Ac to DC powersupply and voltage drop

I have a project that require me to use 120VAC through the tether, and then use a AC-DC converter inside the battery housing to run a Bluerov standard config. I will only run the motors on 25-50% gain.

The tether i have available is a 100m long Deltarov power tether with 2x0,75mm2 power conductors.

My plan is to use this Ac-Dc converter in the rov:

I tried to use the voltage drop calculator by bluerobotics and got this results when i put in 100m length and 7A current to the load.

You can see that i get a output voltage of 83volt.

If i put in 75meters length instead, i get a output voltage of 91,61volt.

On the technical specs on the AC-DC converter, this can have a input voltage of 90-264VAC.

What do you guys think? How long tether can i use on this setup?

I am happy if i can use this setup with minimum 50meters of tether. Just looking for someone to confirm that my theory is right.

How many watts do i really need to use the motors on maximum 50% gain?

Cheers

Hi @Jhans -
Before you go any further, I would strongly push you to identify, obtain, and test adequate safety components for your project. Putting AC voltage in proximity to water can be very dangerous, especially at high voltage (anything above 24V, AC or DC, becomes pretty nasty!) At the minimum you should use a ground fault interruptor circuit, with a ground line in the body of water you’re deployed in. Other ways of detecting leakage current are possible, and worth doing!

When you used the calculator, you increased your wire area above what you said your tether had?

400-600 watts is a good estimate for 50% power consumption of the standard ROV.

Keep in mind that the power loss as heat can cause issues, especially for portions of cable not immersed in the water or coiled on a spool. Divide the energy loss by the length of your tether to understand how much heat energy per meter will need to be removed, to prevent wires from melting.

Be careful!!

Hi Anthony.

Yeah i will have a ground fault interruptor circuit for that goes to a ground point in the water.

Yes on the calculator i put in 0,980mm wire area.
That results in 0,75mm2 wire conductors.

For the power loss as heat, the tether will be on a submersed winch at all times, so it has water cooling before i put power on the system.

Here is a calculation with 50m tether length on 0,75mm2 conductors.

I didn’t quite understand that last bit about how heat energy per meter that needs to be removed.

In this calculation i get 109watt as power lost as heat. If i divide that to 50meter i get: 2,18

What does that tell me?



Dear Jorgen,

Upon reviewing your power supply system, it appears that it may not perform as expected. The issue does not seem to stem from the conductor size; the voltage drop across the 50-meter tether is within the specifications of your AC-DC converter.

However, a significant concern arises when considering the use of brushless motors. When decelerating (e.g., transitioning from full forward to a stop), these motors function as generators. This generates a reverse flow of power that must be properly managed.

In systems using a battery, like the BlueROV2, this reverse power flow is absorbed back into the battery without issues. But in your setup, which uses a direct power supply, there isn’t a straightforward path for this back current to dissipate. Consequently, it will feed back into your AC-DC converter, potentially causing an increase in the output voltage. This spike in voltage could trigger the overvoltage protection mechanism in your converter, leading to a shutdown of your power supply.

It’s crucial to address this reverse power flow to ensure the reliability and safety of your system.

Hi Davide,

Thanks for your reply.
I understand.

What if i install a diode in the circuit to prevent the reverse power to go back into the converter? Will that solve my problem?

ons. 24. jan. 2024 kl. 23:45 skrev SubseaLED via Blue Robotics Community Forums <notifications@bluerobotics.discoursemail.com>:

I would send 200v DC down expecting 30 volts drop or more in the tether, then use a switching regulator in the ROV.
Shock hazard for nearby swimmers.
Galvanic corrosion for any leak to water.
You need a battery in the ROV to handle surges and store the charge.

Dear Jorgen,

Installing a power diode in series with your PSU output could prevent the overvoltage trigger on your PSU. However, this approach has a significant downside. While the diode would block the back current from returning to the PSU, it doesn’t provide an alternative path for this current. As a result, you might see an increase in voltage within your circuit. This heightened voltage could potentially exceed the maximum voltage tolerance of your devices, posing a risk of damage.

Regarding the brushless motors, one avenue to explore could be adjusting the settings in QGroundControl to modify or disable the braking function of the motors. If feasible, this adjustment might help manage the back current issue but I’m not sure if it is possible to do.

I’m also curious about the operational duration and specific tasks your BlueROV2 is intended for. Depending on these factors, incorporating an additional battery could be a viable strategy to extend its runtime. Could you please provide more details about the intended duration and nature of your BlueROV2’s tasks?

P.S. It’s important to note that constructing a reliable and efficient power supply system, especially one that deals with high voltage input and high current output, is a complex task. This complexity extends beyond just preventing overvoltage; it also involves integrating additional components like capacitors and resistors. Moreover, managing heat dissipation is a critical aspect that cannot be overlooked.

At SubseaLED, we devoted two years to research and development to perfect our 1500W power supply system. Given this context, if you’re working within a tight deadline and wish to avoid the extensive investment required in developing a bespoke power supply system, I would strongly recommend considering alternative solutions.

One such alternative is to focus on extending operational time using batteries. This approach is generally simpler and can be more readily implemented without the complexities and risks associated with designing a custom power supply system. If you decide to explore the battery option, I would be happy to discuss potential configurations and strategies to maximize the effectiveness of this solution.

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