Drawing too many amps

[greenkarson]

I have 4 brushed car rad fan motors for thrusters. With 6 inch aluminum fan blades for props. I tested them underwater with out the props. (see my brushed motor post) I never actually tested how many amps they where drawing but they ran fine for 30 mins plus dozens of times. But now with everything installed they are drawing masive amps. Tested the rov in the water with a 15amp fuse and blew it in a split second. bumped up to a 30 amp fuse blew it in about 6 seconds. So i done the unthinkable (i know your all cringing) and wired it direct without a fuse. All was working great!!.. for 40 seconds. Then 2 of the thruster stoped working. So brought the rov back home and open the hatch to a bunch of smoke. I pulled all the electronics out to find i'd melted the rubber off several of the 10 gauge wires(insert i told you so's here)
Why are the motors drawing so many amps now? is it possible that my props are to aggresive? The motors ran fine before underwater so i don't think its a shorting problem
I went with the bigger blades because they are readly available and my rov is quite heavy. I havent weighed it but i'd guess its about 40-50 lbs
Any ideas or thoughts?

[Unorthodox]

big prop means more amps as a rule, just turning the shaft under watter is almost no resistance to rotation, but adding a 6" aluminum prop to the shaft is a lot of propeller and even more water to move. You just overloaded them pure and simple.

What do you think you need 4 6" props for anyway? how big is your ROV and what are you doing with it?

[greenkarson]

My rov is pushing 50 lbs maybe a little more. and is almost 4 feet long and the main hull is .25 think steel. Too withstand the pressure at 500'

[greenkarson]

any better ideas for a prop for this beast?


Oh ya here is the depressing pic.

[Unorthodox]

when they were running, how well did it move?

Maybe a gear reduction box to take the load off the motor and put it on the gears?

[manhattan]

Agree with Unorthodox, sounds like you over-propped. Go down to 50-70mm with a gearbox like:

http://www.hobby-lobby.com/drives.htm?p ... Id=2030785

It does look like a short though, a 10 wire should be able to handle 40A continuously, I think, and you melted several. Even with your motor stalled you should not be pulling 40A/motor? Can you check what stall rating your motors have?

[jason howard]

dude in line fuses. definatly any project that you put time and money in to with sensitive electronics should have some sort of safety. its like a condom better to have it and not need it then to need it and not have it. will save you tons of hrs and $$. as for the power did you do the testing with the teather attached or direct from power to the motor? the addition of the teather may have added too much resistance. is your powersorce top or sub side? if your power is top side it has to use more amps to get down to the sub. did you test all of the motors at once to see their total draw or individually? you may have been able to bench test one or two motors with out too many problems but once in the water using most or maby all at once through the teather under load pushing such large props was drawing massive amperage.