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My subsea power module

(Etienne Demers) #1

Finaly received my PCB for my surface power mod.

This fits inside the 3inch housing. for an easy swap of the battery housing.

My special tether to make it all work.

it has fiber, power and shielded twisted pairs.

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(Jacob) #2

sick!

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(Nikos Maurommatis) #3

@etienne, is that a commercial tether or a custom order tether?

BR
nmau

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(Etienne Demers) #4

Hi Nikos,

Its commercially available from my website. http://www.deltarov.com/new/product/power-tether/

Cheers,
E.

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(Etienne Demers) #5

Its that moment when I fitted the heatsink inside your 3inch housing :slight_smile:

By providing all the drawings you guys really make it easy to design equipment using your products.

Just a couple days ago I had a preliminary meeting to design a CCR (closed circuit rebreather) for diving and I was showing your housings as something we could use. They were really impressed.

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(Oystein Skarholm) #6

What is the spec on the power supply ?

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(Etienne Demers) #7

750W DC at 12V

You can mount in parallel for additional power.

You need my surface power unit and my tether for the safety system to work. The tether has wires around the power conductors and is the reference for the insulation monitoring.

The system will defect if the fault comes from the 0V or 300V wire.

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(Oystein Skarholm) #8

Looks good. As you know I am running 24VDC on my ROV now and I absolutely love the power it gives.
Is it possible for you to make your PCBs in 600AC - 24VDC version and still keep them small enough to fit them inside a, say 4" bottle?

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(Etienne Demers) #9

Hi Oystein,

To answer your question, it depends on your DC-DC converter and whatever else you have in there. Heat dissipation is a big concern which is one of the reasons why I opted for doing my power module this way.

I remember you mentioning this before. I am not really in favor of it and I will explain why.

Going over 310VDC has a lot more factors to consider than just the PCB. You have to get connectors, an IRM, contactors etc that will handle higher voltages and its going to affect the price tag considerably.

At 310Vdc, lots of parts available for cheap.

Most importantly, 24VDC is also going to pull a lot more current and, according to the T200 graph, it might not be giving you much more thrust. (which means a lot of wasted power) Sure, it will give more thrust but at what cost?

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EX:
Have a look at the specs for the T200

At 12V the truster consumes 182w. At 16v (only 4 more volts), the thruster consumes nearly double the power but give only 26% more thrust. https://www.bluerobotics.com/store/thrusters/t100-t200-thrusters/t200-thruster/

You are better off doubling your thrusters for roughly that same power consumption you would get double the thrust instead f 26%… (This is why I am only providing 12V but I may increase to 15V later so people can charge their batteries)

That’s only at 16V. At 24V it would be a lot more wasteful not counting the size of the tether you will need to push that amount of power.

============================

The Bluerobotics guys designed their thrusters for running on battery so the design has maximum efficiency within that voltage range.

They are also saying that maximum voltage is 20V and 350W.

I would refrain from building products outside manufacturer specs as it will bring problems eventually.

At 12.2mm outer diameter, my tether is good for 1000Vdc because I am planning on increasing the power transmission by a factor of 3. This would be done as an add-on with regulation at the ROV end to output 310Vdc regardless of the upstream voltage fluctuations caused by power draw and length of tether. I would be hoping to fit this in a 2inch housing but I haven’t really looked into it past my design theory and initial schematics.

By remaining modular, clients can opt to purchase what is relevant to their application.

I hope you are not taking this the wrong way, to each his own and your system works for you.

Bluerobotics would need to change their thruster windings for me to be interested in 24Vdc but honestly, the real upgrade would be to have the windings for 310Vdc with ESC inside with hall effect sensors for feedback. (@rjehangir :wink: )

Not sure if they are into this as it caters commercial application which isn’t their target audience.

Cheers,
E.

