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Old May 29, 2007, 02:51 PM
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Ebolavoodoo's Avatar
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Is there such a thing as "too hot a motor?"

Hi all,

Was thinking of using an EDF motor such as a HET 2W-20 for a stryker. Of course I would use a small prop to draw 50 amps with 4S lipo for 700 watts. Someone told me not to use such a motor as it is considered "too hot." I do know at 700 watts, this Stryker will be "zoom zoom zooming." Any thoughts all? Thanks! I bet people thought putting a huge motor in a cobra back in the day was "too hot." Now they are priceless!

motor specs: 2980kv 8.4-17V 65amp max draw 700 max watts 90% eff

jerry
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Old May 29, 2007, 03:22 PM
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As long as the prop size is chosen so that the motor can run it without over-doing the specs - there is no such thing as "too hot".

I just went to Motocalc and see that it lists the 2W-20 as 3450Kv????? - but as you have said, elsewhere it is 2980Kv.
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Old May 29, 2007, 04:46 PM
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"90% efficiency"

Baloney. I'd bet that 1/3 of your available power will blow away as heat.

mw
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Old May 29, 2007, 04:54 PM
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definitely too high a KV motor will net you a lot of pitch speed and not much thrust..

Even if you use a fine pitched prop, the actual airspeed over the blades will be very high, which means a lot of drag.

Also, very hot motors have high idle current which tends to mean they are not efficient at part throttle.

For best efficiency the larger slower prop always wins, up to the point at which its simply TOO coarse a pitch to work well at low speeds. Or prop diameter gets too high and you end up with either the prop clipping the ground or unmanageable torque.
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Old May 29, 2007, 06:33 PM
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Quote:
Originally Posted by Mark Wood
"90% efficiency"

Baloney. I'd bet that 1/3 of your available power will blow away as heat.

mw

More likely most of that waste heat will NOT be dissipated and the 100g motor will fry at 700W unless you have VERY good cooling!

I must say, though, that I have seen several reports from guys who reckon that they are consistently using these power levels and that the motor seems able to take it.


In the light of Vintage's post I realize that I hadn't though it through - I guess that having to use a 1.5" prop on a 7800Kv motor would be ridiculous - but in that case one would simply employ a gear box....so, there are ways around an "apparently too hot motor".
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Old May 29, 2007, 10:42 PM
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Well, I remember now that there were two versions of this motor. The old one had the higher kv. The newer versions are the lower kv. Lots of edf guys run this motor at 700watts with no problems. However, they are using an edf. They even run motor softly after a flight, know as a cool down period to prevent overheating. I must tell you, with my edf hooked to it, it screams and scares me just holding the plane at WOT. Many guys have hooked wattmeter and get 700+ watts. As far as prop goes, guess I'll have to use motocalc to see what prop works, hopefully not too small a prop as someone mentioned. Thanks all for your reply. Still pretty new at e-flight.

jerry
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