KV / Gearing choices
 

Okay gang, here's a question for the experienced BL users:

Given the same voltage, and adapt the gearing to get the same speed, what would be a better choice: high or low KV? I mean in regards to the "stress" on ESC and motors.

I know it all boils down to the so-called "most efficient rpm range"... but lately you hear stories here of people having very good luck with uncommon voltage / KV combinations. Such as 1600rpm/V on 12 cells... and then you'll have to use a low FGR to get to some speed... but if it works for people.

Real life experience would be nice to hear. I wonder what really IS the most efficient rpm range.

Thank you
Daf

Well, from my own point of view, you can for example reach let's say 40mph with a 7xl and 12 cells, but you wont be able to reach that speed with a 10xl and 12 cells with the same gearing. Probably you can reach 40mph with a 10xl and 12 cells if you use very tall gearing, but then you'll be tiring up your esc.
So, to reach a 40mph speed run with a 10xl without sacrificing the controller's maxximum performance, you might want to go up in cell counts and look for the most suitable gearing in which both the motor and esc would be running at their highest performance.
IF you specifically want a certain speed, then pick up the motor which would give such a speed at a moderate voltage like a 1940/6 at 17 volts would easily take you to the fourties, but if you want more speed with that motor, then you can get to it with either higher gearing or cell counts, but the controller might suffer from heat and poor effeciency and thus, a higher turn motor with higher cell count can give you higher than 40s speed runs with good effeciency. A 1940/8 with 5s would be much faster than a 1940/6 with 4s.
I might add some more, but I'm afraid I'm not as thorough as a REAL brushless expert. Also, I feel like I'm a bit repeatitive here.

Me too. And I feel you missed the point, too. But nevermind, thank you for your input.

My!!! I did miss the point!

Well, how about revisiting Promod's thread David VS Goliath to refresh your mind, Daf. I believe it answers your question very well.:)

See? :005:

I remember that thread very well. It's about the size of motors, what they pull and how much they have to work... not much to do with my question.

Here it is again, more specific: Given the same voltage, same motor-size, and adapt the gearing for the same speed, what would be a better motor: low KV (with low FGR) or high KV (with high FGR) ???

And, please refer to my first post too. I'm too lazy to type the "most efficient rpm range" thing again.

I'm just looking for real life experiences: Such as "I have tried this motor, then I bought a hotter motor, but geared down, and my temps got cooler"

Thank you
Daf

LOL

I already wanted to start the same thread because everytime when i start building up a Bl-Truck i think about that thingy/problem and don't come to an answer...


So bl-experts out there, pls help us!... That i can sleep better at night without always thinking about the best rpm range^^:005:

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.

I think a low KV motor would be the best to go with in your condition, Daniel.

My condition???



Glassdoctor, just the kind of information I try to collect. Thank you.

There seems to be a sweet spot, though. With some kind of luck I have hit it before. I guess it's a combination of everything, vehicle weight, driving style,... But when you get it right, the motors stay surprisingly cool.

I use to like 40 - 45K rpm......... but now, I like lower KV motor with taller gearing......
But I like to stay above 30K rpms....

Seems that a good rule is to mimic a nitro which is good news when converting to electric, because it makes the gearing no problem... we can basicly run the stock ratios.

I used to think we would have to modify for lower gearing but now I'm actually gearing higher.

30-35K is typical for high-revving nitros I believe, and many don't get a sniff at 30K in reality.

So... 30K give or take seems a good choice.

As a side note, part of the reason I chose the low kv, is that it leaves some room to "volt up"... I expect to be trying 6s lipo at least in the future... so even my 1700kv may be to much eventually.

I've got some thoughts on this, that aren't really backed up by any sort of real-world use, but as Scotty says, "ya canna change tha laws of phyziks!"

1. Higher voltage is good. In the real world, as electric motor horsepower goes up, so does the voltage. Why? That's so the amp draw of the motor doesn't become unmanagable, which would require huge wire gauges. The higher the voltage you run, the less current you need to do the same amount of work. This means less strain on your battery and ESC system. For instance:

2S liPo driving a 1000w motor:

watts/volts = amps

1000/7.4= 135 amps

3S LiPo driving a 1000w motor:

1000/11.1 = 90 amps

The benefits there are obvious. Same amount of work being done, but with 33% less amp draw. The more amps you draw, the more you are fighting the internal resistance of the wires/battery/motor/esc and your system is running less efficiently. Let the voltage do the work, rather than the amperage.

2. Lower speed motors don't have to fight as much frictional resistance. It's not hard to see that a motor spinning at 65,000 rpm is fighting much more frictional resistance than one at 40,000 rpm. Gearing up then puts that high speed load on the driveline, rather than the motor.