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(Oystein Skarholm) #10

First of all I think you misunderstand my set-up. The supply voltage is not 24 volt from topside, its 24 volt subsea, derived from 600volt topside.Which allows me for much better performance through a slim tether. The higher the voltage is, the slimmer the tether can be. The current drawn are less and hence the voltage drop which again gives you better effect in the end. Transforming the voltage from 950volt or so to 310 subsea is also a good solution to avoid voltage drop. The reason I ended up with 24 volt subsea was because I could not find a PCB that could transform 600vdc - to 12 or 15vdc. My old PCB from 400volt to 12 volt caused for a large voltage drop subsea and resulted in poor power effect subsea.
With you 310 volt topside power supply you will need a pretty fat tether to supply 8 thrusters.
My tether has 0,8mm^2 conductors and with 300m of tether the voltage drop was just to much. So I ramped the voltage up to 600, but was then forced to change PCBs as well. But so far so good with 24 volt.
Using higher voltage is offcourse at my own risk, but really, using electrical components under water is normally done at your own risk anyway.

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(Etienne Demers) #11

I do not mean to argue with your logic but you are not considering that the coil resistance is remaining the same.

At 12V you do not reach 350W, you reach max 182.2W and for that power you loose only 26% thrust. Look where the 12V line stops in the graph.

As you increase voltage for a same resistance coil you will increase current. ohm’s law applies here.

For the sake of illustration, lets assume the coil is a 25 ohm resistor and the power is not a pwm signal but pure DC:

V=RI
V/R=I
12V/25ohm=0.48A
24V/25ohm=0.96A

Resulting power:

P=VI
12V0.48A=5.76W
24V
0.96A=23.04W

There is only so much power the coil in the T200 thruster can handle before the lacker in the winding melts and thruster coil goes bad.

Else whats stopping us from applying 300V to the thruster?

Or if we apply 0V, does this mean the thruster has infinite current going through it?

The power loss in the tether will depend on how much your DC-DC converter subsea can handle. As you said, the higher the distribution voltage in this case, the lower the current and heat dissipation requirements in the tether.

cheers,
E.

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(Etienne Demers) #12

Fyi

the graph right above the one you used shows the current draw for 12V and 16V.

I am unsure why you got a huge drop with your initial 400VDC to 12VDC.

I would need to see the specs for everything to explain what was going on.

My DC-DC handles 200 to 425 VDC input at an efficiency of 89%

This means I can take a 110VDC drop before it goes bad.

My tether has 1sqmm power conductors.

I’ll post the results on here when I have everything hooked up.

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(Adam) #13

Hi,

I believe this thread and my answer within will clarify how exactly our thrusters, and all brushed/brushless DC motors in general behave at increasing voltage supply.

Power is not held constant- for a higher voltage input and constant resistance, current will always increase according to Ohm’s law (I = V/R). At 20 V, a T200 will draw about 32-32 A, and produce about 15 lbf thrust. As current draw increases, stator heating increases by P = I2R. Past a certain point, the heating will be great enough that the magnet wire that makes up the windings overheats, burns off the enamel coating, and shorts, destroying the motor. We’ve found that for constant draw, the max safe value is about 30-32 A, leading to the 20 V max voltage rating. This assumes running anywhere in the throttle range, including full throttle.

However, since lower throttle reduces duty cycle and thus the apparent voltage at the ESC, running at 50% throttle at 24 V is effectively the same as running at 100% throttle at 12 V.

0.5*24 =12 *1.

There is a more detailed explanation in the first link above.

-Adam

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(Etienne Demers) #14

Awesome answer Adam. Thanks for your input.

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(Jacob) #15

Please do not use Blue Robotics products in safety-critical applications like a rebreather. They were not designed for this.

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(Oystein Skarholm) #16

Not at all Etienne. Good discussions are very meaningful and a great way to learn new perspectives.

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(Etienne Demers) #17

Its all good. I can talk electronics, software, ROVs non stop if I could :wink:

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(Etienne Demers) #18

Don’t worry Jacob, we will science the shit out of it. I have experts in the field involved.

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(Etienne Demers) #19

Starting to take shape but forgot a cap I’m my RS shopping cart…

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