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Old 09-24-2009, 09:03 PM   #11
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Re: How to calculate watts?

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Originally Posted by AKM-47 View Post
- Will it damage my ESC and /or motor (now I have a 30A - 40A peak ESC) if I use a battery with a higher "C" rating. Let's say I use a battery with a 30 or 40C discharge rate?
C rating and the ESC rating 2 different things. However, if you take 2 similar packs, one with 20C and the other with 30C ratings, the 30C pack will put out more power as they generally hold a higher voltage under load.

- Is there a way to boost motor's output without actually burning it out?
Boost as in higher voltage or higher current? Keeping the motor cool will help somewhat but depending on the manufacturer, they may already be considering airflow when coming up with specs. So it would be prudent to stay within specs.

One other item I'd like to point out is that comparing two different motors with the same KV rating will not give you the same performance. KV rating is determined with no load (essentially a propeller with 0 pitch). The cheaper motors usually will deliver fewer RPMs for the same input than more expensive motors. So a $50-70 900kv may compare to a 950-1000kv Turnigy in terms of performance (prop RPM), but the more expensive motor will deliver those RPMs with less current.

So don't trust the KV rating given by manufacturers, look for actual performance measurements (ie: prop, volts, rpm, thrust) released by the manufacturer or as tested by users.

This guy has tested a few:

Also, Scorpion releases detailed data for most of their motors (I'm not saying that's the right model for your application, just an example of what to look for):

There are several websites that will let you model various power systems. They sometimes already have the data for the plane you want, other times you have to input size & weight yourself:

One more point of advice. Don't overload your ESC. If your power system can draw 35 Amps continuous, you need a 40 Amp continuous ESC. Don't fool yourself by getting a 30 Amp ESC that has a 35 A peak rating and expect that you will use throttle management.

I'm simplifying a bit here but essentially what happens is at 100% throttle, the ESC dumps 100% of the available power to the motor. At 50% throttle, the full power is modulated (on / off) for an average load of aprox. 50%. So the full current is still flowing, just 50% of the time. This is why some people (usually helicopter pilots), who use reduced throttle settings, let's say a 90-85-90 power curve to stay within the ESC rating, still manage to burn up their electronics.
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Old 09-26-2009, 07:27 PM   #12
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Re: How to calculate watts?

Thank you guys, I think I am starting to get it, and thanks for the links as well.

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