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Old 03-28-2011, 03:57 PM
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dumo01
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Originally Posted by RCFlyer44 View Post
I am building this Great Planes Super Sportster EP and I purchased the recommended RimFire .25 42-40-1000 Brushless Outrunner Electric Motor and the Silver Series ESC 45A Brushless with 5V/2A BEC.
Originally Posted by RCFlyer44 View Post

Both the ESC and Motor say anything from a 2S to 4S works great, and the plane recommends a 3S 3500mAh battery.

Haven't purchased the battery yet and I have plenty of 2200mAh batteries but I was wondering if I am correct in assuming if I use a 4S I would basically increase the RPMs by 3400 RPMs since I am increasing voltage by 3.4 volts and the motor has a 1000kV rating??

Also I know Volts X Amps = Watts and this motor is rated for 666watts according the manual but at 11.1 volts I am going to be right around 500 watts and they are getting their 666Watts using a 4S battery, obviously.

So any reason I should not just use a 4S battery vs. a 3S and what this the right formula for figuring the mAh rating if I do switch?

Also a 4S 2200 mAh battery seems to be a little cheaper (not enough to make a difference to me though) than a 3S 3500 mAh battery.

Or am I out in left field and done jumped the fence thinking this way???


You haven't jumped the fence, but there are a number of complicating issues to be aware of.
The KV rating indicates an unloaded ( no prop) RPM that the motor will spin at. The actual RPM you get will depend on the prop you use.
The ESC will work with a 4S battery, but for any given set up ( load and throttle setting) the 4s will result in a higher average current delivery than the 3 S.
The 11.1 v assumption you are making on the the 3S battery is the low end of the charge/ discharge voltage curve, I would use a fully charged battery voltage to think about current draw. If the top end the motor will deal with is 660 watts, at 12.5 v that will be a current draw of 52 to 53 amps, which exceeds the capacity of your ESC ( 45 amp continuous, 50 burst).
The ESC seems to be the limiting factor in your power system, so to use the 4S battery you may need to consider reducing your prop size and or pitch to maintain the same overall current draw on the system.

Because the ESC deliver current in a time gated fashion ( full current on then off with the interval determined by your thottle setting) you cannot depend on the just assuming you can fly at less than full throttle to keep your current delivery in a safe range for your ESC.

Another vital factor to be aware of with the Silver series ESC is that they use a % of initial voltage to determine the LVC, as opposed to the fixed voltage that most other manufactures use. As stated in their instructions, never use a partially discharged battery on thier ESC as it may set the LVC at a point that will result in battery damage if flown to the LVC.
I typically do not fly to the LVC, but especially with this unit I would be very careful in that regard.
This has been sort a rambling post on my part, but like I said there are a lot of issues that complicate going to a 4s from 3 s. As many others have noted if you are going to try this you absolutely should have a wattmeter to get direct measurments and see what effect the changes you are making result in. It is a good practice to do that in any case even if using the stock setups to verify that what you are seeing is in line with what the manufacture says it should be.
Good luck with it, I am sure that there are parts of my reply which are clear to me but clear as mud to you so hopefully others will chime in here as well.
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