Non-cogging motor generator

[team-joker]

Hi Folk

my interest is to sail a robotic 4 metre yacht across the Atlantic. One way to generate electricity is to let the water flow turn 250mm diameter weedless 2 bladed props to drive generators. Many motors have a strong cogging effect and cannot be turned at low speed. Some (expensive) motors do not display this effect. What is it about the design of these motors which reduces this cogging effect? Can anyone recommend a motor suitable for low voltage use but which could generate a few milliamps if turned slowly? I am hoping to charge at about 6V.

Cheers

[green-boat]

The motor would have to have a skewed armature. Some of the Swiss made instrument grade motors have this feature. Try searching ESCAP motors, I think that's the name. Expensive little things but they can be found inexpensively on the surplus side.

[Umi_Ryuzuki]

Maxon DC motors would be another good brand to consider.
I have seen boats set up with a pair of Maxon motors, and
when you spin one propeller, and motor with your finger,
the second motor and propeller will react to the current
generated by the first motor and spin also.

:)

[fooman2008]

joker
Send some pics when you get it built sounds like an interesting idea. 4 meters is about 13 feet, that should be a hell of a boat to be able to stand up to that! A pair of motors (generators) one for each side so that if she is healing on one tack she still has power?
Here is another thought, just for the thought of simplicity, use a pair of small car (or motorcycle) alternators to make power for it. While they are heavy you can get them with built in regulators, and they turn pretty easily, might not even need as big a prop to turn them. I would also consider cross posting this thread on the sailing thread. It certainly will be challenging......
Foo

[mfr02]

The cogging is a sign that the generator is trying to put some energy into its load. While there is the option to reduce the suddenness of the resistance to movement, its always going to be there. There aint no such thing as a free lunch where energy is concerned.
I did see a drawing somewhere (cant remember where) where a brushless motor was used as a generator. Probably preferable, as using a brushed motor tends to generate unwanted interference, a problem with radio, equally unpleasant when upsetting a power supply. A recovery from a floppy disc drive suitably geared might do the job if you are just talking about a few watts.

[Shaun Hendricks]

You can always use a brushless generator. There are plenty of articles on how to build them from scratch using windpower to drive them. I would use windpower and not water power to run a generator. There are actually a great deal of windgenerators for boats on the market. You can custom build your own.

Putting a prop in the water to turn a generator induces a lot more drag than a wind generator which can run any direction the wind is coming from.

This is the best home built one I've seen:

http://www.windstuffnow.com/main/vawt.htm

He also has instructions on how to build the generator. Neat stuff.

[team-joker]

Hi Shaun

Thanks for showing interest in this project. If a yacht excedes its hull speed it tends to burrow its nose into its bow wave and pitchpole so we need ways of slowing the boat down and props will have a small effect on reducing this. We need to induce drag at high wind speeds since automatic reefing is difficult to achieve. Water driven generator will be unaffected by solid water landing on the deck.

Hi Green-boat

A scewed armature is probably the reason for this. Thanks for that.

Hi Umi

I have never carried out that experiment. I take it that the other motor goes on the reverse direction. The motors must be very efficient.

Hi Mrf02

This cogging occurs on open circuit so it is not wasting much energy. We are considering brushless because of reliability in a salty environment.

Hi Footman

I understood that car alternators needed energy in the first place to power the field coils. I did not think that they would work if the batteries got flat completely.


Thanks everyone for the help you have given me.

[Brooks]

I would be worried about the water rotor fouling, either from biological encrustation or from plastic trash. Thus, I'd go with the airmount generator suggested by Shaun.

If your hull is diving, you need more hull volumn forward, not a speedbrake. Or, reduce righting moment to allow the sails to spill the excess wind. Or perhaps use a flexible mast that will shed wind load. Lots of lower tech solutions to reduce sail power than a prop in the water, I think.

With 4 meters hull length, you should have enough weight carrying capacity to install roller reefing if you want to stay techie.

The main design criteria should be strength - the ocean is a rough, tough environment.

[Warrington]

There are water driven generators built for cruising boats in a relatively small and low output. The only one I've seen had about a 50 foot rope and a propeller that looked like it belonged on a three horse outboard.
With some ingenuity you might be able to put a paddlewheel under the boat. I'm thinking something that looks similar in design to a speed meter paddlewheel, here again like full-size runabouts.
If you mounted a propeller under the boat in the same fashion that you would if it had an inboard engine, then sharpen the leading edges or put one of the cutter blades on it cutting the rope that would follow normal prop.
Most automotive alternators do need power to start but and there is always a but, by modifying the field with permanent magnets you don't need it external power to activate. There are some very small alternators power equipment such as diesel riding lawn mowers in a commercial field there are no larger than your fist.
Good luck, Mike