Monday, November 28, 2011

Sturmey Archer Dynohub magnet upgrade: early results

My idea for a GH6 magnet upgrade seems to be working.  I had an adapter machined out of mild steel to space out 20 0.25" x 0.75" x 0.125" neodymium magnets, which I attached with J-B Weld.  It took a while to fit it into the hub shell and I ultimately had to steal a spacer clip from another GH6 to make it fit without the armature rubbing.  Unfortunately, I didn't document any of this with pictures, so you'll just have to take my word for it for now!  The cogging effect is quite strong, but once mounted in the fork the wheel spins fairly easily. Not sure how this will translate into actual drag while riding.  

I connected it to my test circuit, which consists of an Arduino controlled buck converter driving two LEDs in series at 100% duty cycle.  The series forward voltage of the two LEDs is about 5V so you can infer power from the current but I didn't actually measure Vf, which goes up modestly as current rises. The data was acquired on my motorized testing jig. I'm using the Arduino to log the current at different speeds. Speed is calculated using zero-cross detection to measure the frequency of the pulses coming from the hub. My jig has crappy speed control so all I can do is ramp it up and slow it down and catch the current at each speed. At each speed I collect the average current of two complete wheel rotations (again, as measured using hub pulse frequency) four times and plot the average of that. Error bars are standard deviation of the mean.

The hub seems to saturate around 750 mA, although my multimeter is reading 900-1000 mA at higher speeds (update: confirmed with another multimeter that the hub is indeed saturating at 900 mA, so my high side current monitor is not linear :( grrrr!).  Thus far, it looks like the magnet upgrade is a success. I was worried, based on the accounts of others, that the armature would saturate at a much lower current, but that doesn't seem to be the case here.  I'm going to try to use 1/16" block magnets to see if I can get a similar power output with less drag.  There is a chance that the 1/8" magnets are overkill, causing unnecessary drag and reducing efficiency.

Decreasing the gap between the magnets and the claw poles of the armature might also help make the hub more efficient. At the moment they are about 0.050-0.060", whereas the original magnet is probably closer to 0.010-0.020".

Will post photos of the upgrade as soon as I have them!

Update:  I found a photo of the adapter sticking to my fridge.

GH6 magnet adapter upgrade with Neodymium magnets in place
Update 2:  here it is mounted in the hub shell with the armature (the magnet cover plate has been removed)

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