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. Here’s the video:
|Here's the SWXK opening tool in place|
|undoing one of the grub screws|
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.
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|