Update 26.05.14

During the last two months, I have been travelling for three weeks and studying for my exams the rest of the time, so I have not made much progress on the project. Still, I have been able to complete the turning operations on the tailcone and milled the rest of the brackets that will hold the shroud in place around the propeller. Since only some minor machining operations remain on the tailcone, I have also been able to test fit the magnetic coupling and discovered a flaw in my design. More on that later.

Finally the propeller could be test fitted to the tailcone, representing a major milestone in the project. This is when I discovered the flaw in my design. It turns out that the strong magnets in the coupling exhibit a lot of resistance on the coupling when spun. This is due to eddy currents being induced into the barrier. The induced currents generate their own magnetic field which counteracts the field from the magnets, resulting in loss of efficiency. The intensity of the eddy currents depend on the rotational speed of the coupling, barrier thickness, conductivity of the material and magnetic field intensity. Due to the high electrical conductivity of aluminium, this makes aluminium less suited than for example stainless steel in a magnetic coupling barrier.

After discovering this problem, I did some research on the phenomena. There are some literature on how eddy current losses can be calculated in magnetic couplings, but it is generally not straightforward to calculate. This article [link] present an estimation of average eddy current losses in axial permanent magnet couplings, however it assumes that the coupling is made from tile magnets instead of an array of rectangular magnets which I am using. This expression concludes that the coupling will have an average loss of 300W! Although this is probably conservative since I have less magnetic material in my coupling, it is still way too much.

Another way of estimating the eddy current losses is to model the coupling as an eddy current brake. Eddy current brakes are used in high speed trains and rollercoasters to enable braking without the use of brake pads, relieving the brake from any wearing parts. I found a good example [link] on how to model eddy current brakes in FEMM (Finite Element Method Magnetics), the free program I used to calculate the maximum torque for the coupling. This example assumes that the brake is linear, so it is not an exact representation of my problem. By using the mean relative velocity between the magnets and barrier, I obtained a loss of around 80 W at 1000 rpm. When inputting the maximum value of relative velocity (at the periphery of the coupling), the loss is around 200 W at 1000 rpm. I assume that the real loss is somewhere between these two values, and I have concluded that I will not do any modifications before the tail section is complete and I can measure the real loss. If the loss turns out to be unacceptably high, I can make the barrier thinner. The relationship between eddy current loss and barrier thickness is linear, so by reducing the barrier thickness from 3 mm to 2 mm, I can reduce the losses by 33% at the cost of lower depth rating.

Above is the 2D density plot of the magnets with the steel backing plate.

Last, here is a picture of the milled brackets that will hold the shroud. Some grinding, deburring and drilling holes still have to be done, but I'm very happy with how they turned out!