The original phase wires (wires between the motor and controller) on my E-bike are way to thin for the currents they are handling. When I got the motor was equipped with 2 m long 1,5 mm² cables. In low-speed-high-torque situations for example steep hills or starts from a standstill the currents in the phase cable can be several times the battery current. My controller limits the battery current to 27 A but the phase current could sometimes approach 100 A. Doing some calculations with the resistivity ρ=1.68•10^{-8} Ωm giving the original cables a resistance of

[latex]

\frac{1.68\cdot 10^{-8}}{1.5\cdot 10^{-6}} = 0.0112 \: \Omega/m

[/latex]

Judging by the color I’m not really sure that the original cables are made of copper. They could just as well be made of some other metal which would result in even higher resistance. With the 2 m cables the current path to/from the motor is 4 m and the resistance (excluding the motor resistance) is about

[latex]

0.0112 \cdot 4 = 0.0448 \: \Omega

[/latex]

This may seem pretty low but when constantly running 100 A through this cable the voltage drop on the cables will be ~4,5 V and the losses will be about 450W. This would of course instantly cook the cables and luckily enough the current to the motor is not constant and this current levels will only appear for short periods of time at very low speeds.

Anyways, I decided to replace these cables for the thickest I could fit into the axle where the cables are fed into the motor, as well as shortening these cables as much as possible. What I’ve read is that 12 AWG cables (3.31 mm²) is the thickest you could get through the axle without stripping the insulation and adding something thinner. The original cables have a PTFE insulation which I think is a good idea since PTFE cables generally have thin insulation and is very resistant to heat and mechanical wear. I ordered a couple of meters of 12 AWG PTFE cable from Apex Jr which was tho only place I could find that sold this dimension of wires online in small quantities. This cables are definitely made of tin plated copper which can be seen on the cut surfaces.

Doing the same calculations with the new shorter and thicker cables

[latex]

\frac{1.68 \cdot 10^{-8}}{3.31 \cdot 10^{-6}} \cdot 1 = 0.005 \: \Omega

[/latex]

This will result in a voltage drop of 0.5 V and 50 W losses at 100A which is much more manageable. I guess this will give me some additional efficiency but its probably unnecessary with the current performance of my controller. Later this summer there will probably be a post about re-programming current limits and eventually upgrading the MOSFETs of my controller.

Taking the motor apart was easy, I first removed the side cover on the cable-side by removing the nine hex screws and then used a knife to cut the glue and pry the cover off. Before I did this I made a mark on both the cover and the hub so I can put it back exactly the same way. I’m not sure what tolerances are used when manufacturing these but I don’t want to risk a wobbly wheel.

Getting the wires through the axle was hard work and took me more than an hour, the method that worked for me was to put a thin wire through and the used it to pull through the phase and hall wires all at once. It helped a lot to grease the wires with soap to get them through. I kept the original hall sensor wires.