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How to determine safe watts for unknown motor?

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Old 06-16-2010, 11:16 AM
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KB9STD
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Default How to determine safe watts for unknown motor?

Ok,I have a brushless outrunner that I know nothing about.I don't have any specs for it,and iit doesn't have an mfr mark on it.How can I determine the "safe" wattage I can get from it.I want to scratchbuild a plane,and size it to use this motor.
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Old 06-16-2010, 01:40 PM
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Default RE: How to determine safe watts for unknown motor?

You have a pretty much impossible situation there. You might try puttting up a photo to see if any one recognizes it! The things you have to determine are almost beyond the scope of most common available test equipment, you need the motors resistance, for one, and it is below most multi meters to measure being most times in the tenths a/o hundredths of an OHM. Do a size comparision Lenth, diameter and shaft size! This will get you in the same city, but unfortunately that city is the size of LA!
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Old 06-16-2010, 04:39 PM
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Default RE: How to determine safe watts for unknown motor?

Plot a graph of watts vs temp. The wattage at which it is stable at 80C above ambient is you continuous limit.
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Old 06-20-2010, 11:41 PM
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Default RE: How to determine safe watts for unknown motor?

Dave -

The web sites of many magnet vendors say that the maximum operating temperature for neodymium magnets of type N is 80°C. That's not 80° above ambient but a measured temp of 80°C (176°F).

There are other types of neodymium magnets with higher operating temperatures (e.g., type NEH are OK to 200°C), but since you don't know what type are in the unknown motor, the safe assumption would seem to be a flat 80°C, not 80°C above ambient.

Just my $0.02. I'm no magnet expert, but I read spec sheets.

- Jeff
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Old 07-14-2012, 12:40 AM
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Default RE: How to determine safe watts for unknown motor?

A very rough rule of thumb uses the motor's weight. Look at E-Flite motors of similar size, weight, and shaft diameter vs rated wattage as examples.
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