KV / Gearing choices - RC-Monster Forums

Dafni · 06:42 AM

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

captain harlock · 07:32 AM

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.

Dafni · 10.31.2006, 07:36 AM

Quote:

Originally Posted by captain harlock

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.

captain harlock · 10.31.2006, 08:06 AM

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.:)

Dafni · 10.31.2006, 08:49 AM

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

maXXler · 10.31.2006, 12:05 PM

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:

glassdoctor · 10.31.2006, 12:15 PM

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.

captain harlock · 10.31.2006, 12:23 PM

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

Dafni · 12:41 PM

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.

aqwut · 01:02 PM

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....

glassdoctor · 01:15 PM

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.

sleebus.jones · 10.31.2006, 01:31 PM

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

BrianG · 10.31.2006, 01:52 PM

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:

Run a motor on some type of dyno with a fixed gearing. Record the mechanical output power graph as the motor voltage is increased.
At the same time, graph the electrical power by measuring battery voltage and current draw at various points.
Then compare the mechanical power and electrical power. The closer these are to each other, the more efficient the motor is running.
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).

BrianG · 10.31.2006, 02:01 PM

Quote:

Originally Posted by sleebus.jones

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

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.

sleebus.jones · 10.31.2006, 03:19 PM

Quote:

Originally Posted by BrianG

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