KV / Gearing choices

[coolhandcountry]

Well form my experence. You can have to low of kv and makes the motor have to work to hard. I have got just as hot temps on a 7xl on 18 cells as i did on a xl1200 basic on 6s. Some times it just seems some motors like different things. I try to go with a mid set up. Not to low not to hi. Hope this helps you some.

[neweuser]

Well I have something here. I ran 5s on the 9xl and 10xl. The 10xl was a lower kv but seemed to perform better on the 5s than the 9xl. It seemed more efficient too. Better run time, and seemed to "stretch it's legs" and got more speed on the top end. I used the same gearing on both. This comparison isn't much info, but I liked the 10xl better. But, the 9xl stayed cooler than the 10xl and seemed better on the Quark.

I'm wondering with the odd voltage used, and having success, is the software in the controllers now. If you go to BK's website, they seem to still have "old" set ups that could be used with certain esc's.

You hear that people have great success with the mamba now with different set ups. But the technology has changed too.

[starscream]

Originally Posted by glassdoctor

I don't have the answer either, but I can say I no longer believe you need to hit a certain rpm range, like 40-50K to have a setup work well.

I used to think this allowed the motor to "stretch it's legs"....

But after running setups that peak in the mid 20k's and gearing even higher than a nitro, I think it may not matter much which route you take. Now I can't say this is ideal for happpy esc and motor temps, but it runs fine and feels good.

I would like to see how motor temps would be if I bumped up the rpm and geared down to match... I think it might be a little better.

Mike and I both ran 1515's in trucks and were geared for the same speeds. He ran the 2200kv and I ran the 1700kv. He had lower temps on the motor than I did. My temps were not bad, but they got higher than the 140's Mike saw. I don't know if track conditions/driving style etc were a factor, but the edge goes to his setup from what I can tell.

Mike's motor would have been peaked about 31,000rpm and mine at 24,000rpm... again, both geared for 30-32mph.

Hey Dafni,
I have been struggling with this question for some time now. My motor is rated for 2500kv (unloaded) so I'm guessing its probably around 2200kv loaded. I've run this system on 5S and 6S and as expected it gets a much better run time on 6S. I have a 12XL that I've been testing with 5S to compare temps and run time. I have the truck geared about the same or a little slower than the 2200kv and the temps are slightly higher, especially on the Quark ~200F :mad:1


Glassdoctor, I am assuming that both you and Mike were running the same voltage as well (4S 8000mAh in this case)?
If that is correct, it would be interesting to see how the temps and run time changed if you both ran 5S while adjusting the gear ratios accordingly. This would give us a better idea of the kv to voltage difference geared to the same speeds.

If I were a betting man I'd say that the 2200kv motor would run hotter than the 1700kv at higher voltage but would get a better run time.

[Serum]

Daf, remember we talked about the eddy currents affecting the non segmented magnets in the partial load?

This is frequency related. The PWM signals the rotors are 'seeying' are different coming from different controllers. a certain frequency would heat it up quicker than the other. (i know this from my schulze controller) while one motor gets hotter on 9khz, the other has a higher temp on 19khz sampling frequency. This frequency is used to make the PWM signal build up like a signal.

I am convinced that a sinus build up PWM signal would heat up the rotor less than an on/off signal.

The better the signal follows the rotor, the more efficient it will be. If the pulses are 100 percent digital, (on off, 0,1) the magnet would heat up the fastest.

The permanent magnet has resistance to the electric-magnetical signal. Due to this resistance the rotor will spin, but if this signal is 'spastic' the rotor/stator would heat up the fastest.

not only the magnet/stator is responsible for the temperature of the motor, but the controller has got a strong hand in this as well.

A 95 percent efficient motor can run less efficient on the bad controller.


Talk about something impressive;
http://lehner-motoren.com/motordaten...50-6HA.32V.xls

I wondered the same thing that you guys do..
Yes... 94 percent efficiency at allmost 70000 rpm.....

But 94 percent of 6400 watts is 400 watts.. 400 watts in a small motor like the 1950... Captain.. We need cooling..

[Finnster]

I've been wondering about this issue as well, and sometimes I'm wondering if sometimes we tie ourselves in logical knots over it.

While I can't attest to the larger motors, I did quite a bit of testing with my Novaks in my Rustler and vs the sensorless. Let me just comment on the Novaks.

All the GTB systems run on 6 cells (and I even ran it on 7) and in the same car & controller, so the comparisons were direct as possible.

The lower turn motors were always faster. There is no way to gear a 8.5 (lower kv) to go faster than a 5.5, after a certain amount the motor is overgeared and goes no faster (or even slower) and just gets hot. :)

Also, a 8.5 on 7 cells is faster/same than a 5.5 on 6. But on 7 the 5.5 is clearly faster. Same on 6 cells.

