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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Motorman on September 09, 2017, 09:32:12 PM
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Kv is the number of rpm the motor can turn per volt, right? So the battery drains down so you have to set your governor rpm below that number.
Motor 700 Kv
6 cell Battery 25.2v at the start
17640 rpm possible
19.2v after flight (3.2 cell)
13440 rpm possible
So how much head room do you want if you want a 10,000 flight rpm? Someone told me the same 6 cell with a Kv of 820 would be less efficient. What makes it less efficient?
Thanks,
Motorman
So, you got it all simplified, and now you want it complicated right back up. OK, I can do that.
The rule of thumb is to give yourself about 25% headroom -- meaning, multiply the nominal voltage of 3.7V/cell by about 75%. That accounts for the resistance in the motor and wiring, ESC inefficiencies, and the actual voltage drop in the battery under load.
The reason that using more voltage than necessary is inefficient has to do with losses in the system. When an ESC is running at part throttle, it's switching the voltage to the motor (at around 10-20kHz these days -- the exact number depends on the ESC, and I don't follow who does what). So at part throttle, the active pair of wires into the motor is seeing the voltage pulsing between +battery voltage and -battery voltage (or, depending on the ESC, between +battery voltage and shorted). Mechanically, the motor reacts to the average of the voltage it sees, but the more pulsing needed to get the same effect, the more losses there are. The most efficient setup would be one where the motor is running at full throttle, so that there's no pulsing at all -- but this would mean you're running at zero overhead, so you would hope that doesn't actually happen.
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Seems like from an esc standpoint, if you are at full throttle or nearly so at the end of the flight, then you'll be switching on and off the least at the beginning, but not experience any "sag" in power at the end.
That being said, missing that mark (which is guaranteed), its better to err on the side of more headroom than less.
With the cost of cobra motors, it seems like no big deal. Just buy the other wind in the same configuration if you miss. Then sell the other one or keep it for a different plane.
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Thanks Tim, interesting stuff. I guess that's a good reason to keep the leads as short as possible
You can always go overboard. I'd keep them as short as is reasonable, then add an inch or so because I always, always underestimate the length that a cable needs to be. If you get a note higher than a middle "C" when you pluck a battery lead, it's too short.