General questions

General topics and discussions about the VESC and its development.
Dor
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Joined: 03 Mar 2018, 08:22
Location: Israel
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General questions

Postby Dor » 03 Mar 2018, 09:50

Hi everyone,
Little bit on me:
My name is Dor and I am new here.
Right now I am learning robotics at college, and I decided to bulid an electric skateboard as a hooby.

So I have some basic questions in 3 subjcects.
I hope that the questions are clear and easy to understnad, so let's start:

Subject: VESC
1.what is the latest version of the Vesc?Does it come is several hardware forms?
2.What is the latest firmware version?
3.Except for conncetion a motor, what other inputs does he has?for exaple, can I conncet a small LCD screen?or any sensors?

Subject: Electric skateboard
1. I bought this motor: https://diyelectricskateboard.com/colle ... 6355-190kv
and I also have a 6S lipo battery, does it enoght to bring me to 15 mph?
2.In which speed the motor can provide the highest tourqe?
3.Which factor determines the tourqe?Does it the voltage or does it the current?

Subject:Lipo battery
Let's guess I have a lipo battery with these features: 4200mAh 35C 6S 1P 22.2 v count 35C Brust 70C
1. It capacity is 4200 mAh, right? So as long as I am using it ,the capacity is getting lower and lower, right?
Like a smartphone?
So if the answer is "yes", is any damage will be occur if I finish all her energy?Is it dangerous for the cells?Is it necessary means that at least one cell(or more) are lower thant 3V?

2.There is this formula : CxA=Amax ,right?
So as the mili ampers are getting lower and lower due to using, so also the max current is getting lower and lower?

3.What does it mean "6S2P"?
-On the buttom line, how much cells do I have in serial? how much cells do I have in paralell?

4.what are the differences between connceting cells in serial to parallel?

I am really sorry if I have some grammers errors or something was not wrriten right
I hope I was enught clear

I am realy apreciate your help
Thanks!
Dor :)

arvidb
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Re: General questions

Postby arvidb » 03 Mar 2018, 17:32


devin
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Re: General questions

Postby devin » 03 Mar 2018, 20:54


rew
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Re: General questions

Postby rew » 04 Mar 2018, 10:12

Dor, please ignore Devin. He's got some non-scientific ways to get some results that are not scientifically valid.

He starts out with assuming a gear ratio. That's the last thing you need.

The motor can put out 2500 Watts. If we take that to be correct, the main question is: Can you do 15mph on 2500 W?

On my bike I measured the amount of energy I put out to bike 5m/s or 18km/h to be about 70W. If we estimate that this becomes 2.5 times more at 24km/h (15mph, 6.6m/s) we get about 175W to go 15mph.

Now a bike is more efficient (but the air resistance becomes dominant somewhere in the region 20-25km/h) so you might assume a skateboard to be twice as inefficient as the bike. So 350W.

So... Can your 2500W motor deliver 350W? Yes it can.

Next... If you gear it wrong, it won't be able to achieve the theoretical performance. So you need to gear it in such a way that it achieves max RPM at the desired speed. With the current margin (350vs 2500), I would want it to run 15mph at the lowest possible battery voltage. So 6*3V = 18V. Times 190 Kv means it will be running at 3400 RPM. NOW the assumptions made by devin can come in to play. (3:1 gear reduction ratio, 90mm tire diameter) means that your tire will be doing about 1100RPM, with a 282mm tire circumference, that means you'll be doing 5.1 m/s, about 19km/h or below. the target speed. If you want to go 1.5 times faster, you need a 1.5 times lower gearing ratio (or bigger wheels). So... with a 2:1 gearing ratio you'll easily do 15mph, close to 20mph on fully charged battery.

If I cherry-pick a bit more from Devin, 2.8 Nm of torque means that you get about 5.5Nm at the wheel. This gives a force of about 120N of thrust. Should be enough. If you weigh 80kg, you'll reach 5m/s in about 3 seconds.

devin
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Re: General questions

Postby devin » 04 Mar 2018, 14:23

i agree with rew if you lower the gear reduction to 2:1 or increase the battery voltage, then 15mph should be possible, but using 3:1 reduction, 90mm tire, 24v battery, then I believe (as rew seems to confirm) 15mph would not be possible.

I do have a question for rew...

what happens if he uses 3:1 gear reduction so that the board won’t do 15mph on its own on flat ground, but then on a downhill he coasts it up to 20mph, then while at 20mph, he uses the throttle. my understanding is the back emf voltage at this speed will exceed the battery voltage. in this case, will using the throttle act as a brake? if the answer is yes there is a real danger to riders of the board who exceed the no load speed of the motor while coasting downhill.
Last edited by devin on 04 Mar 2018, 17:04, edited 1 time in total.

devin
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Re: General questions

Postby devin » 04 Mar 2018, 14:52

& as rew says, if you change the assumptions I chose from 3:1 to 2:1 gear reduction, then by my calculations and factoring the wind you can reach somewhere between 20-25 mph (32.1-40.2kph) before the wind drag force exceeds the thrust force from looking at the middle, top row chart.

Image

further reducing the gear reduction to 1.5:1, it would appear you can reach about 25mph before the wind drag force exceeds the thrust force. on a test bench with no wind drag load the wheel would turn slightly above 30mph equivalent ground speed (middle, top row chart), assuming 1.5:1 gear reduction-- for example with 20 motor pulley teeth and 30 wheel cog teeth...

Image

devin
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Re: General questions

Postby devin » 05 Mar 2018, 17:11

i made a comparison chart of 1 motor set to 57a max motors amps as previously described [24v battery, 0.05ohm winding, 1.5:1 gear reduction, 90mm tire, 190kv], and 4 identical motors set to 14.25a max motor amps. in theory the riding performance should be almost indistinguishable, but with 4 motors at 14.25a max motor amps, the overall electrical to mechanical conversion efficiency is increased and total battery amps drawn at all rpms is decreased (in theory, improving range).

Image

^notice in the left chart, black line (battery amps-4 motors), battery amps are decreased across the chart, while in the right chart, total mechanical watts at most rpms is identical.

a huge benefit to this increase in efficiency would be a reduction in ohmic heating of the windings per motor. with 1 motor at 57a motor amps we get 57a*57a*0.05ohm=162.45w average ohmic heating per motor. by contrast with 4 motors at 14.25a we get identical riding performance but only 14.25a*14.25a*0.05ohm=10.15w average ohmic heating per motor. in other words achieving identical overall performance with 4 motors instead of 1 motor reduces average ohmic heating of the windings by 152.3w per motor to a very manageable 10.15w heating per motor.

devin
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Re: General questions

Postby devin » 11 Mar 2018, 16:24


rew
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Re: General questions

Postby rew » 14 Mar 2018, 07:22


devin
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Re: General questions

Postby devin » 22 Mar 2018, 16:46



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