Is BLDC motor field weakening bad for the magnets?

My understanding is that field weakening is necessary to run a motor above it's rated speed by reducing the magnetic flux.
In a brushed motor the field winding current can be modified to reduce the magnetic flux, but with a brushless motor with permanent magnets, some electronic trickery is employed in the armature to push back the permanent flux, reducing back EMF and allowing higher speeds (less torque).
Is this bad for the permanent magnets?
Should I be judicious with the flux weakening settings on my controller?
I imagine the motor becomes less efficient and drains the battery disproportionately if pushed past its rated speed with flux weakening? Is this correct?

Field weakening is not bad for a motor's permanent magnets. Only if the magnet temperature rises above its maximum working temperature will the magnetic field from the windings be large enough to permanently lower the magnet strength. This is true with or without field weakening. The max working temperature can be as low as 80C for some grades of NdFeB magnets.

If your motor is a simple SPM (surface permanent magnet) machine (the most common type of ebike motor), then none of the field weakening current produces torque which means yes it is wasting power.

Is this bad for the permanent magnets?
Should I be judicious with the flux weakening settings on my controller?

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The field weakening leads to lower efficiency/higher temperature in most ebike motors and can demagnetize neodym during these conditions.

Yes, you should be careful with field weakening settings but it's not a high risk thing if you don't add crazy amounts of field weakening angle/current. what this is depends on motor and controller but if your motor starts getting hot after a field weakening parameter change then it's time to check that setting and probably back off a bit.

Had to chime in here, this is basically what pronghorn said but it was not overly clear :wink:

Thanks guys