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Use watts to measure motor power?

Old 03-25-2007, 12:25 AM
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BeFlyin
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Default Use watts to measure motor power?

I'm trying to pick brushless motors for all my planes (P-51D, Super Cub, Easy Star, Fledgling & Cermark electric glider). I'm lookin at some on E-bay and the way I am figuring out what will work with each application is to figure out the wattage output/draw for each motor, for instance one outrunner is rated for 3 cell LiPo (11.1v) and 16a constant. That would add up to 177.6w? (11.1v x 16a). They rate it at 250w so I assume that would be at max current @ 11.1v. The RPM is 3200K/v. I don't know if I should factor that in.

Another motor is rated at 20a max, 6-18v (up to 4 cell LiPo?) and 1050Kv. Being consertive I would figure watts using 11.1v x 16a =177.6w. Same as above but lesser RPM.

One I like is 980K/v, 30A max and 6-22v. That would be a lot of watts but low RPM.

Am I on the right track? Please feel free to correct me where needed. I am trying hard to establish an easy formula to caculate electric motor power across all brands and even from brushed to brushless.

Thanks.
Old 03-25-2007, 08:25 AM
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packyj
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Default RE: Use watts to measure motor power?

You're on the right track BeFlyin

As a rough rule of thumb you can use this as a guide to performance with watts per pound:


Watts per pound Performance capability


Less than 50 watts per pound Unlikely to get off the ground
50 to 60 watts per pound Can perform simple aerobatics
60-75 watts per pound Will loop from level flight. More aerobatic capability
75-100 watts per pound Warbird fighting capability. Aggressive climbs
100-150 watts per pound Long vertical climbs. Unlimited aerobatics
More than 150 watts per pound Every type of 3D maneuver
Old 03-25-2007, 10:59 AM
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BeFlyin
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Default RE: Use watts to measure motor power?

Well that's great, thanks pakyj.

I also assume that when a compant rates it motor, it is at the max current and voltage it is capable of, not necessarily what the constant rating, or what you would actually run it at.

Then there is the RPM factor. Seems like some higher torque motors have lower RPM's but not always. If I see a high watt high RPM motor should I choose it over the same watt low RPM motor?
Old 03-26-2007, 09:57 AM
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Fliprob17
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Default RE: Use watts to measure motor power?

RPM's / KV rating should depend on the size prop you want to run, and whether you are going to run a gearbox or not. If you are going to run a gearbox, then a high RPM/KV rated inrunner, like the 3200 you mentioned is the way to go, then the gearbox slows it down before it hits the prop, and you can still run the larger props. If you don't want to deal with a gearbox, then you are looking at an outrunner, and they typically have much lower RMP/KV ratings so you can just mount them to the plane, and ditch the gearbox. Typically with the outrunners, if you are thinking of running a 10x7 prop or larger, you need to stay below a 1250kv rating, for a ballpark number. Otherwise your amps are too high, and things may heat up and your flight times will be low.
Old 03-26-2007, 04:51 PM
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Dr Kiwi
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Default RE: Use watts to measure motor power?

This is worthwhile reading about amps/watts ratings - Dave Radford has done a marvellous job of explanation: http://www.aircraft-world.com/prod_d...cer/bl4-15.htm

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