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 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!

savagecommander

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.

behemas

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?

savagecommander

Multiply your motor constant by your amp draw

phmaximus

No the torque wont be the same, Yes in theroy but no in real life. Motor load effects the Current draw.
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 2-3s 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]

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

yakfish

My Feedback: (27)

Good graph there phmaximus. I'll add the reason torque drops as you get higher into the rpm range is due largely to the ability of the batteries to provide enough "juice" at said rpm. The more power (amps) the battery is able to deliver, the less of a drop in torque you will see at the upper end of the rpm range. A 5000 mah 20C battery is capable of 100 amps. A 5000 mah 40C is capable of 200 amp so you would see less of a drop in torque with the 40C battery than you would with the 20C battery.

savagecommander

torque dropping off as rpm increases is a fundamental of BLDC motors. batteries (of any size) will only deliver what the motor draws.

4wdpunchandslide

I have used VXL, hobbywing Ez run, dynamite, and castle just now. I liked the medium kv and the 6 pole ones because It is more responsive like my ezrun hobbywing (4000kv) and the dynamite fuze (4800kv) And gearing had a lot to do with it had 4x4 gearing on my 2wd slash with the fuze and It was a monster and wheeled. The fuze reved quickly but the hobbywing had the top end with what I assumed was the big motor shaft with an 18T pinion and It was in a slash 4x4. The bigger shaft with the same teeth I feel makes a difference.
my new castle “basher” 4600kv motor I had to put in my 2wd because of lack of torque and I only have a 3s connector matching