3. Here's one that'll probably start everyone arguing: the torque of a brushless motor is determined by its power delivery system, not the motor itself! For instance, if you take the Fiegao line of the same size motors, all can produce the exact same torque, regardless if it's a 6XL or a 10XL. If you do see torque differences, it's because your battery system isn't up to snuff with the current demands of the motor, not because of the motor. This becomes more and more apparent the smaller number motor you pick, because the amp draw goes up as the number goes down. Yes, this goes completely contrary to brushed motor thinking, but then, these aren't brushed motors, are they? :027:

Discuss! :)

Sleebus

Interesting thread. I too am trying to figure out the relationship between high V/low Kv/tall FGR vs low V/high kv/low FGR.

I agree with GD's remarks about being able to "volt up" later on by using a relatively low kv motor with the voltage to get the rpms to the mid 20k's.

Other than experience with various motors, it's very difficult to compare them on paper. Most motors specify the KV and can size and that's it. Maybe the "max amps" is listed too, but it means very little. Is that the max amperage the motor can physically handle constantly without melting? What voltage did it take to produce those "max amps"?

I'd imagine finding the ideal voltage/gearing range for a specific motor would be a bit complex, but this is how I would imagine it could be done:

1. Run a motor on some type of dyno with a fixed gearing. Record the mechanical output power graph as the motor voltage is increased.
2. At the same time, graph the electrical power by measuring battery voltage and current draw at various points.
3. Then compare the mechanical power and electrical power. The closer these are to each other, the more efficient the motor is running.
4. Repeat the above steps for different gearing until you find the max voltage and gearing combination that has the highest efficiency (where the mech and elec power are closest).

That argument makes sense. I agree with your statement about frictional resistance somewhat (I'm sure there IS parasitic loss, but it can't be THAT much). I agree with the power calculations. I also agree about your point about very high currents being unmanageable.

However, physcial load has to be taken into consideration. A heavy physical load, whether from a heavy vehicle or very tall gearing, effectively is trying to stall the motor, or at least slowing it down. The motor's coil resistance is a VERY small part of the current limiting factor. Most of the motor's resistance comes from inductive reactance from the back EMF of the coils and moving magnets.

Yes, I agree completely. However, I guess what I'm getting at is that to do 1000w worth of work, you won't need as much current if you increase your voltage (I know we both agree on this part too.) Comparing the 2S to the 3S example, the current requirement is dropped by 45 amps (!) which is the main advantage. so my thought is with that much less current, there's got to be less resistive heating of the wiring. Drawing less current out of the battery means less heating there too.

I only say this because running my mamba max on a 3S, it runs surprisingly cool compared to my 6 cell NiMH pack. I'm chalking that up to the increased voltage, and that I don't have to have the throttle buried all the time.

Sleebus

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.

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.

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.

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..

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.

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?...

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.

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

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...

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

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:

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.

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?

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?

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.

The problem is that at set voltages lower turn (higher KV) motors produce more power.

However it is possible to go down in kv but still produce the same power by using a larger motor.

My 540 8s and the 540 7l i have ordered have similar resistance and current characteristics which enable them to produce nearly the same power at a certain voltage. The pure difference ignoring the extra weight is the higher torque and lower rpm which should hopefully mean that the 540 7L will be more efficient on 3s lipo than the 8s.

I look forward to receiving my motor I have in fact bought a spare xxxt transmission in anticipation of the strain on the transmission.

What about............

Has anyone considered "driveability" or "control" differences between Kv ratings? Consider the "responsiveness" of a 1700kv motor vs a 2300kv motor or a 4600kv. The higher Kv motor would be far more reactive to throttle (or esc/trigger) inputs than the lower Kv motors.

From what I've gathered from various sources (not personal experience) the higher Kv rated motors have a somewhat higher efficiency.

Also....(although a bit off topic) How do we factor in the performance differences of using 2-pole vs 4-pole (or 8-pole for that matter) motors?

Maybe what we need is info like you'll find on this attached pdf of a Mabuchi 550 motor. It tells alot about the motors "personality".

I think with a higher rpm range the higher kv motors will feel smoother on accelleration. However i think lower kv motors that output the same power will feel more punchy as the rotating mass of the rotor will have less effect on the overal performance.

That would be PERFECT! It shows the various values at max eff, and max power. So, the user can decide what they feel is most important and make the decision based on specific data.

To a large degree, battery quality plays a big role. It looks like the graph assumes an ideal voltage source (constant voltage and zero output impedance). Pretty much ANY battery will sag and react to certain loads.

And let's not mention how the ESC handles itself. Ideally, its output should be stable no matter what, but some loads (particularly high inductive loads) are more difficult to drive due to the voltage and current phase shifting.

Argg! It's enough to give one a headache! There are simply too many variables! The motor selection is hard enough without the user being forced to make assumptions on battery quality and ESC stability.