What is the most efficient RPM for a brushless motor?

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Logan's_Rcs

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Hey everyone!

I'm curious to know what the most efficient RPM range is for brushless motors. I am trying to build a big rock right now, and I want it to run cool on grass with a top speed on packed dirt/pavement of 60+mph. I've done some research, and I've realized that the more watts the motor can produce, the cooler the setup will run as long as the watts are mostly produced by higher voltage and not higher amp draw. I have also read that to run a motor at close to it's max watts, you need to be close to the maximum amount of voltage

In my case I am looking at 2 motors, a TP 4050 2100kv and a Tp 4050 1550kv. I am trying to choose one to run on 6s... the 2100kv will be around 52-53k rpm when it has a maximum of 60k and the 1550kv will be producing somewhere around 38k. Logically speaking, it seems like the 1550kv on 6s would be better, but since the 1550kv has a max volt of 40 it wouldn't be running close to it's maximum watts whereas the 2100kv would be very close to it's maximum watts. Will the 1550kv run hot because it isn't producing a lot of power on 6s, or will the 2100kv run hot because of it's high rpm?

So this puts me in a dilemma trying to figure out which motor to get.

Thank you for all your help,
Have a great day!
 
Hey everyone!

I'm curious to know what the most efficient RPM range is for brushless motors. I am trying to build a big rock right now, and I want it to run cool on grass with a top speed on packed dirt/pavement of 60+mph. I've done some research, and I've realized that the more watts the motor can produce, the cooler the setup will run as long as the watts are mostly produced by higher voltage and not higher amp draw. I have also read that to run a motor at close to it's max watts, you need to be close to the maximum amount of voltage

In my case I am looking at 2 motors, a TP 4050 2100kv and a Tp 4050 1550kv. I am trying to choose one to run on 6s... the 2100kv will be around 52-53k rpm when it has a maximum of 60k and the 1550kv will be producing somewhere around 38k. Logically speaking, it seems like the 1550kv on 6s would be better, but since the 1550kv has a max volt of 40 it wouldn't be running close to it's maximum watts whereas the 2100kv would be very close to it's maximum watts. Will the 1550kv run hot because it isn't producing a lot of power on 6s, or will the 2100kv run hot because of it's high rpm?

So this puts me in a dilemma trying to figure out which motor to get.

Thank you for all your help,
Have a great day!
Hello

I am not a pro at this, but two things come to mind. Some motor makers list the max RPM well on the better motors not all. Second I have seen most say keep it under 50,000 RPMs

You can use an app I have the one for my Android here. Sure it might be in the Apple Store as well:
https://play.google.com/store/apps/details?id=redscrew.gearcalc

You can put in the needed info and get RPMs.

I try to keep my RPMs not at max 50,000 and shoot for 27,000-40,000 as best I can
 
wow, my kids big rock was wicked fast and hard to control even on stock setup 3s, its all plastic and light....on 6s that things gonna be vaporized hittin somthin at 60. id consider lookin into 4s setups
 
There is very little data on this.
I found some old motor test sheets and NeuMotors has some good specs on their motors that might provide some insights.

By my estimates the sweet spot is probably in the 20k to 30k range.

power graph of 3.5t motor.PNG


Motor RPM and current.JPG
 
There is very little data on this.
I found some old motor test sheets and NeuMotors has some good specs on their motors that might provide some insights.

By my estimates the sweet spot is probably in the 20k to 30k range.

View attachment 230659

View attachment 230660
Based on this, would you say the 1550kv would be better? I just want to make sure it is producing enough watts to push the big rock up to 60+ mph and stay cool
 
Based on this, would you say the 1550kv would be better? I just want to make sure it is producing enough watts to push the big rock up to 60+ mph and stay cool

Yep I think 1550kv would be fine. Many people run the Hobbywing 1650kv 4985 motor on 6s and love it. Runs cool and makes crazy power....
 
Hey everyone!

I'm curious to know what the most efficient RPM range is for brushless motors. I am trying to build a big rock right now, and I want it to run cool on grass with a top speed on packed dirt/pavement of 60+mph. I've done some research, and I've realized that the more watts the motor can produce, the cooler the setup will run as long as the watts are mostly produced by higher voltage and not higher amp draw. I have also read that to run a motor at close to it's max watts, you need to be close to the maximum amount of voltage

In my case I am looking at 2 motors, a TP 4050 2100kv and a Tp 4050 1550kv. I am trying to choose one to run on 6s... the 2100kv will be around 52-53k rpm when it has a maximum of 60k and the 1550kv will be producing somewhere around 38k. Logically speaking, it seems like the 1550kv on 6s would be better, but since the 1550kv has a max volt of 40 it wouldn't be running close to it's maximum watts whereas the 2100kv would be very close to it's maximum watts. Will the 1550kv run hot because it isn't producing a lot of power on 6s, or will the 2100kv run hot because of it's high rpm?

So this puts me in a dilemma trying to figure out which motor to get.

