5371
General Mach Discussion / Re: Steppers have no torque.
« on: February 05, 2018, 09:48:58 PM »
Hi,
I have read that stepper magnets can be irrevocably demagnetised if the stepper is disassembled. It always makes me wonder how they make them I the first place?
Do they assemble them real quick while the magnets are still hot and before all the goodness drains out of them?
Any permanent magnet can be demagnetized if a sufficient magnetizing force is applied contrary to its field, if memory serves the parameter which measures that threshold
is called coercivity. Ferrite magnets have vey useful coercivity, about 1/2-3/4 Tesla but rare earth magnets are better again 1-1.5 Tesla. You have to try that much
harder to demagnetize a rare earth magnet.
I think the risk comes when you disassemble a stepper the magnet is now dislocated from its soft iron magnetic circuit. It doesn't suddenly lose magnetism it just that
it doesn't have the easy magnetic conditions it is accustomed to. This is in effect a counter field equal to the remenance of its own field. In the case of ferrites
the difference between it remenance and its coercivity is fairly small, about 0.1 Tesla. Any additional demagnetizing flux due to a current in a winding could cause a
ferrite to demagnetize. Rare earth magnets the difference between remanance and coercivity is about 1/4-1/3 Tesla and so unlikely to encounter flux levels
sufficient to demagnetize certainly by accident.
My contention is that unless a ferrite is removed from its soft iron magnetic circuit or was deliberately spun in generator mode well in excess of it ratings that it
will not demagnetize and not therefore have a 'lifespan'. Whether my contention is correct is another matter!
Craig
I have read that stepper magnets can be irrevocably demagnetised if the stepper is disassembled. It always makes me wonder how they make them I the first place?
Do they assemble them real quick while the magnets are still hot and before all the goodness drains out of them?
Any permanent magnet can be demagnetized if a sufficient magnetizing force is applied contrary to its field, if memory serves the parameter which measures that threshold
is called coercivity. Ferrite magnets have vey useful coercivity, about 1/2-3/4 Tesla but rare earth magnets are better again 1-1.5 Tesla. You have to try that much
harder to demagnetize a rare earth magnet.
I think the risk comes when you disassemble a stepper the magnet is now dislocated from its soft iron magnetic circuit. It doesn't suddenly lose magnetism it just that
it doesn't have the easy magnetic conditions it is accustomed to. This is in effect a counter field equal to the remenance of its own field. In the case of ferrites
the difference between it remenance and its coercivity is fairly small, about 0.1 Tesla. Any additional demagnetizing flux due to a current in a winding could cause a
ferrite to demagnetize. Rare earth magnets the difference between remanance and coercivity is about 1/4-1/3 Tesla and so unlikely to encounter flux levels
sufficient to demagnetize certainly by accident.
My contention is that unless a ferrite is removed from its soft iron magnetic circuit or was deliberately spun in generator mode well in excess of it ratings that it
will not demagnetize and not therefore have a 'lifespan'. Whether my contention is correct is another matter!
Craig