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Here are the details: Weight (skateboard + rider) = 75kgs
Required cruising speed = 25kmph
Required range = 15km

Here are my calculations:
To keep a comfy window, assume speed is 30kmph, and since wheel circumference is 23cm, RPM = 2174.

Here is the motor I plan to use: Turnigy Aerodrive SK3 - 4250-350kv Brushless Outrunner Motor (https://hobbyking.com/en_us/turnigy-aerodrive-sk3-4250-350kv-brushless-outrunner-motor.html)

Thus at 2174rpm, the voltage required to run the motor is 6.2V.

Question 1: Does this mean I cannot go above say 11.1V of LiPo voltage?

Power = (Voltage)x(Current)

Thus this means that at the motor rating of 1000W and our voltage of 6.2V, the current draw will be 161.2A, which is insanely high!

Question 2: What is going wrong in the above calculation?

At the cruising speed of 30kmph, and the corresponding current draw of 161A, the travel time required (to finish travelling 20km) would be 2/3 hours, and therefore the required battery would have to be 108mAh.

Thus, according to my calculations, I will need a 180A ESC and a 2S 110mAh LiPo.

I think I have made a mistake in the fact that I am using a belt drive drivetrain with a 1:3 reduction and I have not accounted for this. Is this the case?

Question 3: Why is the system mass not coming into the picture??

Question 4: How would you design the power system for this project?

It is not necessary to answer all questions, any help will be much appreciated.

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  • \$\begingroup\$ This is more complicated than you think. Voltage, torque, current, power all must be considered. It would also help if you knew or could estimate what is the wind resistance at 30 kph? You should see what information you can find online about existing electric skateboards. Focus on the models which are shipping already. Not the ones in the kickstarter phase. Be aware that motor power ratings may be exaggerated quite a bit. \$\endgroup\$ – mkeith May 20 '17 at 22:11
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    \$\begingroup\$ Wrong motor type. A brushless motor for surface vehicles should have hall sensors for low speed torque; that is an aircraft motor. Your project has been done before; look for writeups. Also consider tha bicyclists toss around numbers in the 200-400 watt range, and 30 kph is not unusual for such riders - different machine and aerodunamics of course, but to get some very rough perspective. \$\endgroup\$ – Chris Stratton May 21 '17 at 2:05
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Question 1: Does this mean I cannot go above say 11.1V of LiPo voltage?

No. Look at your spec sheet. Maximum voltage is 19 volts.

"Thus this means that at the motor rating of 1000W and our voltage of 6.2V, the current draw will be 161.2A, which is insanely high!"

Question 2: What is going wrong in the above calculation?

Not a blessed thing. What is wrong is that you're blindly doing calculations and not paying attention to the data sheet. Note that the maximum voltage is 19 volts and the maximum current is 53 amps. Multiply those together and you get 1007 watts. Which, being lower than the rated power of 1190 watt, means that the data sheet is off. At your much lower 6 volts, the motor is incapable of putting out the power you want. Also note that this RPM/voltage combination only applies with no load, and this is not how you want to run the motor. Among other things, this does not accurately calculate the current, which will be much, much lower than your 161 amps, even if the motor could tolerate it.

And this is a good thing. 1000 watts is about 1 1/3 horsepower. Any reasonable linkage to the wheels will produce so much torque that turning the motor on would dump on your butt.

"therefore the required battery would have to be 108mAh." Nope. That's 108 Ah, 1000 times more than you think.

Question 3: Why is the system mass not coming into the picture??

That's because you are not remotely calculating what you need to calculate. In addition to motor friction, the loads on the motor come from drive train friction, road/wheel friction, aerodynamic drag, and potential energy transfer (which occurs when you go up hill.) System mass only directly affects road/wheel friction and potential energy issues. You need to specify the maximum grade you want to handle without losing speed, make some assumptions about your drive train efficiency, and either calculate or measure the drag produced by a person standing upright at 30 km/hr.

Question 4: How would you design the power system for this project?

I'd use a 60A ESC and a very large battery pack. Then again, I'd start small and try to get a feel for how these things work, especially if I were like you, with no idea of what I was doing.

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