Thank you for all your help,
Have a great day!
Keep in mind, the max RPM is achieved under a no-load situation. Meaning, if you apply any load to the RC at all (any driving), you're max possible RPM will be respectively lower. Higher load (more power used), means more current pulled from the batteries, means voltage dips lower, which means your max possible RPM does too.

Both motors should be producing about the same amount of power if you run on 6s, but one will be using the power to get higher top speeds (higher kv) and one will be using that to get faster acceleration (lower kv)--if using the same gearing. Low speed, high torque motors generally run cooler, but if you gear the lower kv motor to achieve the same top speed as the higher kv motor, everything goes out the window as you'd get more heat due to the thinner wires in the lower kv motor. If gearing both these motors for the same speed, you'd need to balance between heat from higher RPM friction (high kv) versus heat due to thinner wires (low kv).

With that said, in your case, i'd say both motor will likely perform pretty much the same. I'd go with the 2100kv, but that's just me.
 
Keep in mind, the max RPM is achieved under a no-load situation. Meaning, if you apply any load to the RC at all (any driving), you're max possible RPM will be respectively lower. Higher load (more power used), means more current pulled from the batteries, means voltage dips lower, which means your max possible RPM does too.

Both motors should be producing about the same amount of power if you run on 6s, but one will be using the power to get higher top speeds (higher kv) and one will be using that to get faster acceleration (lower kv)--if using the same gearing. Low speed, high torque motors generally run cooler, but if you gear the lower kv motor to achieve the same top speed as the higher kv motor, everything goes out the window as you'd get more heat due to the thinner wires in the lower kv motor. If gearing both these motors for the same speed, you'd need to balance between heat from higher RPM friction (high kv) versus heat due to thinner wires (low kv).

With that said, in your case, i'd say both motor will likely perform pretty much the same. I'd go with the 2100kv, but that's just me.
This is the point in time when i wish i had enough money to get both and test them out! That would be such a neat comparison to do, figure out which one is truly more efficient. The one thing i keep coming back to is that i ran a castle 2200kv on both 4s and 5s, on 5s i geared it down and it still ran hotter than 4s. The only thing i can think that causes the extra heat would be the higher rpm.

The stock 3200kv system produces 35k rpm on 3s, and the 1550kv produces around 38-39k on 6s, so it seems like a good option. If i knew for sure the 2100kv would be more efficient i would jump on it. I just wish that there was some kind of data on this...
wow, my kids big rock was wicked fast and hard to control even on stock setup 3s, its all plastic and light....on 6s that things gonna be vaporized hittin somthin at 60. id consider lookin into 4s setups
I did, but my goal is to have a really cool running truck. To do this i need lower amp draw, aka higher volts. You can still have a lighter 6s setup, im going for a 4000mah 6s pack which will still be lighter than what im running now.
 
This is the point in time when i wish i had enough money to get both and test them out! That would be such a neat comparison to do, figure out which one is truly more efficient. The one thing i keep coming back to is that i ran a castle 2200kv on both 4s and 5s, on 5s i geared it down and it still ran hotter than 4s. The only thing i can think that causes the extra heat would be the higher rpm.

The stock 3200kv system produces 35k rpm on 3s, and the 1550kv produces around 38-39k on 6s, so it seems like a good option. If i knew for sure the 2100kv would be more efficient i would jump on it. I just wish that there was some kind of data on this...

I did, but my goal is to have a really cool running truck. To do this i need lower amp draw, aka higher volts. You can still have a lighter 6s setup, im going for a 4000mah 6s pack which will still be lighter than what im running now.
now only having a 4000 mah thats less run time still right? I didnt think there was a whole lot of room for a bigger motor/esc on the bigrock/ 3s models. my older kid has the big rock on 3s, another has the granite which i wont let the younger kid run 3s, only 2s. all 7200 mah lipos. both the same set up really, and then ive got the 6s kraton. these things are a lot of fun trying to tink on but it does get expensive, which ever you decide on, itl be interesting, keep us updated!
 
now only having a 4000 mah thats less run time still right? I didnt think there was a whole lot of room for a bigger motor/esc on the bigrock/ 3s models. my older kid has the big rock on 3s, another has the granite which i wont let the younger kid run 3s, only 2s. all 7200 mah lipos. both the same set up really, and then ive got the 6s kraton. these things are a lot of fun trying to tink on but it does get expensive, which ever you decide on, itl be interesting, keep us updated!
You would be correct with the lower mah battery generally having less runtime, but being 6s it is more efficient so there are less amps being drawn, therefore the battery doesn't drain as fast. You can see this example more clearly in racing drones, most 4s batteries are 1500mah and 6s batteries are 1000 or 1100 mah and the 6s flight times are usually longer because the whole system is more efficient.

Also with the big rock, i believe the biggest motor you can fit is a 40x107mm can such as a tp 4070. To be able to fit that you would have to dremel off part of the reciever box, but it would still fit. But that motor is too heavy so thats why in looking at getting the 4050 40x82.
 
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