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PostPosted: Nov 7th, 2013, 10:25 pm 
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Joined: Jan 28th, 2013, 11:29 pm
Posts: 251
Location: Milton, Florida
Who here has checked draw on their bilge pump motor derived thrusters?
What were you drawing under load?


I have to keep total current draw under 15amps for the whole rov while running 3 thrusters.
Is there a thrust to current gain by using an esc and thereby pwm power as compared to a relay setup?


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PostPosted: Nov 8th, 2013, 3:45 pm 
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Joined: Jul 18th, 2011, 10:23 pm
Posts: 108
Location: Minnesota
With a relay setup, you either have 100% thrust, or 0% thrust.

When using an ESC, you can limit the about of thrust to each motor. This is especially true for your vertical thruster, once you are on or near the bottom, you only need a small amount of thrust to maintain you position, this also helps redurce the amount od sediment your props blow around.

Running your motors at a lower thrust, help to reduce to total power consumption.

The biggest draw back, is you need a more complex electronics package if you are using ESC's. Relays are easy to wire, no radios or software needed.

Rover


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PostPosted: Nov 8th, 2013, 5:14 pm 
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Joined: Feb 13th, 2011, 9:27 pm
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Location: Pennsylvania
I have measured from 4 to 8 amps current draw at 12 volts, depending on the bilge pump GPH and propeller efficiency. There is no 'thrust to current gain' outright in using variable speed control. However, since you are only using as much thrust (and as a result, current) as you need for any given situation, most of the time your current draw will be lower. If you were using 3 standard 500 gph bilge pumps with reasonable propellers, I don't think you would have to worry that much about exceeding a 15 amp current limit, especially when you factor in the additional resistance of the tether. Where is that limit coming from out of curiosity? A fuse? Power supply rating?


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PostPosted: Nov 8th, 2013, 5:33 pm 
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Joined: Jan 28th, 2013, 11:29 pm
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Location: Milton, Florida
thegadgetguy wrote:
I have measured from 4 to 8 amps current draw at 12 volts, depending on the bilge pump GPH and propeller efficiency. There is no 'thrust to current gain' outright in using variable speed control. However, since you are only using as much thrust (and as a result, current) as you need for any given situation, most of the time your current draw will be lower. If you were using 3 standard 500 gph bilge pumps with reasonable propellers, I don't think you would have to worry that much about exceeding a 15 amp current limit, especially when you factor in the additional resistance of the tether. Where is that limit coming from out of curiosity? A fuse? Power supply rating?


from a specification...
I am mentoring the MATE competition rov team at my kids school and we are limited to 15 amps by way of a fuse.
Just trying to think my way through the build.
Thanks


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PostPosted: Nov 9th, 2013, 12:28 am 
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Joined: Feb 13th, 2011, 9:27 pm
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Location: Pennsylvania
Ah, okay. I went and found my notes from when I did current draw/propeller/thrust tests a few years ago for some 1000 gph bilge pumps. The Robbe 50mm drew ~5.3 amps @~12 Volts, and a similarly sized Octura propeller that I also tested drew 5.6 Amps. I also tested some others that drew a greater current but did not produce as much thrust. If you use 500s or 750s you should be fine. You could probably even use 1000s when you factor in tether resistance, but you would have to test to be sure. Good luck to your team in the competition!


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