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Old 05-30-2012, 05:11 AM
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Join Date: Jan 2008
Location: Glenwood, GA
Posts: 1,025

Electric motors are normally rated for a range of current. Say it is 60A top. The rating will normally read something like 50 to 60 amps constant. 70A peak. When you put a smaller prop on it, then the current will drop down a lot and so will the power as in thrust. If a motor is rated for 600rpm per volt, then it will try to turn that no matter what kind of prop you put on. That's why the current changes with a prop change. It is trying to get to that 600rpm per volt. You notice I said trying. With a prop on it, it can't reach that rpm and the harder it tries the more current it draws. What you aim for is that area around 50 to 60 amps and that is the area the motor gives the best performance. Drop the prop size and sure the current goes down, but so does performance. When you drop down in diameter, you increase pitch to make up for it. You never go from a 14X6 to a 12X6 and expect it to fly good. You may need to go to a 12X12 or something. What you gain in forward speed you loose in vertical climb, but there are always tread-offs no matter what you do.

If you don't have a watt meter, get one. They are NOT something just nice to have. If you try changing props blind or changing anything really, you don't know what you are getting into and can either make a poor choice in operation and get a bad flying plane, or burn something up. The ESC and motor are rated for a given max voltage and current. If you are changing things blind, you don't know if you are safe or not without a watt meter. That is a must have item for anyone using electric power sources and I would say get one that will go to at least 100 amps if you are going to play with the big stuff. The currents may go as high as 100 amps on some motors and they can take it "IF" they are rated for that.

An ESC is rated for current and also voltage. Just because it says it can take 60 amps does not mean you can use it if you are using a 5S or 6S battery. It may only be rated for 4S, so make sure you get the right one. It will start popping like pop corn if you put a 4S ESC on a 6S battery. Needless to say it won't work very long. Also don't use a 100A ESC at 50 amps. You are not in the proper working range of the ESC and it will over heat cutting down on the life of it. Get a 60A instead. A little over size is fine, just don't go overboard. Also make sure you have good air flow through the inside of the plane so the motor, ESC, and battery get cooling air. With these higher amp ratings especially, they need that air flow to cool things off.

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