Need help not shredding gears on small Bafang.

[drutledge]

I bought two super cheap Bafang G320.250 off of ebay to convert bikes for guests who want to tag along on e-bike rides. These are rated at 250W so they are not much. I have identical old (been in a box for at least 7 years?) Grin 20Amp controllers in each setup. One bike I gave a 42V 20Ah battery, the other got an 84V 10Ah one I had around. The 42V one is anemic (15mph top speed and slow off the line) but does fine. The 84V one shedded it's gears on the first big hill I came to; oops. I've ordered new gears but I obviously don't want this to happen again. I want the extra speed that the higher voltages will give me but need to not shred gears. I have an old programmable Grin Phaserunner around and was going to use that to limit the phase amps going into the motor. Any ideas on what these motors can handle safely? Anyone know what the old 20A Grin controllers put out?

Also as a side-note, I ran the numbers on Grin's motor simulator, using the smallest Bafang they have in the list. It shows that the motor amps (Phase amps?) are the same from a standing start (0 mph, full throttle) for either voltage. Is this correct? If phase amps are the same for different voltages, what should I be limiting on the Phaserunner controller to prevent shredded gears.

 

[amberwolf]

While I don't have the number you want, I'll post some possibly-useful generalizations:

There's three potential problems:

Heat, torque, shock loading.

A fourth is that different people pedal more, or less, when the motor is doing work, and if the one that failed had less human assist it would have seen more load and had to work harder, and be more vulnerable to damage.


If the higher voltage battery allows higher power output to the motor (or higher phase currents), it would create higher torque, and also create more waste heat in the motor. A hotter motor means hotter gears, and plastic is easier to reshape the higher it's temperature is. Greater torque on the gears while in this state makes it even more likely to deform or damage them.

On a hill, this would all be exacerbated by the more-continuous higher load.

When using a small geared motor like these, especially for any situations that go beyond their design capabilities, monitoring temperatures inside them is a good idea. They probably don't come with sensors, but you can add one, if you can fit one extra very thin wire (two is better, so the sensor has it's own ground, otherwise just share the one for the halls) in the axle wiring. IIRC the Phaserunner can monitor motor temperature and rollback power input as that goes up, to help prevent motor damage.


Shock loading can come from sudden motor power application. Some controllers have "soft start", which ramps up power to the motor a bit, vs "instant start" which just instantly applies however much power is demanded by the throttle (or PAS, if any) input. Soft-start is less likely to apply a sudden load to the motor gearing.

It can also come from the motor spinning freely (like if the wheel leaves the ground) and then suddenly slowing (like if the wheel touches the ground still spinning especially motor still running at higher speed than it will be once the load is reapplied). Potholes, curb jumping, etc., sometimes cause gear and/or clutch damage (which may not fail at that time). Probably not happening to your systems, but listed for completeness.