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Thread: What is the point of service factor?

  1. #1
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    What is the point of service factor?

    Why would they offer 1/2 hp 1.5 SF instead of just selling it as 3/4 hp at 1.0SF?

  2. #2
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    While horsepower is a unit like watts, the horsepower rating of a motor includes about things like temperature rise, break-away torque, stall torque, etc.

    So a 1/2 hp motor with a 1.5 sf implies 1/2 hp overall characteristics but the ability to deliver 3/4 without overheating.

    -Jon

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    Quote Originally Posted by Electric-Light View Post
    Why would they offer 1/2 hp 1.5 SF instead of just selling it as 3/4 hp at 1.0SF?
    More money? Possibly the frame size?
    Electricians do it until it Hertz!

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    Quote Originally Posted by winnie View Post
    While horsepower is a unit like watts, the horsepower rating of a motor includes about things like temperature rise, break-away torque, stall torque, etc.

    So a 1/2 hp motor with a 1.5 sf implies 1/2 hp overall characteristics but the ability to deliver 3/4 without overheating.

    -Jon
    I think that's the question. If it can continuously deliver 3/4 HP, why not just call it what it is?
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Besoeker View Post
    I think that's the question. If it can continuously deliver 3/4 HP, why not just call it what it is?
    For the record.. IEC does not utilize SF like IEEE

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    Quote Originally Posted by Besoeker View Post
    I think that's the question. If it can continuously deliver 3/4 HP, why not just call it what it is?
    Again the 'horsepower' rating implies a bunch of other characteristics such as peak torque (breakdown torque), starting torque, etc.

    If the motor has the starting characteristics of a 1/2 Hp motor, but is thermally capable of 3/4 Hp output, what would you call it?

    IMHO it probably makes more sense to break out all of these factors, and separately state the starting torque, breakdown torque, thermally limited output, etc. But if you want to dumb it dowm to one number....

    -Jon

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    Quote Originally Posted by winnie View Post
    Again the 'horsepower' rating implies a bunch of other characteristics such as peak torque (breakdown torque), starting torque, etc.

    If the motor has the starting characteristics of a 1/2 Hp motor, but is thermally capable of 3/4 Hp output, what would you call it?

    IMHO it probably makes more sense to break out all of these factors, and separately state the starting torque, breakdown torque, thermally limited output, etc. But if you want to dumb it dowm to one number....

    -Jon
    The horsepower rating and service factor tells you none of these things.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Electric-Light View Post
    For the record.. IEC does not utilize SF like IEEE
    This I know.
    Si hoc legere scis nimium eruditionis habes.

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    The best explanation I've heard for service factor is that it's a way to run the equipment outside of design rating without voiding the warranty.

    Because my understanding is that any extended duty in the service factor is very likely to result in heating that reduces operational life, so it's not like you're getting a better motor just because it's rated at 1.15SF

  10. #10
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    Quote Originally Posted by winnie View Post
    Again the 'horsepower' rating implies a bunch of other characteristics such as peak torque (breakdown torque), starting torque, etc.

    If the motor has the starting characteristics of a 1/2 Hp motor, but is thermally capable of 3/4 Hp output, what would you call it?

    IMHO it probably makes more sense to break out all of these factors, and separately state the starting torque, breakdown torque, thermally limited output, etc. But if you want to dumb it dowm to one number....

    -Jon
    Bingo. I'm not going to bother to look it up, but the official NEMA definition of motor "service factor" states something to the effect that if you use the SF, you can expect that the performance specs will NOT be as stated on the nameplate, including things like starting torque, starting current, efficiency, slip, break down torque, bearing life, temperature rise, etc. In other words you can use it at the SF if you want to, but you can't expect it to act and preform as per spec. A decade (or two?) ago, NEMA MG-1 USED TO also say words to the effect of "... and you will experience shorter motor life", but under pressure from OEMs, they removed those words.

    A Service Factor of 1.5 is kind of rare for off-the-shelf motors, usually it's 1.15, maybe 1.25 on small motors or very large motors, which makes more sense in an OEM world where dollars count. If I need 3.75HP at the shaft, I can use a 5HP motor and have some fudge factor, or I can use a 3HP motor with a 1.25 SF and save some $$. The OEM will save the $$ and hope it outlasts the warranty period, an end user should consider using the 5HP motor and not have to replace it after having their production line go down and lose money.
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