KV / Gearing choices

[Finnster]

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.

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.

[BrianG]

Originally Posted by Finnster

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.
.
.
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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.

Yeah, a couple places have those calculators, but prop loading is kinda comple and hard to draw parallels to road use, but I see what you're saying.

Did you mean to say "This suggests to me, it is best to run a motor with as low as kv rating as possible..." judging by the rest of the paragraph?

[Finnster]

Originally Posted by BrianG

Yeah, a couple places have those calculators, but prop loading is kinda comple and hard to draw parallels to road use, but I see what you're saying.

Did you mean to say "This suggests to me, it is best to run a motor with as low as kv rating as possible..." judging by the rest of the paragraph?

Yeah, the prop loading is imperfect, and it only gives you a fixed point as an output, but I do think it gives some interesting data when it comes to motor load, which we can kinda equate to hard accel or WOT. You can use the max eff point for a given voltage to kinda create a dynamic range, but its "eh".

I was intending to address the question posed by glassDr in previous post.

Basically, if you have a fixed voltage (eg 5S) and are trying to gear to the same speed (prop rpms~=wheel rpms, ie 40mph) is it better to use a lower kv motor and overgear, or a high kv motor and gear lower?

The calc data to me says "lower gear, higher kv" is better. This makes sence to me as well as in my Novak BL example from above as well, that a 8.5 can not be ran as fast as a 5.5 on the same # of cells as after a point the motor becomes overgeared and just becomes hot (data indicates that motor will draw more amps, but eff falls, so output power does not improve.)

=> if you want a truck that does 40 on 14 cells, its more efficient to run a (8)XL and lower the gearing than run a 10xl and gear high.

Obviously there are more factors involved in actual motor selection, but this is what the calc data and my own experience tells me.

[crazyjr]

Originally Posted by Finnster

Yeah, the prop loading is imperfect, and it only gives you a fixed point as an output, but I do think it gives some interesting data when it comes to motor load, which we can kinda equate to hard accel or WOT. You can use the max eff point for a given voltage to kinda create a dynamic range, but its "eh".

I was intending to address the question posed by glassDr in previous post.

Basically, if you have a fixed voltage (eg 5S) and are trying to gear to the same speed (prop rpms~=wheel rpms, ie 40mph) is it better to use a lower kv motor and overgear, or a high kv motor and gear lower?

The calc data to me says "lower gear, higher kv" is better. This makes sence to me as well as in my Novak BL example from above as well, that a 8.5 can not be ran as fast as a 5.5 on the same # of cells as after a point the motor becomes overgeared and just becomes hot (data indicates that motor will draw more amps, but eff falls, so output power does not improve.)

=> if you want a truck that does 40 on 14 cells, its more efficient to run a (8)XL and lower the gearing than run a 10xl and gear high.

Obviously there are more factors involved in actual motor selection, but this is what the calc data and my own experience tells me.

Based on this thread I have been wondering if i needed to run a 7XL instead of the 9XL in my G2R 12 cell mamba max setup. Given the same gearing (17/66) and same cell count (12 cells), Could the 7XL be more efficient in this case?

[Dafni]

CrazyJR, now you feel my pain :) I was wondering the very same thing, but with different motors.
You'd have to change the gearing though. Aim for the same speed, and your motor temps may significantly drop.
Whatever you do, please keep us updated.

Daf

[starscream]

Originally Posted by Dafni

CrazyJR, now you feel my pain :) I was wondering the very same thing, but with different motors.
You'd have to change the gearing though. Aim for the same speed, and your motor temps may significantly drop.
Whatever you do, please keep us updated.

Daf

Here's another question that seems relavent to this conversation. The neu motors seem to be built with both Delta and Wye winds. The Wye wind seem to have a lower kv than the Delta. Is one wind type more efficient than the other?

[Finnster]

Originally Posted by crazyjr

Based on this thread I have been wondering if i needed to run a 7XL instead of the 9XL in my G2R 12 cell mamba max setup. Given the same gearing (17/66) and same cell count (12 cells), Could the 7XL be more efficient in this case?

As daf said, you couldn't gear it the same. Assuming both are properly geared, it doesn't mean the 9xl is inefficient, altho it may be just slightly lower. What you can't do is try to gear the 9xl for as high of speed as the 7xl is capable of w/o overgearing/overloading the motor and thus dropping efficiencies

SScream: you evil man....:mad:1 :005: I doubt and diff would be readily noticeble, all winds seem capable of v high efficiencies.