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Thread: Comparing Power Tool Motors

  1. #1
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    Comparing Power Tool Motors

    I am trying to figure out the differences between motors of the same horsepower rating where the published amperage requirement is different.

    In this regard, it seems that when a power tool manufacturer publishes a horsepower rating for a motor, the specs also identify the AC voltage (usually 120, but sometimes 110 or 130), frequency in Hertz (usually 60, sometimes 50) and the amperage requirements for that same motor. My problem is when the same horsepower rating is claimed from a motor running at the same voltage and frequency but with differing amperage requirements.

    Perhaps some manufacturers "fudge" their horsepower rating up or down a bit, but -- assuming they aren't -- I think seeing different amperages used by different electric motors to produce the same horsepower at the same voltage and frequency should tell me something useful about one motor versus another. But I'm just not sure what that tells me and how it should assist me in comparing the motors for real world use.

    For example, in comparing different drill presses from different manufacturers, I found the following published information for several different motors operating at 120VAC and 60 Hz:

    Motor HP Amps
    A 0.6 3.85
    B 0.67 5
    C 0.75 7.5
    D 0.75 7.2
    E 0.67 8.6
    F 1 13.5
    G 1.5 13


    So I tried to compare them on the basis of how much amperage the motor requires to produce the same amount of horsepower and got this:

    Motor HP Amps Amps/HP
    A 0.6 3.85 6
    B 0.67 5 7
    C 0.75 7.5 10
    D 0.75 7.2 9
    E 0.67 8.6 12
    F 1 13.5 13
    G 1.5 13 8


    The problem for me is that I really don't know what this is telling me. Am I seeing comparable efficiency of these motors or something else (or nothing of any real value)?

    As a person who obviously doesn't have a sufficiently deep understanding of electric motor principles, I would really appreciate any explanations or suggestions about trying to compare electric motors based on their horsepower and amperage ratings.

    By the way, I read the earlier thread about converting AMPS to HP (at https://sawmillcreek.org/showthread....-or-Visa-Versa) and I'm sure it's just my own ignorance keeping me from being able to use that information to understand what I'm looking at above . . . or even whether I'm looking at it in the correct manner. Anyway, I started to post this question there, but thought that thread might be considered too old for new posts.

  2. #2
    My experience is that the motor's efficiency makes a big difference.

    You get what you pay for: a (cheap) inefficient motor may actually cost more in the long run? Depends on how long and what load it carries.

  3. #3
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    Motor current is not an absolute measure of power output.

    A low efficiency, low power factor motor will draw substantially more current to produce the same power output as a high efficiency high power factor motor.......Regards, Rod

  4. #4
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    Also note that universal motors draw more current, and put out more horsepower, the more they're loaded...only up to a point, and only in theory.

    In fact, a router manufacturer can claim their router puts out 350 horsepower all they want, but on a 120V/15A circuit, even if the motor were 100% efficient (which it ain't and never could be), it could never put out more than 2.4 horsepower (120Vx15A=1800W=2.4hp).

  5. #5
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    Some basics:

    The amount of current a motor draws is an indication of the power INPUT. So your table multiplying volts and amps is showing how much goes in.

    But each motor may have a different EFFICIENCY in converting that to OUTPUT power. Its not that easy to measure actual power output (requires a dyno or brake or other equipment designed for it). Torque and speed are key. Output power is whats available at the shaft end to do work with.

    So without knowing the efficiency of a particular motor, input current wont tell you the whole story.

    Then just because a motor can have a high output power for a short burst doesnt mean it can sustain that continuously. Some motors are definitely playing marketing games with their numbers posting a HP level that is not sustainable, or that is input HP and not useful output HP. Things like shop vacs come to mind which have values pasted on them that are essentially, meaningless.
    Last edited by Carl Beckett; 03-11-2019 at 7:39 AM.

  6. #6
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    Perhaps some manufacturers "fudge" their horsepower rating up or down a bit, but -- assuming they aren't --"

    snicker snicker

  7. #7
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    Our "6 HP" Craftsman 16 gallon vac is only fudged by a factor of 3, or 4, or 5. Marketing is a weird business.
    Zach

  8. #8
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    You mean the hp of my shop vac is really not 6hp?

  9. #9
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    Quote Originally Posted by Tom M King View Post
    You mean the hp of my shop vac is really not 6hp?
    Of course it is Tom, for one brief shining moment..................Rod.

  10. #10
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    Quote Originally Posted by Rod Sheridan View Post
    Of course it is Tom, for one brief shining moment..................Rod.
    Should that read "brief glowing moment" Rod?

  11. #11
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    Quote Originally Posted by Curt Harms View Post
    Should that read "brief glowing moment" Rod?
    Yes, glowing is so much more descriptive

    Regards, Rod.

  12. #12
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    Paul,

    This thread has some discussion and info on how some manufacturers lie, er, advertise horsepower ratings, in this case specifically for shop vacs:
    https://sawmillcreek.org/showthread....otor&styleid=3

    I found this on a ShopVac web page:
    https://sawmillcreek.org/showthread....92#post2862692

    JKJ

  13. #13
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    Motors are so strange it seems - especially single phase. I have over the years purchased several Asian manufactured large power tools and several had motors with no horsepower ratings. Also had several with HP ratings that were listed but were physically half the size of other motors of the same HP rating. Most of my tools were switched to Baldor motors as that has been a brand I trust with 35 years of experience with them. Amazing how much larger my 1 1/2 HP Baldor motor is than my Craftsman or Jet motors of same HP.

  14. #14
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    Yes, All 6 H.P. motors can run on a 25 ft. 20 ga. power cord under full load!!. (Ha Ha!) I think they rate them on the power they are drawing just as the smoke gets so thick they can't see the amp. gauge!

  15. #15
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    Quote Originally Posted by Peter Kuhlman View Post
    Motors are so strange it seems - especially single phase. I have over the years purchased several Asian manufactured large power tools and several had motors with no horsepower ratings. Also had several with HP ratings that were listed but were physically half the size of other motors of the same HP rating. Most of my tools were switched to Baldor motors as that has been a brand I trust with 35 years of experience with them. Amazing how much larger my 1 1/2 HP Baldor motor is than my Craftsman or Jet motors of same HP.
    The original motor on my Dad's old Parks planer when he bought it, used, back in the early '70s, was probably 12-14" diameter, 12-15" long, and probably weighed close to 150# yet I think my Dad said it was only about 1.5 hp! Lots of head room before the Magic Smoke leaked out, though...

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