Hi everyone.
Whats yout thoughts on most efficent way to power ROV from topside.
Max Demands are pretty high current wise, 6x25amps@16v for thrusters only+lights+camera+control+some spare for toolings.
We are thinking about 300 V dc@15amps that we could sent thru 6mm2 cable, b
ut that seems allot to play with...
p.s. teather will be around 120m long.
Replies
@Vedran,
Patrick Rowe suggested the following method for powering a ROV from the surface:
Good success with very long power delivery using Variable frequency 3 phase AC with active feedback.
A small onboard battery for surge loads that is recharged when the power load is low. (Capacitors can be used as well but a battery has a higher power density. (LiPo) The key here is the active control of the 3phase voltage and control of the power factor. Changing frequency can also help to mitigate Xc loss in water and help to reduce noise that can interfere with other signals. Also using ungrounded three phase is safer if one phase becomes shorted to water...
Think Brushless motor speed controller and a 3phase transformer and rectifier! (all off the shelf parts)!
I have had fun with this one and some good results powering small ROVs over very long, very small tether.
Remember Harmonics can be your best friend or your worst enemy...
If you go with active feedback then all you need is a 3P rectifier and a filter cap to convert back to DC. As the voltage drops the feedback loop "speeds up" the ESC and the Subsea output goes up. Very simple feedback. Converse when the DC voltage is above a set level the ESC throttle is backed off and the voltage drops. The DC voltage will very within limits of the sample rate and limits set in the feedback loop. But if you have the need for several DC voltages to run things subsea like 3.3, 5, 12, 36. Single chip regulators are available to bust down and Isolate the delivered DC. If you want to get really fancy you could use a 3p Multi drop output transformer and break it down that way. But that means you have to have the extra weight of the transformer subsea.
He continued with the following:
As for the PWM frequency that is what makes it a good solution! Very the frequency will allow to adjust around interference and find a good working frequency. Another nice thing is the ability to program the power curves in the ESC. On the down end (Subsea) use a 3 phase rectifier and a series of 1 chip regulators to get the voltages that are required. Keep in mind that there has to be feedback from the down side end that can control the "A-D" input of the ESC to keep the voltage at the subsea end constant as the load changes. A simple inexpensive PIC processor does that very well.
I have been working with Wild AC devices for many years as a hobby related to alternative energy production. DC transmission is expensive and needs to be heavy duty. AC is much easier to deal with, transmission is much more efficient.
Since my ROV uses T100 Thrusters and has modest power requirements I am going with onboard 3S 5000mah LiPo batteries using two in parallel for 10,000mah.
Regards,
Tom C AVD
Once more thank you very much for your input (time) its much appriciated.
@Vedran,
Have you given any thought to the power conversion in the ROV?
If you assume that you are using all of the available 3300 watts, your bucking regulator, assuming it is a switcher, even if it 90% efficient, which is probably on the high side, will dissipate around 330 watts in heat at full load.
Therefore have you given any thought to your regulator heat sinking requirements?
Regards,
Tom C AVD
I have thinked about it.
We plan to make our WTC out of Aluminium tube so i was thinking about mounting a buck heatsink to the WTC, that way i would have large area to disipate produced heat, but i havent gone in details about it jet. What do u think about it?
@Vedran,
Having the surrounding water cool the surface of the Al WTC to which the heat sink of the down converting regulator is attached is a good idea.
What down converter do you plan to use to convert the 300 vdc to 16 vdc? Do you have a spec sheet that shows its heat sinking requirements?
Regards,
Tom C AVD
I found cable that is rated for 450v dc and is designed to be permanently submerged to 100m. And price is 1.32 eur for meter. (+ tax 20%) that seems pretty fair to me.
http://www.shop.solar-wind.co.uk/acatalog/cable_wire_HO7_RNF.html
Theirs max rated consumption is 25amps but ESCs burst or max consumption is 35amps in water as per documentation.
In scenario when you are trying to lift something 'heavy' from bottom and gain upward and foward or reverse mommentum that is easily 6x35amps burst so u need to have that amount of current available or am i completly wrong...
@Vedran,
Power (P) = Current (I) X Voltage (V)
Power = 300 vdc X 11 amps = 3300 watts
Current (I) = 3300 watts/ 16 vdc = 206 amps at the ROV
The above assumes no power loss in the cable and no conversion losses from 300 vdc to 16 vdc.
However, you are not going to have sufficient current, when taking into consideration the various power losses in the cable and the converter, when using a 300 vdc 11 amp topside power supply.
You might want to try and find a 300 vdc 20 amp topside power supply to provide you with the required current, which is around 215 amps, while accounting for the power losses in the cable and the converter.
Regards,
Tom C AVD
Thats almost the same as some of commercial opsevational class ROVs.
We know that is allot of hardware but we are aiming for 5+ hours of operation...
You really think 1.5mm2 would be enough for 300v dc@11amps? I would think atleast 2.5mm2 or 4mm2 would be needed...
@Vedran,
You need to calculate the resistance per meter for each size of the power cable and make a decision based on the maximum length of the cable.
But if you are going to convert back to 16 vdc, then the copper losses in the power cable become trivial.
I would be more concerned about good insulation between the conductors at 300 vdc to prevent arc over.
Regards,
Tom C AVD
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