perfo wrote:Looks good :--
A couple of comments if you don't mind (not that it needs any)
1.Controlling the speed of a DC motor by varying the voltage doesn't give you the best results in terms of heat or torque, It is generally better to do it with PWM. With PWM you can supply the motor with as higher voltage as it can reliable run at and thus gives you max torque even at very slow speeds. You should be able to run the thing slower with PWM than lower voltage and it not stall.
2.If you start the thing too fast is there a chance the reaction thrust of the impeller will throw it backwards and disengage with your magnets ? could you do with a couple of stuck on rings just to stop the impeller going too far if it try's too ?
3.Rather than making the center bore smooth make it with numerous semi circular (or any shape really) holes in it. That way you reduce the plastic to plastic contact area and force a small flow of water between the surfaces.
4.You could make the motor housing out of ally or stainless and then fill it with demin water that way you'll have a good cooling surface and thermal path to the outside.
Thanks, I've numbered your questions so I can answer easily.
1. I am using PWM. It's going through an IBT-2 relay.
2. I may go with the rings or guides to keep it still, but the acceleration will probably be controlled in software. This will address the issue of starting too fast for the outer hub (thrusters) but also the inner one, which is also printed plastic and is press-fit on the motor shaft. If it starts too fast it will start to work that connection loose. It's a delicate balance, but not something that can't be handled. Also the magnets are CRAZY strong so it's not that much of an issue at he prop pitch I think I'm going to need.
3. Very good idea. I have been planning to add small grooves inside the hub to induce a thin layer of water to flow in there (like brake ducting). It's still a work in progress. As it turns out, when the thruster is spinning it already has a very low amount of friction induced by actual contact. I think the majority of resistance by a great margin has been the force of the water itself, which I need to optimize with prop pitch and size, based on rotation speed of my motors.
4. I could, but my design is based on CHEAP and AVAILABLE which is why I'm working primarily in PVC and plastic. I plan to make a kit of plans and printable parts available when I'm done and so I want to work with things that can be readily obtained and assembled pretty much anywhere.