December 25, 2015


The cast stator

As part of building our axial-flux alternator, I cast the stator in resin. The casting itself went well. However, before the casting, continuity tests between all the terminals showed a good connection, but after the casting all failed. I also tried spinning the rotors to create current in the stator and wiring a 12 Volt battery to each pair of terminals in turn, and but neither created any detectable current through the terminals.

Delta wiring diagram

Since the stator was wired in a delta configuration, the failure must have been either systemic or localized in more than one point. If there was a break in one phase or at one terminal, there would still have been a closed circuit between at least one pair of terminals. For example (using the diagram above), if there was a break at terminal S, there would still be a good connection between terminals R and T, or if there was a break in phase l1, the connections between terminals S-T and T-R would still work.

Stator technical information

From my own observations and consultations with other community members, technical help lines, and online discussion boards (such as I accumulated several theories as to what could have gone wrong. I then came up with ways to test each theory. If I could identify what went wrong, perhaps I could fix it, or at least design the next stator to avoid the problem.

Theory 1

Theory 2

Theory 3

Theory 4

A wire cast in resin

Theory 5

The new joints; some soldered well, others poorly

The new joints cast in resin

The first 4 theories were ruled out, but the last one remains possible. There are plausible reasons why the soldered joints in the stator might have failed while the 7 joints I soldered didn't.

The only thing that I can think to do to shed more light on this problem is to cut away the resin from the coils and examine the joints directly. However, that would take a large amount of work, and it's not certain that I would get definitive results. I also wanted to get the next stator cast before the winter set in and made casting difficult (casting resin requires warm temperatures, and our workshop isn't heated).

Three coils wound from a single wire

On the assumption that the soldered joints were the points of failure in the first stator, I designed the next one to not have any soldered joints that would be inside the casting. Each set of three coils was wound using a single piece of wire, and the 6 wire ends (that connect to the 3 terminals) were left sticking out of the casting. I cast this stator and got positive results for continuity tests between all pairs of terminals.

The new cast stator, with the 6 wire ends protruding from the casting

I might never know for sure what went wrong with the first stator. To find out conclusively, I would want to cast several more stators, with the stator and the casting method being identical except for one variable that is changed each time. But that would require a significant investment of time and money. Given the scale of this project, it is more important for us to focus our resources on building an axial-flux alternator that works, rather than discovering precisely what doesn't.