T-Pain Is On A Boat: Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Saturday, January 5, 2013

Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Yes we do and can and maybe even should design a few motors. Why not? It's fun. There's a world of thruster geometry out there too.

The program "FEMM" is the perfect tool for doing so motors as it can handle the drawings as well as magnetics, permeability, flux paths, and current flow....and then selectively calculates power and forces of all kinds as well. And it's a free program too. Used for teaching the rudiments of FEA in many schools I'm told.

But FEMM does have one wretched flaw in the real world - or at least I think it does. The flaw is that so far I have not discovered any way to incorporate notes into the drawings. So am unable to annotate, or label, or attach the thoughts and results that are so much a part of the design process. I end up printing them out and annotating by hand. This shouldn't be hard to fix...should it? Or is that ability already in the program but just unfound by me?

thanks, Roger L.

...........

Hi Roger.
After reading yours and Craig's inputs, I can see we do have the
technical knowledge here to design a low-speed BLDC motor that can
operate efficiently at the desired prop rpm without needing gearing
down. The axial-flux construction gives a motor design that could be
constructed by the skilled amateur, and by making the motor a 3-phase
design (using Hall-effect sensors for the position sensing) would allow
the use of one of the Chinese BLDC motor controllers that are readily
available.

For medium to larger sized monohulls, this sort of motor, coupled via a
conventional prop-shaft to a high efficiency ducted or integrally
shrouded prop would be a cost-effective and high-performance system.

For my specialist canal boat requirement, and also for ocean going
catamarans with an open bridge deck at the stern, an integrated ring
motor/ducted-prop would be even more useful. I have given my canal boat
reasons in another email, but for the cat, an integrated unit that could
sit in a central well in the bridge deck, on a vertically travelling
carrier would allow the assembly to be raised completely out of the
water for sailing, in a safe single-handed manner. A tilting carrier on
a transom would also be possible, but look more ungainly.

Such a thruster would probably require a radial-flux BLDC motor (ring
stator and inner ring rotor, carrying the multi-blade prop inside the
rotor ring), as this is best suited to fit the hydrodynamic shape
required by the duct holding the stator assembly.
The motor would require two low-friction seals, attached to each end of
the duct, allowing the rotor to turn, without letting water into the
rotor/stator gap. Many years ago I saw electric thrusters on a
remote-controlled inspection submersible that had the motors oil filled,
but kept pressure balanced by a simple flexible oil-filled bag at the
water pressure at whatever depth the RPV was working.

I still think that there is a fair-sized market for such a device,
perhaps in the 3kW to 4kW range. I guess from the fact that no-one has
mentioned any such unit being on sale means that at the moment, they
don't exist (or not at any sensible price)? Anyone else reading this
who might be able to point at a real supplier to the end user?

A Google search for 'Electric Ring Thruster' comes up with lots of
prototype devices, some very expensive units for deep-sea RPVs, and a
few Masters degree and D-Phil dissertations (mainly from India and the
Far-East), so somebody is working on these somewhere!
--
Chris Morriss

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