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For those of you who don't know: electricity can be "stepped up" to carry the same amount of power over a thinner wire, over much longer distances with minimal losses. Once the power reaches the destination (like your home), the power is stepped down to a much safer voltage.

Here's what I'm thinking: A quadcopter being powered by a super thin, high-voltage line to continuously hover in a single location indefinitely.
The power cable would be very thin, like 1 mm in diameter.
The voltage would be static-electricity range (very high), but the amperage would be harmlessly low.

When the power reaches the quadcopter, it is stepped down to a normal DC current.

No, I have no plans to actually do this, but how well would this concept work in theory?

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    $\begingroup$ Interesting idea! My first thought is that the AC transformers and rectification circuitry to provide the amount of power required for the BLDC motors would be really heavy... Remember that we're talking about a power supply that would need to source ~840-1500 W! $\endgroup$
    – ifconfig ♦
    Jun 9, 2020 at 22:54
  • $\begingroup$ Also, what is your metric for "working well"? Power efficency? Thrust-to-weight ratio? $\endgroup$
    – ifconfig ♦
    Jun 10, 2020 at 1:18
  • $\begingroup$ @ifconfig, I'm just curious if such a system would be feasible at all, or if outstanding issues would make it very difficult/impossible. $\endgroup$
    – Botspot
    Jun 10, 2020 at 13:02

3 Answers 3

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Let's work out some numbers.

Picking a DJI Phantom 4 as a 'typical photo drone' example, it's battery is 4s 5350mAh and weighs about 500g, and provides about 20 minutes of flight time.

That implies that the motors are using (5350/1000)x(60/20)=15amps (converting mAh to Ah, and minutes to hours). At 15v that is 225W for flight.

The question talks about using 'static electricity high voltages' - but the most noticeable feature of static electricity is that it can spark across gaps. 10,000v will jump 3mm or so in dry air, and 3000v will jump 0.5mm. However I don't think we need to go that high. Stepping the voltage up 100 times to 1500v would reduce our current to 0.15a.

That means we could use 29AWG wire, which is 0.3mm diameter - at that point the insulation may be heavier than the wire. According to that chart, 29AWG has a breaking strain of 3.6lbs, which is much more than the weight we have saved from the battery so we don't need to worry about the wire breaking under its own weight.

So how long could the tether be? 29AWG weighs 0.3863 lbs per 1000ft (or 175g for 300m). We will need two strands (350g), so 1000ft (300m) of cable uses up most the weight we've saved by eliminating the battery, before adding any insulation or the transformer. 1500ft of wire would be heavier than the battery.

TL;DR:
* yes, you could hover at normal photography drone heights for an unlimited time using a tether.
* The weight of the wire prevents you going much higher than 1500ft.
* The weight of the insulation prevents you using lighter wire.

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3 Open issues:

1) Amazon holds patent for this idea (tethered drone). This way they can launch a drone, saving a lot of battery capacity for initial lift-off. Once the drone is in working altitude - just disconnect the wire. This works especially well for wing-type drones.

2) Wire insulation is rated for a specific breakdown voltage. At certain point your low-current-high-voltage transmission line will hit the limit.

3) You will need to step-down the high voltage in the drone - this DC-DC converter will take up space and add up to weight.

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  • $\begingroup$ Just a guess, but as transformers are just coils of copper, they probably need to weigh as much as the motors. $\endgroup$
    – Robin Bennett
    Jun 10, 2020 at 8:04
  • $\begingroup$ @RobinBennett You cannot use transformer for DC-DC conversion. Transformers work with AC only. For DC-DC converter you would use something called en.wikipedia.org/wiki/Buck_converter $\endgroup$
    – jpou
    Jun 10, 2020 at 9:01
  • $\begingroup$ OK, so a quick google tells me that a 300W buck converter weighs 80g (compared to about 200g of motors), but I've not seen any that would work at 1000v. The highest I could find was 100v $\endgroup$
    – Robin Bennett
    Jun 10, 2020 at 9:42
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    $\begingroup$ For >1kV voltages, you would need DC-DC converter to have special MOSFETS with Drain-Source voltage above the supply voltage. I doubt you can find many ready made solutions, this would be custom project. $\endgroup$
    – jpou
    Jun 10, 2020 at 10:30
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Absolutely this is possible - and commercial examples already exist so there must be demand. Such an arrangement allows for unlimited flight times but limited mobility; great for aerial camera work or surveillance, but not so useful for aerobatics!

Considerations:

  • Weight (as always!)
    • The aircraft module (to drop the voltage) and the teather (at full extension) need to be light enough for the aircraft to lift them.
    • The tether needs to weigh enough that it droops below the aircraft and cannot 'float' up into the propellers.
  • Strength
    • The tether should be able to hold the drone under all operational scenarios, as if it breaks the drone will fall.
  • Safety
    • What happens if the power stops? (Broken tether, ground power cut, etc.) You could include a small emergency battery on the aircraft - just enough to land - or parachute; this adds to your weight.
  • Noise (the RF kind!)
    • Stepping up to high voltage AC (for minimum cable losses) will create RF noise from the long antenna tether; this needs to not affect drone control but also comply with local interference rules
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