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Old Nov 15, 2006, 04:24 AM
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Joined Nov 2006
16 Posts
Motor test bench questions

I would like to measure motor efficiency on small electric motors and also propeller thrust. To do this I need a test bench that can measure not only electric input power and thrust (as most hobby test rigs are done), but also motor torque and rpm. I would like to measure the true power curves of small motors and not simply use the conventional motor constants to plot the power curves.

Is there anyone that has built such a rig and that could help with suggestions on what equipment to use? My interest is small motors such as used for indoor and parkfly. It’s a tricky project but interesting.
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Old Nov 15, 2006, 05:05 AM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,705 Posts
Its a hard call to build a dynamometer at such a small size..personally the derived figure for efficeincy got by simply meausroing motor RPM loss under load is enough for me, and sorets the goats and sheep..

BUT in essence all you will need, is torque and RPM to get Pout, RPM is relatively easy, and torque..well a digital scale and a rotating motor mount with a longish strut pressing on the scales should do the trick.

To present various loads to the motor, probably a prop is as good as it gets..just shove half a dozen props on and measure the RPM and torque. Makes use of an optical tacho not that difficult either.

If you want to be really snazzy, you could integrate the whole thing into one bit of electronics, using a pressure transducer, photo detector and voltage and current mesaurement, and write some softwarre to give you pin, pout, voltage, current, efficiency and RPM...drive that lot from a PC and get a graph out teh other end as well..

Some people use a more or less shorted second motor as a load, but I think thats probably not so good..certainly not if you want to use it to measure output power..as it adds a secnd imponderable. It does however make a useful way of adding an infinitely variable load, and, if a brushless is used, a handy way to pick off pulses for RPM measurement.
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Old Nov 15, 2006, 02:58 PM
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United States, NJ, Frenchtown
Joined Mar 2003
9,451 Posts
About a 3/8" thick of plywood about 3' X 12" wide. Drill a 1/4" hole in the middle for a 1/4" ball bearing setup. 2 nd, bottom bearing plywood piece to clamp in a vice or to a table edge.

Mount the engines 1' from the center. Drill a hole on the opposite end 1' from center. For a 10# digital fish scale- cheap one. make a bent wire hook for it.

You can tape a power pack on the scale side of the arm with a RC receiver, and ESC.

You know the rest.
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Old Nov 15, 2006, 04:25 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
11,208 Posts
Several dynamometer and thrust stand designs:

Vriendelijke groeten Ron
• diy motor building tips & tricks
• diy brushless motor discussion group
• Drive Calculator download & discussion group
• int. E fly-in & diy outrunner meet, Aug.26 2007, Nijmegen, the Netherlands
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Old Nov 15, 2006, 05:08 PM
hot air rises...
jfinch's Avatar
Pleasant Grove, UT
Joined Jul 2005
8,064 Posts
I built a thrust stand a while back and used it a few time before I realized that every single APC12x6E prop produced the same thrust at the same rpm every time... The same can be said for torque. Now I test my motors by measuring RPM/volts/amps at no-load and with a handful of props. Plug the data into DriveCalc (free from www.drivecalc.de) and let it spit out my motor constants (and thrust and torque).
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Old Nov 15, 2006, 05:41 PM
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landru's Avatar
Vancouver, Canada
Joined Mar 2003
2,110 Posts
Hi Mavox,

A small dyno is a fun and very useful project. The ability to test motors at full throttle with different loads — and at different voltages — provides data that I know of no other way to measure or predict reliably.

In my own efforts, it soon became aparent that data-logging to PC is a must-have for accuracy. Collecting a set of precisely simultaneous data is critical.

As Vintage suggested, you could build your own hardware/software for daq. Or you could use the data-logging wattmeter set-up from Medusa. Medusa just released a new version of their wattmeter software which allows for real-time calculations from user-entered formulas — so you could incorporate real-time efficiency measurements. There's a dedicated thread in the Medusa Vendor Forum.

There are some neat ideas in the Very cheap and accurate dyno thread.

Here's what I've been working with: small dyno.

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Old Nov 23, 2006, 05:54 AM
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Joined Nov 2006
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Thank you all! I have been away for some time in my work, and now when I’m back I’m surprised how many inspiring tips you have given in this thread. I was planning to use different propellers to put different loads on the motor, but building a dyno seems just the right way to go.
I have data acquisition hardware that I can borrow at work so I will have the possibility to do accurate logging.

Landru did you ever compare the result between measured power curves and the result one can obtain with the common motor constants (Rm Io, Kv)?
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