# Looking to calculate horsepower by mearuring the change in rpm

1. Jul 7, 2012

### two4one

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!

thanks
Anth

2. Jul 7, 2012

### zekeman

Look at your formula for horsepower..

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

3. Jul 7, 2012

### two4one

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.

4. Jul 7, 2012

### HallsofIvy

This has nothing to do with differential equations so I am moving it to "general math".

5. Jul 7, 2012

### haruspex

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.

6. Jul 8, 2012

### two4one

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: Jul 8, 2012
7. Jul 8, 2012

### rcgldr

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).

8. Jul 8, 2012

### haruspex

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...?