with the medium duty 3450 1HP as a bolt in for ease and low cost, without losing any HP, unless you are running the compressor continuously for hours at a time.

    How much does it run, and how is it to be used?

  7. #7
    PackardV8 is offline Cast Iron
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    Thanks for all the replies. This is a Quincy 206 compressor with a 30-gallon tank and has an operating range of from 500 to 1000 RPM. The 500 RPM is for continuous use and the 1000 is for quick recovery intermittent use. We'll be using it for intermittent use.

    Question. What stats on the motor data plate explain why one 6.1 amp motor is rated at 3/4hp and the other at 1hp?

    thnx, jack vines

  8. #8
    rons is offline Stainless
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    The 3450 would work as long as the pulley arrangement
    keeps the pump turning in the range it was designed for.

    Pulleys and belts do not cost that much. Some experience
    I have had with those small 30 gal compressors is they
    sure make a lot of racket. Would be nice if you could
    quiet things down with the 1140 motor.

  9. #9
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    What stats on the motor data plate explain why one 6.1 amp motor is rated at 3/4hp and the other at 1hp?
    Motor manufacturers are famous for over rating motor hp. The formula for correctly calculating motor hp is:

    HP = V x I x Eff
    746



    So take 120 x 6.1 x .82 / 746 = .80 hp

    One motor truly makes 3/4 hp plus, the other motor really would like to make more but physics will not allow it to - so the manufacturer simply lies about the output

    --------------
    Barry Milton

  10. #10
    HelicalCut is offline Stainless
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    Question. What stats on the motor data plate explain why one 6.1 amp motor is rated at 3/4hp and the other at 1hp?
    Nothing, they are not mentioned on the name plate. Leakage inductance, capacitor size (single phase) and saturation flux density are the determinants of this and they are chosen by the motor designer.

    The lower amp per HP motor will probably have a better power factor and or efficiency.

    Consumer compressor manufacturers usually use 2 pole motors because they are about 1/2 a frame size smaller than 4 pole motors for the same HP and they can use a smaller cheaper pulley.

    6 pole motors are a frame size bigger again, they are rarely used except in direct drive applications.

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