Can’t we limit the erpm through the bldc tool to prevent this issue?
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Sure you can do that. You can also limit the battery current to 30 amps if your battery pack is filled with weak sauce. All kinds of compromises can be made. Not ideal.
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I understand a 200kv would be better on 10s ,…but limiting rpm on a 245kv on 10s to around 9000 rpm would keep things safe no?
In addition to what Chaka wrote, I would point out that you would also give up torque by staying with the higher kv motor. Remember, torque is inversely proportional to kv.
I understand that the torque is going to be lower, but I’m saying this so people don’t have to go out and buy new motors just because their motors’ kv are off by a little bit. The torque is going to be lower due to the erpm limit, but it is going to even out once the voltage goes down a little bit. In my case, I have 245kv at 10s, so the erpm limit is not going to be in effect that long anyway.
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Wow I’m really glad I read this! Was was just about to hook my new vesc from enertion (which I have waited forever for) to a tacon 160 245kv and a 12s lipo setup. I will totally be limiting to voltage to 8s instead of the full 12s.
I think you can still use the 12s setup if you want by limiting the erpm in the bldc tool. You just won’t have all of the power that 12s offer at full voltage, but you still have the benefit of the longer ride time from battery with higher voltage.
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Yup I will have to I guess. I originally set out to May my board a speed board but I guess 25 mph should be fine until I decide to get a different esc that can hand the erpm.
What motor or battery limit are yours set to or what’s considered safe? I’ve been running 60/60 and read 50 is the safe max. Is it solely heat dependent? Maybe 60 for shorter periods? Isn’t there a temp limit that would get tripped before it burned up. Don’t want to go down to 50
You missed a serious factor, the revolutions in kv is per V. Lets say you go with a 200 kv and 24 volts, that gives you 24 * 200 RPM on a fresh battery. What RPM will you have at 1/4 battery charge? You need motors to be around four times too fast and four times too powerful at the top of the battery life, in order to have the performance you want all the way through your battery use range.
You also need to consider your gear down ratio in the calculation.You need to consider the voltage top and bottom of battery life, gear down, kv, circumference of the wheel, desired top speed, and torque for hill climbing. I strongly suggest building a spreadsheet and plugging various numbers in.
What? Your battery voltage will only decrease slightly while discharging.
You typically go from 4.2V to something like 3.3V per cell while spending most time between 3.8 and 3.6V.
So no you do NOT need a motor that is 4 times the speed you actually want.
See for example a Lipo discharge curve:
Or did I misunderstand your post?
Thanks, you know your batteries way better than I do. But voltage is additive in series.
You are showing a 1.4 volt drop per cell. Assume six cells in series. For a top voltage of 22.2v, the last quarter starts with an 8.4 volt drop, 22.2 - 8.4 leaves 13.8.v So you would want about double not four times the motor.
You are correct, with that kind of battery performance my numbers were way off. I do appreciate the feedback. Great stuff. Now I need to refactor my build specs, again, tenth time is the charm.
No you still don’t understand. You will not have a 1.4V drop per cell. It will be 0.7V - from 4.2 to maybe 3.5V. Anything below 3.5V is territory you should not go to with Lipos. There is almost no energy left in your battery after 3.5V anyway (look at the graph). Also you will not start at 22.2V at 6S because you charged them to 6*4.2V = 25.2V. That means you will go from 25.2 at full charge to 21V. That is not much and the reason why most people just use the nominal voltage for their drive train calculations. You will spend most time around the nominal voltage of 3.7V - again look at the graph.
Thank you again, I think I need to meditate on what you just said. That is pure gold to me.
This would apply for all escs more than likely, correct? Other ESC are slower and the VESC switching speed is the likely the same or better.
i was going to use dual diagonal 530kv motor (HTD3m) at 6s which is ~93000 erpm.
But that’s unweighted, and 70% of that is 57500 which should be perfect???
I could run 4s and that would put the unweighted numbers exactly in line with yours.
What’s happening with the drop in eff. Does anyone know? Flying machines don’t seem to be suffering the same way.
http://www.hobbyking.com/hobbyking/store/_65951__NTM_Rotor_Drive_600_Series_510KV_5500W_600_50_size_heli_US_Warehouse.html
This Motor is specced for 12s and also a scary amount of amps. That’s 150,000 Erpm, presumably with some Basic atmel ESC, moving over 3 feet of blades through the air. Minimultirotors are pushing above 350,000 erpm.
The high kv is gonna kill your motors quick if you don’t run high speed most of the time
Gearing with htd3m it easy to have a 5:1 ratio and calculated top speed of 21 mph. That should keep the motors very happy,
i still don’t quite understand @chaka point, or whether I should stick with 6s or switch to 4s. If the weighted top speed is 16, then 6s is the best choice.
Calculation and practice are sometimes two different results
youre going over the erpm limit of 60k which may give you problems and errors when running it with the VESC. Try to stick to a sub 300kv motor
Maybe it works, just try it and put you resusts here. You know, for science.