Author Topic: How to measure power and torque of model steam engine?  (Read 408 times)

Offline Moxis

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How to measure power and torque of model steam engine?
« on: January 05, 2019, 06:47:17 AM »
After having finished my Marcher engine, I would be interested to know how much power (W, HP) it generates. I think you have to measure speed and torque to calculate power. Speed is easy, but how to measure torque?

Offline Roger B

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Re: How to measure power and torque of model steam engine?
« Reply #1 on: January 05, 2019, 07:36:23 AM »
The classical method is a Proney Brake. There is plenty of information on the web. Here is one example:

https://me-mechanicalengineering.com/dynamometer-introduction-types/
Best regards

Roger

Offline Zephyrin

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Re: How to measure power and torque of model steam engine?
« Reply #2 on: January 05, 2019, 08:25:17 AM »
You have to measure the work done in a given time; 
 for an example : with a coil on the axis that winds a string to which different weights are attached to vary the load, simple to do...but not that easy to record precisely rpm, height, weight and time all together !

You may use too a small electric motor coupled with your engine whose load resistance is varied with a potentiometer.
there is the problem of the efficiency of the engine as a generator, much less than 1 of course. But these tests are comparative and make it possible to determine the best combination of rotation speed and torque.

And yes, building a Prony brake is the right way.


Offline MJM460

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Re: How to measure power and torque of model steam engine?
« Reply #3 on: January 05, 2019, 11:32:14 AM »
Hi Maxis,

Great job on that engine.  I have followed your build log from the start as I really like that design.

Measuring speed and torque is the obvious way to measure power, but as you say, measuring the torque is the hard part.  Someone recently described an eddy current brake which may work if it gives an appropriate torque range.  The brake band around the flywheel type is the classic way, but I donít know how well they work with the variation in speed each revolution of our engines.  Some damping in the force measuring system may be required to get a useable force measurement.

Or use an electric motor as a generator.  If you have a double ended motor so you can support the armature each end, then use a torque arm to restrain the stator, you can calculate the torque from the force on the torque arm.  Again, some damping in the force measuring system may be required.  I donít know if this can be done with force sensors and electronic filtering of the output signal, but itís a possibility.

But I think Zephyrinís suggestion of lifting a weight is best.  Use your iPad stopwatch facility to time lifting a pre-weighed mass through a pre-measured distance.  Measuring the work per unit time like this averages out those speed variations so no damping is necessary.  Some thought is needed about the winch design, so the wound string does not change the diameter too much.  If you can arrange a long lift, this will minimise the effect of errors at the start and end of the run in both timing and operating speed.  Some experimenting with the weight allows setting an appropriate range by trial and error.  You could also use meccano gears to vary the winch speed, though the gears introduce friction, so possibility better to just experiment with the winch diameter.

So those are  the theoretical possibilities, I suspect the need for damping of the force measurement might make the winch on a fixed mass for a timed mass the most practical.  I have not made much progress on my own experimentation in that area, itís one of those things on my list. 

I am sure there will be a lot of interest in your results.

MJM460
The more I learn, the more I find that I still have to learn!

Online steamer

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Re: How to measure power and torque of model steam engine?
« Reply #4 on: January 05, 2019, 12:47:18 PM »
Ive done the prony brake, but i think a traditional prony brake in the scale we are speaking will be difficut to control and build.   Lifting a weight would be the first form i'd investigate.  You must minimize friction though.     Id consider a eddy current prony brake using permanent magnets would.work.   just a metal disk on a shaft spun by the engine.  It could be made very friction free.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://m.youtube.com/watch%3Fv%3DmopfuVfeIhc&ved=2ahUKEwj8iIGB1tbfAhVNnlkKHQi_BroQt9IBMA56BAgVEEU&usg=AOvVaw1EgeTM-h_BZnczGzP-5mZe
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Offline Moxis

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Re: How to measure power and torque of model steam engine?
« Reply #5 on: January 05, 2019, 01:32:17 PM »
Thanks guys, very interesting methods. I consider the string, weight and stopwatch method to be the easiest in this scale.

But what would you think of following: Lets take two identical DC electric motors. Connect the axles together and use the first one as motor, and the second one as generator. Now measure voltage and current of the motor and calculate power. At the same time measure voltage and current of the generator and calculate power of this one too. Find out the power difference and divide that by two. So now you have approximate losses of both of them.
The only thing you have to do now is to connect axles of the generator and steam engine together and measure voltage & current, calculate power and substract the losses earlier calculated.
Not a scientific method, but should be quite near anyway.

Offline Ian S C

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Re: How to measure power and torque of model steam engine?
« Reply #6 on: January 05, 2019, 02:28:24 PM »
I use a Proney Brake on my engines, and a no contact tachometer. It is calibrated in inches and 1/10th inches, the weight is 1 oz.
Ian S C
           
         
           
« Last Edit: January 05, 2019, 02:34:29 PM by Ian S C »