One of the things that can happen to brushless motors, specially outrunners, is broken off magnet. Stronger crash, or even extensive vibrations, can cause one one small magnets located in motors bell to tear off. After all, they are only glued to rotating part of motor.
- If magnet stays inside motor, it will either rattle, or more probably block motor completely. It will stop spinning, and will not start again. Do not try to force it, it will only get worse,
- If magnet will somehow leave motor interior, motor will usually continue to spin. More, multirotor or a fixed wing plane will be flyable. Of course, motor will loose some power, there might be some jerkiness, excess vibrations, but it will keep to rotate.
When case one happens, you can easily see it happened: motor blocked completely or very rattly. Immediately disconnect power, magical smoke is very near. In case two, motor when pushed with finger will spin, but jumpy, like this:
Plus, due to weight imbalance, motor will be a source of huge vibrations.
Can I fix a motor with broken off magnet?
Yes, if you have a magnet, it can be glued in place. If it stayed inside motor, you already have it. Just remove shaft clip and pull motor bell. If you have lost broken off magnet, you could get a new one. Maybe to buy it, but better option will be different damaged motor as a donor. The same type of course.
And how to put it in place? Glue is the answer. In case of hurry even CA glue is an option. But, if you have little more time, there are better glues for the job. 10 minutes epoxy for example. Or, specialized anaerobic glue like Loctitle 603 or Loctite 638. Specially, when bell has grooves to fit magnets.
Whole process is nice and easy: clean magnet and bell, apply drop of glue, put magnet in place, let is cure.