Motor as generator at low rpm high torque??

[hmic21]

Hello all,
I am having trouble finding a motor that can be used to power up a propeller to about 12 rpm then shut off and act as a generator with the propeller being driven by an underwater current. The obvious problem here is though we have a lot of torque, what motor can generate any significant amount of power at that low rpm? To make it even harder the motor must be able to fit in a nine inch in diameter pressure vessel. We are trying to avoid large ratio gears as much as possible to minimize loss. We are currently looking at testing a DC motor,Stepper motor, and a DC brushless to check for efficiency and amount of power produced. Has anyone ever delt with motors in a similar application and can give any suggestions?

 

[itsmoked]

You know there are many products that do just this....?

They generally run at pretty high RPMs. The short coming of all this is that in the ocean there are some really, seriously, large fish that will successfully attack them on a regular basis.

Are you talking large? What size propellor? How many horsepower?

 

[hmic21]

Right now, I'm just working on a 1:10 scale of the actual thing. The propellor has a 1.5m radius. Rough calculations suggest hp ratings anywhere from 1/4 to 3/4. This machine is going to be put into an underwater current ranging from speeds as slow as .2m/s to close to .8m/s. As far as fish attacks, I believe studies have been conducted in the area by experts for a couple of years before they actually began designing it. So I don't think that should be much of a problem. The low rpm was something I was hoping I could work with due to loss in gearboxes. Do you suggest a high step up with a gearbox? I am pretty ignorant when it comes to the mechanical aspect of it all. Oh and the motor will have to fit in a 9in diameter pressure vessel, so it has to be relatively small.

 

[itsmoked]

1.5m! Well that rules out just about allll the fish.

The energy you can extract from a motor is directly related to the speed you turn it. A couple of RPM is very hard to get anything from. I don't think you can do it. You could, but you would need a very strange motor with dozens of poles which kinda makes the motor think it's turning a lot faster. But lots of poles also makes the motor's diameter explode rapidly.

In my opinion you need gears. That said since you ars sub 1HP they can be very light cogged belts like you find inside a desktop printer. They are efficient and make tight bends without a lot of energy loss.

Also because you are sub 1HP you can actually use a small high performance permanant magnet motor as your generator.

They are only 1-2 inches in dia.

I'll hunt up a recent post that had this same discussion.

Meanwhile check this:

http://www.solomontechnologies.com/Solomon new/electric wheel.html

 

[itsmoked]

thread237-123130
Had a simular discussion except for the fluid involved.

Not a lot of difference.

 

[hmic21]

How much power are we talking about with that small pm motor and at what rpm more or less. By the way, thanks for helping me out, this is driving me nuts!!

 

[itsmoked]

Hello hmic21,

Check out TowerHobbies. You need to dig around a little.
This motor is 200W. See what else you can find.

Go down the page to R/C Electric motors. AND SPECIFICALLY
R/C Electric Motors - Aircraft

http://www.towerhobbies.com/listings/cat-cat-g.html

Some of the listings say a lot about a motor, others say nada.

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LX3559&P=0

This is the kind of thing I was refering to. And from a prototype point of view these places are way easier to deal with.

You could use more than one as a possibility too. Just stay away from the brushless as I don't think they will work as generators.

 

[itsmoked]

Trying to send the image again.. Weird! Works on preview then doesn't work in the post!