Bafang/8Fun SWXK motor winding failure

December 2012

When my Bafang SWXB failed (see here), it took me some weeks to get replacement nylon gears. Luckily I had some Bafang SWXK 20” front motors in stock so was able to quickly lace up a new wheel and get back on the road. 44km later, however, I was broken down again when the new SWXK failed.

Bafang / 8fun SWXK electric hub motor

Coming home from Mt Nebo, just as I reached the crest of the last big hill, the motor suddenly lost power. When I pulled over to the side I found that throttling the motor would result in nothing more than some shakes and judders.

I thought the problem might be the controller, but after I got home I found other controllers would not make it work. Interestingly I found that one model of sensorless controller (KU63 from BMSBattery.com) would drive the motor around slowly and noisily. The problem was in the motor.

Once I was able to obtain a tool from Greenbikekit.com, I was able to open the motor. However, this was more trouble than I expected. I had watched a youtube video of an SWXK being opened, which showed that there was a grub screw locking the side-plate thread, and that the side-plate thread was left-handed (sadly the video has been taken down).

Here's the SWXK opening tool in place

undoing one of the grub screws

Unfortunately for me, my motor was different – I think it’s a later version. I removed a grub screw as advised, and then tried to rotate the side plate clockwise (LH thread). With great force I was only able to make a few millimetres movement, then decided that something was wrong – it was. This motor had 2 grub screws, one of which I had not noticed, and which was now ripped from its place and jammed into the thread sideways. With some drilling and levering I was able to remove the offending grub screw. The side plate on my motor had a right-hand thread as well (maybe the 2 grub screws was to stop the RH thread undoing in use?). The motor also had an o-ring beside the thread which was destroyed by my rough opening.

Once open, I found the motor looked fine. I connected it to a controller, throttle and battery and it ran fine as well. I wondered if there had simply been some stray piece of manufacturing stuff jammed in somewhere, which I had knocked out with dismantling.

I reassembed and reinstalled the motor on my bike to see if it was fixed. After another 60 or so km (including a lot of steep climbing), and a few seconds repeat of the shudering during that time, it finally failed very decisively during a major ascent of the mountain. This time (after a lot of steep pushing home) I dismantled it again more completely, and found a blackened coil in the motor windings.

See the burnt windings at the top

It seems there is some sort of a short in the windings which makes contact when the motor is warmed up. I don’t understand why this would blacken the wire.

I am wondering if I could rewind this motor to get it back in action. I imagine I could unwind the existing wire very carefully with photos and labelling, to reverse-engineer, then rewind with good quality new wire. I note that the original windings have not been varnished after winding – perhaps this would help improve the insulation?

Extra photos of dismantled motor

Here are some more photos of the Bafang SWXK being dismantled. 

The wire plate has been removed (by unscrewing 3 philips head screws) to show the entry chamber for the 3 phase wires (which have been cut off)

The phase wires enter thru a rubber grommet which slips out of the wire plate

When the side plate has been screwed off (with the special tool) you can see the large wheel bearing

Here is the large wheel bearing viewed from the outside