Up to now, my DIY e-Brompton was equipped with the Suzhou Bafang motor (newer "8-Fun" SWXK unit), and my DIY e-Merc (Brompton copy) was equipped with the little Tongxin / Nano (albeit for experimental purposes only). Both e-bikes are front wheel drive 36V machines, designed so that parts can be easily interchanged.
See my old build log for more information:
http://www.pedelecs.co.uk/forum/electric-bicycles/4682-diy-stage-1-received-bafang-tongxin-kits-photos.html#post59183
One crucial difference is that the settings of the controller firmware are quite conservative in the case of the Tongxin kit, so that the motor runs "safely" below its recommended max power. The Bafang motor is rated at 250W whereas the Tongxin is officially a 180W unit. Obviously, these figures are nominal and do not represent the actual output. For example, I currently enjoy the relatively high torque and power offered by the 8-Fun motor: 400W peak, and happy to cruise at 380W continuous (all measured in real time by the Cycle Analyst).
The Bafang motor would actually happily take more volts (i.e. from a 48V battery+controller) or more Amps (by pushing the 36V 15A controller to the max, or by using a bigger 20A controller), either way this would effectively result in delivering more power (i.e. beyond 400W peak). The Bafang is well-known to handle higher loads than its factory default, due to its robust construction (nylon-geared brushless no-Hall-sensors 250W unit, 255rpm for 16" wheel, 20km/h max). By replacing a single nylon gear (out of 3 in total) with a steel one, the motor would be noisier but it would be more reliable under load.
I decided to make an experiment by using the Tongxin motor with the controller currently configured for the Bafang motor. I would essentially "overclock" the little Tongxin by feeding it more Amps than recommended. Thanks to the 3-power-level switch, I would be able to give the little motor a break by using level 2 (70%) instead of level 1 (full power). I could re-configure the Tongxin's 6-MOSFET 15A-max controller from e-Crazyman (Shenzhen Sucteam, Infineon EB806-XC-C-3 board), using USB and the ParameterDesigner.exe software. However I decided to simply swap the front wheel on the Brompton, as all the hardware is compatible. "Simples" !
The Tongxin is a smaller, more fragile motor based on friction rollers rather than permanently interconnected planetary gears. There is the benign risk of slipping under excessive load, but there is also the more damaging risk of stressing one of the inner parts, such as the rather weak axle wall. Never mind, I decided to go ahead as the motor itself didn't cost much, and it was sitting unused in a shed up to now !
I originally widened my Brompton forks to accommodate the bulky 8-Fun motor (100mm), so I used large steel washers to keep the narrower Tongxin in place (80mm). I then simply connected the 3 phase wires directly to the controller...easy peasy.
RESULT:
I rode 11km with at least 5km of steep hills, and I intentionally pedaled lazily to let the motor take the strain. The little Tongxin happily took 360W continuous (380W peak), without noticeable overheating on the motor's shell. The outside temperature was about 25 Celsius, the temperature inside the controller bag was 40 degrees max...all normal !
The 10Ah LiFePO4 battery from Li Ping behaved normally, as expected. Even with 400W peak and 380W continuous I am way below the recommended discharge rate.
So what's next ? Well, I love how silent and light the Tongxin motor is. I will continue testing this configuration (hopefully not to destruction). I actually enjoy having to pedal harder: the power delivery of the Tongxin is much more "discrete" than the Bafang, which encourages putting-in extra human effort !
Note: the particular Tongxin model I own is a 190rpm motor designed for larger wheels (26"), so I effectively get less top-end speed with my tiny 16" wheels...maybe this explains why the motor tolerates higher levels of torque under load. Anyhow, be aware of the risks incurred when "overclocking" a motor !
