WattFlyer RC Electric Flight Forums - Discuss radio control eflight - View Single Post - Measuring Brushless Motor Resistance (Two Digital Meter Method)
Old 02-06-2010, 02:50 AM
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Originally Posted by dirtybird View Post
How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors
Something I've done over the past few years, is set up a particular motor/prop/esc/battery pack. Then run the motor and prop, checking its current/voltage/watts with a meter such as an Astroflight whattmeter.

Then, assuming the battery is putting out the voltage under load as advertised, I check the actual motor RPM against the motocalc predicted RPM. If they vary substantially, I just save the motor under a different name, then "tweak" the motocalc motor characteristic "KV" number so the motocalc RPM matches the actual motor RPM.

If the battery has a different voltage under load, you can also "tweak" the battery constants to match the real world.

That way, as you change props in motocalc, the predicted RPM's should be a little closer.

Not very scientific, but it gets you a little closer.

Just a note, I've found that sometimes motocalc has the WRONG motor constants, as compared to the motors web pages. This was noted on the Hacker motors several times, as well as Eflites.
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