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I am planning on building a personal transportation device which will either be a skateboard with a motor etc or a Segway-like device.

I will have two wheels and must be capable of getting me to move from still-position to at least 20-30 km/h for fun/fast transportation.

The device will be controlled by an Arduino (for some other cool features)

My problem is selecting an electronic motor. Where should I look? What type of motor? It must probably be a DC motor, but what should I be looking for?

  • Voltage
  • Wattage
  • Amperage
  • Size

Maybe also what type of battery & speedcontroller should be used with it? It needs to be able to go at least 20 min on a cycle - hope it won't become too expensive.

Thanks

Take note: This is just a fun project so I have a very small budget

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    \$\begingroup\$ What's your mass? What's the maximum mass of the whole vehicle? Any ideas on the batteries? Also I'm not 100% sure that Arduino is powerful enough to balance two wheel vehicle. \$\endgroup\$ – AndrejaKo Oct 19 '11 at 15:53
  • \$\begingroup\$ I way 95kg and the device will weigh mostly what the batteries + motor weighs. Regarding arduino, I think it is, check the newest Make Robots mag, a guy built a segway controlled by an Arduino. But I think the skatboard type vehicle is more what I am going for. \$\endgroup\$ – LouwHopley Oct 19 '11 at 16:02
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Too wide a question set, and some don't matter.

How fast?: 30 kph is getting fast for something small and unstable. Energy at 30 kph is \$(\dfrac{30}{20})^2\$ which is over double of that at 20 kph. Probability of pedestrian death in vehicle impact is \$\sim\sim\sim\sim\dfrac{V^2}{5000}\$ (V in kph). At 30 kph, the chance is \$\dfrac{30^2}{5000} = \sim 20%\$. That formula is for cars and pedestrians but gives you some indication. 20 kph is a safer target for playing an takes less than 1/3 of the windage power that 30 kph does ! - see below.


Battery is probably lead acid as available cheap per energy content or surplus. Low energy per mass and energy per size are tolerable.

Voltage probably 12V or 24V. One or two x 12V batteries. More is possible but gets bulky / annoying. 12V is good enough. 24V favoured by commercial designs as lower current so lower IR wiring losses.

12V or 24V DC motors in the hundreds of Watts range are available on the surplus market BUT are not usually cheap as they ar sought after by people who want to do similar to what you wish to do - vehicles, robots, ... .


Windage = power losses due to air drag.

  • Empirical formula - power required to overcome windage for a compact one person vehicle is

$$ Windage \, power = \frac{V^3}{180} $$

Power in Watts. V in kilometres per hour. Examples:

  • 160 kph gives 22,500 Watts
  • 30 kph gives 150 Watts
  • 20 kph gives 44 Watts.

This is simply a translation of the old motorcyclists adage

It takes 30 horsepower to ton for a well tucked in rider with leathers :-)

There are other ways of calculating this, but that is good enough in such an inexact area. For example:

$$Power = 0.5 \times air \, density \times frontal \, area \times drag \, coefficient \times velocity^2$$

The motorcycle formulae is as easy :-).

For something approximating a flat plate this gives

$$\sim\sim\sim\sim Power = A \times V^3 $$

A in \$m^2\$, V in \$m/s\$


Take off one V and you get Drag (Newtons) \$\sim\sim A \times V^2\$.

This works well enough for bowling balls, raindrops, skydivers, field mice and parachutes.


SO

ie you need a say 200 Watt motor at minimum to get something like your desired 30 km/h top speed on the flat. Using a 12V battery that's about 16 amps.

An eg 7Ah "brick" alarm system battery would wilt very quickly under that current. Notionally 7/16 hour = 26 minutes but really quite a bit less.

A say 30 Ah car battery will notionally give you approaching two hours at top speed. Somewhat less in practice.

Look at the many small electric scooters/bikes/xxx around and see what they use and what ranges and speeds they claim.


Uphill

$$Power = \dfrac{Newton \times metres}{second} \text{or} \sim \dfrac{kg \times m}{s} \times 10 $$

To ascent a slope at 100% efficiency you need

$$ \sim Power = \frac{Mass \times height \, change}{second} \times 10 Watts $$

Mass in kg, \$h\$ is increase in vertical height per second.

To lift you up the slope in addition to windage.

For example, at 30 kph ~= 8 m/s if you ascend a 1 in 20 slope (about 3 degrees) your vertical height change per second is ~= 8/20 = 0.4 ms.

If all up vehicle and rider weight is 100 kg, then

$$Power \, needed = M \times h/s \times 10 = 100 \times 0.4 \times 10 = 400 Watts$$

This dominates your windage losses on hills of this slope.

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  • \$\begingroup\$ Wow. ... Wow! Thank you very very much! You practically answered every possible question I could have thought off for this project. Thanks. \$\endgroup\$ – LouwHopley Oct 19 '11 at 17:25
  • \$\begingroup\$ McMahon, but if I'm on speed or give it a little push (skating-style) then that last uphill-calculation might not have such drastic effect. So what it comes down to is: I need to go look for about a 200-300 Watt motor and combine it with a sufficient lead-acid car-battery? \$\endgroup\$ – LouwHopley Oct 19 '11 at 17:34
  • \$\begingroup\$ Uphill calculation was for constant speed driven by motor. | Note that 300 Watt moor is a little under 0.5 horsepower - a respectable motor size. \$\endgroup\$ – Russell McMahon Oct 19 '11 at 20:31
  • \$\begingroup\$ You need a lot of extra power in those motors - for it to be able to prevent you from falling forward when you approach top speed. The Segway does it by accelerating, so that your body tilts backwards. The Segway apparently has about 2 HP in each wheel. \$\endgroup\$ – frodeborli Jun 30 '14 at 19:01

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