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# How to limit amp draw of motors? Answered

I am building an ROV that uses four thrusters, each can draw up to a maximum of 3-5 amps (motor says 3, website says 5 here). However, I will need to step up the voltage and I cant find anything that provides more than 10A continuous (currently using this but open to a higher amp option), so how can I limit the current the thrusters use in order to make sure that I dont blow out the DC-DC converter?

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## Discussions

Supercaps.

You are unlikely to need high currents constantly, charge the supercaps with the excess. If it comes to arguing with the judges, a supercap is NOT a battery.

A motor takes what it needs in amps.
Limit the current it can use and you also limit the power massively.
But if you want real answers then provide real details.
And for obvious reasons it is best to design without booster packs...

The problem is that I have to work over a long cable (30m), so there is a lot of voltage drop, and the motors I am using are set as I already have them. I am planning to use exactly that dc-dc to step it back up to the 12v that the motor requires, however, I cant find any 5-11v step up to 12v that is above 10A. The links are to the specific products I am using.

If you are using long cables, you should already use a high voltage to start with. Losing 10V from 100V to 90V, at some current, that is only 10% power loss. A 10V drop from 24V down to 14V is much more significant. (41% loss!) That is why power companies use AC voltage that they can easily boost up to tens of thousands of volts for long distance power lines. For some required amount of power, always choose many volts and low amps over low volts high amps. Or you use LiPo battery packs locally on your ROV.

My cables are already rather large 4 AWG (voltage drop will make the source anywhere from 9-11V. Sadly, it's for a competition and I can't have above 12v at any point of the robot, and the source is a lead acid battery. I am also not allowed to have any sort of batteries underwater.

In that case I have to say you failed in the initial desing categories.
You need motors that run on 6V or something and if required electronics for around 9V operating voltage.
With that it is a simple matter or using the right motor controller or a good step down regulator.
If you only have 12V as the source you also have to make sure the current draw of the motors (according to the datasheet) is within the range your battery can supply.
No use to have 20AH battery and motors that draw 10amp or more.
If in doubt go highspeed with the corresponding props or use reduction gears.

You started your competition from the wrong end of the power problem ;)

Is it possible to use 120V AC along with a local power supply? That would be much more practical. Step up converters are difficult to make and buy that supply such large current. Currents larger than a few 10's of amps are basicly of no practical use, other than things like welding.

This video explains what I was talking about in my last comment.

You are talking about extension lead style power supply??
If you need serious power there is only one way: Higher voltage.
Get proper marine grade cables rated for at least 250V and do the down transformation inside the vessel.
If you already have trouble with the voltage drop it means you need even thicker cables - no matter what on low voltage!
Simple question of weight, flexibility and how well the thing can move around.
I see no chance with cables thick as those used for welders...

My cables are already rather large 4 AWG (voltage drop will make the source anywhere from 9-11V. Sadly, it's for a competition and I can't have above 12v at any point of the robot, and the source is a lead acid battery. I am also not allowed to have any sort of batteries underwater.

+10 !

The VERY last thing you want is to pump 5 volts down and step it UP ! There is NO way that will work.

Steve

This is an ROV, put batteries on board and make it more bouyant to compensate. life is a compromise but you can never beat the physics.