RC motors speed/torque and acceleration characteristics
Hey guys, im pretty new with all the electric stuff, but need to sort out a couple of things. I've got a Turnigy TrackStar 1/8th Sensored Brushless 2100KV motor with the following specs:
RPM/v: 2100kv Max voltage: 15V (4S) Max Current: 140A Max Watts: 2100W From these numbers i'm able to calculate the torque at the shaft provided at max RPMS, using 4S battery. Torque=Power/Angular speed, angular speed being 2100kv*15V/60*2pi= 3297 rad/s Torque=2100/3297=0.637Nm Now my questions: 1. What is the relationship of rpms and torque in rc brushless motors? Do I get larger torque on my motor at lower revolutions (controlled by esc)? The vendor doesn't provide any speed/torque curves, is there a way I could do some theorical calculations of the torque provided at different revs? 2. If Iwas using 3s battery, which makes 12.6 volts at most, my max RPM would be 26460. Max power at this voltage is P=UI=12.6*140=1764W, right? so my maximum torque T=1764/2770=0.633 should remain the same. That brings it back to the first question  if the ESC controlled the voltage provided to the motor only, it would mean that torque is always the same, but is that true or not? 3. A different question now  how to determine motor acceleration time to the max revs? I guess its not possible to calculate the acceleration time to max revs when the motor is subject to some load? Its possible to get the startup time lower, by adjusting ESC "punch control", however it is not adjusted by seconds, and I need to know the realistic acceleration time, anyone could offer a good approximation with a "soft" setting? Thanks! 
RE: RC motors speed/torque and acceleration characteristics
1. torque is proportional to current(A) measure current at load and multiply by motor torque constant.
2. see above maximum current (in this instance provided by manufacturer, 140A) coincides with peak torque. for BLDC, this is 0 rpm. 3. ESC settings vary by manufacturer to be honest the manufacturer just took the max voltage and a suggested max current draw, multiplied them, and gave that as maximum power. it is common for turnigy to do that. i think youll find real numbers significantly different than the suggested specs. 
RE: RC motors speed/torque and acceleration characteristics
Thanks for the quick answer. To make it simple  i need to know maximum torque my motor is able to provide at the starting moment (need to calculate maximum traction force required), so supposedly my motor draws 140A at the start, the torque is going to be 0,673Nm, just like in my previous calculations?

RE: RC motors speed/torque and acceleration characteristics
Multiply your motor constant by your amp draw

RE: RC motors speed/torque and acceleration characteristics
Quote:
ORIGINAL: behemas Hey guys, im pretty new with all the electric stuff, but need to sort out a couple of things. I've got a Turnigy TrackStar 1/8th Sensored Brushless 2100KV motor with the following specs: RPM/v: 2100kv Max voltage: 15V (4S) Max Current: 140A Max Watts: 2100W From these numbers i'm able to calculate the torque at the shaft provided at max RPMS, using 4S battery. Torque=Power/Angular speed, angular speed being 2100kv*15V/60*2pi= 3297 rad/s Torque=2100/3297=0.637Nm Now my questions: 1. What is the relationship of rpms and torque in rc brushless motors? Do I get larger torque on my motor at lower revolutions (controlled by esc)? The vendor doesn't provide any speed/torque curves, is there a way I could do some theorical calculations of the torque provided at different revs? 2. If I was using 3s battery, which makes 12.6 volts at most, my max RPM would be 26460. Max power at this voltage is P=UI=12.6*140=1764W, right? so my maximum torque T=1764/2770=0.633 should remain the same. That brings it back to the first question  if the ESC controlled the voltage provided to the motor only, it would mean that torque is always the same, but is that true or not? 3. A different question now  how to determine motor acceleration time to the max revs? I guess its not possible to calculate the acceleration time to max revs when the motor is subject to some load? Its possible to get the startup time lower, by adjusting ESC ''punch control'', however it is not adjusted by seconds, and I need to know the realistic acceleration time, anyone could offer a good approximation with a ''soft'' setting? Thanks! so the watts are always changing. and the RPM drops underload Have a look at this video I made, Its a Yeah Racing 35a ESC with a 3900kv motor, rated to burst at 190a. and 23s battery rating. in theroy is can make 2109watts, but in real like it does not. under excessive load (up against a wall doing a burnout) it pulled 553.8w but with the wheels off the ground its only 44.3watts and alot more RPM. So i could fit a more powerfull ESC but its not going to make my setup any more powerfull Have a look at this video, and check out the voltage drop, Current draw, watts and RPM [youtube]http://www.youtube.com/watch?v=xbQcM_qJ5C4[/youtube] 
RE: RC motors speed/torque and acceleration characteristics
oh forgot to mention Brushless motors produce there maximum torque down low in the RPM ramge then as the RPM increases the torque drops off in a linear fashion
here is a example http://www.rctech.net/forum/attachme...rotors005.jpg 
RE: RC motors speed/torque and acceleration characteristics
Quote:
ORIGINAL: phmaximus oh forgot to mention Brushless motors produce there maximum torque down low in the RPM ramge then as the RPM increases the torque drops off in a linear fashion here is a example http://www.rctech.net/forum/attachme...rotors005.jpg 
RE: RC motors speed/torque and acceleration characteristics
torque dropping off as rpm increases is a fundamental of BLDC motors. batteries (of any size) will only deliver what the motor draws.

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