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Relation between torque and rpm

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  1. Feb 2, 2015 #1
    I was reading something and they said i was to decrease the rpm of a dc motor to increase the torque..
    What i dont get is the equation for torque is T=(2*p*N)/60
    So increasing the rpm should only increase the torque right..
    Im a little lost here..please help
     
  2. jcsd
  3. Feb 2, 2015 #2
    Torque = HP X 5252 / RPM
    So as rpm increases torque drops.

    How much TORQUE is required to produce 300 HP at 2700 RPM?
    TORQUE = HP x 5252 ÷ RPM
    Answer: TORQUE = 300 x 5252 ÷ 2700 = 584 lb-ft.

    How much TORQUE is required to produce 300 HP at 4600 RPM?
    TORQUE = 300 x 5252 ÷ 4600 = 343 lb-ft.

    I look at it as Work = Force X Distance
    increase distance (RPM), reduce force
    You ever ride a bicycle with gearing?
    Hope this helps
     
  4. Feb 2, 2015 #3

    billy_joule

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    Look at a DC motor curve or spec sheet- the max torque is produced when rpm is zero: 'stall torque'. Torque decreases down to zero as speed increases to no load rpm.
     
  5. Feb 2, 2015 #4
    You ever drive a car with a manual transmission?
    Low gear, high ratio, a ton of torque.
    High gear, low ratio, minimal torque.

    If you have a small motor running with nothing on the shaft, speed is max, torque is low
    grab the shaft with your fingers, speed drops, torque climbs to try to get the motor back to max speed.

    Have you ever done any practical work with motors (as opposed to theoretical)?
     
  6. Feb 2, 2015 #5

    cjl

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    You're assuming fixed power, when the question seems to be asking about a DC motor.
     
  7. Feb 2, 2015 #6
    Yes power is fixed.
    It has to be if he wants to vary rpm and watch torque vary.

    "So increasing the rpm should only increase the torque right.."
     
  8. Feb 2, 2015 #7

    cjl

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    No, power isn't fixed. He's talking about a DC motor.

    "i was to decrease the rpm of a dc motor to increase the torque"

    Look at the torque/power curves of a DC motor (often provided on the spec sheet) and this will make sense though.
     
  9. Feb 2, 2015 #8

    russ_watters

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    Since we have no idea what the application, we don't know what the appropriate equations are. Maybe it is a direct drive fan? A conveyor? A pulley change without a load change? All are different in terms of their impact on motor torque.
     
  10. Feb 2, 2015 #9
    If I change the speed on this curve, the resultant torque value change will keep hp rating constant, correct?

    If I change my speed and my torque changes,(following the curve) the hp. rating changes?
    That's what the curve is saying, vary the speed, varies the torque for a rated hp.
    (That's how I always understood them)
     

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  11. Feb 2, 2015 #10

    russ_watters

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    Yes, but I don't think we are talking about ratings here, but rather actual output.
     
  12. Feb 2, 2015 #11

    jim hardy

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    From where comes that equation?

    It describes something other than a DC motor.

    I think that's the source of confusion.

    As dennisc pointed out early on
    torque follows power divided by RPM not multiplied.
     
  13. Feb 3, 2015 #12
    Im really sorry i got the equation wrong.. My bad..
    Torque and rpm are inversely related..but i dont understand how..
    Forgive my lack of basics..
     
  14. Feb 3, 2015 #13
    No i havent. I am in the second year of my under grads, so its all theoretical for now. Thats the problem i guess.
     
  15. Feb 3, 2015 #14

    jim hardy

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    in my book takes three variables to make an equation , with just two it's a definition.
    Can you start from

    Work = Force X Distance

    and imagine a rope being pulled at a constant rate against force f
    and what's pulling it is a pulley of radius 1 ft
    so the torque t on that pulley is f ft-lb?
    it'll help you to draw a picture, that'll give you a mnemonic mental image.

    Next can you throw in time
    and figure out the equation that relates

    power, torque, and RPM ?

    Where you are, do they use English or SI units?

    for me this formula made it so obvious i could never forget that simple mental image
    horsepower = 2pi X torque X rpm / 33,000
    hint what's 33,000/60 ?
     
  16. Feb 3, 2015 #15
    ooh i get it. So when they say a dc motor loses rpm and gains torque it means that work is done by the motor on another body, so energy is transferred as torque?
     
  17. Feb 3, 2015 #16
    They use SI units.
    What is 33000/60??
    Is it converting horsepower to joules?
     
  18. Feb 3, 2015 #17

    rcgldr

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    For an idealized DC motor, peak torque occurs at 0 rpm, and 0 torque occurs at maximum rpm, with the torque decreasing linearly with rpm. Peak power occurs at 1/2 of maximum rpm. Link to article:

    http://lancet.mit.edu/motors/motors4.html
     
  19. Feb 3, 2015 #18

    jim hardy

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    okay, so work that thought experiment using a pulley of radius 1 meter, and a force of f Newtons.
    Are you clear on the difference between power and work?
    Power is rate of doing work, so suggestion to convert horsepower to joules gives me a "compile error".

    33,000 / 60 = 550.
    550 ft lbs /sec is one horsepower.
    So 33,000 converts between ft-lbs per minute and horsepower.

    So you use SI ?
    What constant converts between newton-meters per minute and watts?
     
  20. Feb 3, 2015 #19
    I believe 100 nm / min = 1.6 watt (ish)
    1 nm / min = .016 watt
    We use 46,875 nm/min = 1 hp (750 wt = 1hp)


    Will let Jim do the excellent job with calculations, I will try the practical side.

    Have you ever driven a manual transmission in a car Vishal?
    Ever ridden a bicycle with multiple gear ratios?
    Your trading speed for torque. Can't do both at the same time.
     
    Last edited: Feb 3, 2015
  21. Feb 3, 2015 #20

    russ_watters

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    No, energy is energy. Torque is torque. They are two different things.

    I like to think of such scenarios and equations as if-then statements: if power is held constant and rpm goes down, torque goes up. But power isn't necessarily held constant when rpm goes down.
     
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