300m cable: a power transmission plea for help!

Other Hardware Motors, Mechanics, Power and CNC
1

Hi folks,

I?’?m trying to design an underwater? ?ROV which will can take long dives and a small payload.? ?I? ?want to power it from the surface so that it has a reliable and constant supply.

The power? ?requirement? ?would be around? ?1500? ?watts (ish) if all motors were operating at maximum.? ?I want to power the device using a? ?portable? ?generator at the control end,? ?through a cable of? ?300m? (?as light a gauge as possible?)?,? ?then have an SMPS unit at the device to distribute the necessary supply to motors via an arduino and ESCs.

I know i’ll get a power loss because of the cable distance and i’m also worried about interferance as i’ll have a video signal and analogue data going down another set of wires. DC or AC best for this distance.? How best to transmit the power down the cable?

?I’d be super grateful for any help...

?David

2

1500W - You talking like 12V, 125A? 24V, 62.5A?
How long is each dive?

Best method is probably the way the power companies do it - higher voltage AC at lower current, run it thru a transformer at the receive end to get it back to lower voltage, higher current AC, and then rectify it back into DC.

Have you looked at what folks like Robert Ballard do on the tethered submersibles?

I am seeing stuff like this (for 220VAC):
"I would recommend 6ga for runs under 75ft, 4ga for under 150, and 2 for under 225"

"First, is it safe for me to assume that you have selected your 60-Ampere fuse based on the circuit ampacity requirements as stamped on your heat pump's nomenclature plate? If the answer to that question is yes, then according to Article 310, Table 310-16 of the NEC (National Electric Code), you can use either AWG#6 CU (Copper) or AWG#4 AL (Aluminum) wire to run your 60-Ampere circuit. Both AWG#6 CU and AWG#4 AL have an ampacity of 65 Amperes. If you choose to use AL conductors, make sure the terminals of your fused disconnect and the terminals on your heat pump are marked for use with aluminum wire. Personally, I would use copper conductors."

And you're talking 300m = 975 feet.

Sure you can't do a couple of batteries?
http://www.batteryquality.com/12V-100AH-Battery/12v-100ah-U
Make good ballast too...

3

I figure if you are using a generator it is a 120/240v generator. Using ohms law 1500w/120v = 12.5 Amps. Or 1500w/240v = 6.25A
Using 120v (400ft length)would require a large AWG wire #8, voltage drop is 6.6v, 113.33v at the far end of the wire.
Using 240v (400ft length)would require a small AWG wire #14, voltage drop is 13.48v, 226.51v at far end of wire.
In your crawler bot design you need to find/design the size motors that will be used to make the motors power the device. Once you know how much mechanical force is needed to power then the load and power requirements may change from 1500w. Remember you have to take in to account the drag weight of the power wiring and comm lines.
tufrank4

4

Hi Guys,

Thanks for the replies. I guess my rational behind not wanting to use batteries was that they are so dang heavy, take up space, and create hassle in terms of keeping them charged- but i guess i'm going to look into them again. Length of dive maybe 2hours.

I've seen ROVs operate over similar distance (1000ft) using a fairly small looking tether and powered from the surface. My conclusion was that they are running a high voltage with relatively low amps down a cable: Maybe 240 volts and around 8 amps through a 10AWG wire. Would this be likely? Whatever they had it was a very neat setup.

Yeah i've had a think about drag and motor size and think 1500watt is reasonable. What do you guys reckon about the interference thing? I've heard AC is a no no for running alongside camera signals etc?

cheers

5

Oops I used the wrong footage for wiring. I have been around long distance controls for 10,000 foot deep oil wells, using AC communication on 3-phase 2300v power wiring(north Slope). Communication over AC lines is a common practice, X10 uses this to talk to devices over 120-240 power wiring. I am not sure about video though using AC comm, you could use fiber for the video since the fiber is not influenced by EMI and is pretty light weight in terms of drag.

6

Nice one - I think i'm going to have to do some experimenting to figure out the best solution.

7

I expect it's going to take a lot of google-fu trying to find out ampacity ratings for wire when used underwater. As far as cheap cable goes I'd look for the cable used for security cameras which contains both a shielded conductor and power conductors. With respect to amperage on the power conductors you could stop at a hardware store and pick some up and try some torture testing in a small tub of water?

waybeyond:
Yeah i've had a think about drag and motor size and think 1500watt is reasonable.

Good grief 1500 watts? The electric trolling motor I have on my boat is 30A (360W) peak which IIRC translates to 40lbs of thrust. You don't normally use it at full power for maneuvering though because people standing in the boat have a tendency to fall over when you engage it.

8

Boat's not dealing with dragging 1000 ft of cable and undersea currents tho.

9

i think 1500watts is maybe a little more than i need but i wanted to test the theory on worst case scenario. I'll check out the camera cables - sounds like a good bet for my purpose if i need to transmit power alongside video signals.

On reflection: i think i may be overcomplicating things by trying to use a generator, surface-based power supply. I think that i'm going to get the project working using batteries at first then try to move onto the more elegant surface supply option.

I'm still mulling it over though...

10

ROV,

High voltage down, 400 to 800 volts area, dependent upon sub.

comms via fibre optics,

all basically because the cable has to be strong, yet can not take up a lot of space on the boat,
even a small ROV on the end of 300m of line, in a strong tide , and with drag on the cable, needs very strong cable,

Kevlar is the prefered , though others are used.

11

High voltage and fibre optics for comms - this definately sounds like a good solution, i'll look into it.

Strength is something i hadn't really considered. I think whatever i use i'm going to try and back-up the cable with an extra structural wire on the outside...

12

This sounds like a lethal device. You need to consult experts in marine electrical safety and ROVs before even
contemplating this.

13

To be honest i was already thinking the same thing myself! I think for this reason i'm going down the battery road. At least this way it keeps tether size down and gets rid of the need for a surface supply source.

14

Why not both? Use tiny batteries, maybe even a Super Capacitor for short duration high loads. Keep them charged with a thin cable while it's idling. Then your time below will be infinite. I don't think high voltage is a good idea.

15

now there's an interesting idea. Its good to get all the options!

16

Hi

I dont know how your getting on with this,

a pointer,

at 300m under the sea, you are going to have 400 plus pounds per square inch of pressure,

its also going to be pitch black,

ok thats with the line streight down,

but if your ROV fails and is not boyant, that is where your going,

just be careful, there are quite a few documented cases of the explosive effect when a canister fails, and is bought up to the surface. The raising seals the crack, leaving a highly pressurized canister, just waiting for you to un screw and blow in your face. A few eyes and arms have been lost over the decades.

You can also have fun with cable disappearing inside the ROV, the pressure pushes the cable in,
great fun figuring what the molten mess is inside the case after the short has happened.

then at 300m, there is plenty of possibility for the cable to get stuck around the props, of the rov or the surface boat, especially when the rov fails and is positive boyant, as most are. the skipper will not thank you for entangling the prop.

And when the line fails, then the fun starts. you dont want to go steaming around looking for a length of line with a rov on the end of it.

remember to put a sonar reflector on the ROV, and if practical a radar surface marker, you have a chance of recovering it then. A surface light is also a good idea, even a glow stick kind is better than nothing. It always seems to go dark quick when something has gone wrong,

volts are the least of your problems,