Weird motor issue (V8), any ideas?

[alcatraz]

Hi everyone...

I got a second V8 as a backup and it works great except the mileage isn't as good as my old one. Not a huge problem. It's meant to be used by my wife and I was testing it today by riding 10-20km on it at full speed. I didn't notice anything until after I met up with the wife telling her it works fine (typical).

When going slow and up a slope where the motor is producing torque at low speed I can feel a skip that could be once every revolution. It's like at that point there is no torque until the motor rolls past that point.

At speed it's impossible to feel anything but if you happen to be at that specific point and asking for torque it will jerk you forward.

Is there a wire or something that maybe is damaged? Like a hall sensor wire?  Luckily I have a working V8 I can swap parts with to failsearch and I'm not afraid to take things apart. I got a simple multimeter if I need to measure the circuits.

Thanks for reading and commenting.

Edited October 15, 2018 by alcatraz

[meepmeepmayer]

Maybe a magnet is missing or not magnetized in the motor rim? That would explain what you describe. Check if it really is once per revolution, maybe you can isolate a specific spot on the tire/rim assembly.

I don't see how a wiring issue could produce these symptoms. Then something would just work or not, but how would it produce periodic behavior?

Also, two wheels of the same model should behave identically, so you should get the same range (with the same rider). Maybe a missing/failing magnet throws off the hall sensor counting the magnets going by and you get shown a false (lower) range due to this while actually getting the right range? The difference would be tiny, though. Number of magnets divided by number of magnets +1, with something like 50 magnets thats around 2% difference in registered range (while real range is the same). Probably too small to notice, so is it something else? If you have too much time, you could do a very controlled test to see if both wheels show the same range for the same route or 20m difference per km. Maybe you and your wife ride slowly the same, suitable route next to each other.

Is the wheel new enough, can you return it? After all, something seems to be wrong with it. That would be the simplest solution.

If you want to or have to DIY, you could switch motors between wheels and see if the problem moves, to test the missing magnet theory (only thing I can come up with). But that's quite the operation.

[alcatraz]

Great idea to check the magnets. I will analyze some pictures of the motor before I open it. 

Are you familiar with the magnet attachment to the motor body? Are they glued onto it? Maybe one has come loose, become displaced or something.

If two of them touch won't that disrupt the polarity in that section? Sounds like a likely scenario.

Yes you guessed right, I'm not under warranty. Got all the stuff very cheap second hand here in China.

One thing I noticed is that the "new" wheel spins a tad less quiet and that there is a tiny bit more friction when spinning it with the wheel turned off.

Both my old and the second one have done around 4-5k km. My old one is perfect apart from a worn tire.

Range is maybe around 10-20% less on the second wheel. The owner charged it with a 2A charger instead of the original 1.5A so that could maybe explain things. Possibly bringing the voltages a bit too high. I plan to refurbish the battery during the winter. Keep all the good capacity cells and add original LG MH1 cells to build a 60cell battery for both machines. Will be a fun project.

[alcatraz]

Great idea to check the magnets. I will analyze some pictures of the motor before I open it. 

Are you familiar with the magnet attachment to the motor body? Are they glued onto it? Maybe one has come loose, become displaced or something.

If two of them touch won't that disrupt the polarity in that section? Sounds like a likely scenario.

Yes you guessed right, I'm not under warranty. Got all the stuff very cheap second hand here in China.

One thing I noticed is that the "new" wheel spins a tad less quiet and that there is a tiny bit more friction when spinning it with the wheel turned off.

Both my old and the second one have done around 4-5k km. My old one is perfect apart from a worn tire.

Range is maybe around 10-20% less on the second wheel. The owner charged it with a 2A charger instead of the original 1.5A so that could maybe explain things. Possibly bringing the voltages a bit too high. I plan to refurbish the battery during the winter. Keep all the good capacity cells and add original LG MH1 cells to build 60cell batteries for both machines. Also got a 3A charger I can set the end voltage on to 82-83V to increase the lifespan. Will be a fun project.

Edited October 15, 2018 by alcatraz

[meepmeepmayer]

Wow you really want to open the motor? Cool. Only remove one of the motor covers to keep the shape, and be very careful not to damage the very brittle hall sensor wires. @Rehab1 once opened a motor, there's a thread but I can't find it. Maybe that helps. Pls make pictures if you find anything in there

Pretty sure the magnets are glued on. You may be right about the polarity thing, I don't know but it sounds plausible.

If you have more friction in one wheel, that would explain the range difference. Either it's the ball bearings (or (more likely here) a consequence of a magnet issue. 2A vs. 1.5A charging shouldn't make a difference in battery life, both of that is far below problematic charging currents. But maybe here were different end voltages. I'd still bet on the increased motor resistance explanation.

[Rehab1]

7 hours ago, meepmeepmayer said:

Rehab1 once opened a motor, there's a thread but I can't find it. Maybe that helps. Pls make pictures if you find anything in there

Here is the thread for my ACM disassembly. Some of the info might provide insight when dissecting your V8 motor.

I used my business cards as spacers to maintain aligment between the motor windings and magnets.  

I also checked polarity of each magnet using a magnetic tester and labeled them. Btw...this motor was destroyed during my rebuild. 

 

 

 

 

Edited October 15, 2018 by Rehab1

[alcatraz]

Wow, that's a ridiculous small space between the magnets and coils. I almost can't imagine this is spinning at high rpm bouncing around like crazy without much suspension at 30km/h ?.

You guys are the best with your patient and informative replies. I will try to pay the kindness forward.?

 

[Rehab1]

17 minutes ago, alcatraz said:

Wow, that's a ridiculous small space between the magnets and coils. I almost can't imagine this is spinning at high rpm bouncing around like crazy without much suspension at 30km/h

When you disassemble a BLDC motor’s 2 main components (rotor and stator) you immediately realize the imposing strength manifested in the stator’s magnetic ring and the separation power induced on the rotor as it spins in precise equilibrium. 

 

47 minutes ago, alcatraz said:

You guys are the best with your patient and informative replies. I will try to pay the kindness forward.?

 

Thanks. Glad I can offer some knowledge occasionally. 

[v8nice]

On 10/16/2018 at 12:03 AM, Rehab1 said:

Here is the thread for my ACM disassembly. Some of the info might provide insight when dissecting your V8 motor.

I used my business cards as spacers to maintain aligment between the motor windings and magnets.  

I also checked polarity of each magnet using a magnetic tester and labeled them. Btw...this motor was destroyed during my rebuild. 

 

 

 

 

Hi,

 

Do you have the name.or.reference of hall sensor?

 

Thank you