Friday, December 30, 2011

Sturmey Archer Dynohub magnet upgrade: progress report

I designed a magnet adapter to hold 20 neodymium magnets in the same positions as the poles of the original Sturmey Archer GH6 Dynohub ring magnet.  The first adapter I made was for 0.125" block magnets.  This adapter worked better than I expected, more than doubling the power output available from the original magnet.  Well, it turns out that, after calibrating my current monitor, the upgraded magnets work even better than I first thought, nearly tripling the dynamo's power output.  In a short-circuit current reading on my multimeter, the upgraded hub saturates at around 1.0A!  Running two power LEDs in series, it gets to around 0.9A at about 45 km/h.  

Great.  However, this is actually probably more power than I need.  While white power LEDs can take well over 1A, the brightest red LEDs I want to use in tail lamps are rated at a maximum of 700 mA.  I think I need to tone it down a bit and try to get that saturation current below 0.7A.  The other incentive to do this is to reduce the drag. Spinning the wheel by hand in the testing jig doesn't give the impression there is much more resistance from the new magnets, but spinning the armature itself on the upgraded hub is much harder than with the original magnet.  So, I designed a new adapter that places 20 0.0625 (1/16)" magnets with the same spacing from the armature (about 0.04").  The sixteenth inch magnets are N40s instead of the N42 eighth inch magnets and they are a lot cheaper.  This time I got them from these fine folks, just north of Toronto.  As before, they are epoxied on with JB-Weld.

Dynohub magnet adapter with 1/16" Nd magnets in place

Here's how it fits over the armature
I fitted this into the Dynohub wheel on my motorized testing jig and measured the current at different speeds, logging the data with an Arduino:

Current versus speed of GH6 Dynohub

The 1/16" magnets seem to be just about right.  At 45 km/h the hub hits just over 700 mA, although it doesn't look like it's quite plateaued.   I might be wise to incorporate some back up current limiting circuitry to protect my red LED at very high speeds.  The resistance from the 1/16" magnets is about as much as you'd expect from a contemporary hub dynamo.  Based on these results, I think I'm going to stick with 1/16" magnets. They're cheaper, easier to install, offer less resistance and produce a peak current that is mostly suitable for the red LEDs I want to use.

The current design leaves the magnets a little exposed.  I doubt they'd ever come loose, even in the exceptional case of contact with the armature.  Still, I've designed a new version of the magnet holder that leaves them less exposed:

Proposed protective magnet holder

 It's more complicated than my current design, so will be more expensive.  The exterior machining is to reduce the weight, but it doubles the price. This should weigh about 190 grams.  Without the outside machining, it would weigh about 270 grams.  Not sure if the extra cost is worth the reduced weight...

I could also get it made out of aluminium, which would make it very light and a little less expensive.  The advantage of using steel is that the magnets stick to it and the attraction acts as a clamp while the epoxy sets.  An aluminium adapter would require that each magnet be individually clamped to prevent them from jumping out of their slots and sticking to one another.


Anonymous said...

There is a lot of great work here. I'm in a rush but I wanted to say great job with this. I'm a lab technician in a plant biotechnology lab and I'm very refreshed by the quality figures you have in here.

minisystem said...

My very first summer research job was in a plant biotechnology lab. I just wrapped up an 18 year career as a molecular cell biologist last October, which probably explains your observation about the figures. :)

Anonymous said...

You make it look so easy. I wondering if it's possible do same kind thing shimano hubor rewiring for more power and less voltages so switching power-supply and rectifier no longer need zener protection. That test jig is awesome, I use just chair and dremel-tool when testing high speed with dyno. Doing 50VAC is quite scary if bike gets loose