TSDZ2 displacing magnets on the BLDC rotor

stratohunter

10 mW
Joined
Apr 12, 2022
Messages
34
So I ran into some very rare issues with my 52V TSDZ2 and OSF 1.1.0 after 6 months and 1200km on it. Last time when I was riding up a small hill, the engine started to give horrible scratching and colliding noises all of a sudden. Although it did not quit working, I switched it off immediately and pedaled home. Initially I thought it was the blue gear, until I opened it up and found it in perfect shape. There was even no obvious sign of wear on it. Then after two days of debugging I finally narrowed it down to the BLDC motor. I took the BLDC motor apart and to my surprise the culprit was a displaced magnet, which had slid so much axially and radially, that it was rubbing against the stator.

I also discovered at least half of the 16 magnets were more or less moved. You can see it in the first image below, notice the gap between some of the magnets and the iron rim. When I was pulling the rotor out, one magnet slid off the rim right away, being the one shown in the second image below. I glued it back with epoxy and put everything back for the engine, problem solved. My engine was back to working, for the next 20 miles, until probably another magnet had just slid far enough to touch the stator. Now the engine is making scratching noises again, though not as severe as the first time, and no noticeable loss of power output. It's still ridable but who knows what will happen after another 20 miles.

I can glue all magnets onto the rotor rim firmly if I want, but I'm a bit curious why I keep running into this issue. I had only exceeded 700 watts once, in a test ride 2 months ago in freezing weather. For most of the time I stayed under 600w (I set 12A for motor current in my trips), and the weather had almost never exceeded 10 degC or 50 degF here. I saw brownish stuff on the rotor but doubt whether I really had the chance to overheat it? Afterall, the weakest blue gear was in perfect condition at the moment the BLDC motor failed. Also, I often pedal beyond 90 rpm, could this overrev the motor and throw the magnets out?

I really hope this little engine can come back to work again, like in the past 6 months. It was a joy to ride, when not making troubles. :roll:
 

Attachments

  • rotor1.png
    rotor1.png
    1.2 MB · Views: 320
  • rotor2.png
    rotor2.png
    1.3 MB · Views: 320
Is the motor a36v or 48v version? Maybe you oversped it?

The windings are not dark, so I don't think that you overheated it badly. The usual problem when the motor is overheated is that the magnets demagnetize and the motor gets weak.

My initial thought is that it is probably just a bad unit from the factory. Being an inrunner, it will be sensitive to bad adhesive and the magnets will be flung out.

No matter the culprit, you can try to reglue the magnets starting with a good clean and using a high quality epoxy. Alternatively you can buy a new motor.
This time around it's probably a good idea to run the temp sensor so you know if you are cooking it.
 
The motor is a 48v version, and I did use field weakening frequently, as my cadence is most usually around 90. The field weakening engages at a cadence of around 82 if the motor is working hard. So I was often over speeding the motor for about 10%.

Today I took the rotor out again and tested the strength of the magnets with a steel wrench, and found that the magnet came loose last time had lost some of its strength. I did some reading on Google and learned that excessive vibration/collision can also weaken magnets, so it's reasonable.

My next thing is to bring back the full strength of the magnets if I can. Spending $120 for a new BLDC motor sounds a bit too much for me. The magnets are in an odd trapezoid shape thus hard to get replacements. Clearly it's an anti-flinging design but somehow didn't work as nicely. It'll be great if I can re-magnetize the magnets at home, but I don't find lots of people doing that. I wonder if a really strong neodymium magnet, or even an electromagnet, will do the job.

I have just done filling thermal pads into the gap between the BLDC motor and the mount. And yes I will also add a temp sensor.
 
Back
Top