stuntguy13-Dennis
Your 12 oz power package is a very realistic achievable goal, only I think the 42-40 is bigger than you want. Check the Rimfire specs; I think the 35-36-1200 has the same W rating and weighs 3.5 oz.
....<snip>...
I am in a bit of a quandary over the kV rating - Alan's recommendation and your own experience notwithstanding: I tried a 950kV motor once that would NOT give me my target 10,500 rpm on 4S. Since then I have opted for higher (too high?) kV then let the ESC do the work. Based on what I read here, I suspect I am sub-optimized, possibly running "hot" but that is next summer's work to further explore & either affirm or fix..!
..oh, and get ready for the strongest 35 you EVER ran!
(edited to correct some typos)
Dennis,
Of course kV isn't the only parameter that matters. It is pretty easy to show using a simple motor model that the torque constant =k
t~10/k
v. The torque just equals k
t*I(amps) or 10*I(amps)/k
v. So far it appears that all k
v motors are basically equivalent. However the smaller motors have a higher internal resistance, and this is limiting the current. So with the same k
v, but a smaller or "weaker" (for example, lower strength magnets or higher air gap) will have less torque at the same rpm
with the same input voltage than the larger motor (example 3014 vs 3020 vs 3026). So if you want to turn the same prop with the smaller motor, you need to have a higher voltage--to get the same current I.
What happens is that the higher resistance of the smaller motor at the same current for the same torque will make more heat---so it will be less efficient than the bigger motor.
Now there is a limit on how big the motor should be. Of course weight is one issue. But there is a point I left out of the simple motor model above. That is there are also magnetic losses in these motors. What this means is as the ESC commutates and the magnetic field grows and shrinks in the iron laminations of the stator, you lose a bit of energy (hysteresis losses). There are also eddy currents induced by these varying magnetic fields in the laminations (the thinner the lamination the lower the eddy current losses). These iron losses are larger at the higher rpms, while the resistance losses are higher as rpm->0. It is stated that at any particular voltage, where these iron losses equal the resistance losses, that is the most efficient rpm to run the particular motor. You can see the effect of these losses by measuring the no load current, I
o. You will see that the bigger motor will typically have a larger I
o, so it is less efficient as you get close to the no load rpm (a very light load). Primarily this is due to the fact that the bigger (or longer in this case) motor has more iron to magnetize/demagnetize and more laminations with eddy currents. I am basically comparing motors of the same configuration with the same kV--like Scorpion 3014, 3020, 3026,... or AXI 2814, 2820, 2826...
We tend run our motors is in the high rpm range (80-90% of the kV endpoint), because as I have pointed out in other posts, we typically supersize our motor. So it is easy to drop the size a bit (3026 to 3020, 3020 to 3014) and not change the efficiency very much-- since we are working around the optimal rpm for efficiency anyway (nominally ~80% of kV*V). However if you shrink the motor too much, you may start operating it at 60-70% range, and now you are putting a lot of your battery energy into heat, not to turn the prop.
Topic: "35" size motor selections (Read 9045 times)
