Looking to calculate horsepower by mearuring the change in rpm

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Discussion Overview

The discussion revolves around calculating horsepower based on the change in RPM over a specific time period, with torque held constant at 200 lbs/ft. Participants explore the relationship between torque, RPM, and horsepower, and seek clarification on how to apply these concepts in practical scenarios.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes a formula for horsepower based on torque and RPM, suggesting that with constant torque, horsepower can be simplified to a function of RPM.
  • Another participant points out that the horsepower formula depends on instantaneous values of torque and RPM, not on changes over time.
  • There is a question regarding the origin of the constant 5252 in the horsepower formula, with some participants noting it is a unit conversion factor.
  • One participant suggests that if torque is constant and RPM is increasing, then power must also be increasing.
  • A participant inquires whether it is possible to determine horsepower if there is a known increase in RPM over one second.
  • Another participant explains how to calculate power using torque and RPM, mentioning angular acceleration and inertia in the context of a rotating system.
  • There is a discussion about whether to calculate horsepower at the start or end of the time interval, and what specific value (average, maximum, or rate of increase) is desired.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of changes in RPM for calculating horsepower, with some asserting that only instantaneous values matter, while others explore the implications of RPM changes. The discussion remains unresolved regarding the best approach to calculate horsepower in this context.

Contextual Notes

There are unresolved questions about the application of the horsepower formula, particularly concerning the constant 5252 and how it relates to unit specifications. Additionally, the discussion touches on the assumptions made about torque being constant and the implications of RPM changes over time.

two4one
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as the title states I am 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 let's 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!

thanks
Anth
 
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Look at your formula for horsepower..

It only depends on the instantaneous values of torque and RPM, not on any changes.
 
zekeman said:
Look at your formula for horsepower..

It only depends on the instantaneous values of torque and RPM, not on any changes.

i looked quickly on wiki for the formula for hp. I am not sure how the constant 5252 was found.

If you could lead me in the righ direction that would great.
 
This has nothing to do with differential equations so I am moving it to "general math".
 
two4one said:
i looked quickly on wiki for the formula for hp. I am not sure how the constant 5252 was found.

If you could lead me in the righ direction that would great.

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.
 
yes
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:
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).
 
two4one said:
yes
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?
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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