Originally Posted by RONALDO_UK
Hi - I'm not sure if this question belongs in this forum, "Beginners", "Power Systems" or all three??
I know the kv value of a motor indicates its rpm (dependent on voltage applied) and some motors can utilise a 2S or 3S battery - is the motor always turning at this speed (V x kv), regardless of how much power is supplied? And if the same power is supplied by each battery, am I right in thinking that with the lower voltage (lower rpm) battery, the compensatory factor is more thrust/ torque?? If weight is not an issue, is it always preferable to use the higher voltage battery (to give extended rpm/power) - provided the permissible current draw of the motor is not exceeded??
There are a number of computer software programs that will give a lot of information on the type of questions you have raised.
Check out www.motocalc.com
, free for 30 days, then $39. With this program, you can put in a range of propeller sizes, and, a range of difference number of cells on your proposed setup. Then just run the program and see what happens.
If you put in as an example, a propeller size from 10-6 to 14-12, the program will spit out results on every combination. And, if you also add as an example, 2 to 6 series LiPos, that will multiply the above results by four times. Just studying the results, and looking at motocalcs opinions can be a nice learning experience.
The program will give approximate RPM, thrust, amps, volts, and percent efficiency of the motor. A high quality $$$$ motor will usually have efficiencies on the order of 85 to 90%, where a cheap motor might only be in the 60% range. If you've got 60% efficiency, the 40% difference is going to be heat in the motor and its windings. When you get to some of the higher powered motors, that efficiency gets important, when you've got a 1 kilowatt motor, and 400 watts of that 1KW is heat.
This program is only as good as the motor specifications put into it, and unfortunately some of the cheap import motor's specifications are not even in the ballpark. But, motocalc will still give ratios on different prop and cell count setups.
There are also other programs that are free, and provide a lot of good information. Motocalc is the one I use, and have had it for some 4-5 years.