Help me decide on brushless motor test protocol - RC Groups
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Apr 24, 2004, 08:21 PM
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Help me decide on brushless motor test protocol

Time for more advice from the wise and experienced ones.

I have several varieties of 3 different makes of brushless motors that I wish to generate useful data for. Two of the makes use gearboxes while one is an outrunner that doesnít use a gearbox.

Available instruments include the following:

1. Thrust stand that is easily calibrated and utilizes a precision digital scale

2. An Astro Micro Watt Meter

3. An accurate digital tachometer

4. Lambda EMS 20 volt 50 amp power supply with digital display and easy adjustment to precise voltages

5. An excellent supply of gearbox pinions and spur gears

6. A good supply of GWS Slow flyer and Hyper drive props from 9 inches to 12 inches

7. A good quality infrared thermometer

My intent is to take measurements of voltages, amps, thrust, rpm and temp are the end of 2 minute runs with each of the possible gearbox and prop combinations.

Iím most interested in recommendations as to what voltages to complete the testing. The motors will most likely be run with 2 and 3 series lithium polymers so Iím thinking 7.4 volts would simulate a mid discharge 2-S and 11.1 volts would simulate a 3-S. What do you think about using these voltages? The great thing about the power supply is that once dialed in, it maintains a value indefinitely.

Let me know if you have any specific ideas and or recommendations.

Thanks for your informed and thoughtful replies,

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Apr 24, 2004, 08:27 PM
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BEC's Avatar
Wow, sounds like you're well set up. That power supply, in particular, will let you remove a BIG variable in static testing - batteries.

Have you done much testing before? If so you know what sorts of voltages the batteries you normally use sag to under load (thanks to you meter).

In my experience of such things, with current generation cells, your proposed voltages are a bit high. I typically see more like 6 1/2 V on 2s and 10 or so on 3s under the sorts of loads that are close to what cells are rated for. Usually this goes UP a bit as the test progresses and the cells warm up.
Apr 24, 2004, 11:20 PM
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Dr Kiwi's Avatar
Hi hugh - I think you might be seeing some fairly high temperatures if you are going to run those motors for 2 minutes static at WOT. Like BEC, I think that 7.4v and 11.1v are higher than one could expect to see from the typical battery pack, 7.0-7.2v and 10.2-10.5v seem to be my norms at 3-5C on TP 2100's [and around 6.5-6.8/9.8-10.1v at the rated 6C max]. But that doesn't really matter for testing purposes - users can extrapolate for the difference between your constant voltage supply and what they actually get from THEIR PARTICULAR PACK. The great thing is that, unlike GWS who obviously use a constant voltage supply but won't admit to it, with your excellent scientific approach, YOU will be stating your source of voltage - so we'll know how the results were obtained (including rpm, another glaring GWS omission), and your experiments will be repeatable!

Good luck!

Cheers, Phil.
Apr 25, 2004, 05:29 AM
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Bruce Abbott's Avatar
Might as well use voltages that are easy to remember, 7.0V for 2S and 10.0V for 3S. Make the cables from PSU to ESC as short and thick as practicable.

If you are only interested in testing the motors, then thrust is irrelevant, however it wouldn't hurt to collect some prop data at the same time!

Temperature measurements after extended running could be interesting, just be careful to avoid overheating anything. To make the data more useful you should measure current and rpm at both the beginning and end of the run (representing a cold and hot motor).

Also, please measure no-load current (and rpm if practicable), as well as current and rpm with a small prop. This data is useful for calculating motor constants.
Apr 25, 2004, 06:51 AM
Registered User

Great feedback as usual

Bruce, Phil and BEC,

Thanks for your carefully reasoned replies. A quick test with an 850 Tanic 3S and 1200 Etec showed me more voltage sag than I'd expected and does show that 7.4 and 11.1 volts were pretty optimistic for any situation where we're pulling a substantial load.

If you have any more thoughts let me know.

Thanks a bunch,

Apr 25, 2004, 10:38 AM
Registered User
Hey ScienceGuy,

I'd be very interested in seeing comparative data for the APC SF props relative to the GWS RS props...

Apr 25, 2004, 11:07 AM
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Fishnut's Avatar
Originally Posted by grizzlyone
Hey ScienceGuy,

I'd be very interested in seeing comparative data for the APC SF props relative to the GWS RS props...

Not to knock that Watt Meter, but I think it's the least accurate device in your stable. You may want to at least calibrate it with other ammeters. I use the term calibrate loosly, because it can't be calibrated, but maybe you can determine if it has a repeatable error that you can offset.


Last edited by Fishnut; Apr 25, 2004 at 11:10 AM.
Apr 25, 2004, 12:39 PM
Registered User

I also have one of the Sears Clamp on style amp meters. I has always correlated very well with the Astro Unit. Not that I have the greatest faith in either. Could you recommend a specific unit or method to compare the Astro meter with? If I'm going to spend the time, I'd really like to do this right the first time through.

Thanks for the continued addition of good ideas.