I have heard warnings over the years about damage to servo and stepper motors when improperly disassembled. The nature of the damage is related to diminished field strength of the permanent magnets due to loss of coupling of the field with a core (adjacent part of the motor is removed).

What specific motor architectures are affected by this? Magnets on the stator (predominantly brush DC motors) -OR- magnets on the rotor... both types?

Are there any references on the web that describe this?

 

I don't have any references handy for you, but whether this occurs depends upon the type of magnet material used, not on the type of motor. This is probably more commonly seen in servo motors because of the types of magnets which tend to get used in them. You would need to know the magnet material used before you would know whether any particular motor is subject to this.

As I don't recall for certain which magnet materials are subject this, I don't want to state which types are safe to disassemble and have someone ruin an expensive motor based on my statements. However, I do know that this does not happen with most low cost ceramic magnets, so there are a lot of PM motors which can be taken apart without damage to the magnets.

I have seen this actually happen when someone took apart a brushed DC servo motor (which was already scrap due to mechanical damage). Most of the flux was lost from the magnets immediately when the flux path was broken.

I should also state that there is another type of damage which can occur by taking a PM motor apart. Unless you hold the rotor very firmly (i.e. in a holding fixture), then once it is free from the bearings it will inevitably
be pulled very sharply to one side against the magnets. This can break, chip, or crack the magnets quite easily.

 

If you take PM motors apart, it's a good idea to put a wrap of mylar or PET from a 2 liter pop bottle around the armature to prevent chipping and dragging. Some fields are so strong that it's almost impossible to prevent scraping without this simple precaution. It's also good to tediously remove any magnetic "fur" that may have collected.

Regards

cww

 

Go to this link from an on-line book on magnetics:

http://www.consult-g2.com/course/chapter2/chapter.html

The second paragraph explains the phenomenon of self-demagnetization "in air" of some of the older magnetic materials in certain form factors.

The whole book is one of the best sources I've found for understanding practical magnetics.

Curt Wilson
Delta Tau Data Systems

 

Another great resource!
Worth tuning in tonight.

Regards

cww

 

DON'T worry about that, you just short circuit the magnet rotor by wraping with metal sheet but should be over lap each other...

 

Generally, only very old motors with ferrit magnets are capable to be demagnitized. Modern magnets are samarium-cobalt and could not be
demaged by disassembling motor. Look in the motor service manual for exact information.

Mike Virgiliev.