Demagnetizing Mysteries Answered!

Hi,

Ok, I just flew in from San Jose from my visit to Lin Engineering, a
manufacturer of very high grade step motors. I asked the question
regarding motor de-magnetization and I got an unexpected answer from
their cheif motor magnetics designer.

It seems demagnetization of the rotor depends on motor frame size.
It's OK to disassemble a size 23 or smaller motor. DO NOT try it on a
frame 34 or 42 motor though.

The thing that makes the difference is the rotor tooth spacing. The
teeth on a size 23 motor are close enough together that the flux path
from tooth to tooth is enough to act as a magnetic "keeper". The
larger motors do not enjoy this advantage and WILL demagnetize if
removed from the stator.

I got enough magnetics equations to completely agree with their
conclusions. To add to that, there were several tales of accidentally
removing rotors from stators from size 34 motors that required
emergency runs to a facility that magnetizers big enough to restore
the rotor magnetization so that orders could be met.

On a side note. Their on-site size 23 motor magnitizer is a 4' by 4'
by 6' piece of equipment. The meters on it are sobering; 100A at
1,000V. That's a lot of power. All discharged into a water-cooled
coil about 6" OD, 3" ID.

They said someone a few years ago entertained themselves during lunch
by putting 6" nails in the magnetizing coil and pushing the "go"
button. The nails went thru the drywall ceiling 12' above and
embedded themselves in the 2X8s as if they had been nailed with a
hammer. The original magnetic rail gun I guess. Impressed me; fun
day, learned things I didn't know before.

Mariss