Toorbough ULL-Zeveigh wrote:
Biff wrote:but is around 94% efficient (vs the 97% of the CSIRO)
that's a gulf as wide as the grand canyon.
You are right, it is a huge difference, but it depends on how you look at it, and what is important.
In terms of heat production it is significant, as 94% efficient produces 2x the heat as the 97%
In terms of power output vs power input, the difference isn't that large, at 2kW power input you would get 1.94kW out with the 97% efficient drive, and 1.88kW out with the 94% efficient drive.
the biggest difference is where the heat can be shed. With the coreless axial flux (without some fancy heat transfer system) you end up trying to get rid of all the heat between the rotors where you have a 2mm airgap on each side and virtually 0 airflow from an external source. With the iron stator, you have the ability to move the heat out to a location where you can easily pass fresh air, so the extra heat is very easy to remove.
Toorbough ULL-Zeveigh wrote:i'm surprised & confused by the use of alu, i would have bet anything an HA demanded CF.
why isn't it causing eddy interference & parasitic heating the way it did for Thud & luke4phys??
The CSIRO motors that I have are actually not hallbach and have iron backing. The added cost and complexity of manufacturing the Hallback arrarys (and I belive using an aluminum "back iron" for structure) was not worth the added efficiency gain and weight lost, so all of the new kits they are selling are standard magnetic arrays with iron rings. The kit that CSIRO sells for around $10,000 consists of 2 rotor rings (magnets attached to iron rings), a stator (coils made of litz wire encased in epoxy (and I believe a small amount of fibreglass)), hall sensor board and 3 large inductors. Once you get the kit you need to design the rim, shaft casing and everythign else you want to actually build the motor, most of those components that my team came up with are aluminum to save weight and still be easy to manufacture. With CF parts it would be significantly more complex to manufacture, and also difficult to model the potental failurs.
There are little to no eddy currents in the rotor due to the magneic field in the rotor remaining almost constant. Where you end up with eddy currents is when you have magnetic field moving past a large piece of conductor (like Luke mentioned, CF would also form eddy currents in that situation).
Kingfish was interested in why they used Litz wire. I read the paper on the development of the motor and if I remember correctly they did it because of the PWM frequency and frequency in the motor, they wanted to reduce the losses at high frequency. But that doesn't make a lot of sense to me, since the PWM frequency is filtered out by the inductors requred to be able to actually control the motor, and the fundemental frequency of the AC driving the motor is low aswell. It does however make sense to use stranded wire to reduce eddy currents because the large (around #10) wire actually used would offer a fairly large cross section to the moving magnetic field causing eddy currents. Litz wire however is not that fun to work with, and it uses a fairly large amount of area for the insulation between all the little strands, so I would recommend using regular magnet wire for your own designs, but use a few conductors in hand to break up the large cross section of a single wire, the 100's of strands in litz wire is overkill IMHO.
For those of you interested in the efficiency of the motor, I have found this graph my team made, I put in a request for the data used to produce the graph, but I am not sure if anyone still has it. I believe this is the efficiency of the motor and controller (the controller we use is worth around $6,000 and has a maximum output of 10kW, search for tritium wavesculptor if you are interested in it). I think most of the RC efficiency numbers are also motor and controller, so if you estimate the RC controller efficiency at around 94% that would mean a system efficiency of 86% would indicate that the motor efficiency is around 91%