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Originally Posted by BrianG
I think there are a LOT of factors at play and it will be very difficult to find the perfect combo. I still think the procedure I came up with in post 13 would help prove it, but that's a lot of work and I don't even know if there is such a small dyno! :confused:
Gearing up a slow running motor works to a point. There are probably less rotational losses, but there is a point where the mechanical load is outside the motor's ideal range.
Running higher voltage on a lower wind motor also works to a point. When too high, there almost isn't enough of a load for the motor to operate optimally.
A specification that would be VERY nice to have for each motor in addition to KV would be a number representing the ideal amount of mechanical load for optimum running. Then it would simply be a matter of a little math to find the best combo. That said, the three specs I'd like to see on all motors are KV, ideal mechanical load, and max rpm where efficiency starts to decline.
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You can simulate much of this using the motor calc on Neu's homepage, but its a bit difficult to use as you have to input for props to make a load. But, the little I have played with it, it quite interesting.
What I tried to do it take same # of cells, create a load w/ prop values, and then vary gear ratios to create ~= prop rpms for two diff kv motors.
EG:
dia: 4
blades: 4
pitch: 25
5s3p Kokam 3700s (big batts to negate v drop effects)
I take a 1512 1.5D (3200) and a 2D (2600) and use GR of ~1.75 and 1.4. These make nearly equal prop rpms, 25K, which we can just call wheel speed for our truck thought experiment.
The 3200 is more efficent (~1.5%), but both are high (93%+). The interesting part is if you then vary the ptich to simulate a very high load (ex 60), the 2D drops way more in inefficiency versus the 1.5D.
Now look @ a 1.5Y (1900kv, ~.95 gr)) vs a 1D (4875kv, 2.78GR) and the effect is more dramatic. Under a very high load, the eff of the 1.5Y drops hard, 87.7% vs 94.1%. The current load is higher for the 1.5Y.
This suggests to me, it is best to run a motor with as high as kv rating as possible as not to over rev motor, excede esc Amp ratings, and not to create mechanical heat (fric from bearings, etc) that doesn't exceede that of the gains in efficiency. What that point is IDK, but I imagine fairly high, but you'd still get better runtimes as you are drawing less current for same power output. How this translates to real world would be interesting to prove.