Unconventional motor use question

[ProjectRadianRobosub]

Hello, we are a second year robosub team trying to create a small underwater robotic submarine. I realize this isn't really the place for submarines, but we found the multi-rotor community extremely helpful last year, and were hoping for some help with an issue we're having.

We saw a team successfully run brushless motors underwater last year, so we decided to try it. We bought this motor for testing:

amazon.com/dp/B075ZSDR2T/ref=sspa_dk_detail_0?psc=1

It was cheap so we figured it'd be a good disposable motor. It did run well underwater, but was drawing over 20 amps(our multimeter only measured up to 20 amps). On amazon the max current listed is 10 amps(I think) but the only issue I've read about with over amp-ing is overheating, something that won't happen while it's submerged in water. If I'm wrong, please correct me, that's why I'm posting. An issue that arises is that once we have 6 motors running, each drawing 20+ amps, I'm not sure we can fit a suitable battery in the small hull of the sub. I suppose here are our main questions:

- Are there any issues besides overheating with running motors far above their max listed amperage?
- How can we choose motors to reduce current draw in these conditions?
- I assume we should look for a low KV, since the motors will be running slow anyways. Does that sound right?

Thanks.

 

[Dugdog47]

I can give advice on the first question, running a motor that hard will put a strain on the esc that controls it, make sure your using some escs with a much higher amperage than your motors are drawing.

 

[GJH105775]

You are correct in that you are going to want lower KV, higher torque motors. RC cars and helis use higher torque motors, so if you decide you need it that would be the place to look. You might want to looks into those. Luckily being underwater will help a little with getting rid of heat, you're still going to want to watch amperage.


What are you running on the motor? I've seen quadcopters that can fly under water using standard motors; it is possible to do.

 

[Tentoes]

The idea is the brushless motor doesn't have any commutator to get wet.

I'm thinking I would still prefer to keep the motor dry. Maybe magnetically couple it to the wet parts.

 

[ProjectRadianRobosub]

You are correct in that you are going to want lower KV, higher torque motors. RC cars and helis use higher torque motors, so if you decide you need it that would be the place to look. You might want to looks into those. Luckily being underwater will help a little with getting rid of heat, you're still going to want to watch amperage.


What are you running on the motor? I've seen quadcopters that can fly under water using standard motors; it is possible to do.

We're planning on putting a special ROV prop on them designed for underwater use. We did see a team last year use normal quadcopter props cut down to half the radius, and we may do that if we need to reduce the current draw.