It seems to me, as long as you are running a motor with a broad eff range, you are better off getting a lower turn motor and gearing properly than overloading a low-turn motor. Now, how low can you go w/o overheating is always the trick.

Almost seems like the diff between a high tq deisel engine vs a high rev, low TQ formula 1 engine.

currently i am running a fegoa 540 8s on 3s lipo but its getting too hot for my liking and i think its because of the high rpm.
I have ordered a 540 7l which on 3s lipo would run about 39krpm which in my experiance fegoa motors tend to run cooler about the 40krpm mark.

From my calculations it should be able to produce the same power as the 8s just with more torque and less rpm which should hopefull help to keep the motor cooler.
Fingers crossed it will turn up this week then i will be able to report my findings in comparing this motor to the 540 8s in my losi xxxt.

[glassdoctor]

The topic really isn't W=VxA... that's pretty clear cut, voltage is your friend. It's much better/more efficient to make power with volts rather than current. But that's not what we're talking about.

The question here is for a given voltage application (4s lipo for example), and "class" of motor (Neu 1515), with the only variable being the wind of the motor... what's the better way to go:

higher kv and lower gear?
lower kv and gear up?

The idea is that you can take the 1500, 1700, 2200, 2600kv motors and get them to run the same in the same car by using the gear ratio to compensate for the rpm differences. But which one is better? more efficient? how big a difference is there exactly?

It's not easy to get answers here.. I even tried to ask the guys at DMA (distributor for Neu motors) this and didn't get anything from them. I even tried to use airplane speak... props size and pitch etc and I got the feeling the guy I was talking to had no idea what was better etc....

I'm sure Steve Neu could shed some light on this if he understood what we wanted to know....

[starscream]

Originally Posted by glassdoctor

The topic really isn't W=VxA... that's pretty clear cut, voltage is your friend. It's much better/more efficient to make power with volts rather than current. But that's not what we're talking about.

The question here is for a given voltage application (4s lipo for example), and "class" of motor (Neu 1515), with the only variable being the wind of the motor... what's the better way to go:

higher kv and lower gear?
lower kv and gear up?

The idea is that you can take the 1500, 1700, 2200, 2600kv motors and get them to run the same in the same car by using the gear ratio to compensate for the rpm differences. But which one is better? more efficient? how big a difference is there exactly?

It's not easy to get answers here.. I even tried to ask the guys at DMA (distributor for Neu motors) this and didn't get anything from them. I even tried to use airplane speak... props size and pitch etc and I got the feeling the guy I was talking to had no idea what was better etc....

I'm sure Steve Neu could shed some light on this if he understood what we wanted to know....

This question seems to be more complex than just kv differences however,
You gave a real world example of a neu 1515 2200kv and a neu 1515 1700kv on 4S. In this example the 1700kv motor ran hotter correct? So, the question remains, which one (the 2200kv or 1700kv) has a longer run time at 4S? This would tell us which is more efficient at that voltage. I was also curious if the heat/run time ratio changes given a higher voltage (geared for the same speed). Hence the next question, is there a sweet spot in the RPM band for a BL motor? if so, what is it based on kv, voltage, both etc?...

[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.

[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.

[Finnster]

Start changing motor sizes and things get even more complex..

[aqwut]

man...this is gettin' technical... there are way too many factors...... but I do agree W = V * A * Effieciency.... but every motor has their own sweet spots.... Lehner has a whole bunch of excel files with motor data on different voltages/amps etc........ some motors runs 94% effiency @ 24000 rpms and some at 70000 rpms..... but lower KV ratings motor has a more broad effiency range at lower amps etc.... for example some hi-amps motors have a good effieciency range at hi-amps...

[Dafni]

That's the stuff boys, keep it coming!

Actually I wanted to hear experiences and theories about setups ON THE SAME VOLTAGE, but it's all good. I appreciate each and any post here.

I feel the same questions are bothering many of us. Nice to have a collection of opinions.

Thank you, fellas
DAF

[glassdoctor]

Originally Posted by starscream

This question seems to be more complex than just kv differences however,
You gave a real world example of a neu 1515 2200kv and a neu 1515 1700kv on 4S. In this example the 1700kv motor ran hotter correct? So, the question remains, which one (the 2200kv or 1700kv) has a longer run time at 4S? This would tell us which is more efficient at that voltage. I was also curious if the heat/run time ratio changes given a higher voltage (geared for the same speed). Hence the next question, is there a sweet spot in the RPM band for a BL motor? if so, what is it based on kv, voltage, both etc?...

correct, although this is only a rough comparison because it was from two different people in different trucks 1000 miles apart on different tracks, etc etc....

but it's all I got right now.... :)

We definitely have more questions than answers don't we? :032:

[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.
.
.
.
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?