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Looking to calculate horsepower by mearuring the change in rpm

  1. Jul 7, 2012 #1
    as the title states im looking to solve how to calculate power (horsepower) by measuring the change in rpm over a few seconds. torque will be kept constant at 200lbs/ft.

    hp= (torque x rpm) / 5252

    since torque will be held constant it reduces to:
    hp= C x rpm where C is 200/5252

    so lets say i measure the change in rpm over 1 second, how do i use this to calculate horsepower?

    ..i used to be good at solving problems like this, and i feel crappy that i've lost that ability

    if you could shed some light as to how i can set this question up and how to go about finding its solution tht would be great!

  2. jcsd
  3. Jul 7, 2012 #2
    Look at your formula for horsepower..

    It only depends on the instantaneous values of torque and RPM, not on any changes.
  4. Jul 7, 2012 #3
    i looked quickly on wiki for the formula for hp. Im not sure how the constant 5252 was found.

    If you could lead me in the righ direction that would great.
  5. Jul 7, 2012 #4


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    This has nothing to do with differential equations so I am moving it to "general math".
  6. Jul 7, 2012 #5


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    The 5252 is just a conversion of units. It is not appropriate unless you specify the units everywhere.
    If the torque is constant and the rpm increasing then the power must be increasing, as HofI said.
  7. Jul 8, 2012 #6
    so if i find that over 1second there is an increase of 250rpm, ex from 4000 to 4250; can i determine the engines/sources hp?
    Last edited: Jul 8, 2012
  8. Jul 8, 2012 #7


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    If you know the torque and the rpm, then you can calculate the power in units of horsepower:

    power / hp = torque (ft lb) x rpm (rev / minute) x (1 minute / 60 sec) x (1 sec / (550 ft lb) )

    If the engine is accelerating a rotating drum over time, then torque = (angular acceleration) x (angular inertia).
  9. Jul 8, 2012 #8


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    You can calculate it at the start of that second, and again at the end of that second, and you will find the hp has increased. So what do you want - the average, the max, the rate of increase...?
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