An RC motor dynamometer

My 3800KV brushless motor is only spinning up to 20,000rpm when paired with a 64mm fan which is no where near it's 40,000rpm no load speed (@11V). At the same time, the most power I'm drawing from it is about 100W instead of its rated 500W. This doesn't make sense to me; I'd expect that at 100W, the motor is lightly loaded. Why am I not able to spin faster and hence draw more power? The faster the fan turns, the more torque that must be overcome. I need to see this motor's power curves.

This problem has got me wanting a dynamometer for measuring RC motor performance. The attached picture shows how I created and mounted a flywheel made of CDs reinforced with fiberglass/epoxy to the EDF rotor adapter. I got the idea to use CDs for the flywheel from sci-spot.com/mechanical/dyno.htm. The inertia of a uniform disc like a CD is easy to determine. Using Newton's Second Law, we can determine torque from acceleration.

At 40,000rpm, the stresses in a CD(s) should be much less than yield, but there are plenty of videos out there of CDs exploding around 20,000-25,000 rpm. I'm assuming this might be due to vibrations when nearing one of the natural frequency harmonics. To prevent this, I layered sheets of fiber glass and epoxy between CDs and compressed them as it cured. This formed a solid 5 CD thick disc. Once cured, I was able to run it to very near full speed (in a safe area where people and property were not in harm's way). Now I'm just waiting for some IR LEDs to use with my IR Phototransistors to arrive so I can measure RPM and a Hall effect current sensor for measuring input power.

Am I the only one that is interested in this stuff? Why are there no hobby dynamometer's available? In order to properly design an EDF system, I definitely need to know the power characteristics of the motor so I can predict its operating envelope.

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2

thank you very much, thanks to the article above insights and information I became more widespread

Info Harga Kendaraan Otomotif dan Elektronik

Hi Irfran,

I remember when I was working on this a few years back. What I found is that the CD actually caused enough drag to significantly affect my power measurements.

So, the plan is to eventually get a local machine shop to making me a smaller diameter flywheel made of a denser material. Future plans. ;)

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